U.S. patent application number 11/511162 was filed with the patent office on 2007-08-16 for system for and method of restraining a subsurface exploration and production system.
Invention is credited to Charles H. King, Eric E. Maidla, Keith K. Millheim.
Application Number | 20070187109 11/511162 |
Document ID | / |
Family ID | 37591829 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070187109 |
Kind Code |
A1 |
Millheim; Keith K. ; et
al. |
August 16, 2007 |
System for and method of restraining a subsurface exploration and
production system
Abstract
A system for and method of limiting and controlling the
unintended subsurface release of an exploration or production riser
system is provided including one or more means for anchoring the
riser or casing stack at one or more pre-determined points upon the
length of the riser, and/or on the housing of an associated
buoyancy chamber or the like, and/or on a particular portion of the
riser as dictated by the operational environment, and/or on an
anchor portion secured in the sea floor; and a network of
restraining members disposed on the anchoring means. A lower
anchoring portion includes one or more anchors disposed in
communication with a wellhead, or with the sea floor or below the
sea floor mud line, or with a well casing portion. A network of
restraining members forms an essentially continuous connection from
the buoyancy member portion to said bottom anchor portion. In a
particular, though, non-limiting embodiment of the invention, a
means for anchoring the system using pairs of anchors disposed at
one or more predetermined points along the riser portion of the
system is provided. Also disclosed is a variety of means and
devices by which a surface vessel or a rig, etc., servicing a
subsea well equipped with the present system may absorb or deflect
impact forces originating from portions of the system that
unexpectedly break free and rush upwards toward the surface vessel
or rig.
Inventors: |
Millheim; Keith K.; (The
Woodlands, TX) ; Maidla; Eric E.; (Sugar Land,
TX) ; King; Charles H.; (Austin, TX) |
Correspondence
Address: |
ARNOLD & FERRERA, L.L.P.
2401 FOUNTAIN VIEW DRIVE, SUITE 630
HOUSTON
TX
77057
US
|
Family ID: |
37591829 |
Appl. No.: |
11/511162 |
Filed: |
August 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60772078 |
Feb 10, 2006 |
|
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|
Current U.S.
Class: |
166/367 |
Current CPC
Class: |
E21B 41/0021 20130101;
E21B 17/012 20130101 |
Class at
Publication: |
166/367 |
International
Class: |
E21B 17/01 20060101
E21B017/01 |
Claims
1. A method for restraining the release of a subsurface riser
system, said method comprising the steps of: disposing one or more
anchoring members at one or more predetermined points along a
length of said riser system; and disposing one or more restraining
members in communication with said one or more anchoring
members.
2. The method of claim 1, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members in communication with an associated buoyancy
member.
3. The method of claim 2, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one surface of said buoyancy
member.
4. The method of claim 2, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one longitudinal portion of an
upper riser segment disposed above said buoyancy member.
5. The method of claim 2, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one longitudinal portion of a
lower riser segment disposed beneath said buoyancy member.
6. The method of claim 1, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one portion of an associated
well casing.
7. The method of claim 1, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one portion of an associated sea
floor.
8. The method of claim 7, wherein said step of disposing one or
more anchoring members further comprises a step of disposing one or
more anchoring members on at least one portion of the sea floor
disposed beneath the mud line.
9. The method of claim 1, wherein said step of disposing one or
more restraining members further comprises a step of disposing a
restraining member between a first predetermined point and a second
predetermined point disposed along a length of the riser
system.
10. The method of claim 1, wherein said step of disposing a
restraining member further comprises a step of disposing at least
one restraining member between a buoyancy member and a
predetermined point along a length of said system.
11. The method of claim 1, wherein said step of disposing one or
more restraining members further comprises a step of disposing at
least one restraining member between a predetermined point along a
length of said riser system and a wellhead disposed in
communication with said system.
12. The method of claim 1, wherein said step of disposing one or
more restraining members further comprises a step of disposing at
least one restraining member between a predetermined point along a
length of said riser system and a predetermined point disposed
beneath a wellhead associated with said system.
13. The method of claim 1, wherein said step of disposing one or
more restraining members further comprises a step of disposing at
least one restraining member between a predetermined point along a
length of said riser system and a predetermined point disposed
beneath the sea floor mud line.
14. The method of claim 1, wherein said step of disposing one or
more anchoring members further comprises a step of disposing at
least one anchoring member between a first predetermined point and
a second predetermined point located along one or more lengths of
said riser system, wherein said first predetermined point and said
second predetermined point are disposed in functionally close
proximity to one another, thereby constituting an effective
anchoring pair.
15. The method of claim 14, wherein said step of disposing one or
more restraining members further comprises a step of disposing at
least one additional restraining member between said first
predetermined point and said second predetermined point of said
anchoring pair.
16. A system for restraining the release of a subsurface riser
system, said system comprising: one or more anchoring members
disposed at one or more predetermined points along a length of said
riser system; and one or more restraining members disposed in
communication with said one or more anchoring members.
17. The system of claim 16, wherein said system further comprises
one or more anchoring members disposed in communication with an
associated buoyancy member.
18. The system of claim 17, wherein said system further comprises
one or more anchoring members disposed on at least one surface of
said buoyancy member.
19. The system of claim 17, wherein said system further comprises
one or more anchoring members disposed on at least one longitudinal
portion of an upper riser segment disposed above said buoyancy
member.
20. The system of claim 17, wherein said system further comprises
one or more anchoring members disposed on at least one longitudinal
portion of a lower riser segment disposed beneath said buoyancy
member.
21. The system of claim 16, wherein said system further comprises
one or more anchoring members disposed on at least one portion of
an associated well casing.
22. The system of claim 16, wherein said system further comprises
one or more anchoring members disposed on at least one portion of
an associated sea floor.
23. The system of claim 22, wherein said system further comprises
one or more anchoring members disposed on at least one portion of
the sea floor disposed beneath the mud line.
24. The system of claim 16, wherein said system further comprises
at least one restraining member disposed between a first
predetermined point and a second predetermined point disposed along
a length of the riser system.
25. The system of claim 16, wherein said system further comprises
at least one restraining member disposed between a buoyancy member
and a predetermined point along a length of said system.
26. The system of claim 16, wherein said system further comprises
at least one restraining member disposed between a predetermined
point along a length of said riser system and a wellhead disposed
in communication with said system.
27. The system of claim 16, wherein said system comprises at least
one restraining member disposed between a predetermined point along
a length of said riser system and a predetermined point disposed
beneath a wellhead associated with said system.
28. The system of claim 16, wherein said system further comprises
at least one restraining member disposed between a predetermined
point along a length of said riser system and a predetermined point
disposed beneath the sea floor mud line.
29. The system of claim 16, wherein said system further comprises
at least one anchoring member disposed between a first
predetermined point and a second predetermined point located along
one or more lengths of said riser system, wherein said first
predetermined point and said second predetermined point are
disposed in functional proximity to one another, thereby
constituting an effective anchoring pair.
30. The system of claim 29, wherein said system further comprises
at least one additional restraining member disposed between said
first predetermined point and said second predetermined point of
said anchoring pair.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The instant application claims the benefit of prior U.S.
Provisional Application No. 60/772,078, filed Feb. 10, 2006.
FIELD OF THE INVENTION
[0002] The present invention relates generally to methods and means
for improving the stability and safety of offshore exploration and
production systems, and, in a particular, though non-limiting
embodiment, to a system for and method of restraining a
self-standing casing riser system deployed in conjunction with an
adjustable buoyancy chamber, or a functional equivalent
thereof.
BACKGROUND OF THE INVENTION
[0003] Innumerable systems and methods have been employed in
efforts to find and recover hydrocarbon reserves around the world.
At first, such efforts were limited to land operations involving
simple but effective drilling methods that satisfactorily recovered
reserves from large, productive fields. As the number of known
producing fields dwindled, however, it became necessary to search
in ever more remote locales, and to move offshore, in the search
for new resources. Eventually, sophisticated drilling systems and
advanced signal processing techniques enabled oil and gas companies
to search virtually anywhere in the world for recoverable
hydrocarbons.
[0004] Initially, deepwater exploration and production efforts
consisted of expensive, large scale drilling operations supported
by tanker storage and transportation systems, due primarily to the
fact that most offshore drilling sites are associated with
difficult and hazardous sea conditions, and thus large scale
operations provided the most stable and cost-effective manner in
which to search for and recover hydrocarbon reserves. A major
drawback to the large-scale paradigm, however, is that explorers
and producers have little financial incentive to work smaller
reserves, since potential financial recovery is generally offset by
the lengthy delay between exploration and production (approximately
3 to 7 years) and the large capital investment required for
conventional platforms and related drilling and production
equipment. Moreover, complex regulatory controls and industry-wide
risk aversion have led to standardization, leaving operators with
few opportunities to significantly alter the prevailing paradigm.
As a result, offshore drilling operations have traditionally been
burdened with long delays between investment and profit, excessive
cost overruns, and slow, inflexible recovery strategies dictated by
the operational environment.
[0005] More recently, deepwater sites have been found in which much
of the danger and instability present in such operations is
avoided. For example, off the coast of Brazil, West Africa and
Indonesia, potential drilling sites have been identified where
surrounding seas and weather conditions are relatively mild and
calm in comparison to other, more volatile sites such as the Gulf
of Mexico and the North Sea. These recently discovered sites tend
to have favorable producing characteristics, yield positive
exploration success rates, and admit to production using simple
drilling techniques similar to those employed in dry land or
near-shore operations.
[0006] However, since lognormal distributions of recoverable
reserves tend to be spread over a large number of small fields,
each of which yield less than would normally be required in order
to justify the expense of a conventional large-scale operation,
these regions have to date been underexplored and underproduced
relative to its potential. Consequently, many potentially
productive smaller fields have already been discovered, but remain
undeveloped due to economic considerations. In response, explorers
and producers have adapted their technologies in an attempt to
achieve greater profitability by downsizing the scale of operations
and otherwise reducing expense, so that recovery from smaller
fields makes more financial sense, and the delay between investment
and profitability is reduced.
[0007] For example, in published Patent Application No. US
2001/0047869 A1 and a number of related pending applications and
patents issued to Hopper et al., various methods of drilling
deepwater wells are provided in which adjustments to the drilling
system can be made so as to ensure a better recovery rate than
would otherwise be possible with traditional fixed-well
technologies. However, the Hopper system cannot be adjusted during
completion, testing and production of the well, and is especially
ineffective in instances where the well bore starts at a mud line
in a vertical position. The Hopper system also fails to support a
variety of different surface loads, and is therefore self-limiting
with respect to the flexibility drillers desire during actual
operations. The Hopper system also fails to contemplate any
significant safety measures to protect the welfare of operating
crews or the capital expenditure of investors.
[0008] In U.S. Pat. No. 4,223,737 to O'Reilly, a method is
disclosed in which the problems associated with traditional,
vertically oriented operations are addressed. The method of
O'Reilly involves laying out a number of interconnected,
horizontally disposed pipes in a string just above the sea floor
(along with a blow out preventer and other necessary equipment),
and then using a drive or a remote operated vehicle to force the
string horizontally into the drilling medium. The O'Reilly system,
however, is inflexible in that it fails to admit to practice while
the well is being completed and tested. Moreover, the method fails
to contemplate functionality during production and workover
operations. As would therefore be expected, O'Reilly also fails to
teach any systems or methods for improving crew safety or
protecting operator investment during exploration and production.
In short, the O'Reilly reference is helpful only during the initial
stages of drilling a well, and would therefore not be looked to as
a systemic solution for safely establishing and maintaining a
deepwater exploration and production operation.
[0009] Other offshore operators have attempted to solve the
problems associated with deepwater drilling by effectively "raising
the floor" of an underwater well by disposing a submerged wellhead
above a self-contained, rigid framework of pipe casing that is
tensioned by means of a gas filled, buoyant chamber. Generally,
this type of solution falls in the class of self-standing riser
systems, since it typically includes a number of riser segments
fixed in a rigid, cage-like structure likely to remain secure or
else fail together as a integrated system. For example, as seen in
prior U.S. Pat. No. 6,196,322 B1 to Magnussen, the Atlantis
Deepwater Technology Holding Group has developed an artificial
buoyant seabed (ABS) system, which is essentially a gas filled
buoyancy chamber deployed in conjunction with one or more segments
of pipe casing disposed at a depth of between 600 and 900 feet
beneath the surface of a body of water. After the ABS wellhead is
fitted with a blowout preventer during drilling, or with a
production tree during production, buoyancy and tension are
imparted by the ABS to a lower connecting member and all internal
casings. The BOP and riser (during drilling) and production tree
(during production), are supported by the lifting force of the
buoyancy chamber. Offset of the wellhead is reasonably controlled
by means of vertical tension resulting from the buoyancy of the
ABS.
[0010] The Atlantis ABS system is relatively inefficient, however,
in several practical respects. For example, the '322 Magnussen
patent specifically limits deployment of the buoyancy chamber to
environments where the influence of surface waves is effectively
negligible, i.e., at a depth of more than about 500 feet beneath
the surface. Those of ordinary skill in the art will appreciate
that deployment at such depths can be an expensive and relatively
risk-laden solution, given that installation and maintenance can
only be carried out by deep sea divers or remotely operated
vehicles, and the fact that a relatively extensive transport system
must still be installed between the top of the buoyancy chamber and
the bottom of an associated recovery vessel in order to initiate
production from the well.
[0011] The Magnussen system also fails to contemplate multiple
anchoring systems, even in instances where problematic drilling
environments are likely to be encountered. Moreover, the system
lacks any control means for controlling adjustment of either
vertical tension or wellhead depth during production and workover
operations, and expressly teaches away from the use of lateral
stabilizers that could enable the wellhead to be deployed in
shallower waters subject to stronger tidal and wave forces. The
Magnussen disclosure also fails to contemplate any safety features
that would protect the crew and equipment associated with an
operation in the event of a sudden, unintended release of the fluid
transport cage.
[0012] In published Patent Application US 2006/0042800 A1 to
Millheim, et al., however, a system and method of establishing an
offshore exploration and production system is disclosed in which a
well casing is disposed in communication with an adjustable
buoyancy chamber and a well hole bored into the floor of a body of
water. A lower connecting member joins the well casing and the
chamber, and an upper connecting member joins, the adjustable
buoyancy chamber and a well terminal member. The chamber's
adjustable buoyancy enables an operator to vary the height or depth
of the well terminal member, and to vary the vertical tension
imparted to drilling and production strings throughout exploration
and production operations. Also disclosed is a system and method of
adjusting the height or depth of a wellhead while associated
vertical and lateral forces remain approximately constant. A
variety of well isolation members, lateral stabilizers and
anchoring means, as well as several methods of practicing the
invention, are also disclosed. There is, however, little detailed
discussion of safety features useful in the event of an unintended
release of system components.
[0013] Thus, presently known offshore exploration and production
systems, especially those relying on the so-called self-standing
riser type configuration, can be susceptible to a variety of
potentially catastrophic system failures that could lead to damage
or destruction of the drilling platforms and surface vessels
disposed overhead (e.g., a pontoon type drilling rig floating on
the surface of the ocean and disposed in communication with the
riser system).
[0014] For example, casing connections, wellhead connections,
buoyancy chambers connected to the riser stack, etc., can all fail,
thereby creating an unsafe condition in which buoyancy and tension
forces are suddenly released from a submerged captured system
toward the surface of the water. When such a release of forces
occurs, the components of the system--for example, a buoyancy
chamber disposed in communication with several thousand feet of
casing riser--are released toward the surface and can impact the
rig and/or associated surface vessels servicing an offshore well.
For purposes of this disclosure, it should be noted that while many
of the detailed embodiments described below relate specifically to
a single riser system and its functional equivalents, those of
ordinary skill in the art should appreciate that aspects of the
present invention are applicable to virtually any type of
subsurface exploration and production system insofar as they relate
to features drawn to limiting and controlling the deleterious
effects of system components suddenly and unexpectedly released
from tension.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a side view of an offshore exploration and
production system in which a floating mobile offshore drilling unit
is connected to an upper riser stack and a blowout preventer
assembly; the blowout preventer assembly is in turn connected to a
conventional self-standing casing riser. The self-standing casing
riser employs a buoyancy device to support the casing riser from a
sea-floor wellhead.
[0016] FIG. 2 is a side view of a self-standing casing riser
employing a buoyancy device without an upper riser and blowout
preventer assembly, wherein the casing riser is extended from a
sea-floor wellhead, with a mobile offshore drilling or production
unit or disposed overhead.
[0017] FIG. 3 is a side view of an offshore exploration and
production system, with an upper riser and blowout preventer
assembly, shown while undergoing catastrophic failure or release
along a length of the casing riser, illustrated here by upward
lines of force.
[0018] FIG. 4 is a side view of an offshore exploration and
production system, depicted without an upper riser and blowout
preventer assembly, undergoing catastrophic failure or
unintentional release along the self-standing casing riser, further
illustrating potential impact points of the buoyancy device into
the overhead floating unit.
[0019] FIG. 5 is a side view of a self-standing casing riser
employing a buoyancy device but without a riser and blowout
preventer assembly, supporting the casing riser from a sea-floor
wellhead, with an example of the restraining devices of the present
invention.
[0020] FIG. 6 is a side view of an offshore exploration and
production system in which a floating mobile offshore unit is
connected to an upper riser and blowout preventer assembly, which
is, in turn, connected to a self-standing casing riser. In an
example of the present invention, both the floating unit and the
self-standing casing riser employ independent restraining and
control systems.
[0021] FIG. 7 is a side view of an offshore exploration and
production system in which a floating mobile offshore drilling or
production unit is mechanically connected to an upper riser and
blowout preventer assembly; the blowout preventer assembly is in
turn connected to a self-standing casing riser. In a further
example of the present invention, one or more restraining and
control devices are connected between the floating unit and the
upper riser.
SUMMARY OF THE INVENTION
[0022] According to a first aspect of the invention, there is
provided a method for restraining and, at least to some degree,
controlling the unintended subsurface release of exploration and
production riser systems, in which the method comprises the steps
of disposing one or more means for anchoring a riser system to
either the sea floor or an underwater wellhead system; and
disposing a network of associated restraining members in
communication with the anchoring means.
[0023] Also provided is a system for restraining and controlling
the unintended subsurface release of a riser system, the system
generally comprising one or more restraining elements disposed
along the length of the riser stack at predetermined points along
the sea floor or beneath the mud line.
[0024] Also disclosed is a system for and method of restraining and
controlling the unintended subsurface release of a subsurface riser
system, in which a receiving station having one or more means for
absorbing or deflecting force carried by an unintentionally
released system component is disposed in a fluid transport
system.
DETAILED DESCRIPTION
[0025] As seen in the attached FIGS. 1-4, some offshore exploration
and production systems, especially those relying on self-standing
casing riser type configurations, are potentially susceptible to a
variety of system failures that could lead to the damage or
destruction of associated drilling platforms and surface vessels
disposed overhead (e.g., a pontoon type drilling rig floating on
the surface of the ocean and disposed in communication with the
riser system).
[0026] For example, casing connections, wellhead connections,
buoyancy chambers connected to a riser stack, etc., can all fail,
thereby creating an unsafe condition in which buoyancy and tension
forces are suddenly released from a submerged exploration or
production system back toward the surface of the water. When such a
release occurs, the components of the system--for example, a
buoyancy chamber disposed in communication with several thousand
feet of casing riser--are released toward the surface and can
impact an associated rig or surface vessel servicing the well.
[0027] FIG. 1, for example, is a side view of an offshore
exploration and production system in which a floating mobile
offshore drilling unit 1 is connected to an upper riser 2 and
blowout preventer 3, which is in turn connected to a self-standing
casing riser system 4. The riser system 4 employs a buoyancy device
5 to support the casing riser stack 6 from a sea-floor wellhead
member 7. Wellhead member 7 is connected to the top of a well
casing member 8. Well casing member 8 enters the mud line or sea
floor 9.
[0028] In practice, the floating unit 1 may comprise any number of
vessels or structures used as surface stations for receiving
hydrocarbons produced from offshore wells. In addition to a mobile
offshore drilling unit (or "MODU"), some other examples of
receiving station members include: ships or other sea vessels;
temporary or permanent exploration and production structures such
as rigs and the like; rig pontoons; tankers; a floating production,
storage and offtake ("FPSO") vessel; a floating production unit
("FPU"); and other representative receiving units as would be known
to one of ordinary skill in the art.
[0029] It should be appreciated that upper riser 2 may comprise any
number of structural or functional equivalents having a purpose of
facilitating hydrocarbon transfer from casing riser stack 6 to the
receiving station. For example, riser 2 may comprise flexible drill
tubing, casing, a string of rigid pipe, etc., either contained
within the interior of an outer pipe or sheath, or instead serving
as a direct hydrocarbon transfer means. For purposes of this
application, all such fluid communication means will generally be
referred to as a "riser."
[0030] Like upper riser 2, self-standing riser system 4 also
facilitates connection of one or more wellheads to one or more
subsurface wells, and/or to a riser stack, a buoyancy member, etc.,
as dictated by operational requirements. The riser system 4 can
comprise any of a number of structural or functional equivalents
having a purpose of facilitating the transfer of fluids from a well
to a surface or near-surface receiving station, which in some
embodiments is self-standing and disposed under essentially
continuous buoyant tension. The riser stack is typically made up of
one or more known fluid communication devices, for example, casing
riser or another suitable connecting member, such as a tubular, a
length of coiled tubing, or a conventional riser pipe assembly. The
buoyancy member is typically submerged in the sea, and may comprise
a buoyancy chamber located in an upper portion of the riser stack.
The relative buoyancy of the buoyancy member applies tension to the
riser stack, thereby establishing a submerged platform of sorts
from which a wellhead, blowout preventer, riser stack, etc.,
connected to the receiving station member may be assembled or
affixed.
[0031] FIG. 2 is a side view of a self-standing riser system 4
disposed in communication with a buoyancy device 5, which lacks a
conventional riser or blowout preventer and is instead capped by a
well isolation member such as a ball valve, or a shear ram, etc.
The buoyancy device 5 will be used to connect riser stack 6 from a
sea-floor wellhead member 7 to a mobile offshore drilling unit 1 or
another representative exploration or production unit floating
overhead. As seen, the tension forces associated with riser stack 6
as a result of its communication with buoyancy device 5 are
restrained by only wellhead member 7, which is anchored by well
casing member 8 to the sea floor.
[0032] FIG. 3 is a side view of an offshore exploration and
production system having an upper riser 2 and a blowout preventer
3, depicted during the initiation of an unintentional subsurface
release along a length of riser stack 6, the direction of
associated released forces being illustrated by upward pointing
lines 10. As is clear from the depiction, this particular single
point failure will cause buoyancy device 5 to launch suddenly and
forcefully toward the surface. In fact, any such failure or release
of the riser system 4 occurring between buoyancy device 5 and the
well casing 8 will cause a buoyant, projectile-like release of the
disconnected system components directly toward the mobile offshore
drilling unit 1. For example, failure or release of the casing
wellhead connection from the sea floor, or wellhead member 7 from
well casing member 8, will set free some portion of riser stack 6
and the entirety of buoyancy device 5, thereby transferring the
associated buoyancy forces to blowout preventer 3 and upper riser
2. Major damage can obviously ensue when upper riser 2 accelerates
and crashes into mobile offshore drilling unit 1, thereby creating
a tightly concentrated damage impact point 11 that is poorly
equipped to handle the sudden and unexpected application of such
enormous force. Other example points of failure or release events
might include a failure point 12 occurring near the base of riser
stack 6, a failure point 12' anywhere along the length of riser
stack 6, and a failure point 12'' occurring near the top of riser
stack 6, which is also in close proximity to buoyancy device 5. In
short, sudden release of the riser stack will also release all of
the previously restrained buoyant and tension forces present in the
system, thereby causing upper riser 2 to rush upward and possibly
causing significant damage to mobile offshore drilling unit 1.
[0033] FIG. 4 is a side view of a receiving station unit 1',
depicted prior to installation of an upper riser and blowout
preventer assembly and while undergoing a catastrophic failure or
other unintentional release along the length of the riser system 4,
and further illustrating potential impact points 13, 13' of the
buoyancy device 5 into the body or support members of the receiving
station 1'. As seen, the riser system 4 has suffered a catastrophic
system failure in which the riser stack 6 has broken off at failure
point 14''. Depending on the orientation of the stack 6 at the time
of system failure, the buoyancy chamber 5, which was attached to
riser stack 6 in order to provide tension during exploration and
production, is suddenly released together with up to several
thousand feet of trailing casing riser back toward the surface of
the water, where it impacts vertical impact point 13 disposed near
a bottom portion of a receiving station, again causing an unsafe
condition in which the entire receiving station, and perhaps all or
a significant percentage of associated equipment and personnel, are
lost.
[0034] In the alternative, or in combination, other points of
failure may occur, such as, for example, failure at points 14
and/or 14'. As those of ordinary skill in the art will readily
recognize, such failures can occur as a result of mechanical
failure, material decomposition attributable to corrosion, etc., or
in response to bending forces applied to casing stack 6. Lateral
forces, such as those resulting from cross currents associated with
particular water depths, can also cause bending or breakage, and
may also cause lateral deviation or inclination of the angle at
which the otherwise upwardly directed forces occur in practice. As
seen, a riser 6' so inclined or laterally deviated could impact a
pontoon or a cross-brace, thereby creating an impact point 13' and
severely damaging the receiving station member 1' and/or other
floating units such as workboats or floating transmission
lines.
[0035] As seen in the example embodiments of FIGS. 5-6, a
catastrophic release control system is provided, comprising a
network of restraining members (e.g., chains, cables, adjustable
tension lines, etc.) disposed between an anchoring means and one or
more predetermined points along the length of the riser stack. A
number of possible connection points and means by which connection
may be affected are expressly disclosed in the drawings, though one
of ordinary skill in the art will appreciate that a great many
other connection means and attachment points are presently
contemplated, the precise nature of each being determined by
operational variables, for example, the sea conditions in which
operations occur, the various materials used to construct the
system, the extent and significance of wave and tidal forces, etc.
By pairing appropriate system, the extent and significance of wave
and tidal forces, etc. By pairing appropriate connection means and
attachment points together with an understanding of related
operational variables, a system is achieved in which the riser or
casing stack is restrained even in the event of an otherwise
catastrophic system failure.
[0036] Referring now to the specific, non-limiting embodiment of
the invention depicted in FIG. 5, a system for controlling the
unintended release of self-standing riser systems is provided,
comprising a plurality of anchor points 100 through 109 disposed on
the riser system with restraining members 200 through 209 connected
to the anchor points. In the present depiction, the self-standing
system 4 is not yet connected to overhead surface unit 1', and thus
no connecting riser or blowout preventer is present. Buoyancy
chamber 5 connects riser stack 6 to a sea-floor wellhead member 7,
and one manner in which the restraining devices may be deployed in
practice is depicted for purposes of illustration of the
invention.
[0037] For example, one or more means for anchoring are illustrated
by anchor points 100 through 109. In this particular embodiment,
anchoring is disposed on the casing riser, buoyancy member, and
bottom portions of the riser system 4. Anchor points 101 through
106 are shown in this instance as disposed on the riser stack 6
portion of the riser system 4. Anchor points 100 are disposed on
the buoyancy device 5, and anchor points 107 are disposed on the
wellhead member 7. Redundant or alternative anchoring may also be
deployed on the sea floor, such as by connection to a template or a
weighted mass, or into the sea floor or mud line using suction
anchors, etc., as illustrated by anchor points 109. Additional or
alternative anchoring may also be deployed on well casing member 8,
as illustrated by anchor points 108.
[0038] Restraining members may be formed from any of several
previously known components and materials, depending on the
specific engineering, environmental, and weight bearing
requirements dictated by the operational environment. Examples
include, but are not necessarily limited to, chains, cable, rope,
elastic cord, extension springs, and limited travel extension
springs, etc. In any event, the various restraining members are
attached between anchor points such that one end of a restraining
member is attached to a first anchor point, while the other end of
the restraining member is connected to a second anchor point. A
plurality of restraining members 200 through 209 connects various
portions of riser stack 6 from wellhead member 7 to buoyancy device
5, thereby affecting a network of restraining members tying points
along the riser system together.
[0039] The aforementioned network of restraining members can be
variably deployed in a variety of configurations. As shown in the
example embodiment of FIG. 5, restraining members 201 through 209
are disposed in an interconnected, "daisy-chain" like manner, with
at least two restraining members disposed upon or proximate to each
of the anchor points. For example, restraining member 201 is
connected to anchor point 101 and anchor point 102, while
restraining member 202 is connected to anchor point 102 and anchor
point 103. Similarly, restraining member 203 is connected to anchor
point 103 and anchor point 104, restraining member 204 is connected
to anchor point 104 and anchor point 105, restraining member 205 is
connected to anchor point 105 and anchor point 106, restraining
member 206 is connected to anchor point 106 and anchor point 107,
etc. In the depicted embodiment, a terminal restraining member 200
is disposed on anchor point 100 of buoyancy device 5. Restraint of
the riser system using chains, cables or adjustable tension lines,
etc., attached to both an anchor and one or more predetermined
points along the stack will prevent the chamber and casing riser
from releasing and impacting an associated rig or surface vessel.
In the depicted embodiment, redundant terminal restraining members
are disposed on one or more of anchor points 106, 107, 108 and 109.
The network forms a continuous linkage from the buoyancy member
back to the sea floor foundation, in this example, a chain like
assembly 20 disposed in mutual interconnection along the
longitudinal entirety of casing or riser stack 6.
[0040] Continuing with reference to FIG. 5, two separate chains of
restraining members are depicted, namely, chains 20 and 20',
although it will be appreciated by one of ordinary skill in the art
that both a single chain 20 can suffice, whereas additional
restraining member chains (not illustrated) can be disposed to
connect separate restraining chains in a net-like manner. For
example, a number of restraining members may be disposed on a
single anchor point, or in relatively close physical proximity to
one another. Thus, the network of restraining members can be used
to form multiple continuous linkages, wherein any particular
linkage may or may not be linked to any other. In a further
embodiment, some of restraining members are disposed in a staggered
pattern so that various individual restraining members need not
share a common anchoring point, while still forming a continuous
connection along the length of the casing riser. In yet another
embodiment, the network of restraining members covers only a
partial span of the overall riser system.
[0041] In a still further embodiment, FIG. 5 depicts a pair of
anchoring means and corresponding connections for various
restraining members. For example, anchor points 101 and 102 are
disposed in relatively close physical proximity with one another.
Complementary restraining member 201 then connects between anchor
point 101 and anchor point 102. In at least one embodiment, the
portion of casing or riser stack 6 between anchor point 101 and
anchor point 102 represents the location of a flange or coupling,
an intentionally engineered breaking point, or a potential bending
point requiring redundant anchoring for additional safety.
[0042] In short, the modified riser system, once secured by one or
more networks of restraining members, prevents the unintentional,
projectile-like release of a buoyancy device and associated casing
riser, thereby preventing release toward the surface and avoiding
possible impact with a receiving station, or with an associated rig
or proximately disposed sea vessel.
[0043] As seen in FIGS. 6-7, redundant safety features are also
provided for attendant surface vessels and rigs, so that additional
safety is provided for operators in the event an unintended
subsurface release of casing, etc., reaches the surface despite the
subsurface safety features disclosed above. For example, one or
more pistons or other shock absorbing devices can be disposed near
a bottom portion of a rig or platform in order to absorb and
dissipate the upward energy of one or more released riser system
components. Appropriate force absorbing devices may comprise a
system of springs, hydraulic or gas filled cylinders, etc., and
optimally are disposed in such a manner that as few of the devices
as possible are required to absorb and diminish even the maximum
force a sudden, uncontrolled riser release might deliver. For
example, a system of springs or cylinders can be disposed on the
bottom portion of a rig at an angle of approximately forty-five
degrees or so (measured relative to the direction of likely riser
impact) in order to absorb and dissipate incoming forces. However,
any force absorbing system suitable for installation on a rig or
platform, or even the bottom of a vessel, and as many such devices
and angles of inclination and declination as may be required to
absorb and diminish an impact force can be employed in place of the
optimal configuration.
[0044] FIG. 6 is a side view of an example offshore exploration and
production system in which an overhead floating production unit 1'
is connected to an upper riser 2 and a blowout preventer 3. The
blowout preventer 3 is disposed in mechanical communication with a
self-standing casing riser system 4. In one embodiment of the
invention, both the overhead floating production unit 1' and the
riser system 4 employ separate restraining systems. In the event of
a release or failure of the riser system, and in the absence or
failure of the riser system 4 restraining member network to retard
the unintended projectile-like release of subsurface system
components toward the surface, one or more absorbing means disposed
on overhead floating production unit 1' are employed to absorb,
deflect, and otherwise reduce or intercept the force of impact
associated with the released buoyancy device 5 and attendant riser
stack 6. As shown in the depicted example, hydraulic springs 300
are disposed at an angle of approximately forty-five degrees on the
lower infrastructure of overhead floating production unit 1', and
may be employed either alone or in combination with a plurality of
lower restraining members 200 through 209 (see FIG. 5) disposed on
the riser system 4. Other absorbing means are also contemplated,
e.g., springs, gas-filled cylinders, hydraulic cylinders, extension
springs, limited travel extension springs, ventable gas-filled
cylinders, etc.
[0045] In an alternate example, hydraulic springs 300 are disposed
at an approximate angle of between thirty and forty-five degrees
measured relative to the direction of likely riser impact. In this
example, likely riser impact is approximately measured from a
vertical location situated directly beneath the overhead floating
production unit 1', as the wellhead member 7 in this example is
directly beneath overhead floating unit 1'. Hydraulic springs 300
are therefore disposed on the underside of overhead floating
production unit 1' at an angle of approximately thirty to
forty-five degrees measured relative to the vertical, longitudinal
axis of the subsurface riser stacks 2, 6. It should be appreciated,
however, that a wellhead member 7 or an associated riser system 4
may also be laterally displaced from a receiving station member,
and the direction of likely riser impact to a particular receiving
station member may well originate from various other released
system component ascension angles.
[0046] Still further means may be employed to reduce or eliminate
upward, projectile-like forces in the event of a sudden, unintended
riser system release. For example, a mechanical means for directly
stabilizing an unintentionally released buoyancy member will help
to constrain the angular sweep of potential impact locations, and
reduce the incoming projectile-like forces prior to impact. Such
means, when disposed in communication with either a means disposed
on the receiving station member for absorbing impact or a network
of restraining members disposed on the riser network, or both, will
cumulatively reduce the chance for serious damage from failure or
unintended release of the riser system.
[0047] One means for stabilization of the buoyancy member comprises
a means to reduce rotation of the buoyancy member in the event of
inadequate anchoring or the unintended projectile-like motion of
the buoyancy member. In one example, a plurality of baffling
members (not shown) is disposed around the periphery of the
cylindrical outer surfaces of buoyancy device 5. In another
example, a plurality of fin-like planes are disposed on and extend
outwardly from the outer surfaces of buoyancy device 5. In one
particular example, a plurality of plane-like or curved fin members
are disposed around the periphery of the cylindrical surfaces of
buoyancy device 5, thereby providing resistance to otherwise
uncontrolled rotational forces, which can result in excessive
stress forces on the restraining members 200 through 209 (see FIG.
5). In short, baffling, fins and other such devices lend additional
stability to both dynamically positioned and relatively fixed
buoyancy chamber systems by controlling lateral underwater
currents, and retarding rotation of the buoyancy chamber, which in
turn can greatly reduce or prevent shearing forces on riser stack 6
and subsurface wellhead member 7.
[0048] Yet another means for stabilizing the unintended release of
a buoyancy chamber comprises a means for swamping the buoyancy
member upon detection of release of the riser system. In one
example, a series of pressure sensitive latches are disposed on the
upper surfaces of the buoyancy member. The latches collapse when
pressure outside the buoyancy member greatly exceeds the pressure
inside the buoyancy member, as would be the case when a riser
system having a buoyancy member is suddenly released toward the
surface in an uncontrolled manner. In this embodiment, seawater
swamps the buoyancy member and retards the buoyant force with which
the released riser system approaches the surface of the water. The
means for facilitating the swamping of the chamber can function
either directly (for example, in the case where latches are formed
from a material sufficiently weaker than the surrounding chamber
materials that the latches will collapse during the normal course
of sudden release) or indirectly (as when collapse of the latches
is initiated by a differential pressure sensor or the like).
[0049] FIG. 7 is a side view of an offshore exploration and
production system in which the overhead floating production unit 1'
is connected to an upper riser 2 and a blowout preventer assembly;
the blowout preventer is in turn mechanically connected to a lower
riser stack 6. In still another example of the invention, a
plurality of restraining devices can be connected between the
overhead floating unit 1' and the upper riser 2. As shown in the
depicted example, hydraulic springs 300' are disposed on the
underside infrastructure of overhead floating production unit 1'.
Other means may be employed, such as the use of springs, gas-filled
cylinders, hydraulic cylinders, extension springs, limited travel
extension springs, ventable gas-filled cylinders, etc. In this
particular example, hydraulic springs 300' are disposed at a
declination angle of approximately thirty to forty-five degrees
measured relative to the direction of likely riser impact.
[0050] The foregoing specification is provided for illustrative
purposes only, and is not intended to describe all possible aspects
of the present invention. Moreover, while the invention has been
shown and described in detail with respect to several exemplary
embodiments, those of ordinary skill in the pertinent arts will
appreciate that changes to the description, and various other
modifications, omissions and additions may also be made without
departing from either the spirit or scope thereof.
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