U.S. patent application number 12/606986 was filed with the patent office on 2010-05-06 for tractor with improved valve system.
This patent application is currently assigned to Western Well Tool, Inc.. Invention is credited to Duane Bloom, Robert Levay, Norman Bruce Moore.
Application Number | 20100108387 12/606986 |
Document ID | / |
Family ID | 22949395 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100108387 |
Kind Code |
A1 |
Bloom; Duane ; et
al. |
May 6, 2010 |
TRACTOR WITH IMPROVED VALVE SYSTEM
Abstract
A hydraulically powered tractor includes an elongated body, two
gripper assemblies, at least one pair of aft and forward propulsion
cylinders and pistons, and a valve system. The valve system
comprises an inlet control valve, a two-position propulsion control
valve, a two-position gripper control valve, two cycle valves, and
two pressure reduction valves. The inlet control valve spool
includes a hydraulically controlled deactivation cam that locks the
valve in a closed position, rendering the tractor non-operational.
The propulsion control valve is piloted on both ends by fluid
pressure in the gripper assemblies. The propulsion control valve
controls the distribution of operating fluid to and from the
propulsion cylinders, such that one cylinder performs a power
stroke while the other cylinder performs a reset stroke. Each end
of the gripper control valve is piloted by a source of
high-pressure fluid selectively admitted by one of the cycle
valves. The gripper control valve controls the distribution of
operating fluid to and from the gripper assemblies. The cycle
valves are spring-biased and piloted by fluid pressure in the
propulsion cylinders, so that the gripper control valve shifts only
after the cylinders complete their strokes. The pressure reduction
valves limit the pressure within the gripper assemblies. These
valves are spring-biased and piloted by the pressure of fluid
flowing into the gripper assemblies. Some or all of the valves
include centering grooves on the landings of the spools, which
reduce leakage and produce more efficient operation. The propulsion
control and gripper control valves include spring-assisted detents
to prevent inadvertent shifting.
Inventors: |
Bloom; Duane; (Anaheim,
CA) ; Moore; Norman Bruce; (Aliso Viejo, CA) ;
Levay; Robert; (Yorba Linda, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
Western Well Tool, Inc.
Anaheim
CA
|
Family ID: |
22949395 |
Appl. No.: |
12/606986 |
Filed: |
October 27, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12046283 |
Mar 11, 2008 |
7607495 |
|
|
12606986 |
|
|
|
|
11717467 |
Mar 12, 2007 |
7353886 |
|
|
12046283 |
|
|
|
|
11418546 |
May 3, 2006 |
7188681 |
|
|
11717467 |
|
|
|
|
10759664 |
Jan 19, 2004 |
7080700 |
|
|
11418546 |
|
|
|
|
10004965 |
Dec 3, 2001 |
6679341 |
|
|
10759664 |
|
|
|
|
60250847 |
Dec 1, 2000 |
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Current U.S.
Class: |
175/51 |
Current CPC
Class: |
E21B 4/18 20130101; E21B
23/001 20200501; E21B 23/04 20130101 |
Class at
Publication: |
175/51 |
International
Class: |
E21B 4/04 20060101
E21B004/04 |
Claims
1. A tractor assembly, comprising a tractor for moving within a
borehole, said tractor configured to be powered by a pressurized
operating fluid, said tractor comprising: an elongated body having
a thrust-receiving portion longitudinally fixed with respect to
said body, said body having an internal passage configured to
receive the operating fluid; a gripper assembly longitudinally
movably engaged with said body, said gripper assembly having an
actuated position in which said gripper assembly limits relative
movement between said gripper assembly and an inner surface of said
borehole, and a retracted position in which said gripper assembly
permits substantially free relative movement between said gripper
assembly and said inner surface, said gripper assembly configured
to be actuated by receiving operating fluid from said internal
passage of said body; a valve system housed within said body, said
valve system configured to receive operating fluid from said
internal passage of said body and to selectively control the flow
of operating fluid to at least one of said gripper assembly and
said thrust-receiving portion; a pressure reduction valve; a first
gripper fluid passage extending from said valve system to said
pressure reduction valve; and a second gripper fluid passage
extending from said pressure reduction valve to said gripper
assembly; wherein said pressure reduction valve is configured to
provide a flow path for operating fluid to flow from said first
gripper fluid passage to said second gripper fluid passage when the
pressure within said second gripper fluid passage is below a
threshold, and wherein said pressure reduction valve is configured
to prevent fluid from flowing from said first gripper fluid passage
to said second gripper fluid passage when the pressure within said
second gripper fluid passage is above said threshold.
Description
CLAIM FOR PRIORITY
[0001] This application is a continuation of and claims priority to
U.S. application Ser. No. 12/046,283, filed Mar. 11, 2008, now U.S.
Pat. No. 7,607,495, which is a continuation of U.S. application
Ser. No. 11/717,467, filed Mar. 12, 2007, now U.S. Pat. No.
7,353,886, which is a continuation of U.S. application Ser. No.
11/418,546, filed May 3, 2006, now U.S. Pat. No. 7,188,681, which
is a continuation of U.S. patent application Ser. No. 10/759,664,
filed Jan. 19, 2004, now U.S. Pat. No. 7,080,700, which is a
continuation of U.S. application Ser. No. 10/004,965, filed Dec. 3,
2001, now U.S. Pat. No. 6,679,341, which claims the benefit under
35 U.S.C. .sctn.119(e) of U.S. Provisional Patent Application Ser.
No. 60/250,847, filed Dec. 1, 2000.
INCORPORATION BY REFERENCE
[0002] This application incorporates by reference the entire
disclosures of (1) U.S. Pat. No. 6,347,674 to Bloom et al.; (2)
U.S. Pat. No. 6,241,031 to Beaufort et al.; (3) U.S. Pat. No.
6,003,606 to Moore et al.; (4) U.S. Pat. No. 6,464,003 to Bloom et
al.; (5) U.S. Provisional Patent Application Ser. No. 60/250,847,
filed Dec. 1, 2000; and (6) U.S. Pat. No. 6,715,559 to Bloom et
al.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates generally to tractors for moving
equipment within passages.
[0005] 2. Description of the Related Art
[0006] The art of moving equipment through vertical, inclined, and
horizontal passages plays an important role in many industries,
such as the petroleum, mining, and communications industries. In
the petroleum industry, for example, it is often required to move
drilling, intervention, well completion, and other forms of
equipment within boreholes drilled into the earth.
[0007] One method for moving equipment within a borehole is to use
rotary drilling equipment. In traditional rotary drilling, vertical
and inclined boreholes are commonly drilled by the attachment of a
rotary drill bit and/or other equipment (collectively, the "Bottom
Hole Assembly" or BHA) to the end of a rigid drill string. The
drill string is typically constructed of a series of connected
links of drill pipe that extends between ground surface equipment
and the BHA. A passage is drilled as the drill string and drill bit
are together lowered into the earth. A drilling fluid, such as
drilling mud, is pumped from the ground surface equipment through
an interior flow channel of the drill string to the drill bit. The
drilling fluid is used to cool and lubricate the bit, and only
recently for drilling to remove debris and rock chips from the
borehole, which are created by the drilling process. The drilling
fluid returns to the surface, carrying the cuttings and debris,
through the annular space between the outer surface of the drill
pipe and the inner surface of the borehole. As the drill string is
lowered or raised within the borehole, it is necessary to
continually add or remove links of drill pipe at the surface, at
significant time and cost.
[0008] Another method of moving equipment within a borehole
involves the use of a downhole tool, such as a tractor, capable of
gripping onto the borehole and thrusting both itself and other
equipment through it. Such tools can be attached to rigid drill
strings, but can also be used in conjunction with coiled tubing
equipment. Coiled tubing equipment includes a non-rigid, compliant
tube, referred to herein as "coiled tubing," through which
operating fluid is delivered to the tool. The operating fluid
provides hydraulic power to propel the tool and the equipment and,
in drilling applications, to lubricate the drill bit. The operating
fluid also can provide the power for gripping the borehole. In
comparison to rotary equipment, the use of coiled tubing equipment
in conjunction with a tractor should be generally less expensive,
easier to use, less time consuming to employ, and should provide
more control of speed and downhole loads. Also, a tractor, which
thrusts itself within the passage and pushes and pulls adjoining
equipment and coiled tubing, should move more easily through
inclined or horizontal boreholes. In addition, due to its greater
compliance and flexibility, the coiled tubing permits the tractor
to perform much sharper turns in the passage than rotary
equipment.
[0009] A tractor can be utilized for drilling boreholes as well as
many other applications, such as well completion and production
work for producing oil from an oil well, pipeline installation and
maintenance, laying and movement of communication lines, well
logging activities, washing and acidizing of sands and solids,
retrieval of tools and debris, and the like.
[0010] One type of tractor comprises an elongated body securable to
the lower end of a drill string. The body can comprise one or more
connected shafts in addition to a control assembly housing or valve
system. This tractor includes at least one anchor or gripper
assembly adapted to grip the inner surface of the passage. When the
gripper assembly is actuated, hydraulic power from operating fluid
supplied to the tractor via the drill string can be used to force
the body axially through the passage. The gripper assembly is
longitudinally movably engaged with the tractor body, so that the
body and drill string can move axially through the passage while
the gripper assembly grips the passage surface. A gripper assembly
can transmit axial and even torsional loads from the tractor body
to the borehole wall. Several highly effective designs for a
fluid-actuated gripper assembly are disclosed in U.S. Pat. No.
6,464,003, which is incorporated by reference herein. In one
design, the gripper assembly includes a plurality of flexible toes
that bend radially outward to grip onto the passage surface by the
interaction of ramps and rollers.
[0011] Some tractors have two or more sets of gripper assemblies,
which permits the tractor to move continuously within the passage.
Forward longitudinal motion (unless otherwise indicated, the terms
"longitudinal" and "axial" are herein used interchangeably and
refer to the longitudinal axis of the tractor body) is achieved by
powering the tractor body forward with respect to an actuated first
gripper assembly (a "power stroke" with respect to the first
gripper assembly), and simultaneously moving a retracted second
gripper assembly forward with respect to the tractor body (a "reset
stroke" of the second gripper assembly). At the completion of the
power stroke with respect to the first gripper assembly, the second
gripper assembly is actuated and the first gripper assembly is
retracted. Then, the tractor body is powered forward while the
second gripper assembly is actuated (a power stroke with respect to
the second gripper assembly), and the retracted first gripper
assembly executes a reset stroke. At the completion of these
respective strokes, the first gripper assembly is actuated and the
second gripper assembly is retracted. The cycle is then repeated.
Thus, each gripper assembly operates in a cycle of actuation, power
stroke, retraction, and reset stroke, resulting in longitudinal
motion of the tractor. A number of highly effective tractor designs
utilizing this configuration are disclosed in U.S. Pat. No.
6,003,606 to Moore et al., which discloses several embodiments of a
tractor known as the "Puller-Thruster Downhole Tool;" U.S. Pat. No.
6,241,031 to Beaufort et al., which discloses an
"Electro-Hydraulically Controlled Tractor;" and U.S. Pat. No.
6,347,674 to Bloom et al., which discloses an "Electrically
Sequenced Tractor" ("EST").
[0012] The power required for actuating the gripper assemblies,
longitudinally thrusting the tractor body during power strokes, and
longitudinally resetting the gripper assemblies during reset
strokes may be provided by pressurized operating fluid delivered to
the tractor via the drill string--either a rotary drill string or
coiled tubing. For example, the aforementioned Puller-Thruster
Downhole Assembly includes inflatable engagement bladders and uses
hydraulic power from the operating fluid to inflate and radially
expand the bladders so that they grip the passage surface.
Hydraulic power is also used to move forward cylindrical pistons
residing within sets of propulsion cylinders slidably engaged with
the tractor body. Each set of cylinders is secured with respect to
a bladder, so that the cylinders and bladder move together
longitudinally. Each piston is longitudinally fixed with respect to
the tractor body. When a bladder is inflated to grip onto the
passage wall, operating fluid is directed to the proximal side of
the pistons in the set of cylinders secured to the inflated
bladder, to power the pistons forward with respect to the borehole.
The forward hydraulic thrust on the pistons results in forward
thrust on the entire tractor body. Further, hydraulic power is also
used to reset each set of cylinders when their associated bladder
is deflated, by directing drilling fluid to the distal side of the
pistons within the cylinders.
[0013] A tractor can include a valve system for, among other
functions, controlling and sequencing the distribution of operating
fluid to the tractor's gripper assemblies, thrust chambers, and
reset chambers. Some tractors, including several embodiments of the
Puller-Thruster Downhole Tool, are all-hydraulic. In other words,
they utilize pressure-responsive valves and no electrically
controlled valves. One type of pressure-responsive valve shuttles
between its various positions based upon the pressure of the
operating fluid in various locations of the tractor. In one
configuration, a spool valve is exposed on both ends to different
fluid chambers or passages. The valve position depends on the
relative pressures of the fluid chambers. Fluid having a higher
pressure in a first chamber exerts a greater pressure force on the
valve than fluid having a lower pressure in a second chamber,
forcing the valve to one extreme position. The valve moves to
another extreme position when the pressure in the second chamber is
greater than the pressure in the first chamber. Another type of
pressure-responsive valve is a spring-biased spool valve having at
least one end exposed to fluid. The fluid pressure force is
directed opposite to the spring force, so that the valve is opened
or closed only when the fluid pressure exceeds a threshold
value.
[0014] Other tractors utilize valves controlled by electrical
signals sent from a control system at the ground surface or even on
the tractor itself. For example, the aforementioned EST includes
both electrically controlled valves and pressure-responsive valves.
The electrically controlled valves are controlled by electrical
control signals sent from a controller housed within the tractor
body. The EST is preferred over all-hydraulic tractors for drilling
operations, because electrical control of the valves permits very
precise control over important drilling parameters, such as speed,
position, and thrust. In contrast, all-hydraulic tractors,
including several embodiments of the Puller-Thruster Downhole Tool,
are preferred for so-called "intervention" operations. As used
herein, "intervention" refers to re-entry into a previously drilled
well for the purpose of improving well production, to thereby
improve fuel production rates. As wells age, the rate at which fuel
can be extracted therefrom diminishes for several reasons. This
necessitates the "intervention" of many different types of tools.
Hydraulic tractors, as opposed to electrically controlled tractors,
are preferred for intervention operations because intervention, as
opposed to drilling, does not require precise control of speed or
position. The absence of electrically controlled valves makes
hydraulic tractors generally less expensive to deploy and
operate.
[0015] Tractors in combination with coiled tubing equipment are
particularly useful for intervention operations because, in many
cases, the wells were originally drilled with rotary drilling
equipment capable of drilling very deep holes. It is more expensive
to bring back the rotary equipment than it is to bring in a coiled
tubing unit. However, the coiled tubing unit may not be capable of
reaching extended distances within the borehole without the aid of
a tractor.
[0016] In one known design, exemplified by FIG. 3 of U.S. Pat. No.
6,003,606 (which discloses the Puller-Thruster Downhole Tool), a
tractor includes a spool valve whose spool has two main positions.
In one main position, the valve directs pressurized fluid to a
first gripper and to propulsion chambers of a first set of
propulsion cylinders. In this position of the spool, the pressure
is permitted to decrease in a second gripper and in reset chambers
of a second set of propulsion cylinders. In the other main
position, the valve does the opposite--it directs pressurized fluid
to the second gripper and propulsion chambers of the second set of
cylinders, and permits pressure to decrease in the first gripper
and in propulsion chambers of the first set of cylinders. The spool
of the valve is piloted by fluid pressure on both ends of the
spool. A pair of cycle valves selectively administers high pressure
to the ends of the spool. Each cycle valve is in turn piloted by
the pressure in the fluid passages to the cylinders and
grippers.
[0017] The Puller-Thruster all-hydraulic tractor design has proven
to be a major advance in the art of tractors for moving equipment
within boreholes. However, it operates most effectively within a
limited zone of parameters, including the pressure, weight, and
density of the operating fluid, the geometry of the tractor
components, and the total weight of the equipment that the tractor
must pull and/or push. Thus, it is desirable to provide an improved
design for a tractor, which will work within a much larger zone of
such parameters.
[0018] Another prior design consists of a wellbore tractor having
wheels that roll along the surface of the well casing. This design
is problematic because the wheels do not have the ability to
provide significant gripping force to move heavier downhole
equipment. Also, the wheels can lose traction in certain
conditions, such as in regions including sand.
[0019] A typical process of extracting hydrocarbons from the earth
involves drilling an underground borehole and then inserting a
generally tubular casing in the borehole. In order to access oil
reserves from a given underground region through which the well
passes, the casing must be opened within that region. In one
method, perforation guns are brought to the desired location within
the well and then utilized to cut openings through the casing wall
and/or the earth formation. Oil is then extracted through the
openings in the casing up through the well to the surface for
collection. Perforation guns can also be used to penetrate the
formation in an "open hole" to access desired oil reserves. An open
hole is a borehole without a casing. Perforation guns can be
ignited by different means, such as by pressurized operating fluid
or electricity provided through electrical lines ("e-lines").
However, the practice of igniting the perforation guns with e-lines
poses the risk of a spark leading to explosion and potential loss
of life. Thus, it is desirable to fully hydraulic tractors, without
e-lines, for operations that involve the use of perforation
guns.
[0020] Perforation guns are commonly used in conjunction with
rotary drilling equipment, due to the large weight of the guns.
Long strips of perforation guns can weigh up to 20000 pounds or
more. The rotary drilling equipment, consisting of the rigid drill
string formed from connected links of drill pipe, has been used
because of its ability to absorb the weight in tension. However,
the use of rotary equipment is very expensive and time-consuming,
due in part to the necessity of assembling and disassembling the
portions of drill pipe.
[0021] In the prior art, shafts designed for downhole tools used in
drilling and intervention applications have been formed from more
flexible materials, such as copper beryllium (CuBe). This is
because in drilling it is not uncommon to experience sharp turns,
and the tool is preferably capable of turning at sharp angles.
Also, shafts have been formed with relatively large internal
passages for the flow of operating fluid to the valves and other
equipment of the BHA. This is because in drilling the operating
fluid is typically drilling mud, which often contains larger solids
and necessitates a larger flow passage. The drilling mud is
preferred because it provides better lubrication to the drill bit
and more effectively carries the drill cuttings up through the
annulus back to the ground surface.
[0022] The shaft of a downhole tool typically must include multiple
internal passages (e.g., for fluid to the gripper assemblies,
propulsion chambers, and the other downhole equipment) that extend
along the shaft length. In the past, such passages have been formed
by gun-drilling, which is well known. Unfortunately, it is
typically not possible to gun-drill the entire length of the shaft
(in most applications, the length of a shaft for a downhole tool
can be anywhere in the range of 50 to 168 inches). The distance
that a passage can be gun-drilled is limited by (1) the inherent
length limitations of known gun-drilling tools, and (2) the
limitations imposed by the geometry and material characteristics of
the shaft. In the past, it has been necessary to limit the length
of gun-drilled passages in shafts of downhole tools to a relatively
great degree. This is because the larger internal passage required
for drilling mud leaves less room for other fluid passages. This
shortage of available "real estate" in the shaft requires higher
precision gun-drilling and increases the risk of inadvertent damage
to other passages caused by the gun-drilling process. These
problems are exacerbated by the fact that the more flexible
materials used for the shaft (e.g., CuBe) are softer, more
difficult to drill through, and more prone to damage.
[0023] The limitations on the length that passages can be
gun-drilled have necessitated forming the shafts from a plurality
of shaft portions of reduced length. The fluid passages are
gun-drilled in each shaft portion, and then the shaft portions are
attached to each other. Due in large part to the use of CuBe, shaft
portions have been attached together by electron beam welding.
Electron beam welding is favored because it maintains the
structural integrity of the material and of the fluid passages
contained therein. Unfortunately, electron beam welding is a very
expensive process. Most conventional welding processes have not
been used because they do not facilitate the welding together of
thick objects (i.e., the weld does not fuse completely through the
objects). In shaft manufacturing for downhole tools, it is
necessary to soundly fuse together all of the mating surfaces in
order to maintain zero leakage between the various internal fluid
passages and to provide structural integrity.
SUMMARY OF THE INVENTION
[0024] The present invention seeks to overcome the aforementioned
limitations of the prior art by providing a hydraulically powered
and substantially or completely hydraulically controlled tractor to
be used preferably with coiled tubing equipment. This invention
represents a major advancement in the art of tractors, and
particular in the art of well intervention tools. Compared to the
prior art, the preferred embodiments of the tractor of the
invention operate very effectively within a much larger zone of
parameters, such as the pressure, weight, and density of the
operating fluid, the geometry of the tractor components, and the
total weight of the equipment that the tractor must pull and/or
push.
[0025] As explained below, the tractor preferably includes a
two-position propulsion control valve that directs fluid to and
from the tractor's propulsion cylinders. In order for the
propulsion control valve spool to shift, two cycle valves are
provided for sensing the completion of the strokes of the
propulsion cylinders. The cycle valves shift in order to begin a
sequence of events that results in a fluid pressure force causing
the propulsion control valve spool to shift, so that the propulsion
cylinders can switch between their power and reset strokes.
However, rather than administering high pressure fluid directly to
the propulsion control valve spool, the cycle valves shift to send
a pressure force to an additional two-position valve. The
additional valve controls the flow of pressurized fluid to control
the position of the propulsion control valve spool. Thus, the
additional valve isolates the propulsion control valve from direct
interaction with the cycle valves. Advantageously, the shift action
of the additional valve creates a longer time lag between the shift
action of either cycle valve and the shift action of the propulsion
control valve spool. Due to the time lag, the propulsion cylinders
are more likely to complete their strokes before the propulsion
control valve shifts. In addition, better shifting can be effected
by spring-assisted detents on the propulsion control valve spool.
In the illustrated embodiments of the invention, the additional
valve comprises a gripper control valve that controls the
distribution of fluid to and from the gripper assemblies.
[0026] The preferred embodiments include an inlet control valve
having a feature that allows the valve to be hydraulically
restrained in a closed position, so that the tractor is assured of
being non-operational and in a non-gripping state. This permits the
operation of downhole equipment adjoined to the tractor or other
portions of the bottom hole assembly, such as perforation guns,
substantially without the risk of inadvertent movement of the
tractor. It also assures that the gripper assemblies are retracted
from the borehole surface during the operation of other downhole
equipment, thus reducing the risk of damage to the gripper
assemblies.
[0027] In addition, the invention provides a new method of
manufacturing the shafts that form the body of the tractor, which
is much less expensive than prior art shaft manufacturing methods.
According to this method, shaft portions are silver brazed together
to form the shafts. Silver brazing is less expensive than prior art
welding methods, such as electron beam welding. Also, the preferred
material characteristics and internal fluid passage configuration
permits longer gun-drilled holes. Advantageously, fewer shaft
portions are necessary.
[0028] In one aspect, the present invention provides a tractor
assembly comprising a tractor for moving within a borehole. The
tractor comprises an elongated body, first and second gripper
assemblies, first and second elongated propulsion cylinders, and a
valve system. The body has first and second pistons longitudinally
fixed with respect to the body. Each piston has aft and forward
surfaces configured to receive longitudinal thrust forces from
fluid from a pressurized source. The body has a flow passage.
[0029] Each gripper assembly is longitudinally movably engaged with
the body. Each gripper assembly has an actuated position in which
the gripper assembly limits relative movement between the gripper
assembly and an inner surface of the borehole, and a retracted
position in which the gripper assembly permits substantially free
relative movement between the gripper assembly and said inner
surface. Each gripper assembly is configured to be actuated by
fluid.
[0030] The first propulsion cylinder is longitudinally slidably
engaged with respect to the body and has an elongated internal
propulsion chamber enclosing the first piston. The first piston is
slidable within and fluidly divides the internal propulsion chamber
of the first cylinder into an aft chamber and a forward chamber.
Similarly, the second propulsion cylinder is longitudinally
slidably engaged with respect to the body and has an elongated
internal propulsion chamber enclosing the second piston. The second
piston is slidable within and fluidly divides the internal
propulsion chamber of the second cylinder into an aft chamber and a
forward chamber.
[0031] The valve system comprises a propulsion control valve and a
gripper control valve. The propulsion control valve has a first
position in which it provides a flow path for the flow of fluid to
the aft chamber of the first cylinder. The propulsion control valve
also has a second position in which it provides a flow path for the
flow of fluid to the aft chamber of the second cylinder. The
gripper control valve has a first position in which it provides a
flow path for the flow of fluid to the first gripper assembly. The
gripper control valve also has a second position in which it
provides a flow path for fluid to the second gripper assembly. When
the gripper control valve is in its first position and the
propulsion control valve is in its first position, the gripper
control valve must move from its first position to its second
position before the propulsion control valve can move from its
first position to its second position.
[0032] In another aspect, the present invention provides a method
of moving the tractor assembly (described immediately above) within
a borehole. The method comprises providing pressurized fluid from a
source, directing the pressurized fluid toward the gripper control
valve, directing the pressurized fluid toward the propulsion valve,
and, when the gripper control valve and propulsion control valves
are in their first positions, preventing the propulsion control
valve from moving from its first position to its second position
until the gripper control valve moves from its first position to
its second position.
[0033] In another aspect, the invention provides a tractor
assembly, comprising a tractor for moving within a borehole. The
tractor comprises an elongated body, first and second gripper
assemblies, first and second elongated propulsion cylinders, and a
valve system. The elongated body has first and second pistons
longitudinally fixed with respect to the body. Each of the pistons
has aft and forward surfaces configured to receive longitudinal
thrust forces from fluid from a pressurized source. The body also
has a flow passage. Each of the first and second gripper assemblies
is longitudinally movably engaged with the body, and has actuated
and retracted positions as described above. The first and second
propulsion cylinders are configured as described above.
[0034] The valve system comprises a propulsion valve and a control
valve. The propulsion valve has a first position in which it
provides a flow path for the flow of fluid to the aft chamber of
the first cylinder, and a second position in which it provides a
flow path for the flow of fluid to the aft chamber of the second
cylinder. The control valve has a first position in which it
provides a flow path for the flow of fluid to urge the propulsion
valve toward the first position of the propulsion valve. The
control valve has a second position in which it provides a flow
path for the flow of fluid to urge the propulsion valve toward the
second position of the propulsion valve. When the control valve and
the propulsion valve are in their first positions, the control
valve must move from its first position to its second position
before the propulsion valve can move from its first position to its
second position.
[0035] In another aspect, the invention provides a method of moving
the tractor assembly (described immediately above) within a
borehole. The method comprises providing pressurized fluid from a
source, directing the pressurized fluid toward the gripper control
valve, directing the pressurized fluid toward the propulsion valve,
and, when the control valve and the propulsion valve are in their
first positions, preventing the propulsion valve from moving from
its first position to its second position before the control valve
moves from its first position to its second position.
[0036] In another aspect, the invention provides a tractor
assembly, comprising a tractor for moving within a borehole. The
tractor is configured to be powered by operating fluid received
from a conduit extending from the tractor through the borehole to a
source of the operating fluid. The tractor comprises an elongated
body, a gripper assembly, a valve system housed within the body, a
pressure reduction valve, and first and second gripper fluid
passages. The elongated body has a thrust-receiving portion
longitudinally fixed with respect to the body. The body also has an
internal passage configured to receive the operating fluid from the
conduit. The gripper assembly is longitudinally movably engaged
with the body and has actuated and retracted positions as described
above. The valve system is configured to receive operating fluid
from the internal passage of the body and to selectively control
the flow of operating fluid to at least one of the gripper assembly
and the thrust-receiving portion. The first gripper fluid passage
extends from the valve system to the pressure reduction valve,
while the second gripper fluid passage extends from the pressure
reduction valve to the gripper assembly. The pressure reduction
valve is configured to provide a flow path for operating fluid to
flow from the first gripper fluid passage to the second gripper
fluid passage when the pressure within the first gripper fluid
passage is below a threshold. The pressure reduction valve is also
configured to prevent fluid from flowing from the first gripper
fluid passage to the second gripper fluid passage when the pressure
within the first gripper fluid passage is above the threshold.
[0037] In another aspect, the invention provides a method of moving
a tractor assembly within a borehole. The tractor assembly includes
a tractor having an elongated body, a gripper assembly
longitudinally movably engaged with the body, a valve system housed
within the body, and first and second gripper fluid passages. The
body has a thrust-receiving portion longitudinally fixed with
respect to the body. The body also has an internal passage
configured to receive the operating fluid from the conduit. The
gripper assembly has actuated and retracted positions as described
above, and is configured to be actuated by receiving operating
fluid from the internal passage of the body. The valve system is
configured to receive operating fluid from the internal passage of
the body and to selectively control the flow of operating fluid to
at least one of the gripper assembly and the thrust-receiving
portion. The first gripper fluid passage extends from the valve
system, and the second gripper fluid passage extends to the gripper
assembly. According to the method of this aspect of the invention,
pressurized fluid is provided from a source. The pressurized fluid
is permitted to flow from the first gripper fluid passage to the
second gripper fluid passage when the pressure within the first
gripper fluid passage is below a threshold. Fluid is prevented from
flowing from the first gripper fluid passage to the second gripper
fluid passage when the pressure within the first gripper fluid
passage is above the threshold.
[0038] In another aspect, the invention provides a tractor
assembly, comprising a tractor for moving within a borehole. The
tractor is configured to be powered by pressurized operating fluid
received from a conduit extending from the tractor through the
borehole to a source of the operating fluid. The tractor comprises
an elongated body, a gripper assembly longitudinally movably
engaged with the body, and a valve system housed within the body.
The body has a thrust-receiving portion longitudinally fixed with
respect to the body, and an internal passage configured to receive
the operating fluid from the conduit. The gripper assembly has
actuated and retracted positions as described above.
[0039] The valve system is configured to receive fluid from the
internal passage of the body and to selectively control the flow of
operating fluid to at least one of the gripper assembly and the
thrust-receiving portion. The valve system includes an entry
control valve controlling the flow of operating fluid from the
internal passage of the body into the valve system. The entry
control valve comprises a valve passage and a body movably received
therein. The valve passage has at least two secondary passages and
is configured to conduct the operating fluid between the secondary
passages. The entry control valve has first and third position
ranges in which it provides a flow path for operating fluid within
the valve system to flow through the entry control valve to the
exterior of the tractor, and in which the valve body prevents the
flow of operating fluid from the internal passage of the tractor
body into the valve system. The entry control valve also has a
second position range in which it provides a flow path for
operating fluid from the internal passage of the tractor body to
flow into the valve system, and in which the valve body prevents
the flow of operating fluid within the valve system to the exterior
of the tractor. The entry control valve is in its first position
range when the fluid pressure in the internal passage of the
tractor body is below a lower shut-off threshold. The entry control
valve is in the second position range when the fluid pressure in
the internal passage is above the lower shut-off threshold and
below an upper shut-off threshold. The entry control valve is in
the third position range when the fluid pressure in the internal
passage is above the upper shut-off threshold.
[0040] In another aspect, the invention provides a method of moving
a tractor assembly within a borehole, the tractor assembly
including a tractor having an elongated body and gripper assembly
configured as in the previously described aspect of the invention.
The tractor also comprises a valve system housed within the body,
the valve system including an entry control valve. According to the
method, fluid is received from the internal passage of the body,
and the flow of operating fluid from the internal passage of the
body into the valve system is controlled with the entry control
valve. The flow of operating fluid from the internal passage of the
body into the valve system is prevented with the entry control
valve when the fluid pressure in the internal passage of the body
is below a lower shut-off threshold and when the fluid pressure in
the internal passage is above an upper shut-off threshold. The flow
of operating fluid from the internal passage of the body into the
valve system is permitted when the fluid pressure in the internal
passage is above the lower shut-off threshold and below the upper
shut-off threshold.
[0041] In another aspect, the present invention provides a tractor
assembly, comprising a tractor for moving within a borehole. The
tractor is configured to be powered by pressurized operating fluid
received from a conduit extending from the tractor through the
borehole to a source of the operating fluid. The tractor comprises
an elongated body, a gripper assembly longitudinally movably
engaged with the body, and a valve system. The elongated body has a
thrust-receiving portion longitudinally fixed with respect to the
body. The body also has an internal passage configured to receive
the operating fluid from the conduit. The gripper assembly has
actuated and retracted positions as described above.
[0042] The valve system of the tractor is configured to receive
fluid from the internal passage of the body and to selectively
control the flow of operating fluid to at least one of the gripper
assembly and the thrust-receiving portion. The valve system
includes an entry control valve controlling the flow of operating
fluid from the internal passage of the body into the valve system.
The entry control valve comprises a housing defining a valve
passage, a body movably received within the passage, and at least
one spring. The housing has at least two side passages, the valve
passage being configured to conduct the operating fluid between the
side passages. The valve body has a first surface configured to be
exposed to operating fluid from the internal passage of the tractor
body, the first surface being configured to receive a longitudinal
pressure force in a first direction. The valve body has first and
third position ranges in which the body provides a flow path for
operating fluid within the valve system to flow through the entry
control valve to the exterior of the tractor, and in which the
valve body prevents the flow of operating fluid from the internal
passage of the body into the valve system. The valve body has a
second position range between the first and third position ranges
in which the valve body provides a flow path for operating fluid
from the internal passage of the tractor body to flow into the
valve system, and in which the valve body prevents the flow of
operating fluid within the valve system to the exterior of the
tractor.
[0043] The at least one spring biases the valve body in a direction
opposite to that of the pressure force received by the first
surface of the valve body, such that the magnitude of the fluid
pressure in the internal passage determines the deflection of the
at least one spring and thus the position of the valve body. The at
least one spring is configured so that the valve body occupies a
position within the first position range when the fluid pressure in
the internal passage of the tractor body is below a lower shut-off
threshold, so that the valve body occupies a position within the
second position range when the fluid pressure in the internal
passage is above the lower shut-off threshold and below an upper
shut-off threshold, and so that the valve body occupies a position
within the third position range when the fluid pressure in the
internal passage is above the upper shut-off threshold.
[0044] In another aspect, the invention provides a tractor
assembly, comprising a tractor for moving within a borehole while
connected to an injector by a drill string. The tractor comprises
an elongated body, first and second gripper assemblies, elongated
first and second propulsion cylinders, and a valve system. The body
has first and second pistons longitudinally fixed with respect to
the body. Each of the pistons has aft and forward surfaces
configured to receive longitudinal thrust forces from fluid from a
pressurized source. The body also has a flow passage. The first
gripper assembly is longitudinally movably engaged with the body
and has actuated and retracted positions as described above.
Similarly, the second gripper assembly is longitudinally movably
engaged with the body and has actuated and retracted positions as
described above. The first propulsion cylinder is longitudinally
slidably engaged with respect to the body. The first cylinder has
an elongated internal propulsion chamber enclosing the first
piston. The first piston is slidable within and fluidly divides the
internal propulsion chamber of the first cylinder into an aft
chamber and a forward chamber. Similarly, the second propulsion
cylinder is longitudinally slidably engaged with respect to the
body. The second cylinder has an elongated internal propulsion
chamber enclosing the second piston. The second piston is slidable
within and fluidly divides the internal propulsion chamber of the
second cylinder into an aft chamber and a forward chamber.
[0045] The valve system of the tractor comprises a propulsion
control valve and a gripper control valve. The propulsion control
valve has a first position in which it provides a flow path for the
flow of fluid to the aft chamber of the first cylinder, and a
second position in which it provides a flow path for the flow of
fluid to the aft chamber of the second cylinder. The gripper
control valve has a first position in which it provides a flow path
for the flow of fluid to the first gripper assembly, and a second
position in which it provides a flow path for fluid to the second
gripper assembly. The speed of movement of the tractor is
controlled by the pressure and flow rate of the operating fluid and
the tension exerted on the tractor by the drill string.
[0046] In another aspect, the invention provides a tractor
assembly, comprising a tractor for moving within a borehole. The
tractor comprises an elongated body, a first gripper assembly
longitudinally movably engaged with the body, an elongated first
propulsion cylinder longitudinally slidably engaged with respect to
the body, and a valve system. The body has first and second pistons
longitudinally fixed with respect to the body. Each of the pistons
has aft and forward surfaces configured to receive longitudinal
thrust forces from fluid from a pressurized source. The body also
has a flow passage. The first gripper assembly has actuated and
retracted positions as described above. The first propulsion
cylinder has an elongated internal propulsion chamber enclosing the
first piston. The first piston is slidable within and fluidly
divides the internal propulsion chamber of the first cylinder into
an aft chamber and a forward chamber.
[0047] The valve system comprises a propulsion valve and a control
valve. The propulsion valve has a first position in which it
provides a flow path for the flow of fluid to the aft chamber of
the first cylinder, and a second position in which it does not
provide a flow path for the flow of fluid to the aft chamber of the
first cylinder. The control valve has a first position in which it
provides a flow path for the flow of fluid to urge the propulsion
valve toward the first position, and a second position in which it
provides a flow path for the flow of fluid to urge the propulsion
valve toward the second position. When the control valve and the
propulsion valve are in their first positions, the control valve
must move from its first position to its second position before the
propulsion valve can move from its first position to its second
position.
[0048] For purposes of summarizing the invention and the advantages
achieved over the prior art, certain objects and advantages of the
invention have been described above and as further described below.
Of course, it is to be understood that not necessarily all such
objects or advantages may be achieved in accordance with any
particular embodiment of the invention. Thus, for example, those
skilled in the art will recognize that the invention may be
embodied or carried out in a manner that achieves or optimizes one
advantage or group of advantages as taught herein without
necessarily achieving other objects or advantages as may be taught
or suggested herein.
[0049] All of these embodiments are intended to be within the scope
of the invention herein disclosed. These and other embodiments of
the present invention will become readily apparent to those skilled
in the art from the following detailed description of the preferred
embodiments having reference to the attached figures, the invention
not being limited to any particular preferred embodiment(s)
disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a schematic diagram of the major components of one
embodiment of a tractor of the present invention, utilized in
conjunction with a coiled tubing system;
[0051] FIG. 2 is a front perspective view of a preferred embodiment
of the tractor of the present invention;
[0052] FIG. 3 is a schematic diagram illustrating a preferred
configuration of the tractor and the valve system of the present
invention;
[0053] FIG. 4 is a front perspective view of the control assembly
of the tractor of FIG. 2, shown partially disassembled;
[0054] FIG. 5 is a longitudinal sectional view of the control
assembly of FIG. 4, illustrating the inlet control valve of the
tractor;
[0055] FIG. 6 is an exploded view of the inlet control valve shown
in FIG. 5;
[0056] FIG. 7 is an exploded view of the deactivation cam shown in
FIG. 6;
[0057] FIG. 8 is a longitudinal sectional view of the deactivation
cam of FIG. 7;
[0058] FIG. 9 is a longitudinal sectional view of the control
assembly of FIG. 4, illustrating the propulsion control valve of
the tractor;
[0059] FIG. 10 is an exploded view of the propulsion control valve
shown in FIG. 9;
[0060] FIG. 11 is a perspective view of a portion of the propulsion
control valve spool;
[0061] FIG. 12 is a longitudinal sectional view of the aft cycle
valve shown in FIG. 4;
[0062] FIG. 13 is a longitudinal sectional view of the aft pressure
reduction valve of the control assembly shown in FIG. 4;
[0063] FIG. 14 is a perspective view of a forward shaft assembly a
tractor according to one embodiment of the invention, with the
gripper assembly not shown for clarity;
[0064] FIG. 15 is a perspective view of a male braze joint of a
shaft portion of the shaft of FIG. 14;
[0065] FIG. 16 is a longitudinal sectional view of a braze joint of
the shaft of FIG. 14, as well as a connection of a preferred
embodiment of a piston to the shaft;
[0066] FIG. 17 is a schematic diagram illustrating a valve system
according to an alternative embodiment of a tractor of the
invention, which includes a hydraulically controlled reverser valve
that toggles in response to a pressure spike to permit the tractor
to power out of a borehole;
[0067] FIG. 18 is a schematic diagram illustrating a valve system
according to another alternative embodiment of a tractor of the
invention, which includes an electrically controlled reverser
valve;
[0068] FIG. 19 is a schematic diagram illustrating a valve system
according to yet another alternative embodiment of a tractor of the
invention, which includes a pair of inlet control valves, one
hydraulically controlled and the other electrically controlled to
provide electric starting or stopping of the tractor;
[0069] FIG. 20 is a schematic diagram illustrating a valve system
according to yet another alternative embodiment of a tractor of the
invention, which includes both the pair of inlet control valves of
the valve system of FIG. 19 and the electrically controlled
reverser valve of the valve system of FIG. 18;
[0070] FIG. 21 is a perspective view of a preferred embodiment of a
gripper assembly having flexible toes with rollers;
[0071] FIG. 22 is a longitudinal sectional view of the toe
supports, slider element, and a single toe of the gripper assembly
of FIG. 21, shown at a moment when there is substantially no
external load applied to the toe;
[0072] FIG. 23 is an exploded view of the aft end of the toe shown
in FIG. 22;
[0073] FIG. 24 is an exploded view of one of the rollers of the toe
shown in FIG. 22;
[0074] FIG. 25 is an exploded view of the forward end of the toe
shown in FIG. 22;
[0075] FIG. 26 is a longitudinal sectional view of the toe
supports, slider element, and a single toe of the gripper assembly
of FIG. 21, shown at a moment when an external load is applied to
the toe;
[0076] FIG. 27 is an exploded view of the aft end of the toe shown
in FIG. 26;
[0077] FIG. 28 is an exploded view of one of the rollers of the toe
shown in FIG. 26;
[0078] FIG. 29 is an exploded view of the forward end of the toe
shown in FIG. 26;
[0079] FIG. 30 is a partial cut-away side view of the toe supports,
slider element, and a single toe of the gripper assembly of FIG.
21, shown at a moment when the toe is relaxed;
[0080] FIG. 31 is an exploded view of one of the spacer tabs of the
toe shown in FIG. 30;
[0081] FIG. 32 is an exploded view of one of the rollers of the toe
shown in FIG. 30;
[0082] FIG. 33 is a side view of the slider element and a portion
of one of the toes of the gripper assembly of FIG. 21, shown at a
moment when the toe is radially deflected or energized; and
[0083] FIG. 34 is an exploded view of one of the alignment tabs of
the toe shown in FIG. 33.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0084] FIG. 1 shows a hydraulic tractor 100 for moving equipment
within a passage, configured in accordance with a preferred
embodiment of the present invention. In the embodiments shown in
the accompanying figures, the tractor of the present invention may
be used in conjunction with a coiled tubing drilling system 20 and
adjoining downhole equipment 32. The system 20 may include a power
supply 22, tubing reel 24, tubing guide 26, tubing injector 28, and
coiled tubing 30, all of which are well known in the art. The
tractor 100 is configured to move within a borehole having an inner
surface 42. An annulus 40 is defined by the space between the
tractor 100 and the inner surface 42 of the borehole.
[0085] The downhole equipment 32 may include various types of
equipment that the tractor 100 is designed to move within the
passage. For example, the equipment 32 may comprise a perforation
gun assembly, an acidizing assembly, a sandwashing assembly, a bore
plug setting assembly, an E-line, a logging assembly, a bore casing
assembly, a measurement while drilling (MWD) assembly, or a fishing
tool. Also, the equipment 32 may comprise a combination of these
items. If the tractor 100 is used for drilling, the equipment 32
will preferably include an MWD system 34, downhole motor 36, and
drill bit 38, all of which are also known in the art. Of course,
the downhole equipment 32 may include many other types of equipment
for non-drilling applications, such as intervention and completion
applications. While the equipment 32 is illustrated on the forward
end of the tractor, it will be understood that such downhole
equipment can be connected both aftward and forward of the
tractor.
[0086] It will be appreciated that a hydraulic tractor of a
preferred embodiment of the present invention may be used to move a
wide variety of tools and equipment within a borehole or other
passage. For example, the tractor can be utilized for applications
such as well completion and production work for producing oil from
an oil well, pipeline installation and maintenance, laying and
movement of communication lines, well logging activities, washing
and acidizing of sands and solids, retrieval of tools and debris,
and the like. Also, while preferred for intervention operations,
the tractor can be used for drilling applications, including
petroleum drilling and mineral deposit drilling. The tractor can be
used in conjunction with different types of drilling equipment,
including rotary drilling equipment and coiled tubing
equipment.
[0087] For example, one of ordinary skill in the art will
understand that oil and gas well completion typically requires that
the reservoir be logged using a variety of sensors. These sensors
may operate using resistivity, radioactivity, acoustics, and the
like. Other logging activities include measurement of formation dip
and borehole geometry, formation sampling, and production logging.
These completion activities can be accomplished in inclined and
horizontal boreholes using a preferred embodiment of the hydraulic
tractor of the invention. For instance, the tractor can deliver
these various types of logging sensors to regions of interest. The
tractor can either place the sensors in the desired location, or it
can idle in a stationary position to allow the measurements to be
taken at the desired locations. The tractor can also be used to
retrieve the sensors from the well.
[0088] Examples of production work that can be performed with a
preferred embodiment of the hydraulic tractor of the invention
include sands and solids washing and acidizing. It is known that
wells sometimes become clogged with sand, hydrocarbon debris, and
other solids that prevent the free flow of oil through the borehole
42. To remove this debris, specially designed washing tools known
in the industry are delivered to the region, and fluid is injected
to wash the region. The fluid and debris then return to the
surface. Such tools include acid washing tools. These washing tools
can be delivered to the region of interest for performance of
washing activity and then returned to the ground surface by a
preferred embodiment of the tractor of the invention.
[0089] In another example, a preferred embodiment of the tractor of
the invention can be used to retrieve objects, such as damaged
equipment and debris, from the borehole. For example, equipment may
become separated from the drill string, or objects may fall into
the borehole. These objects must be retrieved, or the borehole must
be abandoned and plugged. Because abandonment and plugging of a
borehole is very expensive, retrieval of the object is usually
attempted. A variety of retrieval tools known to the industry are
available to capture these lost objects. The tractor can be used to
transport retrieving tools to the appropriate location, retrieve
the object, and return the retrieved object to the surface.
[0090] In yet another example, a preferred embodiment of the
tractor of the invention can also be used for coiled tubing
completions. As known in the art, continuous-completion drill
string deployment is becoming increasingly important in areas where
it is undesirable to damage sensitive formations in order to run
production tubing. These operations require the installation and
retrieval of fully assembled completion drill string in boreholes
with surface pressure. The tractor of the invention can be used in
conjunction with the deployment of conventional velocity string and
simple primary production tubing installations. The tractor can
also be used with the deployment of artificial lift devices such as
gas lift and downhole flow control devices.
[0091] In a further example, a preferred embodiment of the tractor
of the invention can be used to service plugged pipelines or other
similar passages. Frequently, pipelines are difficult to service
due to physical constraints such as location in deep water or
proximity to metropolitan areas. Various types of cleaning devices
are currently available for cleaning pipelines. These various types
of cleaning tools can be attached to the tractor so that the
cleaning tools can be moved within the pipeline.
[0092] In still another example, a preferred embodiment of the
tractor of the invention can be used to move communication lines or
equipment within a passage. Frequently, it is desirable to run or
move various types of cables or communication lines through various
types of conduits. The tractor can move these cables to the desired
location within a passage.
Overview of Tractor Components
[0093] FIG. 2 shows a preferred embodiment 100 of a tractor of the
present invention, shown with the aft end on the right and the
forward end on the left. The tractor 100 comprises a central
control assembly 102, an uphole or aft gripper assembly 104, a
downhole or forward gripper assembly 106, an aft propulsion
cylinder 108, a forward propulsion cylinder 114, tool joint
assemblies 116 and 129, shafts 118 and 124, and flex joints or
adapters 120 and 128. The tool joint assembly 116 connects a drill
string, such as coiled tubing, to the shaft 118. The aft gripper
assembly 104, aft propulsion cylinder 108, and flex joint 120 are
assembled together end-to-end and are all axially slidably engaged
with the shaft 118. Similarly, the forward gripper assembly 106,
forward propulsion cylinders 114, and flex joint 128 are assembled
together end-to-end and are axially slidably engaged with the shaft
124. The tool joint assembly 129 couples the tractor 100 to
downhole equipment 32 (FIG. 1). The shafts 118 and 124 and control
assembly 102 are axially fixed with respect to one another and are
sometimes referred to herein as the body of the tractor. The body
of the tractor is thus axially fixed with respect to the drill
string and the downhole tools.
[0094] The tractor 100 can be made to have the capability of
pulling and/or pushing downhole equipment 32 of various weights. In
one embodiment, the tractor 100 is capable of pulling and/or
pushing a total weight of 100 lbs, in addition to the weight of the
tractor itself. In three other embodiments, the tractor is capable
of pulling and/or pushing a total weight of 500, 3000, and 15,000
lbs.
[0095] In order to prevent damage to a surrounding formation or
casing wall, the tractor can be designed to limit the radial
gripping load that it exerts on a surface surrounding the tractor.
In one embodiment, the tractor exerts no more than 25 psi on a
surface surrounding the tractor. This embodiment is particularly
useful in softer formations, such as gumbo. In three other
embodiments, the tractor exerts no more than 100, 3000, and 50,000
psi on a surface surrounding the tractor. At radial gripping loads
of 50,000 psi or less, the tractor can be used safely in steel tube
casing.
[0096] The tractor components shown in FIG. 2 are assembled in a
manner similar to the components of the aforementioned EST,
disclosed and illustrated in U.S. Pat. No. 6,347,674. Two notable
differences between the tractor 100 shown in FIG. 2 and the EST are
(1) the tractor 100 of the present invention utilizes gripper
assemblies of a different type, and (2) the control assembly 102 of
the tractor 100 is different than the control assembly of the EST.
In the preferred embodiment, the gripper assemblies 104 and 106 of
the tractor 100 are preferably of a design similar to a gripper
assembly disclosed and illustrated in U.S. Pat. No. 6,464,003, with
a number of improvements described below. The control assembly 102
houses a valve system that controls the distribution of operating
fluid to and from the gripper assemblies and propulsion cylinders.
The control assembly 102 is described below.
[0097] The control assembly 102 includes internal fluid passages
for flow between the valves and flow to the gripper assemblies,
propulsion cylinders, and downhole equipment. In a preferred
embodiment, some of the fluid passage sizes are similar to or
larger than the fluid passages of the control assembly of the EST.
As in the EST design, the fluid passages are sized and located to
fit within the available space constraints of the tractor. The
sizes of the various components (e.g., the shafts, propulsion
cylinders, pistons, control housing, valves, etc.) are generally
similar to the sizes of analogous components of the EST. Using
principles of design and space management made apparent by U.S.
Pat. No. 6,347,674 (which discloses the EST) in combination with
the specification and figures of the present application, one of
ordinary skill in the art will understand how to build a tractor
according to the present invention.
[0098] The tractor 100 can be any desirable length, but for typical
oilfield applications the length is approximately 25 to 30 feet.
The maximum diameter of the tractor will typically vary with the
size of the hole, thrust requirements, and the restrictions that
the tractor must pass through. The gripper assemblies can be
designed to operate within boreholes of various sizes, but
typically can expand to a diameter of 3.75 to 7.0 inches.
[0099] The flex adapters 120 and 128 are hollow structural members
that provide a region of reduced flexural rigidity in the tractor.
This region of increased flexibility facilitates the negotiation of
sharp turns. The adapters are preferably formed of a relatively low
modulus material such as Copper Beryllium (CuBe) and Titanium.
Occasionally, there are applications that require the use of
non-magnetic materials for the tractor. Otherwise, depending on the
required turning capability of the tractor and resultant stresses,
it is possible that various stainless steels may be used in many
areas of the tractor.
[0100] In the preferred embodiment, the tool joint assembly 116
couples the shaft 118 to a coiled tubing drill string, preferably
via a threaded connection. However, downhole tools can also be
placed aftward of the tractor, connected to the tool joint assembly
116. The tool joint assembly 129 will normally be coupled to
downhole tools. The interface threads of the tool joint assemblies
are preferably API threads or proprietary threads (such as Hydril
casing threads). The tool joint assemblies can be prepared with
conventional equipment (tongs) to a specified torque (e.g.,
1000-3000 ft-lbs). The tool joint assemblies can be formed from a
variety of materials, including CuBe, steel, and other metals.
[0101] The shafts 118 and 124 can be formed from any suitable
material. In one embodiment, the shafts are formed from a flexible
material, such as CuBe, in order to permit the tractor 100 to
negotiate sharper turns. In other embodiments CuBe is not used, as
it is relatively expensive. Other acceptable materials include
Titanium and steel (when low flexibility is sufficient). In a
preferred configuration, each shaft includes a central internal
bore (forming a portion of the passage 44 discussed below and shown
in FIG. 3) for the flow of pressurized operating fluid to the
downhole equipment and to the valve system of the tractor. This
bore extends the entire length of each shaft. Each shaft also
includes numerous other passages for the flow of fluid to the
gripper assemblies and propulsion cylinders. These fluid passages
range in length and are equal to or less than the overall length of
the tractor. Multiple fluid passages can be drilled in the shaft
for the same function, such as to feed a single propulsion chamber.
Preferably, the bore and the other internal fluid passages are
arranged so as to minimize stress and provide sufficient space and
strength for other design features, such as the pistons within the
cylinders. Each shaft is preferably provided with threads on one
end for connection to the tool joint assemblies 116 and 129, and
with a flange on the other end to allow bolting to the control
assembly 102.
[0102] In one embodiment, the tractor 100 is specifically designed
for intervention applications. While intervention tractors can be
made any size, they are typically operated within 5-inch or 7-inch
casing. The inside diameter of a 5-inch casing can range from 4.5
to 4.8 inches. The inside diameter of a 7-inch casing can range
from 5.8 to 6.4 inches. The primary structural components of the
tractor 100 are the shafts 118 and 124. In a preferred embodiment,
the shafts have an outside diameter of 1.75 inches and an inside
bore diameter of 0.8 inches. The remaining fluid passages of the
shafts are preferably smaller. The pistons can have varying outside
diameters.
[0103] For intervention applications, the tractor 100 saves time
and money. Prior art intervention tools that utilize rotary drill
strings are as much as 150% more expensive than the illustrated
tractor 100 using coiled tubing equipment. In addition, the tractor
100 is more time-conservative, as the longer rig-up time associated
with rotary equipment is avoided. The use of coiled tubing is
particularly advantageous when operating perforation guns.
[0104] FIG. 3 schematically illustrates a preferred configuration
of the major components of the tractor 100. The tractor 100
includes an internal passage 44 extending from the aft end of the
aft shaft 118 through the control assembly 102 to the forward end
of the forward shaft 124. In use, pressurized operating fluid is
pumped through the drill string into the internal passage 44. The
operating fluid can be used for various applications to be
undertaken by the downhole equipment, such as for powering
perforation guns utilized for cutting holes in a casing wall of an
oil well. The valve system 133 is configured to receive a portion
of the operating fluid flowing through the internal passage 44.
[0105] FIG. 3 also schematically illustrates a preferred
configuration of the valve system 133 of the tractor 100. The valve
system 133 is housed within the control assembly 102 shown in FIG.
2. The valve system 133 selectively controls the flow of operating
fluid to and from the gripper assemblies 104 and 106 and to and
from the propulsion cylinders 108 and 114. The operation of the
valve system 133 is described in detail below.
[0106] In the aft shaft assembly, the aft propulsion cylinder 108
is longitudinally slidably engaged with the aft shaft 118 and forms
an internal annular chamber surrounding the shaft. An annular
piston 180 resides within the annular chamber formed by the
cylinder 108, and is at least longitudinally fixed to the shaft
118. The piston 180 fluidly divides the internal annular chamber
formed by the cylinder 108 into an aft chamber 154 and a forward
chamber 156. Preferably, the chambers 154 and 156 are fluidly
sealed to substantially prevent fluid flow between the chambers or
leakage to the annulus 40. The piston 180 is longitudinally
slidable within the cylinder 108.
[0107] In the forward shaft assembly, the forward propulsion
cylinder 114 is configured similarly to the aft propulsion cylinder
108. The cylinder 114 is longitudinally slidably engaged with the
forward shaft 124. An annular piston 186 is at least longitudinally
fixed to the shaft 124, and is enclosed within the cylinder 114.
The piston 186 fluidly divides the internal annular chamber formed
by the cylinder 114 into a rear chamber 166 and a front chamber
168. The piston 186 is longitudinally slidable within the cylinder
114.
[0108] Thus, the chambers 154, 156, 166, and 168 have varying
volumes, depending upon the positions of the pistons 180 and 186
within the cylinders. It will be understood that the cylinders and
pistons can have any of a variety of different shapes and sizes
(including non-circular cross-sections), preferably keeping in mind
the goals of providing an elongated thrust chamber for a suitable
power stroke, as well as concerns of simplicity, prevention of
leakage, ease of manufacturing, and compatibility with existing
downhole tools.
[0109] Although one aft propulsion cylinder 108 and one forward
propulsion cylinder 114 (along with a corresponding aft piston and
forward piston) are shown in the illustrated embodiment, any number
of aft cylinders and forward cylinders may be provided. The
hydraulic thrust provided by the tractor increases as the number of
propulsion cylinders increases. In other words, the hydraulic force
provided by the cylinders is additive. Thus, the number of
cylinders is selected according to the desired thrust. It will be
understood that the number of cylinders may be limited by the
capability of the gripper assemblies to transfer radial loads to
the borehole wall. In other words, the thrust produced by the
cylinders should not be so high as to cause the gripper assemblies
to slip in their actuated positions. In a preferred embodiment, the
cylinder outside diameter is 3.75 inches. In this embodiment, the
gripper assemblies are designed to transmit a radial gripping force
of approximately 6,500 pounds, and each piston is designed to
produce a stall force of 8,835 pounds at 1500 psi. Thus, in this
embodiment, only one aft and one forward cylinder are preferred.
The load transmission capability of the gripper assemblies varies
by design of the gripper assembly.
[0110] The tractor 100 is hydraulically powered by an operating
fluid pumped down the drill string, such as brine, sea water,
drilling mud, or hydraulic fluid. In a preferred embodiment, the
same fluid that may operate downhole equipment 32 (FIG. 1) powers
the tractor. This avoids the need to provide additional fluid
channels in the tool for the fluid powering the tractor.
Preferably, liquid brine or sea water is used in an open system.
Alternatively, fluid may be used in a closed system, if desired.
Referring to FIG. 1, in operation, operating fluid flows from the
drill string 30 through the tractor 100 and down to the downhole
equipment 32. Referring again to FIG. 3, a diffuser or filter 132
in the control assembly 102 diverts a portion of the operating
fluid into the valve system 133 to power the tractor. Preferably,
the diffuser 132 filters out larger fluid particles that can damage
internal components of the valve system, such as the valve
spools.
Preferred Configuration of Valve System
[0111] With reference to FIG. 3, a preferred embodiment of the
valve system 133 includes an inlet or entry control valve 136, a
propulsion control valve 146, a gripper control valve 148, an aft
cycle valve 150, and a forward cycle valve 152. In addition,
pressure reduction valves 244 and 246 are preferably provided to
limit the fluid pressure in the gripper assemblies, as described in
further detail below. The operation of each of these valves is
discussed below.
[0112] Fluid diverted to the valve system 133 through the diffuser
132 enters an inlet galley 134 upstream of the inlet control valve
136. As used herein, the terms "galley," "chamber," and "passage"
refer to regions of the tractor that are configured to contain
operating fluid, and are not limited to any particular shape. Some
of these regions are illustrated as flow paths or lines in FIG.
3.
[0113] The inlet control valve 136 is preferably a spool valve, a
preferred embodiment of which is illustrated in FIGS. 4-8. The
valve 136 serves as a gateway for fluid to flow into a main galley
144 of the valve system 133. The spool of the valve 136 has first,
second, and third position ranges, the second range being
interposed between the first and third ranges. In the first and
third position ranges, the spool provides a flow path (represented
by arrow 174 for the first position range and arrow 176 for the
third position range) for fluid within the main galley 144 to flow
through the valve 136 to the annulus 40 on the exterior of the
tractor. Also, in the first and third position ranges, the spool
prevents the flow of fluid from the inlet galley 134 through the
valve 136 into the main galley 144. Thus, in the first and third
position ranges of the inlet control valve spool, fluid exits the
valve system 133 to render the tractor non-operational. In the
second position range, the spool provides a flow path (represented
by arrow 172) for fluid in the inlet galley 134 to flow into the
main galley 144. In the second position range, the spool also
prevents the flow of fluid from the main galley 144 through the
valve 136 to the annulus 40. Thus, in the second position range of
the inlet control valve spool, fluid enters the valve system 133
such that the tractor is operational. In FIG. 3, the spool of valve
136 is shown in its second position range. When shifted vertically
downward in FIG. 3, the spool occupies its first position range.
When shifted vertically upward in FIG. 3, the spool occupies its
third position range.
[0114] The spool of the inlet control valve 136 has a first end or
surface 139 biased by one or more springs 140 and a second end or
surface 138 exposed to fluid in the inlet galley 134. In the
illustrated embodiment, the spring 140 is also in fluid
communication with the annulus 40, as indicated by the broken lines
142. The spring 140 imparts a spring force on the first end surface
139 that tends to push the spool toward its first position range.
In the illustrated embodiment, fluid from the annulus 40 also
imparts a pressure force onto the first end surface 139. The fluid
in the galley 134 imparts a pressure force on the second surface
138 that tends to push the spool toward its third position range.
Thus, the spring force and fluid pressure force on the first end
surface 139 act against the fluid pressure force on the second
surface 138. The differential fluid pressure in the inlet galley
134 required to move the spool from the first position range to the
lower endpoint of the second position range (i.e., the position at
which the valve opens a flow path between the galleys 134 and 144)
depends upon the effective spring constant of the spring 140 and is
defined as the lower shut-off threshold. Likewise, the differential
fluid pressure required to move the spool from the second position
range to the lower endpoint of the third position range (i.e., the
position at which the valve closes the flow path between the
galleys 134 and 144) also depends upon the effective spring
constant of the spring 140 and is defined as the upper shut-off
threshold. Unless otherwise indicated, as used herein,
"differential pressure" or "pressure" at a particular location
within the tractor refers to the difference between the pressure at
that location and the pressure in the annulus 40. Advantageously,
the inlet control valve 136 thus permits the fluid pressure within
the valve system 133 to be limited to within a specific range. In a
preferred embodiment, the lower shut-off threshold is 800 psid and
the upper shut-off threshold is 2100 psid.
[0115] It will be understood that the spring 140 can bear against
any suitable surface of the spool or any component having a fixed
relationship with the spool. It will also be understood that the
spring 140 can be configured to operate primarily in tension or
primarily in compression, keeping in mind the goal of biasing the
spool toward its first position.
[0116] In the preferred embodiment, discussed in greater detail
below, the inlet control valve 136 includes a locking feature to
lock the valve spool in its third position range and to thus
prevent fluid from entering the valve system 133. The locking
feature is schematically represented in FIG. 3 by a latch 137. The
purpose and preferred configuration of the locking feature is
discussed below.
[0117] The main galley 144 fluidly communicates with and provides
incoming pressurized operating fluid to the propulsion control
valve 146, the gripper control valve 148, the aft cycle valve 150,
and the forward cycle valve 152. The propulsion control valve 146
is preferably a two-position spool valve. The spool of the valve
146 has a first position, shown in FIG. 3, in which the valve 146
provides a flow path (represented by arrow 192) for the flow of
fluid from the main galley 144 into a chamber or passage 196. The
chamber 196 leads from the valve 146 to the aft chamber 154 of the
aft cylinder 108, and also to the forward chamber 168 of the
forward cylinder 114. When the spool of the valve 146 is in its
first position, the valve 146 also provides a flow path
(represented by arrow 194) for the flow of fluid within a chamber
or passage 198 to the annulus 40. The chamber 198 leads from the
valve 146 to the forward chamber 156 of the aft cylinder 108, and
also to the aft chamber 166 of the forward cylinder 114.
[0118] The spool of the propulsion control valve 146 also has a
second position, shifted to the left in FIG. 3. When the spool of
the valve 146 is in its second position, the valve 146 provides a
flow path (represented by arrow 200) for the flow of fluid from the
main galley 144 to the chamber 198. When the spool of the valve 146
is in its second position, the valve 146 also provides a flow path
(represented by arrow 202) for the flow of fluid from the chamber
196 to the annulus 40.
[0119] With continued reference to FIG. 3, the spool of the
propulsion control valve 146 has a first end surface 188 and a
second end surface 190. The first end surface 188 is exposed to
fluid within a chamber 204 that leads to the aft gripper assembly
104 (or, if present, to an aft pressure reduction valve 244). The
second end surface 190 is exposed to fluid within a chamber 206
that leads to the forward gripper assembly 106 (or, if present, to
a forward pressure reduction valve 246). The first and second end
surfaces 188 and 190 are configured to receive respective fluid
pressure forces that act against each other. The first end surface
188 receives a pressure force from the fluid in the chamber 204
that tends to move the spool of the valve 146 toward its first
position, as shown in FIG. 3. The second end surface 190 receives a
pressure force from the fluid in the chamber 206 that tends to move
the spool toward its second position, which would be shifted to the
left in FIG. 3. Preferably, the valve 146 includes detents
(mechanical catches or restraints) for retaining the spool in its
first and second positions until the pressure difference between
the chambers 204 and 206 reaches a shifting threshold. In a
preferred embodiment, the detents include resilient elements, such
as springs, that interact with tapered surfaces of the spool
landings, as described in further detail below and illustrated in
FIG. 10. Alternatively, the detents may be conventional mechanical
detents.
[0120] Like the propulsion control valve 146, the gripper control
valve 148 is preferably a two-position spool valve. The spool of
the valve 148 has a first position, shown in FIG. 3, in which the
valve 148 provides a flow path (represented by arrow 208) for the
flow of fluid from the main galley 144 into the chamber 204. When
the spool of the valve 148 is in its first position, the valve 148
also provides a flow path (represented by arrow 210) for the flow
of fluid within the chamber 206 to the annulus 40. The spool of the
gripper control valve 148 also has a second position, not shown in
FIG. 3. The second position is that which the spool would be in if
it is shifted to the left in FIG. 3. When the spool of the valve
148 is in its second position, the valve 148 provides a flow path
(represented by arrow 212) for the flow of fluid from the main
galley 144 to the chamber 206. When the spool of the valve 148 is
in its second position, the valve 148 also provides a flow path
(represented by arrow 214) for the flow of fluid from the chamber
204 to the annulus 40.
[0121] The spool of the gripper control valve 148 has a first end
surface 216 and a second end surface 218. The first end surface 216
is exposed to fluid within a chamber or passage 220 that leads to
the aft cycle valve 150. The second end surface 218 is exposed to
fluid within a chamber or passage 222 that leads to the forward
cycle valve 152. The first and second end surfaces 216 and 218 are
configured to receive respective fluid pressure forces that act
against each other. The first end surface 216 receives a pressure
force from the fluid in the chamber 220 that tends to move the
spool of the valve 148 toward its first position, as shown in FIG.
3. The second end surface 218 receives a pressure force from the
fluid in the chamber 222 that tends to move the spool toward its
second position, which would be shifted to the left in FIG. 3.
Preferably, the valve 148 includes detents for retaining the spool
in its first and second positions until the pressure difference
between the chambers 220 and 222 reaches a shifting threshold. In a
preferred embodiment, the detents include resilient elements, such
as springs, that interact with tapered surfaces of the spool
landings. Alternatively, the detents may be conventional mechanical
detents.
[0122] The aft cycle valve 150 is preferably a two-position
spring-biased spool valve. The spool of the cycle valve 150 has a
first position, shown in FIG. 3, in which the valve 150 provides a
flow path (represented by arrow 224) for the flow of fluid from the
chamber 220 to the annulus 40. The spool also has a second
position, not shown in FIG. 3. The second position is that which
the spool would be in if it is shifted vertically downward in FIG.
3. When the spool of the cycle valve 150 is in its second position,
the valve 150 provides a flow path (represented by arrow 226) for
the flow of fluid from the main galley 144 to the chamber 220.
[0123] The spool of the cycle valve 150 has an end surface 228
exposed to fluid in the chamber 198. The fluid in the chamber 198
imparts a pressure force onto the end surface 228, which tends to
move the spool toward its second position. An opposite end surface
230 of the spool is biased by one or more springs 232. In the
illustrated embodiment, the end surface 230 is also in fluid
communication with fluid in the annulus 40. The spring 232 imparts
a spring force onto the spool, which tends to move the spool to its
first position. Thus, the fluid pressure force on the end surface
228 and the spring force on the end surface 230 act against each
other. When the differential fluid pressure in the chamber 198 is
below a threshold, the fluid pressure force is less than the spring
force and the spool occupies its first position. When the
differential fluid pressure in the chamber 198 exceeds the
threshold, the fluid pressure force exceeds the spring force and
the spool moves to its second position. Any desired threshold can
be achieved by careful selection of the spring 232. It will be
understood that the spring 232 can bear against any suitable
surface of the spool or any component having a fixed relationship
with the spool. It will also be understood that the spring 232 can
be configured to operate primarily in tension or primarily in
compression, keeping in mind the goal of biasing the spool toward
its first position.
[0124] The forward cycle valve 152 is preferably configured
similarly to the aft cycle valve 150. The valve 152 is preferably a
two-position spring-biased spool valve. The spool of the cycle
valve 152 has a first position, shown in FIG. 3, in which the valve
152 provides a flow path (represented by arrow 234) for the flow of
fluid from the chamber 222 to the annulus 40. The spool also has a
second position, not shown in FIG. 3. The second position is that
which the spool would be in if it is shifted vertically downward in
FIG. 3. When the spool of the cycle valve 152 is in its second
position, the valve 152 provides a flow path (represented by arrow
236) for the flow of fluid from the main galley 144 to the chamber
222.
[0125] The spool of the cycle valve 152 has an end surface 238
exposed to fluid in the chamber 196. The fluid in the chamber 196
imparts a pressure force onto the end surface 238, which tends to
move the spool toward its second position. An opposite end surface
240 of the spool is biased by one or more springs 242. In the
illustrated embodiment, the end surface 240 is also in fluid
communication with fluid in the annulus 40. The spring 242 imparts
a spring force onto the end surface 240, which tends to move the
spool to its first position. Thus, the fluid pressure force on the
end surface 238 and the spring force on the end surface 240 act
against each other. When the differential fluid pressure in the
chamber 196 is below a threshold, the fluid pressure force is less
than the spring force and the spool occupies its first position.
When the differential fluid pressure in the chamber 196 exceeds the
threshold, the fluid pressure force exceeds the spring force and
the spool moves to its second position. Any desired threshold can
be achieved by careful selection of the spring 242. It will be
understood that the spring 242 can bear against any suitable
surface of the spool or any component having a fixed relationship
with the spool. It will also be understood that the spring 242 can
be configured to operate primarily in tension or primarily in
compression, keeping in mind the goal of biasing the spool toward
its first position.
[0126] The gripper control valve 148 acts as a pilot for the
propulsion control valve 146, which would stall without this pilot.
The pilot action of valve 148 improves the operation of valve 146
since the operation of valve 146 controls the pressure signal to
the cycle valves 150 and 152. Without the gripper control valve 148
to isolate the valve 146 from the cycle valves 150 and 152, the
valve 146 would stall or oscillate. For example, consider a
configuration in which the valve 146 controls fluid flow to the
passages 196, 198, 204, and 206 (which is not the case in the
illustrated embodiment), and in which the valve 148 is eliminated.
In a worst-case scenario, the system would operate as follows. When
the piston 180 reaches the end of its stroke, rising pressure in
the passage 196 would "open" the valve 152 (i.e., would cause the
valve 152 to shift to its second position, downward in FIG. 3).
This would cause a pressure rise in the passage 222, causing the
spool of valve 146 to shift toward the left position (in FIG. 3).
As the flow path 192 begins to close, the pressure in passage 196
would decrease, causing the cycle valve 152 to close. The high
pressure force on the end surface 190 of the spool of the valve 146
would be lost. Without a pressure force on the surface 190, the
spool of the valve 146 would not be able to finish the shift and
would either stall in a partially shifted position or return to the
first position (i.e., to the right in FIG. 3). If the spool of the
valve 146 returns to its first position, the pressure signal would
be restored to the cycle valve 152, which would again shift to
provide a pressure signal to the spool of the valve 146. The spool
would again start to shift. This cycle would continue without the
spool of the valve 146 ever completing a full shift. In the
illustrated embodiment of the valve system 133, the gripper control
valve 148 ensures that the spool of the propulsion control valve
146 completes each of its shifts. A complete sequence of operation
is described below.
[0127] As shown in FIG. 3, the valve system 133 preferably includes
two pressure reduction valves 244 and 246. The pressure reduction
valves limit the pressure of the fluid in the gripper assemblies,
and thus provide a means for preventing possible failure of the
gripper assembly components.
[0128] The aft pressure reduction valve 244 preferably comprises a
spool valve. In a first position of the spool, shown in FIG. 3, the
valve 244 provides a flow path (represented by arrow 250) for the
flow of fluid within the chamber 204 to a chamber or passage 248
that leads to the aft gripper assembly 104. The valve spool is
designed to be in its first position when the gripper assembly 104
is being purposefully actuated or retracted according to the
operational cycle of the valve system 133. A second position of the
spool is that in which the spool is shifted partially to the left
in FIG. 3. In the second position of the spool, the valve 244
blocks communication between the chambers 204 and 248. The valve
spool is designed to be in its second position when the gripper
assembly 104 is actuated during the normal operational cycle of the
valve system 133. The second position of the spool prevents fluid
from exiting the gripper assembly 104.
[0129] A third position of the spool of the pressure reduction
valve 244 is that in which the spool is shifted further to the
left. In the third position, the valve 244 provides a flow path
(represented by arrow 252) for the flow of fluid within the chamber
248 to the annulus 40. In the preferred embodiment, the valve spool
is designed to shift to the third position when the toes 612 (see
FIG. 21) of the preferred gripper assembly experience external
forces, such as sliding friction between the toes and the borehole
surface. These external forces can cause over-pressurization of the
fluid in the gripper assembly 104. The third position of the spool
of the valve 244 allows the excess pressure to bleed to the annulus
40. The spool has a surface 254 exposed to fluid within the chamber
248, and an opposing surface 256 biased by one or more springs 258.
Fluid within the chamber 248 imparts a fluid pressure force onto
the surface 254, which tends to move the spool toward its third
position. The spring 258 exerts a spring force that counteracts the
fluid pressure force and tends to move the spool toward its first
position. When the pressure in the chamber 248 exceeds a threshold
determined by the spring 258, the spool shifts to its third
position. Thus, the valve 244 imposes an upper limit on the
pressure in the passage 248 and thereby prevents
over-pressurization of the aft gripper assembly 104 by bleeding
excess pressure to the annulus 40.
[0130] It will be understood that the spring 258 can bear against
any suitable surface of the spool or any component having a fixed
relationship with the spool. It will also be understood that the
spring 258 can be configured to operate primarily in tension or
primarily in compression, keeping in mind the goal of biasing the
spool toward its first position.
[0131] The forward pressure reduction valve 246 is preferably
configured similarly to the aft pressure reduction valve 244. The
forward pressure reduction valve 246 preferably comprises a spool
valve. In a first position of the spool, shown in FIG. 3, the valve
246 provides a flow path (represented by arrow 262) for the flow of
fluid within the chamber 206 to a chamber or passage 260 that leads
to the forward gripper assembly 106. The valve spool is designed to
be in its first position when the gripper assembly 106 is being
purposefully actuated or retracted according to the operational
cycle of the valve system 133. A second position of the spool is
that in which the spool is shifted partially to the left in FIG. 3.
In the second position of the spool, the valve 246 blocks
communication between the chambers 206 and 260. The valve spool is
designed to be in its second position when the gripper assembly 106
is actuated during the normal operational cycle of the valve system
133. The second position of the spool prevents fluid from exiting
the gripper assembly 106.
[0132] A third position of the spool of the pressure reduction
valve 246 is that in which the spool is shifted further to the
left. In the third position, the valve 246 provides a flow path
(represented by arrow 264) for the flow of fluid within the chamber
260 to the annulus 40. In the preferred embodiment, the valve spool
is designed to shift to the third position when the toes 612 (see
FIG. 21) of the preferred gripper assembly experience external
forces, such as sliding friction between the toes and the borehole
surface. These external forces can cause over-pressurization of the
fluid in the gripper assembly 106. The third position of the spool
of the valve 246 allows the excess pressure to bleed to the annulus
40. The spool has a surface 266 exposed to fluid within the chamber
206, and an opposing surface 268 biased by one or more springs 270.
Fluid within the chamber 260 imparts a fluid pressure force onto
the surface 266, which tends to move the spool toward its third
position. The spring 270 exerts a spring force that counteracts the
fluid pressure force and tends to move the spool toward its first
position. When the pressure in the chamber 260 exceeds a threshold
determined by the spring 270, the spool shifts to its third
position. Thus, the valve 246 imposes an upper limit on the
pressure in the passage 260 and thereby prevents
over-pressurization of the forward gripper assembly 106 by bleeding
excess pressure to the annulus 40.
[0133] It will be understood that the spring 270 can bear against
any suitable surface of the spool or any component having a fixed
relationship with the spool. It will also be understood that the
spring 270 can be configured to operate primarily in tension or
primarily in compression, keeping in mind the goal of biasing the
spool toward its first position.
[0134] It will also be understood that some of the illustrated
valves of the valve system 133 can be combined to provide a more
condensed configuration of the valve system. The valves can be
formed from various different materials, but are preferably made of
a hard erosion-resistant material such as Tungsten Carbide,
Ferrotic (a proprietary metal formulation), or possibly a ceramic
blend.
Valve System Operation
[0135] With reference to FIG. 3, when the inlet control valve 136
is open, i.e., in its second position range, pressurized operating
fluid flows from the inlet galley 134 to the main galley 144 of the
valve system 133. With the valves in the positions shown in FIG. 3,
the pressurized operating fluid in the main galley 144 flows
through the gripper control valve 148, the chamber 204, the aft
pressure reduction valve 244, the chamber 248 (which extends
through the aft shaft 118), and into the aft gripper assembly 104.
Thus, the aft gripper assembly 104 becomes actuated and grips onto
the borehole surface 42. At the same time, fluid within the forward
gripper assembly 106 flows through the chamber 260 (which extends
through the forward shaft 124), the forward pressure reduction
valve, the chamber 206, the gripper control valve, and into the
annulus 40. Thus, the forward gripper assembly 106 becomes
retracted from the borehole surface 42.
[0136] With the aft gripper assembly 104 actuated and the forward
gripper assembly 106 retracted, pressurized fluid within the main
galley 144 flows through the propulsion control valve 146, the
chamber 196 (which extends through both shafts), and into the aft
chamber 154 of the aft cylinders 108, as well as into the forward
chamber 168 of the forward cylinder 114. Simultaneously, fluid
within the forward chamber 156 of the aft cylinder 108, as well as
fluid within the aft chambers 166 of the forward cylinder 114,
flows through the chamber 198 (which extends through both shafts)
and the propulsion control valve 146 into the annulus 40. This
causes the aft piston 180, and thus the entire tractor body, to be
thrust forward (to the right in FIG. 3) with respect to the
actuated aft gripper assembly 104. In other words, the aft cylinder
108 performs a power stroke. Simultaneously, the forward cylinder
114 is thrust forward with respect to the piston 186 and the
tractor body. In other words, the forward cylinder 114 performs a
reset stroke.
[0137] During the above strokes of the cylinders, note that the
fluid within the chamber 204 is pressurized and the fluid within
the chamber 206 is depressurized. Thus, the fluid pressure force
acting on the first end surface 188 of the spool of the propulsion
control valve 146 is significantly larger than the fluid pressure
force acting on the second end surface 190 of the spool. As a
result, the spool of the valve 146 is maintained in its first
position (the position shown in FIG. 3).
[0138] Also, during the above strokes of the cylinders, the cycle
valves 150 and 152 remain in their first positions (the positions
shown in FIG. 3). Since there is flow into the valve system 133
filling the cylinders, there is a pressure drop from the full
system pressure available in the central passage 44. This decrease
in pressure maintains the cycle valves in their first positions.
Thus, the chambers 220 and 222 remain in fluid communication with
the annulus 40. In this state, the fluid pressure forces on the end
surfaces 216 and 218 of the spool of the gripper control valve 148
are approximately equal (the pressure within the annulus 40 may
vary depending upon position). Hence, the gripper control valve 148
will remain in the position shown in FIG. 3, particularly since the
detents (described below) require a threshold force to shift the
valve spool.
[0139] When the cylinders complete their respective strokes, the
fluid pressure in the chamber 196 will begin to rise. In contrast
to when the cylinders are still stroking, the incoming flow of
fluid into the system is halted. As a result, the pressure in the
tractor valve system 133 will rise to the full pressure available
in the center passage 44. When the pressure in the chamber 196
exceeds a threshold associated with the spring(s) 242 of the
forward cycle valve 152, the spool of the valve 152 will shift to
its second position (downward in FIG. 3), permitting pressurized
fluid from the main galley 144 to enter the chamber 222. At this
point, the spool of the aft cycle valve 150 is still in its first
position, due to the low pressure in chamber 198. Due to the
pressure imbalance on the end surfaces 216 and 218, the spool of
the gripper control valve 148 overcomes the retaining forces of the
detents and shifts to its second position (to the left in FIG. 3).
As a result, pressurized fluid within the galley 144 flows through
the gripper control valve 148, the chamber 206, the forward
pressure reduction valve 246, the chamber 260, into the forward
gripper assembly 106. This causes the forward gripper assembly to
actuate and grip onto the borehole surface 42. Simultaneously,
fluid within the aft gripper assembly 104 flows through the chamber
248, the aft pressure reduction valve 244, the chamber 204, the
gripper control valve 148, into the annulus 40. This causes the aft
gripper assembly to retract from the borehole surface 42. Thus,
when the gripper control valve 148 switches positions, both gripper
assemblies switch between their actuated and retracted
positions.
[0140] After the gripper control valve 148 switches its position,
the fluid within the chamber 204 becomes depressurized and the
fluid within the chamber 206 becomes pressurized. The resulting
pressure imbalance on the end surfaces 188 and 190 causes the spool
of the propulsion control valve 146 to overcome the retaining
forces of its detents and shift to its second position (to the left
in FIG. 3). This happens when the flow of fluid into the valve
system 133 stops, which occurs when the gripper assembly has come
into contact with the borehole wall. When the flow stops, there is
no longer a pressure drop (due to flow), and the pressure will rise
to full system pressure. As a result of the shifting of the spool
of the valve 146, pressurized fluid within the main galley 144
flows through the propulsion control valve 146, the chamber 198,
and into the forward chamber 156 of the aft cylinder 108 and the
aft chamber 166 of the forward cylinder 114. Simultaneously, fluid
within the aft chamber 154 of the aft cylinder 108, as well as
fluid within the forward chamber 168 of the forward cylinder 114,
flows through the chamber 196 and the propulsion control valve 146
into the annulus 40. This causes the forward piston 186, and thus
the entire tractor body, to be thrust forward (to the right in FIG.
3) with respect to the actuated forward gripper assembly 106. In
other words, the forward cylinder 114 performs a power stroke.
Simultaneously, the aft cylinder 108 is thrust forward with respect
to the piston 180 and the tractor body. In other words, the aft
cylinder 108 performs a reset stroke. The depressurization of the
chamber 196 causes the spool of the forward cycle valve 152 to
shift back to its first position (the position shown in FIG.
3).
[0141] During the above strokes of the cylinders, the fluid within
the chamber 206 is pressurized and the fluid within the chamber 204
is depressurized. Thus, the fluid pressure force acting on the
second end surface 190 of the spool of the propulsion control valve
146 is significantly larger than the fluid pressure force acting on
the first end surface 188 of the spool. As a result, the spool of
the valve 146 is maintained in its second position (shifted to the
left in FIG. 3).
[0142] Also, during the above strokes of the cylinders, with the
cycle valves 150 and 152 in their first positions (the positions
shown in FIG. 3), the chambers 220 and 222 are in fluid
communication with the annulus 40. In this state, the fluid
pressure forces on the end surfaces 216 and 218 of the spool of the
gripper control valve 148 are again equal. Hence, the gripper
control valve 148 will remain in its position, particularly since
the detents (described below) require a threshold force to shift
the valve spool.
[0143] When the cylinders complete their respective strokes, the
fluid pressure in the chamber 198 will begin to rise. When the
pressure in the chamber 198 exceeds a threshold associated with the
spring(s) 232 of the aft cycle valve 150, the spool of the valve
150 will shift to its second position (downward in FIG. 3),
permitting pressurized fluid from the main galley 144 to enter the
chamber 220. At this point, the spool of the forward cycle valve
152 is still in its first position, due to the low pressure in
chamber 196. Due to the pressure imbalance on the end surfaces 216
and 218, the spool of the gripper control valve 148 overcomes the
retaining forces of the detents and shifts back to its first
position (the position shown in FIG. 3). As a result, pressurized
fluid flows from the galley 144 through the gripper control valve
148, the chamber 204, the aft pressure reduction valve 244, the
chamber 248, into the aft gripper assembly 104. This causes the aft
gripper assembly to actuate. Simultaneously, fluid within the
forward gripper assembly 106 flows through the chamber 260, the
forward pressure reduction valve 246, the chamber 206, the gripper
control valve 148, into the annulus 40. This causes the forward
gripper assembly 106 to retract.
[0144] After the gripper control valve 148 switches its position,
the fluid within the chamber 204 again becomes pressurized and the
fluid within the chamber 206 again becomes depressurized. The
resulting pressure imbalance on the end surfaces 188 and 190 causes
the spool of the propulsion control valve 146 to overcome the
retaining forces of its detents and shift back to its first
position (the position shown in FIG. 3). With the valve 146 back in
its first position, pressurized fluid again flows into the aft
chamber 154 of the aft cylinder 108, and into the forward chamber
168 of the forward cylinder 114. Simultaneously, fluid within the
forward chamber 156 of the aft cylinder 108, as well as fluid
within the aft chamber 166 of the forward cylinder 114, flows into
the annulus 40. This causes the aft cylinder 108 to perform a new
power stroke. Simultaneously, the forward cylinder 110 performs a
new reset stroke. The depressurization of the chamber 198 causes
the spool of the aft cycle valve 150 to shift back to its first
position (the position shown in FIG. 3).
[0145] At this point, all of the valves have returned back to their
original positions (the positions shown in FIG. 3). Thus, the above
describes a complete cycle of operation of the valve system during
forward motion. Note that during forward (or backward) motion, the
gripper assemblies shuttle between two extreme positions: First,
the gripper assemblies move as far apart as possible toward
opposite ends of the tractor. Second, the gripper assemblies move
as close together as possible (with the propulsion cylinders and
control assembly between them). During most of the operation of the
tractor, one gripper assembly is in a power stroke while the other
is in a reset stroke. When they switch directions they also switch
gripper action. Hence, the tractor continually moves in one
longitudinal direction.
[0146] A significant advantage of the preferred configuration of
the valve system 133 is that the cylinders are assured of
completing their respective strokes before the gripper assemblies
are switched between their actuated and retracted positions. This
result is achieved by (1) the provision of separate valves for
controlling the flow of fluid to the gripper assemblies and to the
propulsion cylinders (in the illustrated embodiment, these are the
propulsion control valve 146 and the gripper control valve 148),
and (2) piloting the gripper control valve by cycle valves that are
themselves piloted by the pressure in the cylinders. This ensures
that the cycle valves will open only when the pressure in the
cylinders increases significantly, which in turn will occur only
when the cylinders complete their strokes or when the tractor is
stalled by an overload.
[0147] In a preferred embodiment, the valve system 133 requires an
incoming flow of operating fluid of about 16 gallons per minute.
Typically, large positive displacement pumps are utilized at the
ground surface to pump fluid down the coiled tubing and through the
internal passage 44 of the tractor. Such pumps usually supply a
flow rate of about 80 to 120 gpm. Thus, since the valve system only
requires a relatively small portion of the flow, the operation of
the tractor has little effect on the pressure in the passage 44.
This makes the system more stable. Preferably, an orifice is
provided downstream of the tractor. The orifice is designed to
provide the desired back pressure (which the tractor utilizes to
push/pull a specified load) at a predetermined flow rate within the
passage 44.
[0148] The speed of the tractor is determined by the pressure and
flow rate of fluid pumped through the coiled tubing, as well as the
loads experienced by the tractor. The pressure and flow rate of the
fluid in the coiled tubing, which are substantially controlled by
the actions of surface equipment operators, together determine the
amount of hydraulic energy available in the tractor. The loads
experienced by the tractor include the weight of equipment (such as
the equipment 32 shown in FIG. 1) pushed and pulled by the tractor,
tension in the coiled tubing from the surface, frictional drag
forces between the coiled tubing and the borehole, etc. The surface
operators also control the injector and coiled tubing reel and thus
the feed rate of the coiled tubing into the borehole.
[0149] Because the valve system 133 is all-hydraulic, its maximum
speed is greater than an electrically controlled tractor. The valve
system does not include electrical conductors and other electrical
elements, which allows for larger internal fluid passages, greater
flow rates, and improved power density. The faster maximum speed of
the tractor results in lower operational costs, especially for
intervention applications. In a preferred embodiment of the
invention, the tractor is capable of moving at speeds greater than
or equal to 1350 feet per hour.
Control Assembly
[0150] According to the preferred embodiment, the tractor 100
includes a control assembly 102 which houses the valve system 133
described above. One embodiment of the control assembly 102 is
shown partially disassembled in FIG. 4. The illustrated control
assembly includes a control housing 280, an aft transition housing
282, and a forward transition housing 284.
[0151] The control housing 280 houses the inlet control valve 136,
the propulsion control valve 146, the gripper control valve 148
(not visible, as it is located on the backside of the view of FIG.
4), and the cycle valves 150 and 152. Each valve includes an
elongated valve housing defining a spool passage, and a spool. The
valves are positioned within recesses in the outer surface of the
control housing 280.
[0152] For example, the inlet control valve 136 includes a housing
290 having a spool passage 292 sized to receive a spool. The valve
housing 290 also has an external vent 294 configured to vent
operating fluid into the annulus 40 between the tractor and the
borehole surface. The housing 290 is positioned within a recess 296
in the outer surface of the control housing 280. In contrast to the
housings of the other valves, the inlet control valve housing 290
includes two pin receiving side portions 298 configured to receive
pins or slot engagement portions 300, for purposes described below.
The ends of the housing 290 are slightly inclined from the radial
direction, such that the housing has a trapezoidal axial
cross-section. Two valve housing clamp elements 304 are secured
into the recess 296 at each end of the valve housing 290 by bolts
306. The clamp elements have surfaces 308 that mate closely with
the inclined surfaces 302 of the valve housing 290, thus securing
the valve housing rigidly onto the control housing 280. The aft
clamp element has a vent 305, and the forward clamp element has a
vent 307. The inner configuration of the valve housing 290 and the
spool of the inlet control valve 136 are described below.
[0153] The propulsion control valve 146, gripper control valve 148,
and cycle valves 150 and 152 are configured somewhat similarly to
the inlet control valve 136. Specifically, the valve housings of
the valves 146, 148, 150, and 152 are include similarly configured
spool passages and vents and are secured to the control housing 280
in similar fashion. In the illustrated embodiment, the housings of
the valves 146, 148, 150, and 152 include two vents as opposed to
one. Also, each of the clamp elements for the valves 146, 148, 150,
and 152 receives a single bolt as opposed to two bolts.
[0154] The control housing 280 includes numerous internal fluid
passages for the controlled flow of operating fluid to the downhole
equipment 32 (FIG. 1), between the valves, to the gripper
assemblies, and to the propulsion cylinders. The fluid passages are
configured to effect the hydraulic circuit shown in FIG. 3. Some of
the fluid passages extend to openings 312 in the end surfaces 310
of the control housing 280, where they connect to openings of
corresponding fluid passages in the end surfaces 316 of the
transition housings 282 and 284. Some of these fluid passages
extend through the shafts 118 and 124 (FIG. 2) to the gripper
assemblies, the propulsion cylinders, or to downhole equipment
connected to the tractor. As in the EST, within the housing 280 the
internal passage 44 is shifted to one side (i.e., it is not in the
center of the housing), to maximize available space for the various
valves and internal fluid passages. Also, if liquid brine is used
as the operating fluid, the passage 44 is not required to be as
large as in the EST design, further maximizing the available
space.
[0155] The control housing 280 is bolted to the transition housings
282 and 284 by a plurality of studs 318 and nuts 319. The studs
extend though holes 322 in the end surfaces 310 of the housing 280
into holes 324 in the end surfaces 314 of the transition housings.
Recesses 320 are provided in the outer surfaces of the housing 280,
which facilitate access to the studs 318. In the illustrated
embodiment, five studs 318 are provided in the end surfaces of the
housing 280 and the transition housings.
[0156] The aft transition housing 282 houses the diffuser 132 and
the aft pressure reduction valve 244. The aft end 326 of the
housing 282 receives the internal passage 44 from the aft shaft 118
at the center axis of the tractor. Within the housing 282, the
passage 44 transitions toward one side of the housing. Thus, the
housing 282 moves the passage 44 to one side to maximize space for
the valves and various fluid passages within the control housing
280. The diffuser 132 is positioned on the forward end 314 of the
housing 282. As in the EST, the diffuser 132 is generally
cylindrical and has a plurality of side holes 328 for directing the
flow from the passage 44 into the inlet galley 134 of the inlet
control valve 136. In one embodiment, the side holes 328 are angled
so that the fluid passing forward through the diffuser must turn
somewhat aftward to enter the inlet galley 134. This prevents
larger particles within the operating fluid from entering the valve
system 133, as it is more difficult for the larger particles to
overcome forward momentum and flow through the side holes 328.
Those of ordinary skill in the art will understand that any of a
variety of different types of filters can be used instead of the
illustrated diffuser 132.
[0157] The aft pressure reduction valve 244 includes a valve
housing 330. The valve housing 330 is configured similarly to the
housings of the valves within the control housing 280.
Specifically, the valve housing 330 includes a similarly configured
spool passage 332 and vents 334. In the illustrated embodiment, the
valve housing 330 includes two vents 334. Also, the valve housing
330 is secured into a recess 338 of the aft transition housing 282
by the use of clamp elements 336, in similar fashion as the
aforementioned valve housings are secured to the control housing
280. The recess 338 includes several openings 344. The openings 344
comprise ends of fluid passages that conduct fluid to and from
corresponding side passages in the valve housing 330 of the valve
244 (such as the side passages 477 and 479 shown in FIG. 13), as
described in further detail below. It will be understood that the
corresponding recesses for all of the valve housings of the
housings 280 and 284 (such as the recess 296 of the inlet control
valve 136) have openings of fluid passages that communicate flow
through the valves.
[0158] The forward transition housing 284 is configured generally
similarly to the aft transition housing 282. One difference is that
the aft housing 282 is configured to accommodate the diffuser 132
and has a fluid passage for the inlet galley 134, whereas the
forward housing 284 does not require these features. Also, the
forward housing 284 transitions the internal passage 44 back to the
center axis of the tractor.
[0159] FIG. 5 shows a longitudinal cross-section of the assembled
control assembly 102 of FIG. 4, with the aft end on the right and
the forward end on the left. This particular section shows the
configuration of the inlet control valve 136. Also shown in FIG. 5
are several internal fluid passages, which comprise some of the
flow lines, chambers, passages, and galleys schematically
illustrated in FIG. 3. One of skill in the art will understand that
the internal fluid passages can have any of a large variety of
configurations.
Inlet Control Valve
[0160] FIG. 6 is an exploded view of the inlet control valve 136
shown in FIG. 5, which includes the valve housing 290, an elongated
spool 346, and a set of springs 140 biasing the spool to the right
of the figure. The valve housing 290 defines an elongated generally
cylindrical spool passage 292 that receives the spool 346. The
inner surface of the passage 292 has annular recesses 362, 364, and
366 (commonly referred to as "galleys"), in which the passage has a
slightly enlarged inner diameter. The valve housing 290 also
includes side passages or fluid ports 348, 350, 352, and 354 that
are open to the spool passage 292. When the valve housing 290 is
secured onto the control housing 280, these ports align with
openings of fluid passages in the housing 280. The ports 348 and
352 are in fluid communication with the main galley 144 of the
valve system 133. The ports 350 and 354 are in fluid communication
with the inlet control galley 134. The ports 348, 350, and 352 are
located within the annular recesses 362, 364, and 366,
respectively. The port 354 is located aftward of the second end
surface 138 of the spool 346. The port 354 permits fluid within the
inlet galley 134 to impart a pressure force against the end surface
138, which tends to move the spool 346 toward its second and third
position ranges (to the left in FIG. 6). The housing 290 further
includes the aforementioned vents 294, 305, and 307. The port 305
is non-functional in this configuration. It exists only because it
is desirable to have identical designs for the clamp elements 304,
and because a vent is desired within the forward clamp element. On
the aft end of the valve housing 290, a plug 374 and an O-ring seal
are provided to prevent fluid on the second end surface 138 of the
spool 346 from flowing out to the annulus 40 through the vent
305.
[0161] As described above, the first end surface 139 of the spool
346 is in contact with a set of springs 140 that bias the spool 346
aftward, or to the right in FIG. 6. In a preferred embodiment,
Belleville springs are stacked in 30 sets in series, each set
containing three springs in parallel. This configuration provides a
desired spring rate and resultant deflection. The spool 346 has
three "landings" 356, 358, and 360. These landings comprise larger
diameter portions that effect a fluid seal of the spool passage
292, as known in the art. In other words, each landing slides
within the passage and prevents fluid on one side of the landing
from flowing to the other side of the landing. The spool 346 also
includes a locking feature to lock the spool in its third position
range, in which the inlet control valve 136 is closed at high
pressure. In the illustrated embodiment, the locking feature
comprises a deactivation cam 368, described in further detail
below.
[0162] As explained above, the spool 346 has first, second, and
third position ranges. In the first and third ranges, the inlet
control valve 136 provides a flow path for fluid from the main
galley 144 of the valve system to vent into the annulus 40, and
prevents fluid within the inlet galley 134 from flowing through the
valve 136 into the main galley 144. In the second range, the valve
136 provides a flow path for fluid within the inlet galley 134 to
flow into the main galley 144, and prevents fluid within the main
galley 144 from flowing through the valve 136 into the annulus
40.
[0163] In FIG. 6, the spool 346 is shown in its first position
range, shifted to the right. In this position, fluid from the main
galley 144 flows through the fluid port 348, past the forward end
of the landing 356, through the spool passage 292, and out to the
annulus 40 through the vent 307. The spool 346 occupies this
position when the pressure in the inlet galley 134 is below a lower
shut-off threshold (e.g., 800 psid). As the pressure in the galley
134 rises, the fluid pressure force acting on the second end
surface 138 of the spool 346 increases and pushes the spool to the
left in FIG. 6, until the fluid pressure force is equalized by the
spring force from the springs 140. When the pressure in the inlet
galley 134 exceeds the lower shut-off threshold, the spool 346
moves to the left in FIG. 6 until it occupies a position within its
second range. In this position, the landing 356 blocks flow between
the port 348 and the vent 307, and permits flow between the ports
348 and 350. Fluid now flows from the inlet control galley 134
through the port 350, the spool passage 292, the port 348, and into
the main galley 144. Fluid within the galley 144 is prevented from
flowing through the valve 136 into the annulus 40. When the
pressure in the inlet galley 134 exceeds an upper shut-off
threshold (e.g., 2100 psid), the spool 346 moves further left in
FIG. 6 until it occupies a position within its third range. In this
position, the landing 358 blocks flow through the port 350 but
permits flow between the port 352 and the vent 294. Fluid flows
from the main galley 144 through the port 352, the spool passage
292, the vent 294, into the annulus 40.
[0164] A spring adjustment screw 370 is preferably provided to
adjust the compression of the springs 140. In the illustrated
embodiment, the screw 370 is accessible via a recess 372 in the
control housing 280, which is also shown in FIG. 4. Adjustment of
the screw 370 permits the shut-off threshold pressures of the inlet
control valve 136 to be adjusted.
[0165] As shown in FIG. 6, the landings 356, 358, and 360 include
"centering grooves" 376. The grooves 376 comprise circumferential
grooves oriented generally perpendicular to the spool passage 292.
The grooves 376 reduce leakage across the landings by providing a
series of expansions and contractions in the leak path. Also, the
grooves effectively equalize pressure around the circumference of
the landing. During operation, fluid within the valve tends to push
the spool against the side of the spool passage. By equalizing the
pressure around the landings, the centering grooves cause the spool
to remain more accurately centered within the spool passage. As a
result, less energy is required to move the spool, and the valve
operates more efficiently and reliably. Further, the centering
function reduces leakage. The concentric relationship between the
landings and the spool passage minimizes the largest width of the
leak path. The grooves 376 also provide a region for small
particles to deposit, which further prevents jamming of the spool
within the spool passage. Any number of centering grooves can be
provided on each of the landings of the spool 346. In the preferred
embodiment, the grooves have a depth between 0.010 and 0.030
inches, and a width between 0.010 and 0.020 inches.
[0166] FIGS. 7 and 8 further illustrate the deactivation cam 368 of
the spool 346 of the inlet control valve 136. The cam 368 forms a
portion of the spool 346 and is preferably axially fixed, but
rotationally free, with respect to the remainder of the spool. The
cam 368 comprises a large diameter portion 378 having a first
portion 382 and a second portion 384 separated by an annular cam
path recess 380. The peripheral surface of the first portion 382
includes at least one slot 386 oriented parallel to the spool
passage 292 and extending into the recess 380. In the preferred
embodiment, four slots 386 are provided in the peripheral surface
of the first portion 382 and are spaced at 90.degree. intervals
(with respect to the longitudinal axis of the spool 346) around the
circumference of the cam 368. Each slot 386 is sized and configured
to receive a slot engagement portion of the valve housing 290. At
least one slot engagement portion is provided within the spool
passage 292. The slot engagement portion extends radially inward
from an inner surface of the spool passage 292. Preferably, there
are two slot engagement portions, on opposite sides of the spool
passage separated by 180.degree.. In the preferred embodiment, the
slot engagement portions comprise pins 300 (FIG. 4) received within
side walls of the valve housing 290.
[0167] The cam path recess 380 of the deactivation cam 368 is
defined partially by a first annular sidewall 388 and a second
annular sidewall 390. The sidewalls 388 and 390 include a plurality
of cam surfaces 392 and valleys 394. As used herein, a "valley"
refers to a region of the sidewall in which one of the slot
engagement portions can become restrained within when the slot
engagement portion bears against the sidewall 388 or 390. The cam
surfaces 392 are angled with respect to the axis of the spool 346.
In the preferred embodiment, the cam surfaces 392 are oriented at
angles of about 60.degree. with respect to the axis of the spool
346. The valleys 394 are configured to receive the slot engagement
portions, such as the pins 300. When the pins 300 are not received
within the slots 386, the cam 368 can freely rotate about the
longitudinal axis of the spool passage 292. In a less preferred
embodiment, the spool 346, including the deactivation cam 368, is
rotatable about its longitudinal axis within the spool passage
292.
[0168] When the spool 346 is in its first position range, as
defined above, the pins 300 are received within the slots 386 of
the deactivation cam 368, preventing the cam from rotating. In the
first position range, the pins 300 are positioned near the first
ends 396 of the slots 386. As the spool 346 moves to its second
position range, the cam 368 moves toward the springs 140 (FIG. 6)
and the cam path recess 380 moves closer to the pins. However, the
pins 300 remain within the slots 386. When the spool 346 moves to
the lower endpoint of its third position range (i.e., when the
pressure in the inlet galley 134 reaches the lower shut-off
threshold pressure, as explained above), the pins 300 are still
within the slots 386. As the pressure within the inlet galley 134
continues to rise, the pins 300 eventually enter the cam path
recess 380, at which point the cam 386 becomes free to rotate. When
the pressure in the inlet galley 134 reaches an upper cam
activation pressure (e.g., 2500 psid), which is above the upper
shut-off threshold pressure (e.g., 2100 psid), cam surfaces 392 of
the first sidewall 388 bear against the pins 300. This causes the
cam 368 to rotate in a first direction (so that the labeled slot
396 moves upward in FIG. 7) until each pin 300 is nestled in a
valley 394 of the first sidewall 388. In a preferred embodiment,
the cam surfaces 392 are configured similarly, such that the spool
346 rotates 22.5.degree.. If the pressure in the inlet galley 134
increases beyond the upper cam activation pressure, the pins 300
nestled within the valleys 394 of the first sidewall 388 prevent
the spool 346 from moving further toward the springs 140.
[0169] With the cam 368 in this rotated position, the pins 300 are
no longer aligned with the slots 386. If the fluid within the inlet
galley 134 (or in the passage 44--it will be understood that the
pressure within the passage 44 is very closely equal to the
pressure in the galley 134) is depressurized only once, the pins
300 will not re-enter the slots 386. Rather, the pins 300 are now
restrained within the cam path recess 380. In this locked position
of the valve 136, the spool 346 is in its third position range,
such that the fluid within the valve system 133 is free to vent to
the annulus 40. In this position, the tractor is in a failsafe
mode, i.e., a mode in which the gripper assemblies are
depressurized and retracted from the borehole surface 42. A
significant advantage of this failsafe mode is that equipment
connected to the tractor can undertake activities without risking
damage to the gripper assemblies. For example, perforation guns can
be operated with the gripper assemblies assured of being retracted,
thus preventing or minimizing any possible damage to the gripper
assemblies. Also, with the gripper assemblies assured of being
retracted, they cannot cause the perforation guns to be erroneously
moved. The failsafe mode also makes it possible to pull the tractor
out of the borehole in case of an emergency.
[0170] After the cam surfaces 392 of the first sidewall 388 bear
against the pins 300 for the first time and cause the cam 368 to
initially rotate in the first direction, a subsequent first
depressurization of the fluid within the inlet galley 134 below a
lower cam-activation pressure (which is above the upper shut-off
threshold) causes the deactivation cam 368 to move to the right in
FIG. 7, so that cam surfaces 392 of the second sidewall 390 bear
against the pins 300. This causes the cam 368 to rotate further in
the first direction, until each pin 300 is nestled within a valley
394 of the second sidewall 390. In the preferred embodiment, the
cam surfaces 392 of the second sidewall 390 are configured so that
the cam rotates another 22.5.degree.. At this point, the cam has
rotated a total of 45.degree. from the time the spool 346 was last
in its first or second position ranges. The spool 346 is still
restrained within its third position range. If the fluid in the
inlet galley 134 is further depressurized, the pins 300 nestled
within the valleys 394 of the second sidewall 390 will prevent the
spool 346 from moving into its second (or "operating") position
range.
[0171] Thus, as described above, a single pressure spike of the
fluid in the inlet galley 134 to the upper cam activation pressure
causes the entry control valve 136 to move to its locked position,
in which the gripper assemblies are assured of being retracted.
[0172] The deactivation cam 368 is preferably configured so that,
in order to move the spool 346 back into its second or first
position ranges, it is necessary to again pressurize the fluid
within the inlet galley 134. In the illustrated embodiment, this
repressurization must occur after the pressure was first lowered
from the upper cam activation threshold to the lower cam activation
threshold. With the pins 300 restrained within the cam path recess
380 and nestled within valleys 394 of the second sidewall 390, a
repressurization of the fluid within the inlet galley 134 to the
upper cam activation pressure causes the spool 346 to move to the
left in FIG. 7, so that the pins 300 again bear against cam
surfaces 392 of the first sidewall 388. The cam 368 again rotates
in the first direction (again, preferably 22.5.degree., such that
the cam will have rotated a total of 67.5.degree. since the spool
346 was last in its first or second position ranges) until each pin
is again nestled within a valley 394 of the first sidewall 388.
Then, a subsequent second depressurization of the fluid within the
inlet galley 134 causes the spool 346 to move to the right in FIG.
7. When the pressure decreases to the lower cam activation level,
each pin 300 bears against a partial cam surface 398 just "above"
(see FIG. 7) one of the slots 386. As the pressure in the galley
134 continues to drop, the pins 300 slide along the cam surfaces
398 such that the cam rotates another 22.5.degree. in the first
direction. At this point, the cam 368 will have rotated a total of
90.degree. since the spool 346 was last in its first or second
position ranges. This causes the pins 300 to reenter the slots 386,
although each pin is now in a different slot than before. The
reengagement of the pins 300 within the slots 386 prevents the cam
368 from rotating further and permits the spool 346 to move into
its second and first position ranges.
[0173] The spool 346 of the inlet control valve 136 can have
variable diameter sections to allow some degree of throttling of
the fluid into the tractor. This configuration provides some
control over the pressure drop and speed of the tractor. In one
embodiment, the landings of the spool 346 include notches, such as
the notches 438 shown in FIG. 11 and described below. Thus, it will
be understood that, in industry parlance, the valve 136 is commonly
referred to as a "four-way valve," as it has a throttling
position.
[0174] If desired, the cam 368 could be made to be completely rigid
with respect to the remainder of the spool. However, such a
configuration would require more force to rotate the cam and is
thus less desirable than the preferred configuration described
above.
Propulsion Control and Gripper Control Valves
[0175] The propulsion control valve 146 and the gripper control
valve 148 function similarly. They are both piloted by fluid
pressure on both sides. In a preferred embodiment, the valves 146
and 148 are configured substantially identically. Thus, only the
propulsion control valve 146 is herein described.
[0176] Preferably, the propulsion control valve 146 almost has a
"critically lapped spool design." A critically lapped valve has no
"center" position (or third position), which would allow the valve
to be closed. In this case, a closed propulsion control valve would
render the tractor non-operational. Instead, the valve 146 is
preferably "overlapped," which assures that fluid flows to only one
of the chambers 196 and 198 (FIG. 3). An overlapped design also
keeps leakage to a minimum. In contrast, an "under lapped" design
would allow fluid to simultaneously flow to both of the chambers
196 and 198. Preferably, the valve 146 is not under lapped.
[0177] FIG. 9 is a longitudinal sectional view of the preferred
embodiment of the control assembly 102, with the aft end shown on
the left and the forward end on the right. FIG. 9 shows the
propulsion control valve 146 in cross-section. The valve 146 is
located toward the forward end of the control housing 280. FIG. 10
is an exploded view of the valve 146 as depicted in FIG. 9. In the
preferred embodiment, the valve 146 functions as a two-position
spool valve with detents that tend to retain the spool within one
of its two main positions. In reality, it is a three-position valve
with a center (blocked) position. However, the spool resides within
its center position for only about 0.005 inches of a total spool
stroke of 0.35 inches, which makes the center position relatively
insignificant. In the illustrated embodiment, the valve 146
includes a valve housing 410 having an internal cylindrical spool
passage 412. Plugs 414 with O-rings seal the ends of the spool
passage 412. The valve housing 410 includes two vents 416 and 418.
Two clamp elements 440 secure the ends of the valve housing 410 to
the control housing 280 via bolts 426.
[0178] In the illustrated embodiment, the valve housing 410
includes fluid ports 430, 422, 420, 424, and 432, which align with
openings of fluid passages within the control housing 280. The
ports 430 and 432 provide pilot pressures that control the position
of the spool 400. The ports 430 and 432 fluidly communicate with
chambers 204 and 206, respectively. Fluid from the chamber 204
flows through the port 430 into the spool passage 412 and imparts a
pressure force against the end surface 188 of the spool 400. Fluid
from the chamber 206 flows through the port 432 into the spool
passage 412 and imparts a pressure force against the end surface
190 of the spool 400. The ports 422, 420, and 424 fluidly
communicate with the chamber 198, the main galley 144, and the
chamber 196, respectively.
[0179] Near the ends of the valve housing 410, the inner surface of
the spool passage 412 includes two grooves 442. Each groove 442 is
preferably circular and sized to receive a resilient stop 434, 436.
The stops 434 and 436 perform a detent function; they tend to
retain the spool 400 in one of its two main positions. Each stop
434, 436 preferably defines an inner diameter and is positioned at
least partially within the groove 442. Each stop 434, 436 has a
relaxed position in which it has a first inner diameter and in
which at least an inner radial portion of the stop is positioned
outside of the groove 442. Each stop 434, 436 also has a deflected
position in which it has a second inner diameter larger than the
first inner diameter. Preferably, in its deflected position,
substantially all of the stop is in the groove 442. In a preferred
embodiment, each stop 434, 436 comprises an expandable ring-shaped
spring. However, various other configurations are possible. For
example, each stop could alternatively comprise a plurality of
(e.g., three) circumferentially separated stop portions that extend
radially inward from the inner surface of the spool passage
412.
[0180] The valve 146 includes a spool 400 having four landings 402,
404, 406, and 408. In the preferred embodiment, each of the two
ends of each of the outer landings 402 and 408 have a radially
tapered section followed by a generally constant diameter section
that intersects the bottom of the taper. The tapered section has a
tapered peripheral or radial surface 428. The tapered or conical
surfaces 428 operate in conjunction with the stops 434, 436 to
provide the detent function. The tapered surfaces 428 also function
to prevent the stops 434, 436 from falling out or being washed out
of the grooves 442. In their relaxed positions, each stop 434, 436
is configured to bear against or be in very close proximity to one
of the tapered peripheral surfaces 428 of the landings 402 and 408,
while being immediately radially outside of the reduced constant
diameter section that intersects the bottom of the taper. It is
this reduced diameter section that retains the stop from
inadvertently being removed from the groove 442. The resilient
stops are configured so that the landings 402 and 408 cannot move
across the stops until the net longitudinal movement force on the
spool 400 (from the fluid pressure on the end surfaces 188 and 190)
reaches a threshold at which the tapered surfaces 428 of the
landings cause the stops to move to their deflected positions. In
their deflected positions, the stops 434, 436 permit the landings
402 and 408 to move across the stops. As used in this context, the
terms "longitudinal" and "axial" refer to the longitudinal axis of
the spool 400. Preferably, the shifting threshold of the valve 146
is relatively low, preferably between 250 and 800 psid.
[0181] As described above, the spool 400 of the propulsion control
valve 146 has two main positions. The position shown in FIG. 10
corresponds to the above-described first position (shown in FIG.
3). In this position, fluid flows from the main galley 144 through
the port 420, the spool passage 412, the port 424, and into the
chamber 196. Simultaneously, fluid in the chamber 198 flows through
the port 422, the spool passage 412, the vent 416, and into the
annulus 40. As the fluid pressure forces against the end surfaces
188 and 190 fluctuate, the stops 434 and 436 bear against tapered
surfaces 428 of the landings 402 and 408, respectively, to maintain
the spool 400 in the position shown in FIG. 10. When the pressure
differential acting on the end surfaces 188 and 190 (the force
acting on end surface 190 being larger) reaches a threshold, the
pressure force on the spool 400 exceeds the retaining forces of the
stops 434, 436. The tapered surfaces 428 force the stops to move to
their deflected positions, such that the spool 400 is permitted to
shift to its second main position (to the left in FIGS. 3 and 10).
After the spool 400 shifts, the stops 434, 436 move back to their
relaxed positions and bear against or come in close proximity to
the tapered surfaces 428 on the opposite sides of the landings 402
and 408. The spool 400 is thus maintained in its second position by
the stops' contact with or close proximity to the tapered surface.
The spool is prevented from moving away from the stop by the spool
ends bearing against or being in close proximity to the end plugs
414. In the second position of the spool, fluid flows from the main
galley 144 through the port 420, the spool passage 412, the port
422, and into the chamber 198. Simultaneously, fluid in the chamber
196 flows through the port 424, the spool passage 412, the vent
418, and into the annulus 40. The spool 400 will not shift back to
its first position until the pressure differential acting on the
end surfaces 188 and 190 (the force acting on end surface 188 being
larger) reaches the aforementioned threshold necessary to again
overcome the retaining forces of the stops 434, 436.
[0182] The landings of the spool 400 preferably include centering
grooves 326, similar to those of the inlet control valve spool 346
described above. In the illustrated embodiment, the center landings
404 and 406 each include three centering grooves, and the outer
landings 402 and 408 each include two centering grooves. Any number
of centering grooves can be provided on each landing.
[0183] The center landings 404 and 406 preferably include a
plurality of notches 438 (preferably between 3 and 8) at each end.
The notches 438 permit a small amount of fluid flow past the
landings when the landings are almost in a completely closed
position with respect to a fluid port. The notches 438 help to
reduce hydraulic shock caused by the sudden flow of fluid into a
valve (commonly referred to as "hammer"). Thus, the notches help
decrease wear on the valves. The skilled artisan will understand
that notches can be included on some or all of the landings of the
valves of the tractor 100. The notches 438 are preferably V-shaped.
FIG. 11 shows an exemplary notch 438, having an axial length L
extending inward from the edge of the landing, a width W at the
edge of the landing, and a depth D. In one embodiment, L is about
0.055-0.070 inches, W is about 0.115-0.150 inches, and D is about
0.058-0.070 inches. Preferably, the positions of the notches 438
are carefully controlled, as the notches provide the lapping
function of the valve 146.
[0184] As mentioned above, the gripper control valve 148 is
preferably configured substantially identically to the propulsion
control valve 146. One difference is that, in the valve 148, the
fluid ports analogous to the fluid ports 430, 422, 424, and 432 of
the valve 146 are in fluid communication with the chambers 220,
206, 204, and 222, respectively. Also, the gripper control valve
148 can be significantly smaller than the propulsion control valve
146, because the flow through the valve 148 can be significantly
less.
[0185] In a preferred embodiment, the stops 434, 436 of the
propulsion control valve 146 have about twice the detent force of
analogous stops within the gripper control valve 148. In one
embodiment, only one stop is provided within the valve 148, as
opposed to two in the valve 146. Also, it is possible to use stops
of differing stiffness or grooves 442 of differing diameter to
adjust the detent force, keeping in mind the goal of ensuring that
upon the completion of the strokes of the propulsion cylinders the
gripper assemblies switch between their actuated and retracted
positions before the valve 146 switches positions. It will also be
understood that the detent force can be modified by adjusting the
angles of the tapered sections 428 of the spools.
Cycle Valves
[0186] In the preferred embodiment, the cycle valves 150 and 152
are configured substantially identically. Thus, only the aft cycle
valve 150 is herein described.
[0187] FIG. 12 shows a longitudinal sectional view of the aft cycle
valve 150, according to a preferred embodiment, with the aft end
shown on the left and the forward end shown on the right. With
reference to the inlet control valve 136 and the propulsion control
valve 146 described above, the cycle valve 150 includes a generally
similarly configured valve housing 444. The housing 444 has an
internal cylindrical spool passage 445 and includes vents 446 and
448. The housing 444 also includes fluid ports 450, 452, and 454
that fluidly communicate with the chamber 198, the main galley 144,
and the chamber 220, respectively. The valve 150 includes a spool
456 with landings 458, 460, and 462 as shown. One or more of the
landings preferably include centering grooves 376 as described
above. The spool 456 has end surfaces 228 and 230. The end surface
228 is in fluid communication with the fluid in the chamber 198,
via the port 450. A spring, and more preferably a set of springs
232 (preferably Belleville springs), bears against the end surface
230, such that the springs bias the spool 456 to the left in FIG.
12.
[0188] As explained above, the spool 456 of the valve 150 has a
first position and a second position. The spool 456 is shown in its
first position in FIG. 12. In this position, fluid within the
chamber 220 flows through the port 454 and the spool passage 445,
within the springs 232, through the vent 448, and out into the
annulus 40. The fluid from the chamber 198 imparts a pressure force
against the end surface 228, which tends to push the spool 456 to
its second position (to the right in FIG. 12). When the fluid
pressure force on the end surface 228 exceeds an actuation
threshold, the spool 456 moves such that the landing 462 blocks the
flow of fluid between the port 454 and the vent 448, and permits
flow between the ports 452 and 454. When the spool 456 is in its
second position, fluid within the main galley 144 flows through the
port 452, the spool passage 445, the port 454, and into the chamber
220. Preferably, the actuation threshold of the valve 150 is
between 800 and 1500 psid, or possibly even as high as 2000 psid.
The vent 446 is non-operational. It exists only because of a
preference that all of the valve housings have the same
configuration, to keep manufacturing costs down.
[0189] As mentioned above, the forward cycle valve 152 is
preferably configured substantially identically to the aft cycle
valve 150. One difference is that, in the valve 152, the fluid
ports analogous to the fluid ports 450 and 454 of the valve 150 are
in fluid communication with the chambers 196 and 222, respectively.
If desired, the valves 150 and 152 can be provided with screws to
permit adjustment of the spring forces of the springs. Such screws
can compensate for variance in manufacturing tolerances.
Pressure Reduction Valves
[0190] In a preferred embodiment, the pressure reduction valves 244
and 246 are configured substantially identically. Thus, only the
aft pressure reduction valve 244 is herein described.
[0191] FIG. 13 shows a longitudinal sectional view of the aft
pressure reduction valve 244, according to a preferred embodiment,
with the aft end shown on the right and the forward end shown on
the left. The valve 244 includes a valve housing 330 configured
generally similarly to those of the valves described above. The
housing 330 has an inner cylindrical spool passage 332 with an
annular recess 478. The housing 330 also includes two vents 334, as
well as fluid ports 477 and 479 that fluidly communicate with the
chambers 248 and 204, respectively. Each of the ports 477 and 479
is aligned with a fluid passage opening 344 in the aft transition
housing 282 (FIG. 4). The port 477 is open to the annular recess
478 of the valve 244. The valve housing 330 is secured via clamp
elements 336 and bolts to the aft transition housing 282.
[0192] The valve 244 includes a spool 458 comprising a first spool
portion 460 and a second spool portion 462. The second spool
portion 462 is preferably a spring guide. The spool portion 460
includes landings 470, 472, and 474 as shown. In some embodiments,
one or more of the landings include centering grooves as described
above. The spool portion 460 also includes a center-drilled passage
482 and a side passage 480. The passage 482 extends from the aft
end of the spool portion 460 to the longitudinal position (in this
context, the term "longitudinal" refers to the axis of the spool
passage) of the side passage 480. The spool portion 460 is
configured so that in normal operation the side passage 480 is
positioned within the annular recess 478 of the spool passage 332.
The side passage 480 is fluidly open to the center-drilled passage
482 so that fluid within the chamber 248 can flow into the passage
482. The fluid within the center-drilled passage 482 imparts a
pressure force against the surface 254, which tends to push the
spool 458 to the left in FIG. 13. As referred to herein, the
surface 254 can include the aft end surface of the spool portion
460, outside of the passage 482.
[0193] The spool portion 462 has a flange 484 that defines an
annular surface 256. A spring 258 is positioned between the surface
256 and an end plug 476. The spring 258 biases the spool portion
462 to the right in FIG. 13. In the illustrated embodiment, the
spring 258 comprises a coil spring (only one coil is shown in FIG.
13) coiled around an elongated portion of the spool portion 462. In
the preferred embodiment, there is always a clearance between a
flange 484 of the spool portion 462 and an annular step 486 formed
within the spool passage 332.
[0194] The spool portions 460 and 462 have opposing end surfaces
with partially tapered and preferably partially conical
ball-receiving recesses 466 and 468, respectively. A ball 464 is
interposed between the spool portions 460 and 462, partially within
the ball-receiving recesses 466 and 468. Preferably, the recesses
466 and 468 are configured to only partially receive the ball 464,
so that the ball makes contact with both spool portions. The
presence of the ball 464 and the ball-receiving recesses 466 and
468 results in improved alignment of the spool 458 within the spool
passage 332, which in turn results in reduced leakage and more
efficient operation.
[0195] As explained above, the spool 458 of the valve 244 has
first, second, and third positions. The spool 458 is shown in its
first position in FIG. 13. In this position, fluid within the
chamber 204 flows through the port 479 across the forward end of
the landing 472, and through the spool passage 332, the port 477,
and into the chamber 248. When the fluid pressure force on the
surface 254 exceeds an actuation threshold, the spool 458 moves to
its second position (shifted partially to the left in FIG. 13). In
this position, the landing 472 blocks fluid flow between the ports
477 and 479, which stops the flow into the aft gripper assembly 104
(FIG. 3). This spool will normally be in the second position when
the gripper assembly is actuated. If the pressure in the chamber
248 is further increased, such as by an external friction force on
the gripper assembly, the spool shifts further left to its third
position. In the third position, excess pressure in the chamber 248
bleeds past the aft end of the landing 472 through the aft vent 334
into the annulus 40. The forward vent 334 accommodates volume
changes on the left side of the landing 470 as the spool moves to
the left.
[0196] As mentioned above, the forward pressure reduction valve 246
is preferably configured substantially identically to the aft
pressure reduction valve 244. One difference is that, in the valve
246, the fluid ports analogous to the fluid ports 477 and 479 of
the valve 244 are in fluid communication with the chambers 260 and
206, respectively.
Shaft Configuration and Manufacturing Process
[0197] With reference to FIG. 2, a process for manufacturing the
shafts 118 and 124 of the tractor 100 is herein described.
[0198] As explained above in the Background section, prior art
shafts designed for downhole tools used in drilling and
intervention applications have been formed from more flexible
materials, such as copper beryllium (CuBe), in order to facilitate
turning at sharper angles in the bore of a well. Due to the various
constraints of CuBe and other materials, prior art individually
gun-drilled shaft portions have been attached to one another by
electron beam welding, a very expensive process. The geometry of
prior art shafts (e.g., larger internal passages necessitated by
drilling mud) and the constraints of softer materials like CuBe
have limited the possible length of gun-drilled passages and
required a relatively large number of gun-drilled shaft
portions.
[0199] In one aspect, the present invention provides a shaft design
and manufacturing method for a tractor to be used primarily for
intervention. In contrast to drilling, intervention applications
are typically undertaken in cased boreholes and do not require the
ability to negotiate sharp turns. In contrast to drilling tools,
which typically use drilling mud having larger solid particles, an
intervention tractor can use an operating fluid such as clean
brine, and thus does not require as large an internal flow passage
for fluid to the downhole equipment and valve system. Accordingly,
a preferred embodiment of a tractor of the present invention
includes a shaft with a relatively smaller internal flow passage
for fluid to the downhole equipment and valve system. Also, the
shaft is preferably formed from a stronger, more rigid material.
The combination of a smaller diameter flow passage, which leaves
more space for gun-drilled passages, and a stronger material of the
shaft makes it possible to gun-drill longer passages. This in turn
allows for fewer shaft portions. In a preferred embodiment of the
invention, each shaft 118 and 124 (FIG. 2) includes only two shaft
portions and an end flange.
[0200] FIG. 14 shows a preferred embodiment of the forward shaft
124 of the tractor of the invention. In this embodiment, the
tractor includes only a single forward propulsion cylinder 112
enclosing a single piston. The forward gripper assembly is not
shown for clarity, but would typically be located generally at
position 490. Attached to the forward end of the shaft 124 is a
tool joint assembly 129 for attachment to downhole equipment. The
assembly 129 includes an internal bore for the passage 44 for
operating fluid to the downhole equipment. The aft end of the shaft
124 is welded to a flange 488 for connection to the forward end of
the control assembly 102 (FIG. 2). The shaft 124 preferably
includes a first shaft portion 494 and a second shaft portion 496.
The shaft portions are preferably brazed together, as described
below. The braze joint is located, for example, at about the
position 492. The braze joint is enclosed by the cylinder 112.
[0201] FIG. 15 shows the forward end of a preferred embodiment of
the first shaft portion 494 of FIG. 14. Preferably, the end
surfaces of the first shaft portion 494 and the second shaft
portion 496 are configured to mate with each other. The illustrated
forward end of the first shaft portion 494 comprises a male
connection, while a conforming aft end of the second shaft portion
496 is female. The shaft portion 494 includes an elongated end
portion 498 having a reduced width (which may include non-circular
configurations) or diameter (for circular configurations). The
portion 498 has a peripheral surface 500 and an end surface 502,
and is preferably about one inch long. A connecting annular surface
504 is formed between the end portion 498 and the remainder of the
shaft portion 494. In the illustrated embodiment, the end surface
502 and the connecting surface 504 are generally flat and
perpendicular to the longitudinal axis of the first shaft portion
494. However, other configurations are possible, such as tapered
surfaces.
[0202] A "mating surface" of the first shaft portion 494 comprises
the surfaces 502, 500, and 504. The second shaft portion 494
preferably has a "mating surface" that mates with that of the first
shaft portion 494. Other mating surface configurations are
possible, giving due consideration to the goal of forming a strong
joint that is capable of withstanding combined tensile, shear, and
bending loads experienced downhole. At the outside diameter of the
shaft portion 494, an edge 506 is formed between the connecting
surface 504 and the remainder of the shaft portion 494. The
illustrated edge 506 is circular and forms an outer interface
between the first and second shaft portions when they are attached
together. Bores 508 form fluid passages within the shaft portion
494 (for the flow to the gripper assemblies and propulsion
chambers), while a larger center bore forms the main passage 44
(FIG. 3). In the illustrated embodiment, the outside diameter of
the end portion 498 interrupts the passages.
[0203] Preferably, a stress-relief groove 510 is formed proximate
the mating surface of the first shaft portion 494. The groove 510
provides a stress concentration point to reduce the stresses felt
at the outside diameter of the joint between the first and second
shaft portions. Thus, the groove 510 further reduces the risk of
failure at the joint by taking the stress away from the outside
diameter of the shaft, where stresses are typically at a maximum.
Preferably, the groove 510 extends along the entire or
substantially the entire circumference of the outer diameter of the
shaft portion 494. The groove 510 is preferably circular. The
longitudinal position, as well as the width and depth, of the
groove 510 can vary, keeping in mind the goal of pulling stress
away from the outermost edge of the brazed connection. The groove
510 is desirably positioned within 0.060 inches of the edge 506.
Preferably, the groove 510 has a width between 0.080 and 0.120
inches, and a depth between 0.050 and 0.060 inches.
[0204] In the preferred embodiment, the mating surfaces of the
first and second shaft portions are silver brazed together. The
silver braze connection is formed by placing a brazing shim on the
end surface 502 and then mating together the mating surfaces of the
first and second shaft portions. The connected shafts are then
heated to melt the brazing shim. The brazing shim contains silver
alloy which, when melted, flows along the mating surfaces of the
shaft portions by capillary action. Advantageously, the silver
generally does not flow into the bores 508 or the passage 44 it
remains substantially along the mating surfaces. Since the heat
will normally be applied from the exterior surfaces of the shaft
portions, the surface 502 will be heated last. Thus, the surfaces
500 and 504 will be slightly hotter than the surface 502. This
ensures that when the brazing shim melts at the surface 502 it will
flow to the warmer surfaces 500 and 504 and remain in liquid form
to effect a better connection. The emergence of excess silver at
the external interface 506 signals that the silver has fused
completely through the mating surfaces. Preferably, the shaft
portions 494 and 496 are formed from stainless steel, such as
17-4PH steel, a high-strength corrosion-resistant steel that is
readily brazed. Furthermore, in the H-1150 condition, the strength
is sufficient and is not significantly affected by the silver braze
process. In experimental testing, silver braze joints of the
illustrated configuration have withstood multiply administered
tension loads greater than 100,000 pounds.
[0205] FIG. 16 is a longitudinal sectional view of the braze joint
of the shaft 124 of FIG. 14. Preferably, the piston 184 is fitted
over the interface 506 between the first and second shaft portions
494 and 496. Advantageously, the piston 184 provides additional
strength to the joint, reducing the risk of failure. FIG. 16 also
illustrates a preferred embodiment of a piston 184, which comprises
two ring-shaped compression clamps 514 and 516, a spacer ring 518,
and a locking assembly 521. The compression clamps 514 and 516 each
apply a radial inward compression force onto the shaft 124. The
compression clamps rigidly lock onto the shaft and, along with the
spacer ring 518 described below, provide the majority of the
piston's resistance to moving with respect to the shaft 124. In the
illustrated embodiment, each compression clamp comprises a pair of
ring-shaped clamp members with tapered annular surfaces that
interact with one another to produce the compression force. For
example, the clamp 514 includes an inner clamp member 530 and an
outer clamp member 532. The members 530 and 532 have inclined
annular surfaces that mate with one another. As the members 530 and
532 are forced axially together with respect to the shaft axis, the
axial force is converted into a radial inward compression force
that locks the compression clamp 514 onto the shaft. The
compression clamp 516 is preferably configured substantially
similarly to the compression clamp 514. In a preferred embodiment,
the clamps 514 and 516 comprise Ringfeder.RTM. clamps, available
from Ringfeder Corporation of Westwood, N.J., U.S.A.
[0206] The spacer ring 518 is not a necessary element of the
illustrated piston 184. However, the spacer ring advantageously
provides additional resistance to axial movement or sliding of the
compression clamps 514 and 516 with respect to the shaft 124. The
spacer ring, preferably a two-piece part to facilitate
installation, includes an annular lip 520 on its inner surface. The
lip 520 is sized and adapted to fit within the stress-relief groove
510 of the first shaft portion 494 of the shaft. The reception of
the lip 520 within the groove 510 resists axial sliding of the
spacer ring 518, and thus of the entire piston 184, with respect to
the shaft 124. Another advantage of the groove 510 and the spacer
ring 518 is that the groove provides a convenient method for
locating and properly positioning the piston 184 during assembly of
the shaft 124.
[0207] The locking assembly 521 imparts an axial compression force
onto each pair of clamp members of the compression clamps 514 and
516. The clamps 514 and 516 convert the axial compression force of
the locking assembly 521 into the aforementioned radial inward
compression force onto the shaft 124. In the illustrated
embodiment, the locking assembly 521 comprises a pair of
ring-shaped locking members 522 and 524, which are clamped axially
together by one or more bolts 526 extending through holes in the
member 522 and into threaded holes in the member 524. As the
locking members 522 and 524 are clamped together, they increase the
radial compression force of the compression clamps 514 and 516. The
locking assembly 521 also comprises a majority of the volume of the
piston 184. Preferably, the locking assembly 521 extends radially
to the inner surface 523 of the propulsion cylinder 112. Seals 528
are provided within recesses in the peripheral surface of the
locking member 524. The seals 528 effect a fluid seal between the
piston 184 and the inner surface 523 of the cylinder 112. Also, at
least one seal 531 is provided between the piston 184 and the shaft
124. The seals 528 and 531 may comprise O-ring type or lip type
seals. It will be understood that seals can alternatively or
additionally be positioned within recesses in the peripheral
surface of the locking member 522. Seals 529 are also provided
within recesses at the ends of the cylinder 112 adjacent the shaft
124 to prevent leakage of fluid from within the cylinder to the
annulus 40. The aforementioned Ringfeder Corporation sells locking
assemblies. However, in the preferred embodiment, the locking
assembly 521 is custom sized and shaped.
[0208] It will be understood that each of the shafts 118 and 124
(FIG. 2) may comprise any number of shaft portions silver brazed
together, preferably configured as shown in FIGS. 15 and 16. Also,
some or all of the joints can be strengthened by positioning the
pistons so as to enclose the interfaces of the joints, as shown in
FIG. 16. Also, some or all of the pistons of the shafts can
comprise compression clamps (preferably with spacer rings) and
locking assemblies, as shown in FIG. 16.
Hydraulically Controlled Reverser Valve
[0209] FIG. 17 illustrates a valve system 540 for a tractor
according to an alternative embodiment of the invention. As
explained below, the valve system 540 permits the direction of
travel of the tractor to be controlled. With the exception of a
number of modifications discussed below, the valve system 540 is
configured substantially similarly to the valve system 133 shown in
FIG. 3. Elements of the valve system 540 are labeled with the
reference numbers of analogous elements of the valve system 133.
The valve system 540 includes a propulsion control valve 146,
gripper control valve 148, aft cycle valve 150, forward cycle valve
152, aft pressure reduction valve 244, and forward pressure
reduction valve 246, all configured similarly to corresponding
elements of the valve system 133. However, the inlet galley 541 and
the inlet control valve 542 of the valve system 540 are configured
differently than the inlet galley 134 and inlet control valve 136
of the valve system 133. The valve system 540 also includes a
hydraulically controlled reverser valve 550, as well as fluid
chambers 564 and 566, described below.
[0210] The inlet galley 541 of the valve system 540 extends to the
inlet control valve 542 and the reverser valve 550. The inlet
control valve 542 preferably comprises a spool valve. The valve
spool has a first position (shown in FIG. 17) in which fluid is
prevented from entering the remainder of the valve system 540, and
a second position (shifted vertically downward in FIG. 17) in which
fluid does enter the remainder of the valve system. In the first
position of the spool, the valve 542 provides a flow path
(represented by arrow 549) for fluid within the main galley 144 to
flow into the annulus 40. In the first position of the spool, fluid
within the inlet galley 541 is prevented from flowing through the
valve 542 into the main galley 144. In the second position of the
spool, the valve 542 provides a flow path (represented by arrow
548) for fluid within the inlet galley 541 to flow into the main
galley 144. In the second position of the spool, fluid within the
main galley 144 is prevented from flowing through the valve 542
into the annulus 40.
[0211] The inlet control valve 542 is piloted by the fluid pressure
within the inlet galley 541. The spool has a surface 544 exposed to
fluid within the inlet galley 541. At least one spring 546 biases
the spool in a direction opposite to the fluid pressure force
received by the surface 544. In this respect, the operation of the
valve 542 is effectively similar to that of the cycle valves 150
and 152 and the pressure reduction valves 244 and 246. The valve
spool of the valve 542 moves to its second position when the
pressure in the inlet galley 541 exceeds a threshold determined by
the characteristics of the at least one spring 546. Thus, the valve
542 effectively has an "off" position (as shown in FIG. 17) and an
"on" position (shifted vertically downward in FIG. 17).
[0212] The reverser valve 550 controls the direction that the
tractor travels within the passage or borehole. The valve 550
permits the sequence of operations for forward motion of the
tractor (to the right in FIG. 13) to be modified so that the
actuation and retraction of the gripper assemblies are reversed.
During the operational cycle of the valves associated with forward
motion of the tractor (described above), fluid is distributed to
and from the gripper assemblies and to and from the chambers of the
propulsion cylinders according to a specific sequence. At certain
stages of the sequence, the aft gripper assembly is actuated and
the forward gripper assembly is retracted. At other stages of the
sequence, the aft gripper assembly is retracted and the forward
gripper assembly is actuated. If this operational sequence is
modified so that each gripper assembly is actuated during stages
when it was previously retracted, and so that each gripper assembly
is retracted during stages when it was previously actuated, the
tractor will travel backward (to the left in FIG. 13). The reverser
valve 550 accomplishes this task.
[0213] In the illustrated embodiment, the reverser valve 550
communicates with the chambers 204 and 206. Unlike in the valve
system 133, the chambers 204 and 206 do not extend to the pressure
reduction valves. The reverser valve 550 also communicates with the
chambers 564 and 566. The chamber 564 extends from the valve 550 to
the aft pressure reduction valve 244. The chamber 566 extends from
the valve 550 to the forward pressure reduction valve 246. The
valves 244 and 246 communicate with the chambers 564 and 566,
respectively, in the same manner that the valves 244 and 246
communicate with the chambers 204 and 206 in the valve system 133
(FIG. 13).
[0214] In the preferred embodiment, the reverser valve 550
comprises a two-position spool valve. The valve spool has a first
position (shown in FIG. 17) in which the tractor travels forward,
and a second position (shifted to the right in FIG. 17) in which
the tractor travels backward. In the first position of the spool,
the valve 550 provides a flow path (represented by arrow 560) for
fluid within the chamber 206 to flow into the chamber 564. In the
first position of the spool, the valve 550 also provides a flow
path (represented by arrow 562) for fluid within the chamber 566 to
flow into the chamber 206. In the second position of the spool, the
valve 550 provides a flow path (represented by arrow 558) for fluid
within the chamber 204 to flow into the chamber 566. In the second
position of the spool, the valve 550 also provides a flow path
(represented by arrow 556) for fluid within the chamber 564 to flow
into the chamber 206.
[0215] In the illustrated embodiment, the fluid pressure in the
inlet galley 541 controls the position of the spool of the reverser
valve 550. The spool has a surface 552 exposed to the fluid from
the inlet galley 541. The surface 552 receives a pressure force
that tends to move the spool to its second position. At least one
spring 554 biases the spool toward its first position and opposes
the pressure force on the surface 552. Thus, the spool shifts to
its second position, to effect backward travel of the tractor, when
the fluid within the inlet galley 541 exceeds a shifting threshold
pressure determined by the characteristics of the at least one
spring 554. Preferably, the shifting threshold pressure (e.g., 2000
psid) required to move the spool of the reverser valve 550 to its
second position is greater than the threshold pressure (e.g., 800
psid) required to move the spool of the inlet control valve 542 to
its second position. The skilled artisan will understand that the
greater the variance between these threshold pressures, the easier
it will be to open the inlet control valve 542 (i.e., to move the
spool to its second position) without inadvertently reversing the
direction of tractor motion.
[0216] In the preferred embodiment, the reverser valve 550 includes
a locking feature, schematically represented by a latch 568, which
locks the spool in its second (or first) position. Preferably, the
locking feature comprises a cam such as the deactivation cam 368
(FIGS. 5-8) described above. In this embodiment, in order to shift
and lock the spool within its second (or first) position, it is
necessary to increase the pressure in the inlet galley 541 above
the upper cam-activation threshold of the cam (e.g., 2000 psid). In
order to unlock the spool, it is necessary to (1) reduce the
pressure below the lower cam-activation threshold of the cam (e.g.,
1000 psid), (2) increase the pressure back above the upper
cam-activation threshold, and (3) reduce the pressure below the
shifting threshold of the valve 550. Refer to the discussion of the
deactivation cam 368 above.
[0217] Thus, the illustrated reverser valve 550 provides a
convenient means for reversing the direction of the tractor, while
preserving an all-hydraulic design for the valve system of the
tractor.
[0218] An alternative embodiment of a tractor of the invention
includes a hydraulically controlled reverser valve configured to be
actuated only once. When the reverser valve is actuated, the
tractor will walk backward out of the passage or borehole. A
preferred configuration of the valve system of this embodiment is
herein described with reference to FIG. 17. The valve system is
substantially identical to that shown in FIG. 17, with the
following exceptions. First, the reverser valve 550 is modified so
that the toggle feature 568 and the spring 554 are removed. Second,
a burst disc or rupture disc device is provided in the pilot line
that extends from the inlet galley 541 to the end surface 552 of
the spool of the reverser valve 550. The burst disc is configured
to burst or open when the pressure in the inlet galley 541 reaches
a burst pressure of the disc.
[0219] It will be understood that this configuration is useful if
the tractor gets stuck in the borehole or if any downhole equipment
of the BHA needs assistance in being removed, the reverser valve
can be actuated. In this configuration, the tractor will normally
be inserted into a borehole with the reverser valve 550 in its
first position (the position shown in FIG. 17). The burst disc
prevents fluid within the inlet galley 541 from exerting a pressure
force on the spool of the valve 550. When it is desirable to
reverse the direction of tractor motion, the pressure in the inlet
galley 541 can be increased to the burst pressure of the burst
disc. The burst disc will then burst or open to allow the fluid
pressure within the inlet galley to move the spool of the valve 550
to its second position (shifted to the right in FIG. 17). Since the
spring 554 is removed from this design, the valve 550 will not
change its position. Optionally, stops or detents can be provided
to prevent inadvertent shifting of the spool, such as the stops
434, 436 illustrated in FIG. 10. The burst pressure of the burst
disc is preferably between 2500 and 7000 psid, and more preferably
about 3200 psid. Preferably, the burst pressure of the disc is
greater than the shifting threshold of the inlet control valve
542.
Electrically Controlled Reverser Valve
[0220] FIG. 18 illustrates a valve system 570 for a tractor
according to another alternative embodiment of the invention. Like
the valve system 540 of FIG. 17, the valve system 570 permits the
direction of travel of the tractor to be controlled. With the
exception of a number of modifications discussed below, the valve
system 570 is configured substantially similarly to the valve
system 540. Elements of the valve system 570 are labeled with the
reference numbers of analogous elements of the valve system 540.
However, the inlet galley 574 of the valve system 570 is different
than the inlet galley 541 of the valve system 540. Also, the
reverser valve 550 is controlled differently.
[0221] The inlet galley 574 of the valve system 570 does not extend
to the reverser valve, as in the valve system 540. This is because
the reverser valve 550 of the system 570 is not piloted by fluid
pressure. Instead, a motor 572 controls the position of the spool
of the reverser valve. In a preferred configuration, the output
shaft of the motor 572 is coupled to a leadscrew, and a traversing
nut is threadingly engaged with the leadscrew. The nut is coupled
to the spool of the reverser valve 550, preferably via a flexible
stem. As the leadscrew rotates with the motor output, the nut
traverses the leadscrew and thereby moves the spool. The position
of the spool can be controlled by controlling the amount of
rotation of the motor output shaft. An assembly for controlling the
position of a valve spool with a motor, within a tractor, is
illustrated and described in U.S. Pat. No. 6,347,674.
[0222] Preferably, the motor 572 is controlled by electronic
signals sent from a remote location (such as from ground surface
equipment) or even from a programmable logic controller on the
tractor itself.
[0223] It will be understood that the position of the spool of the
reverser valve 550 can alternatively be controlled via solenoids or
other electronic means.
[0224] Electrical Control of Fluid Entry
[0225] FIG. 19 illustrates a valve system 574 for a tractor
according to yet another alternative embodiment of the invention.
As explained below, the valve system 574 provides electronic
control of whether the tractor is "on" or "off." With the exception
of a number of modifications discussed below, the valve system 574
is configured substantially similarly to the valve system 133 shown
in FIG. 3. Elements of the valve system 574 are labeled with the
reference numbers of analogous elements of the valve system
133.
[0226] The valve system 574 includes an inlet galley 578, a pair of
inlet control valves 576 and 577, and a fluid chamber 582. The
inlet galley 578 extends to both of the valves 576 and 577. The
chamber 582 extends between the valves 576 and 577. Preferably, the
valve 576 comprises a spool valve. The valve 576 is controlled by a
motor 580, and can be configured similarly to the reverser valve
550 of the valve system 570 (FIG. 18). It will be understood that
the position of the spool can alternatively be controlled via
solenoids or other electronic means. The spool of the valve 576 has
a first "closed" position (shown in FIG. 19) in which the valve 576
provides a flow path (represented by arrow 586) for fluid within
the chamber 582 to flow into the annulus 40, and in which fluid
within the inlet galley 578 is prevented from flowing through the
valve 576 into the chamber 582. The spool of the valve 576 also has
a second "open" position (shifted vertically downward in FIG. 19)
in which the valve 576 provides a flow path (represented by arrow
584) for fluid within the inlet galley 578 to flow into the chamber
582, and in which fluid within the chamber 582 is prevented from
flowing through the valve 576 into the annulus 40.
[0227] The valve 577 preferably comprises a spool valve and is
preferably configured substantially similarly to the valves 542 of
FIGS. 17 and 18. The spool of the valve 577 has a first "closed"
position (shown in FIG. 19) in which the valve 577 provides a flow
path (represented by arrow 590) for fluid within the main galley
144 to flow into the annulus 40, and in which fluid within the
chamber 582 is prevented from flowing into the main galley 144. The
spool of the valve 577 also has a second "open" position (shifted
vertically downward in FIG. 19) in which the valve 577 provides a
flow path (represented by arrow 588) for fluid within the chamber
582 to flow into the main galley 144, and in which fluid within the
main galley 144 is prevented from flowing through the valve 577
into the annulus 40.
[0228] The pair of inlet control valves 576 and 577 operate to
control the flow of fluid into the remainder of the valve system
574. The hydraulically controlled valve 577 shifts to its "open"
position only when the fluid in the inlet galley 578 exceeds the
threshold pressure associated with the valve 577. Regardless of the
position of the valve 576, when the valve 577 is closed the fluid
within the main galley 144 flows through the valve 577 into the
annulus 40. Thus, when the pressure in the inlet galley 578 is
below the threshold associated with the valve 577, the tractor is
"off." In other words, the valve 577 is a failsafe valve to
deactivate the tractor in case of control system failure. The
electrically controlled valve 576 provides additional control. When
the valve 576 is closed, the tractor is "off," regardless of the
position of the valve 577. Even if the valve 577 is open when the
valve 576 is closed, fluid within the main galley 144 flows through
the valve 577, the chamber 582, the valve 576, and into the annulus
40. The tractor is "on" only when both the valves 576 and 577 are
open. In such a condition, fluid within the inlet galley 578 flows
through the valve 576, the chamber 58, the valve 577, and into the
main galley 144. Thus, fluid flows into the remainder of the valve
system 574 only when (1) the pressure in the inlet galley 578
exceeds the threshold associated with the valve 577 and (2) the
valve 576 is shuttled to its "open" position.
Electrical Control of Fluid Entry and Reverse Motion
[0229] FIG. 20 illustrates a valve system 592 for a tractor
according to yet another alternative embodiment of the invention.
The valve system 592 comprises a combination of the valve systems
570 (FIG. 18) and 574 (FIG. 19). The valve system 592 includes a
pair of inlet control valves 576 and 577, configured similarly to
analogous valves of the valve system 570. In particular, the valve
576 is electrically controlled and the valve 577 is hydraulically
controlled. The valve system 592 also includes an electrically
controlled reverser valve 550, configured similarly to the
analogous valve of the valve system 574. Thus, the valve system 592
permits electrical control of (1) the on/off state of the tractor
and (2) the direction of tractor motion.
Gripper Assemblies
[0230] As mentioned above, the gripper assemblies 104 and 106 are
preferably configured in accordance with a design illustrated and
described in a U.S. patent application Ser. No. 10/004,963,
entitled "GRIPPER ASSEMBLY FOR DOWNHOLE TRACTORS," filed on Dec. 3,
2001, now U.S. Pat. No. 6,715,559. FIGS. 21-34 illustrate a
preferred configuration of such a gripper assembly. Below is a
brief description of the configuration and operation of the
illustrated gripper assembly. For a more detailed description,
please refer to the above-referenced application.
[0231] In a preferred embodiment, the gripper assemblies 104 and
106 are substantially identical. Thus, the gripper assembly
configuration shown in FIGS. 21-34 describes both assemblies 104
and 106. In FIG. 21, the gripper assembly is shown with its aft end
on the left and its forward end on the right. The gripper assembly
includes an elongated mandrel 600, a cylinder 602 engaged on the
mandrel, toe supports 608 and 610, a tubular piston rod 604, a
slider element 606, and three flexible toes or beams 612. The
mandrel 600 surrounds and is free to slide longitudinally with
respect to the shafts 118 and 124 (FIG. 2) of the tractor. When
used for non-drilling applications, the mandrel 600 is preferably
also free to rotate with respect to the shafts (i.e., there are no
splines that prevent rotation). This is because it is generally not
necessary to transmit torque to the borehole wall for non-drilling
applications. The ends 614 and 616 of the toes 612 are pivotally
secured to the toe supports 608 and 610, respectively. The cylinder
602 and the toe support 608 are fixed with respect to the mandrel
600, while the toe support 610 is free to slide longitudinally
along the mandrel. The piston rod 604 and the slider element 606
are fixed with respect to each other and are together slidably
engaged on the mandrel 600. The cylinder 602 encloses an annular
piston (not shown) that is fixed with respect to the piston rod 604
and slider element 606 and also slidably engaged on the mandrel
600. The piston is biased in the aft direction by a return spring
(not shown) that is also enclosed within the cylinder 602.
[0232] With reference to FIGS. 21-25, the central region of each
toe 612 has a recess 624 (FIG. 24) formed in the inner radial
surface of the toe. The recess 624 is formed between two axial
sidewalls 618 of the toe 612. The recess 624 includes two rollers
626 on axles 628 secured within the sidewalls 618. The slider
element 606 includes three pairs of ramps 630, each pair aligned
with one of the toes 612. The ramps 630 are radially interior of
the toes 612. As the slider element 606 slides forward, each roller
626 rolls up one of the ramps 630, causing the central regions of
the toes 612 to bend radially outward to grip onto a borehole
surface. As the slider element 606 slides aftward, the rollers 626
roll down the ramps 630, causing the toes 612 to relax back to the
position shown in FIGS. 21 and 22.
[0233] The gripper assembly is actuated by pressurized operating
fluid supplied to the cylinder 602, on the aft side of the enclosed
piston. The pressurized fluid causes the piston, piston rod 604,
and the slider element 606 to slide forward against the force of
the return spring. As explained above, this causes the rollers 626
to roll up the ramps 630 and deflect the toes 612 radially outward.
The toe support 610 freely slides aftward to accommodate the
deflection of the toes 612. The gripper assembly is retracted by
reducing the pressure aft of the piston, which causes the return
spring to push the piston, piston rod 604, and slider element 606
aftward. The rollers 626 roll down the ramps 630, allowing the toes
612 to relax.
[0234] FIGS. 22-29 illustrate the design of the toes 612, toe
supports 608 and 610, and the slider element 606. The ends 614 and
616 of the toes 612 include elongated slots 607 and 609,
respectively. The slots receive axles 611 secured to the toe
supports 608 and 610. The slots 607 and 609 reduce potentially
dangerous compression loads in the toes 612 when the toes
experience external forces (e.g., sliding friction against the
borehole surface). FIGS. 22-25 show a toe 612 in a normal position
with respect to the (retracted) slider element 606 and toe supports
114 and 116, as the toe will shift forward due to gravity. FIGS.
26-29 show the toe 612 in a shifted position, which occurs when the
toe experiences an aftwardly directed external force. As shown in
FIGS. 24 and 28, as the toes 612 shift axially between these
positions, the aft rollers 626 remain between the ramps 630 without
rolling up the aft ramps. In other words, external forces applied
to the toes do not cause the gripper assembly to self-energize.
[0235] As shown in FIGS. 30 and 31, each toe 612 includes four
spacer tabs 620 that extend radially inward from the toe's
sidewalls 618. Two spacer tabs 620 are positioned on each sidewall
618, one tab near each end of the sidewall. The spacer tabs 620 are
configured to bear against the slider element 606 when the toes 612
are relaxed. Also, as shown in FIG. 32, when the toes 612 are
relaxed the rollers 626 do not contact the slider element 606.
Thus, when the toes 612 are relaxed, the spacer tabs 620 absorb
radial loads between the toes and the slider element 606 and also
prevent undesired loading of the rollers 626 and roller axles
628.
[0236] As shown in FIGS. 33 and 34, each toe 612 includes four
alignment tabs 622 that, like the spacer tabs 620, extend radially
inward from the toe's sidewalls 618. A pair of alignment tabs 622
is provided for each of the ramp/roller combinations, one tab on
each sidewall 618. Each pair of alignment tabs 622 straddles one of
the ramps 630 and thus maintains the alignment between the roller
626 and the ramp. The alignment tabs 622 prevent the rollers 626
from sliding off of the sides of the ramps 630, particularly when
the rollers are near the radial outward ends or tips of the
ramps.
[0237] With reference to FIG. 33, each ramp 630 of the slider
element 606 is configured to have a relatively steeper initial
inclined surface 632 followed by a relatively shallower inclined
surface 634. This causes the toes 612 to deflect radially outward
at an initially high rate, followed by a low rate of deflection.
Advantageously, during actuation of the gripper assembly, the toes
612 quickly approach the borehole surface. Before the toes 612
contact the borehole, the rate of expansion is slowed as the
rollers roll along the shallower surfaces 634, to permit a degree
of fine tuning of the radial expansion.
[0238] The gripper assemblies 104 and 106 are preferably formed of
CuBe, but other materials can be employed. For example, the
flexible toes can be formed of Titanium, and the mandrel can be
formed of steel.
[0239] It will be understood that the tractor 100 can be utilized
with any of a variety of different types of gripper assemblies. For
example, U.S. Pat. No. 6,464,003 discloses a compatible gripper
assembly in which toggles are utilized to radially expand flexible
toes that grip a passage surface. Many compatible gripper designs
comprise packerfeet. For example, U.S. Pat. No. 6,003,606 to Moore
et al. discloses packerfeet that include borehole engagement
bladders. Another reference, U.S. Pat. No. 6,347,674, discloses one
packerfoot design having bladders strengthened by attached flexible
toes and another packerfoot design in which the bladders and toes
are not attached. Yet another reference, U.S. Pat. No. 6,431,291,
discloses an improved packerfoot design.
[0240] Although this invention has been disclosed in the context of
certain preferred embodiments and examples, it will be understood
by those skilled in the art that the present invention extends
beyond the specifically disclosed embodiments to other alternative
embodiments and/or uses of the invention and obvious modifications
and equivalents thereof. Further, the various features of this
invention can be used alone, or in combination with other features
of this invention other than as expressly described above. Thus, it
is intended that the scope of the present invention herein
disclosed should not be limited by the particular disclosed
embodiments described above, but should be determined only by a
fair reading of the claims that follow.
* * * * *