U.S. patent application number 16/961453 was filed with the patent office on 2021-03-18 for downhole anchoring device.
The applicant listed for this patent is Equinor Energy AS. Invention is credited to Bjorn Torstein Bruun, Gaute Grindhaug.
Application Number | 20210079747 16/961453 |
Document ID | / |
Family ID | 1000005254026 |
Filed Date | 2021-03-18 |
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United States Patent
Application |
20210079747 |
Kind Code |
A1 |
Grindhaug; Gaute ; et
al. |
March 18, 2021 |
Downhole anchoring device
Abstract
There is disclosed an apparatus for axially anchoring a tool
downhole in a well casing, the apparatus comprising: an anchor
configured to be disposed in, and actuatably and rotatably engaged
with, the casing; and a rotatable tool being configured to be
disposed within the casing; whereby the anchor, when rotatably
engaged with the casing, prevents movement of the rotatable tool in
the axial direction A whilst allowing for rotation of the tool
about the axial direction. A corresponding method of anchoring a
tool downhole is also disclosed.
Inventors: |
Grindhaug; Gaute;
(Stavanger, NO) ; Bruun; Bjorn Torstein;
(Stavanger, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Equinor Energy AS |
Stavanger |
|
NO |
|
|
Family ID: |
1000005254026 |
Appl. No.: |
16/961453 |
Filed: |
January 11, 2019 |
PCT Filed: |
January 11, 2019 |
PCT NO: |
PCT/NO2019/050004 |
371 Date: |
July 10, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 23/0415 20200501;
E21B 29/005 20130101; E21B 23/0411 20200501 |
International
Class: |
E21B 23/04 20060101
E21B023/04; E21B 29/00 20060101 E21B029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2018 |
GB |
1800475.4 |
Claims
1. An apparatus for axially anchoring a tool downhole in a well
casing, the apparatus comprising: an anchor configured to be
disposed in, and actuatably and rotatably engaged with, the casing;
and a rotatable tool being configured to be disposed within the
casing; whereby the anchor, when rotatably engaged with the casing,
prevents movement of the rotatable tool in the axial direction
whilst allowing for rotation of the tool about the axial
direction.
2. The apparatus of claim 1, wherein the rotatable tool is
connected below the anchor in the axial direction.
3. The apparatus of claim 1 or 2, wherein the anchor and the
rotatable tool are rotationally locked to one another.
4. The apparatus of any preceding claim, wherein the anchor is
arranged to be actuated into rotatable engagement with the casing
by a fluid, preferably drilling mud.
5. The apparatus of any preceding claim, wherein the anchor
comprises a substantially hollow body.
6. The apparatus of any preceding claim, wherein the anchor
comprises a plurality of extendable casing engagement blocks.
7. The apparatus of claim 6, wherein the blocks are actuatable from
a retracted position in which the blocks are substantially housed
within the body to an actuated position in which the blocks extend
radially from the body.
8. The apparatus of claim 6 or 7, wherein the plurality of blocks
are disposed circumferentially around the anchor in at least one
plane perpendicular to the axial direction, each plane comprising
at least two blocks.
9. The apparatus of any of claims 6 to 8, wherein the plurality of
blocks are disposed in more than one plane perpendicular to the
axial direction, and preferably in at least two planes
perpendicular to the axial direction.
10. The apparatus of any of claims 7 to 9, wherein the blocks are
biased towards the retracted position.
11. The apparatus of any of claims 7 to 10, wherein the body
comprises at least one opening through which the plurality of
blocks extends through in the actuated position.
12. The apparatus of claim 11, wherein a plurality of openings is
provided, and wherein there is preferably one opening for every
block of the plurality of blocks.
13. The apparatus of any of claims 6 to 12, wherein the blocks
comprise at least one engagement wheel rotatably mounted thereto in
a plane perpendicular to the axial direction, the at least one
wheel being configured to rotatably engage with the inner surface
of the casing.
14. The apparatus of claim 13, wherein each of the at least one
wheels on each of the plurality of blocks has a knife-edge
circumference arranged to cut into the surface of the casing.
15. The apparatus of claim 13 or 14, wherein each of the at least
one wheels on each of the plurality of blocks has a circular outer
circumference.
16. The apparatus of claims 13 to 15, wherein each of the wheels is
formed of high carbon steel or tungsten carbide.
17. The apparatus of any of claims 13 to 16, wherein the engagement
of the wheels with the casing forms a groove in the casing, whereby
the wheel may freely rotate against the casing in a plane of the
groove whilst preventing movement of the wheels in the axial
direction.
18. The apparatus of any of claims 13 to 17, wherein a wheel on
each of the plurality of blocks is axially aligned with at least
one other wheel on another block.
19. The apparatus of any preceding claim, wherein the rotatable
tool is a casing cutter for cutting through the casing.
20. The apparatus of claim 19, wherein the casing cutter comprises
a plurality of blades, the blades being hydraulically actuatable
into contact with the casing.
21. The apparatus of claim 20, wherein the blades comprise high
carbon steel, or tungsten carbide.
22. A drillstring comprising the apparatus of any preceding claim,
wherein the drillstring extends downwardly from a rig or vessel,
and wherein rotation of the drillstring drives the apparatus into
simultaneous rotation.
23. The drillstring of claim 22, configured to allow fluid to pass
down therethrough to engage the apparatus.
24. The drillstring of claim 22 or 23, further comprising a length
compensating device (LCD) disposed above the apparatus in the axial
direction.
25. The drillstring of claim 24, wherein the LCD is a bumper
sub.
26. The drillstring of claim 24 or 25, further comprising a
controller associated with the LCD, the controller being configured
to alter the length of the LCD.
27. The drillstring of any of claims 22 to 26, wherein the
drillstring is connected to a surface heave compensator disposed at
the rig or vessel.
28. A method of anchoring a tool downhole within a casing of a
subsea oil well comprising the steps of: providing: a drillstring
extending downwardly from a rig or vessel; an actuatable anchor
rotatably mounted on the drillstring; and a rotatable tool disposed
on the drillstring; locating the tool in a desired position
downhole within the casing; actuating the anchor into rotatable
engagement with the casing; and actuating the tool; whereby the
anchor prevents axial movement of the actuated tool.
29. The method as claimed in claim 28, wherein the tool is a casing
cutter.
30. The method as claimed in claim 29, further comprising extending
the casing cutter into rotational contact with the casing whilst
allowing the casing cutter to rotate about the axial direction so
as to cut through a depth of the casing.
31. A method of cutting a casing of a subsea oil well comprising
the steps of: providing the drillstring of any of claims 22 to 27;
lowering the drillstring from the rig or vessel to align the anchor
and the casing cutter in a desired position within the casing; and
passing a fluid down the drillstring to activate the anchor and the
casing cutter, wherein, the fluid actuates the anchor into
rotatable engagement with the casing and the casing cutter into
contact with the casing such that the anchor prevents movement of
the casing cutter in the axial direction whilst allowing the casing
cutter to rotate about the axial direction to cut through a depth
of the casing in a radial plane perpendicular to the axial
direction.
32. The method of claim 31, further comprising the use of a length
compensation device controlled by a controller.
33. An apparatus for cutting a casing of a subsea oil well, the
apparatus comprising: a drillstring extending downwardly from a rig
or vessel; an anchor rotatably mounted on the drillstring and
configured to be disposed in, and rotatably and actuatably engaged
with, the casing; and a rotatable casing cutter disposed on the
drillstring and configured to be disposed within and cut through
the casing; whereby the anchor, when rotatably engaged with the
casing, prevents movement of the casing cutter in the axial
direction whilst allowing for rotation of the casing cutter about
the axial direction.
Description
[0001] The present invention relates to an apparatus and method for
preventing axial motion of a subsea tool caused by heave felt at
the rig or vessel from which the apparatus extends, and in
particular to preventing axial motion of a casing cutter caused by
heave to allow for improved casing cutting. The apparatus and
method are also applicable to other subsea tools and operations
where relative axial motion of the tool and well structure should
be avoided.
[0002] In the oil and gas industry it is commonplace to carry out
subsea, downhole operations and/or cutting or otherwise machining
of subsea components using a tool attached to a distal end of a
drillstring, the drillstring extending from a rig or vessel at sea
level.
[0003] For instance, the use of a drillstring, having a cutting
element disposed thereon, to cut and manipulate a subsea well
casing or other component that is disposed below the seabed, is
commonplace.
[0004] One such category of downhole operations that typically
employ the above combination of a drillstring and cutting element
are plug and abandonment (P&A) operations. An exemplary P&A
operation is depicted in FIGS. 5A-5D, in which a distal portion 110
of a drillstring 104 is deployed downhole in a subsea casing 105 in
order to cut said casing 105, specifically an inner casing 105a
positioned within an outer casing 105b in the depicted example,
using a hydraulically actuated casing cutter 111.
[0005] As seen in FIG. 5B, the drillstring 104 is rotated, which in
turn rotates the casing cutter 111. The casing cutter 111 is
hydraulically actuated to engage with the inner casing 105a and,
due to its rotation, cuts through the inner casing 105a. This
enables removal of a section 105c of said inner casing 105a as
shown in FIGS. 5C and 5D. After the inner casing 105a or the
section 105c of the inner casing 105a is removed, the remaining
part of casing 105a is then plugged, usually with cement.
[0006] Similarly and as equally represented by FIGS. 5A-5D, slot
recovery operations employ a casing cutter 111 incorporated on a
drillstring 104 to remove a section 105c of a casing 105 (an inner
casing 105a in the example depicted) from a non-producing, plugged
well within a slot. Following the removal of such a section, a new
wellbore is typically sidetracked from the existing well to allow
for the continued recovery of resources from the previously
non-producing slot. The slot production lifetime is thus extended
through the recovery of resources from the new, sidetracked
wellbore.
[0007] A requirement for the successful cutting of the casing that
occurs in both slot recovery and P&A operations is a steady
drillstring as there needs to be little, or no, axial displacement
of the cutting element in order to provide a successful cut of the
casing. Indeed, in many downhole operations, as well as the cutting
operations discussed above, it is often a requirement that there is
little or no axial displacement of the relevant downhole tool to
ensure successful completion of the operation. As such, clear
operational limits for slot recovery, P&A, and other downhole
operations exist due to the effects of heave felt by the rig or
vessel at sea level caused by surface waves. As a result, operation
may be delayed significantly because of poor sea conditions.
[0008] The movement of the rig or vessel can be accommodated to
some extent by flexing or stretching of the drillstring on which
the downhole tool is disposed. However, to further counteract the
motion of the rig or vessel that the drillstring extends from, it
is known to employ surface heave compensators disposed on the rig
or vessel. These surface heave compensators counteract the
up-and-down motion experienced by the drillstring due to motion of
the rig or vessel by providing a temporary displacement in the
drillstring to counterbalance this upward or downward motion. The
aim of the surface heave compensators is to keep the position of
the drillstring, and hence the downhole tool, constant in an axial
direction. In the exemplary P&A and slot recovery operations
discussed above, keeping the cutting element 111 in a constant
position in the axial direction ensures that a cut can be
successfully performed, smoothly and in a single plane
perpendicular to the axis of the casing 105.
[0009] However, surface heave compensators have only been found to
be partly effective and they are unable to completely negate axial
displacement experienced by the downhole tool due to the effects of
heave felt at the surface, particularly in severe weather
conditions. Moreover, the length compensation provided to the
downhole tool by the surface heave compensator has been proven to
be imprecise, often over-shooting or under-shooting the required
length compensation that is required to keep the downhole tool
steady. Thus, there are often long waiting times associated with
downhole operations (e.g. both P&A and slot recovery operations
where a precise cut is required) whilst the rig or vessel
experiences high degrees of heave, especially in the autumn and
winter seasons.
[0010] It is also known to provide a second length compensating
device, similar in functionality to the surface heave compensator,
elsewhere along the length of the drillstring in order to try and
further counteract the effects of heave, such as a bumper sub, an
accelerator or an intensifier. However, these additional length
compensating devices typically only provide partial improvements to
both precision and effectiveness due to their limited stroke. As
such, drillstrings that make use of both a surface heave
compensator and an additional length compensating device are still
unable to completely negate the axial displacement experienced by
the downhole tool due to the effects of heave felt at the
surface.
[0011] U.S. Pat. No. 2,534,858 discloses a device adapted to be
lowered into a casing within a well bore for the purpose of
severing and removing a portion of the casing from the well. The
device comprises a cutter assembly and an anchor assembly. The
anchor assembly comprises an anchor casing that house slips and
blocks which are adapted to be expanded radially outwardly through
openings or from recesses in the anchor casing. Once expanded
radially outwardly, the slips and blocks of the anchor assembly
frictionally engage with the casing so as to prevent rotation of
the anchor assembly relative to the casing, similar in action to a
drum brake, whilst also preventing axial movement of the anchor
assembly. The cutter assembly of the device is axially connected to
the anchor assembly and, by virtue of a bearing, is able to rotate
relative to the anchor assembly. During a cutting operation, the
slips and blocks of the anchor assembly engage with the casing so
as to prevent any axial and rotational movement of the anchor
assembly relative to the casing, whilst the cutter assembly rotates
about the axial direction so as to cut through the casing. The
connection between the cutter assembly and the anchor assembly
allows for the relative rotation between the two to ensure that a
cut of the casing may be produced, whilst it also ensures that
axial movement of the cutter assembly during the cutting operation
is prevented by virtue of the frictional engagement between the
slips/blocks and the casing.
[0012] The device of U.S. Pat. No. 2,534,858 provides an
improvement in negating axial displacement of a downhole tool
(cutter assembly) during operation as compared to those other
devices discussed above. However, the solution proposed in U.S.
Pat. No. 2,534,858 is mechanically complex, not least because it
requires two separate downhole assemblies to be rotatably connected
with one another via a bearing mechanism.
[0013] According to the invention, there is provided an apparatus
for axially anchoring a tool downhole in a well casing, the
apparatus comprising: an anchor configured to be disposed in, and
actuatably and rotatably engaged with, the casing; and a rotatable
tool configured to be disposed within the casing; whereby the
anchor, when rotatably engaged with the casing, prevents movement
of the rotatable tool in the axial direction whilst allowing for
rotation of the tool about the axial direction.
[0014] Thus, by means of the invention, the tool is held in the
desired position in the axial direction by the anchor so that it
may perform its function effectively and precisely. Furthermore,
where a drillstring is potentially affected by heave of a surface
vessel, this may be accommodated in the known manner by
flexing/stretching of the drillstring and/or by means of heave
compensators, or similar devices, whilst the anchor ensures that
the tool remains in its desired axial location. Moreover, since the
anchor is disposed within the casing in a rotatable manner, the
apparatus may be used to anchor rotatable tools without the need
for additional bearing mechanisms. Thus, it will appreciated be
that the apparatus of the present invention provides a mechanically
simpler, and thus potentially more reliable means for negating
axial displacement of a downhole tool during operation than the
device disclosed in U.S. Pat. No. 2,534,858.
[0015] Both the rotatable tool and the anchor of the invention are
positioned on the apparatus along the axial direction, and it is
preferable that the rotatable tool is connected below the anchor in
the axial direction.
[0016] Although the invention is useful in connection with any
rotary tool, and in particular any downhole rotary tool, that needs
to be retained in a given axial position, it is particularly useful
in the context of casing cutting operations and so the tool is
preferably a casing cutter.
[0017] As noted above, the anchor is arranged to rotate with the
drillstring relative to the casing, whilst axially anchored
thereto, and so it and the rotatable tool may be rotationally
locked to one another. However, the anchor and the rotatable tool
may be arranged such that the tool rotates relative to the anchor
or relative to a part of it, e.g. so that the tool may rotate at a
different speed to the drillstring.
[0018] Any suitable mechanism may be used to actuate the engagement
of the anchor--e.g. where electrical power is available solenoid(s)
may be used. However, the anchor is preferably arranged to be
actuated into engagement with the casing by a fluid, which will
typically be drilling fluid ("mud") since it is conventional for a
flow of drilling mud to be provided from a platform or surface
vessel via a drillstring to downhole tools. For example, drilling
mud-driven hydraulic actuators may be used. Likewise, whilst the
tool may be driven by an electric motor, or simply, and preferably,
driven by the rotation of the drill string, it is optionally caused
to rotate by the fluid, e.g. by means of a drilling mud-driven
hydraulic motor. Furthermore, it is convenient to arrange this so
that the tool is driven simultaneously with the actuation of the
anchor.
[0019] In order to allow the anchor to engage with the casing, it
will typically comprise an expandable engagement system. A
convenient arrangement is for the anchor to comprise a (e.g.
substantially hollow) body housing a plurality of extendable casing
engagement members, referred to here as blocks. Thus, these blocks
may be actuatable from a retracted position in which the blocks are
substantially within the body to an actuated position in which the
blocks extend radially from the body. Suitable openings in the body
may be provided through which the blocks may extend into the
actuated position.
[0020] In order to ensure sound engagement and to enable the
apparatus to rotate, the blocks are preferably disposed
circumferentially around the anchor in at least one plane
perpendicular to the axial direction, with each plane comprising at
least two blocks. However, preferably at least three or more blocks
are provided in each plane to distribute the loads more widely.
[0021] Indeed, in applications where the anchor has to deal with
significant axial loading, the plurality of blocks are preferably
disposed in more than one plane perpendicular to the axial
direction and preferably in two or three such planes.
[0022] Whilst the blocks may be configured to require positive
actuation and retraction, in order to simplify design and to ensure
that the drillstring may be recovered in the event of a loss of
fluid communication or other failure, the blocks are biased towards
the retracted position. Thus, cessation of the flow of drilling mud
will result in retraction of the blocks under the bias force and
hence deactivation of the anchor.
[0023] It is preferable that the blocks have rotary engagement
means, which may also act as a bearing (i.e. similarly to a roller
bearing). Thus, preferably they each comprise at least one
engagement wheel rotatably mounted thereto. Such wheels are most
preferably arranged in a plane perpendicular to the axial
direction. The wheels may then be configured to engage with the
inner surface of the casing when the blocks are in the actuated
position.
[0024] Whilst in some cases sufficient purchase on the casing to
anchor the apparatus may be provided by frictional engagement, it
is preferred that the rotary engagement means on each of the
plurality of blocks is a wheel arranged to cut into the surface of
the casing, e.g. by having knife-edge circumference. Such an
arrangement obviously requires the engagement surface of the wheels
to be harder than the casing. Accordingly, each of the wheels may
be formed of high carbon steel or comprise tungsten carbide
elements.
[0025] The wheels may have any suitable circumferential geometry
that allows for rotation of the anchor, and that ensures the casing
is sufficiently cut into. For instance, the outer circumference of
the wheel may be elliptical, triangular, star-shaped, or indeed any
suitable geometrical shape. However, it is preferred that the
wheels have a circular outer circumference to provide uniformity to
both the rotation of the anchor and the cut imprinted into the
casing.
[0026] Thus, the engagement of the wheels with the casing may form
a groove in the casing within which they may then run, whereby the
wheels may freely rotate against the casing in a plane of the
groove, whilst preventing movement of the wheels in the axial
direction because of their engagement in the groove.
[0027] Where multiple blocks are provided, a wheel on each of the
plurality of blocks may conveniently be axially aligned with at
least one other wheel on another block.
[0028] As discussed above, the rotatable tool may be a casing
cutter for cutting through the casing. Any cutter suitable for
cutting through a depth of the casing may be used, for instance
laser cutters or abrasive cutters. However, it is preferred that
the casing cutter is a mechanical cutter comprising a blade,
preferably a plurality of blades. Like the other active components
of the apparatus, the blades may be extended radially outwardly
hydraulically, e.g. by drilling fluid. Thus, the flow of the fluid
may result in the blades extending into contact with the
casing.
[0029] As is known in the art, the casing cutter blades should be
harder than the casing itself and so the blades may be formed, for
example, from a high carbon steel or tungsten carbide.
[0030] Viewed from another aspect of the invention, there is
provided an apparatus for axially anchoring a tool downhole in a
well casing, the apparatus comprising: an anchor configured to be
rotatably disposed in, and actuatably engaged with, the casing; and
a rotatable tool being configured to be disposed within the casing;
whereby the anchor, when engaged with the casing, prevents movement
of the rotatable tool in the axial direction whilst allowing for
rotation of the tool about the axial direction.
[0031] The apparatus of this further aspect may employ any of the
features outlined above in relation to the first aspect of the
invention.
[0032] The apparatuses of the above aspects are intended for use in
combination with a drillstring and so, viewed from another aspect,
there is provided a drillstring comprising an apparatus as
described above, wherein the drillstring extends downwardly from a
rig or vessel. As is well known in the art, the drillstring may be
configured to permit drilling fluid ("mud") to pass down
therethrough to engage the apparatus.
[0033] Since a particularly useful application of the invention is
in the context of casing cutting, another aspect of the invention
provides an apparatus for cutting a casing of a subsea oil well,
the apparatus comprising: a drillstring extending downwardly from a
rig or vessel; an anchor rotatably mounted on the drillstring and
configured to be disposed in and actuatably and rotatably engaged
with the casing; and a rotatable casing cutter disposed on the
drillstring and configured to be disposed within and cut through
the casing; whereby the anchor, when rotatably engaged with the
casing, prevents movement of the casing cutter in the axial
direction whilst allowing for rotation of the casing cutter about
the axial direction.
[0034] Whilst a certain degree of heave may be accommodated by the
bend and/or stretch of the drillstring (particularly in the case of
long drillstrings), preferably the drillstring further comprises a
length-compensating device (LCD). This may be a bumper sub, an
accelerator or an intensifier. The LCD can form part of the
apparatus, though it is preferably disposed above the apparatus in
the axial direction. The system may further comprise a controller
associated with the LCD, the controller being configured to alter
the length of the LCD to compensate for heave of the surface vessel
or platform and/or allow for precise placement of the casing cutter
in the axial direction. In addition, the drillstring may be
connected to a surface heave compensator disposed at the rig or
vessel.
[0035] The invention also extends to a corresponding method and
thus, viewed from another aspect, there is provided a method of
anchoring a tool downhole within a subsea casing comprising the
steps of providing: a drillstring extending from a rig or vessel
toward the seabed; an actuatable anchor rotatably mounted on the
drillstring; and a rotatable tool disposed on the drillstring;
locating the tool in a desired position downhole within the casing;
actuating the anchor into rotatable engagement with the casing; and
actuating the tool; whereby the anchor prevents axial movement of
the actuated tool.
[0036] The method preferably comprises the use of the apparatus
(and particularly its preferred forms) described above. Thus, for
example, the tool may be a casing cutter.
[0037] In such a case, the method may further comprise extending
the casing cutter into rotational contact with the casing whilst
allowing the casing cutter to rotate about the axial direction so
as to cut through a depth of the casing.
[0038] The method preferably comprises the steps of: lowering the
drillstring from the rig or vessel to align the anchor and the
casing cutter in a desired position within the casing; and passing
a fluid down the drillstring to activate the anchor and the casing
cutter; wherein the fluid actuates the anchor into rotatable
engagement with the casing and the casing cutter into rotational
contact with the casing such that the anchor prevents movement of
the casing cutter in the axial direction whilst allowing the casing
cutter to rotate about the axial direction to cut through a depth
of the casing in a radial plane perpendicular to the axial
direction.
[0039] As previously described, the method may further comprise the
use of a length compensation device controlled by a controller.
[0040] Certain embodiments of the present invention will now be
described, by way of example only, and with reference to the
accompanying drawings in which:
[0041] FIG. 1 is a schematic view of a rig or vessel carrying out
an exemplary downhole operation in a subsea well;
[0042] FIG. 2 is an enlarged schematic view of a distal end portion
of a drillstring, disposed within a casing of a subsea well, that
incorporates an anchor of a first embodiment used in a P&A or
slot recovery operation;
[0043] FIG. 3 shows a close up view of an embodiment of an anchor
disposed on the drillstring;
[0044] FIG. 4 shows a close up view of another embodiment of the
anchor disposed on the drillstring;
[0045] FIGS. 5A-5D are sequential enlarged schematic views of a
distal end portion of a drillstring during the various stages
involved in cutting of a casing in a P&A or slot recovery
operation as is known from the prior art.
[0046] FIG. 1 illustrates a floating rig or vessel 3 carrying out a
downhole operation. Specifically, FIG. 1 depicts a cutting process
on a casing 5 of a subsea well that is typically done in a P&A
or slot recovery operation. The rig or vessel 3 is positioned at
sea level 1 in a position that is approximately axially aligned
with the casing 5 extending from the seabed 2. A drillstring 4
extends down from the rig or vessel 3 to the casing 5 and is partly
received within a casing 5 of the subsea well, such that the distal
end portion 10 is disposed hundreds of metres downhole within the
casing 5 (see FIG. 2). The distal end portion 10 is the portion of
the drillstring predominantly responsible for cutting the casing 5
and will be described further below with reference to FIGS.
2-4.
[0047] It can clearly be seen in FIG. 1 how the effects of heave at
the rig or vessel 3 due to surface waves would lead to an axial
displacement of the drillstring 4. In FIG. 1 the surface of the sea
1 is depicted as level and, at the particular moment in time
depicted in FIG. 1, there is no requirement for any length
compensation to be provided to the distal end portion 10 of the
drillstring 4. However, a surface heave compensator (not shown) is
provided at the rig or vessel 3 to counteract some of the potential
axial displacement of the drillstring 4 that may be caused by the
heave of the rig or vessel 3. The remainder of the length
compensation that is required to counteract the potential effects
of heave felt by the drillstring 4 is provided by flexing or
stretching of the drillstring 4 and/or elements contained in the
distal end portion 10, which are described in more detail
below.
[0048] Turning now to FIG. 2, the distal end portion 10 of the
drillstring 4 of FIG. 1 is shown. The distal end portion 10 is
disposed downhole within a section of the casing 5. The distal end
portion 10 comprises a casing cutter 11, an anchor 12 and a
length-compensating device (LCD) 13.
[0049] The LCD 13 is located above the anchor 12 and the casing
cutter 11 on the drillstring 4 and it provides length compensation
to both the anchor 12 and the casing cutter 11 such that when the
drillstring 4 is caused to move upward in an axial direction A due
to the effects of heave felt by the rig or vessel 3 at the surface,
the LCD 13 extends in length along the axial direction A so that
the anchor 12 and the casing cutter 11 experience no, or very
little, displacement in the axial direction A. Following any upward
motion of the rig or vessel 3 along the axial direction due to
heave, there will be a consequential downward motion of comparable
magnitude of the rig or vessel 3. During said downward motion the
LCD 13 will contract from its extended length to ensure that the
anchor 12 and casing cutter 11 again experience little, or no,
displacement along the axial direction A. Thus, the LCD 13 provides
some of the required length compensation to ensure that, regardless
of the heave of the rig or vessel 3, the anchor 12 and the casing
cutter 11 are maintained approximately, or precisely, in the same
position relative to the axial direction A.
[0050] In the embodiments depicted herein the LCD 13 is a bumper
sub. The bumper sub not only compensates for movement of the
drillstring 4 in an axial direction A due to the upward and
downward heave caused by typical surface waves, it also compensates
for downward movement of the drillstring 4 due to a sudden downward
displacement of the rig or vessel 3 below the resting sea level,
for instance when heavy cargo is placed on the rig or vessel 3.
This additional length compensation is achieved by the ability of
the bumper sub to contract from its resting length along the axial
direction A. This contraction in length of the bumper sub can
accommodate for some, or all, of the downward displacement of the
drillstring in the axial direction A caused by a downward
displacement of the rig or vessel 3. Thus, as well as providing
length compensation to the anchor 12 and the casing cutter 11 when
the drillstring 4 is displaced in the axial direction A due to the
effects of heave, the bumper sub also limits or prevents
displacement of both the anchor 12 and the casing cutter 11 along
the axial direction A when the drillstring 4 is displaced suddenly
downward in the axial direction A.
[0051] As can also be seen in FIG. 2, the casing cutter 11 on the
drillstring 4 is positioned below both the LCD 13 and the anchor
12. In the embodiment of FIG. 2, the cutter 11 has a plurality of
blades 16, the blades 16 being formed from a material that is
harder than the casing 5, e.g. a high carbon steel or tungsten
carbide. The casing cutter 11 of the depicted embodiments is a
hydraulically actuated cutter. It is configured such that when
drilling mud is passed down through the drillstring 4 toward the
distal end portion 10, the relatively high fluid pressure within
the drillstring 4 forces the blades 16 of the casing cutter 11
radially outward from the drillstring 4 and into contact with an
inner circumference of the casing 5. Thus, when the casing cutter
is driven into rotation due to rotation of the drillstring 4, the
engagement of the casing cutter 11 with the casing 5 enables the
blades 16 to cut through the casing 5.
[0052] The anchor 12 is positioned on the drillstring 4 in a
position below the LCD 13 but above the casing cutter 11. As can be
seen in more detail in FIG. 3, the anchor 12 comprises a
predominantly hollow body 21 that houses a plurality of extendable
blocks 22. Hollow body 21 is in fluid communication with the
drillstring 4 disposed above the anchor 12 and is also in fluid
communication with the casing cutter 11 disposed below the anchor
12. Thus, when drilling mud flows down the drillstring 4, towards
the distal end portion 10, the drilling mud hydraulically engages
both the anchor 12 and the casing cutter 11 simultaneously. Hence,
the anchor 12 and the casing cutter 11 are hydraulically actuated
simultaneously.
[0053] The blocks 22 are movable between a retracted position, in
which the blocks 22 are predominantly housed within the body 21,
and an actuated position in which the blocks 22 are extended
radially from the body 21 through openings 23 as is shown in FIG.
3. The blocks 22 are configured such that their movement is limited
between the retracted position and the actuated position, and they
are biased toward the retracted position. The blocks 22 are
actuated by drilling mud that is passed down the drillstring 4
which causes the blocks 22 to extend against their bias into their
actuated position.
[0054] In the embodiment depicted in FIG. 3, two blocks 22 are
shown disposed diametrically opposed to one another about the
anchor 12 to form a single axial row of blocks 22. Each block 22 is
aligned with an opening 23 in the body 21 that allows for the
blocks 22 to move from the retracted position to the actuated
position.
[0055] As shown in FIG. 3, each block 22 has mounted thereon a set
25 of wheels 24, each set 25 comprising two wheels 24. Each wheel
24 is disposed in a plane perpendicular to the axial direction A
and, in the present embodiment, each wheel 24 is axially aligned
with one other wheel 24 on the block 22 diametrically opposed to
the block 22 on which it is disposed. Those wheels 24 that share a
common plane are said to form a plane of wheels 26.
[0056] The wheels 24 are rotatably supported on the blocks 22 in a
manner that allows for rotation of each wheel 24 about its axis.
The ability of the wheels 24 to rotate about their own axis means
that the wheels 24 provide bearing support to the anchor 12 when
disposed downhole within the casing 5 in the manner of roller
bearings. The wheels 24 are formed from a material that is harder
than the steel of the casing 5, for instance tungsten carbide or
high carbon steel and their outer circumference is a sharp
knife-edge. Thus, when the blocks 22 are hydraulically actuated
into engagement with the casing 5 they score a groove into the
inner circumference of the casing 5. These grooves provide a track
for wheels 24 that allows the wheels 24 to freely rotate around the
inner circumference of the casing 5 in their own plane that is
perpendicular to the axial direction A whilst inhibiting any
movement of the wheel 24 in the axial direction A.
[0057] All of the components in the distal end portion 10, i.e. the
LCD 13, anchor 12 and cutter 11, are rotationally locked to the
drillstring 4. Thus, when the drillstring 4 is driven into rotation
at the rig or vessel 3, the casing cutter 11, the anchor 12 and the
LCD 13 are also driven into rotation about the axial direction
A.
[0058] The process of cutting the casing 5 begins with alignment of
the casing cutter 11 into a desired position along the axial
direction A in the casing 5. This is achieved by lowering the
drillstring 4 from the rig or vessel 3 until the casing cutter 11
is positioned as desired, downhole within the casing.
[0059] Once the cutter 11 is placed in the desired downhole
position, the drillstring 4 is driven into rotation, which in turn
drives rotation of the distal end portion 10. Jointly, or shortly
afterwards, drilling mud is passed down the drillstring 4 toward
the distal end portion 10. The drilling mud causes the blocks 22 of
the anchor to extend from their retracted position through openings
23 and into their actuated position, thereby forcing the wheels 24
disposed on each block into engagement with the casing.
Simultaneously, the drilling mud causes the blades 16 of the casing
cutter 11 to extend radially outwards and into contact with the
inner circumference of the casing 5.
[0060] As the anchor 12 rotates, the wheels 24 move about their own
axes around the inner circumference of the casing 5 (i.e. they
rotate counter-synchronously to the anchor 12). This provides the
necessary bearing support to the anchor 12 within the casing 5 such
that the rotation of the anchor 12 and the casing cutter 11 about
the axial direction A occurs relatively freely. As it does so, each
plane of wheels 26 scores a circumferential groove in the casing 5
in which each plane of wheels 26 is free to run. The depth of the
groove that each plane of wheels 26 forms in the casing 5 depends
upon the drilling mud pressure and the relative dimensions of the
anchor 12 and casing 5. It will be appreciated that the engagement
of each plane of wheels 26 with each groove allows for the anchor
12 and the casing cutter 11 to freely rotate about the axial
direction A whilst also preventing movement of the anchor 12 in the
axial direction A.
[0061] As a result, the blades 16 are maintained in a constant
position in the axial direction relative to the casing during the
cutting process. Cutting may therefore proceed until a section of
the casing 5 is separated from the remainder thereof.
[0062] The cutting process may then be terminated by stopping
rotation of the drillstring 4 and stopping the flow of drilling mud
to the distal end portion 10 of the drillstring 4. As such, the
blades 16 retract radially inward under the bias force and the
blocks 22 also withdraw into their retracted position within the
body 21 such that there is no longer any engagement of the wheels
24 or blades 16 with the casing 5. The drillstring 4 may then be
removed upwardly along the axial direction A from the casing 5 and
be recovered and the portion of the casing 5 that has been cut from
the remainder of the casing 5 may be removed.
[0063] During the cutting process, if the rig or vessel 3 is caused
to move in an axial direction A due to the effects of heave, the
interaction of the wheels 24 with the grooves that are formed in
the inner circumference of casing 5, in combination with the length
compensation provided by the surface heave compensator, the
internal flexibility of the drillstring 4 and/or the LCD 13,
ensures that the casing cutter 11 remains in a constant position
along the axial direction A. For instance, when the rig or vessel 3
undergoes upward heave while the anchor 12 is actuated into
engagement with an inner circumference of casing 5, the distal end
of the drillstring 4 is held in place by anchor 12 whilst the
remainder of drillstring 4 will extend in length by means of an
extension in the LCD 13 and/or by means of the surface heave
compensator, along with a certain amount of extension of
drillstring 4 due to its inherent flexibility. Thus, the casing
cutter 11 is kept in a constant axial position for the duration of
the upward heave of the rig or vessel 3 and cutting of the casing 5
can be maintained in a single plane even during upward heave of the
rig or vessel 3.
[0064] Subsequently, after said upward heave, the rig or vessel 3
will undergo a corresponding downward heave, which is accommodated
in a similar manner except that the relevant components contract.
As such, the casing cutter 11 is kept in a constant axial position
for the duration of the heave cycle of the rig or vessel 3.
[0065] In cases where the axial forces due to heave may be larger,
it is beneficial to provide additional wheels 24 that allow for the
anchor 12 to tolerate larger axial loads whilst maintaining a
constant axial position. An embodiment having additional wheels is
depicted in FIG. 4. This embodiment is the same as the first
embodiment, except that it comprises four blocks 22 arranged in two
rows. This provides additional wheels 24 that interact with the
casing 5 and thereby increases the axial resistance of the anchor
12 when engaged with the casing 5.
[0066] As the anchor 12 and the cutter 11 are only ever actuated
simultaneously or almost simultaneously by the drilling mud that is
passed through the drillstring 4, if the rig or vessel 3
experiences an amount of heave that displaces the drillstring 4
along the axial direction A by an amount that falls outside of the
limits that the surface heave compensator, the inherent flexibility
of the drillstring 4 and/or the LCD 13 can tolerate, temporarily
terminating the provision of drilling mud to the distal end portion
10 allows the anchor to disengage from the casing 5 and the cutter
11 will retract and stop cutting. This prevents damage to the
drillstring 4, anchor 12 and cutter 11, whilst also ensuring that
the quality of the cut in the casing 5 is maintained in a single
plane. Once the axial displacement of the drillstring 4 is back
within tolerable limits of length compensation, the process of
cutting the casing 5 as outlined above may be resumed.
* * * * *