U.S. patent application number 10/306316 was filed with the patent office on 2004-05-27 for motion compensation system for watercraft connected to subsea conduit.
This patent application is currently assigned to EXPRO AMERICAS, INC.. Invention is credited to Trewhella, Ross J..
Application Number | 20040099421 10/306316 |
Document ID | / |
Family ID | 32325652 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040099421 |
Kind Code |
A1 |
Trewhella, Ross J. |
May 27, 2004 |
Motion compensation system for watercraft connected to subsea
conduit
Abstract
A safety device 30 for compensating for ocean heave motion of a
vessel or watercraft connected to a conduit 12 or drill string
extending to the subsea. The safety device comprises at least one
vertical piston and cylinder assembly 40,41, the piston rod 40a of
which connects to an upper beam 35 of the device 30 and the
cylinder 40b of which connects to a horizontal beam 42 below the
upper beam 35 and supported by vertical tension legs 51,52, each
including a gas expansion chamber 51a,52a. A valve controlled
conduit system 61,62 connects the chambers 51a,52a and cylinders
40b,41b to a source of pressurized gas whereby a fixed volume of
gas and gas pressure positions the pistons at the mid-lengths of
the cylinders 40b,41b The safety device 30 is mounted on a frame or
rig 11 mounted on the watercraft and the base 33 of the safety
device 30 is connected to the bottoms of the tension legs 51,52. A
connecting means 34 connects the safety device 30 in coaxial
alignment with the conduit or drill string 12 which extends through
an opening 13 in the base 14 of the frame 11. A second connector 24
connects the top of the safety device 30 to the vessel or
watercraft or to another motion compensator used in series
therewith. When the conduit or drill string 12 stress is
acceptable, the length of the safety device 30 remains constant.
However, should the stress exceed acceptable levels, the pistons
40a,41a move into the cylinders 40b,41b to shorten its length and
thereby reduce tension stress to an acceptable level or the pistons
40a,41a move outwardly of the cylinders to increase the safety
device 30 length and thereby reduce the compression stress to an
acceptable level.
Inventors: |
Trewhella, Ross J.;
(Houston, TX) |
Correspondence
Address: |
Marvin J. Marnock
13630 Indian Creek Rd.
Houston
TX
77079-7029
US
|
Assignee: |
EXPRO AMERICAS, INC.
|
Family ID: |
32325652 |
Appl. No.: |
10/306316 |
Filed: |
November 27, 2002 |
Current U.S.
Class: |
166/355 ;
166/363 |
Current CPC
Class: |
E21B 19/09 20130101 |
Class at
Publication: |
166/355 ;
166/363 |
International
Class: |
E21B 007/12 |
Claims
What is claimed is:
1. A safety device for compensating for the ocean heave motion of a
floating watercraft connected to a conduit extending downwardly to
beneath the ocean floor in order to maintain the stress imposed on
the conduit by the watercraft to within acceptable levels, said
safety device comprising: at least one piston and cylinder assembly
disposed vertically with the end of the piston rod connected to an
upper beam of the device and the cylinder connected to a horizontal
support beam below said upper beam, said horizontal support beam
supported by a pair of vertically extending tension leg members,
each of which includes a gas expansion chamber therein; a
valve-controlled conduit system connecting the gas expansion
chambers and said cylinder to a source of pressurized gas whereby a
fixed volume of gas and gas pressure may be supplied to said
cylinder and piston to position the piston substantially at the
mid-length of the cylinder, said safety device having a base member
connected to the bottom ends of said tension leg members and said
safety device being mounted in a frame mounted on said watercraft
and having a frame base provided with an opening therein through
which said conduit extends; a first connecting means for connecting
said base member of said safety device to the upper end of said
conduit which extends through said opening in said frame base; a
second connecting means for connecting said upper beam of the
safety device to a connection member provided on said floating
watercraft, said safety device being adapted to respond to stress
on said conduit which exceeds a predetermined level by movement of
said piston relative to said cylinder to alter the length of said
safety device in a direction to relieve said stress and to maintain
the stress within an acceptable limit.
2. A safety device as set forth in claim 1 wherein said safety
device is provided with more than one piston and cylinder assembly
with the piston rods thereof connected to said upper beam and the
cylinders of said assemblies connected to said horizontal support
beam and to said conduit system whereby a fixed volume of gas and
gas pressure may be supplied to said cylinders to position the
pistons substantially at the mid-lengths of said cylinders.
3. A safety device as set forth in claim 1 wherein said safety
device is adapted to respond to a level of tension stress on said
conduit which exceeds a predetermined level by movement of said
pistons outwardly of said cylinders to thereby increase the length
of said safety device and thereby relieve and maintain said tension
stress within an acceptable level.
4. A safety device as set forth in claim 2 wherein said safety
device is adapted to respond to a level of tension stress on said
conduit which exceeds a predetermined level by movement of said
pistons outwardly of said cylinders to thereby increase the length
of said safety device and thereby relieve and maintain said tension
stress within an acceptable level.
5. A safety device as set forth in claim 1 wherein said safety
device is adapted to respond to a level of compression stress on
said conduit which exceeds a predetermined level by movement of the
pistons inwardly of said cylinders to thereby decrease the length
of said safety device and thereby relieve and maintain said
compression stress within an acceptable limit.
6. A safety device as set forth in claim 2 wherein said safety
device is adapted to respond to a level of compression stress on
said conduit which exceeds a predetermined level by movement of the
pistons inwardly of said cylinders to thereby decrease the length
of said safety device and thereby relieve and maintain said
compression stress within an acceptable limit.
7. A safety device as set forth in claim 1 wherein said watercraft
and said safety device are connected to a drilling string and said
safety device is connected thereto in order to maintain the stress
levels imposed on said drilling string by movement of said
watercraft by the ocean heave movements to within acceptable
limits.
8. A safety device as set forth in claim 2 wherein said watercraft
and said safety device are connected to a drilling string and said
safety device is connected thereto in order to maintain the stress
levels imposed on said drilling string by movement of said
watercraft by the ocean heave movements to within acceptable
limits. 9. A safety device as set forth in claim 1 wherein said
second connecting means connects said upper beam of the safety
device to a traditional motion compensator provided on said
watercraft and whereby said safety device becomes operative to
relieve and maintain stress imposed on said conduit within
acceptable limits in the event of failure of the traditional motion
compensator.
Description
FIELD OF THE INVENTION
[0001] The invention relates to systems for compensating for the
motion imparted by the ocean heaves to floating drilling rigs or
barges, and more particularly to a safety system for compensating
for the ocean heave motion of a floating vessel or barge when
connected to a conduit or drilling string extending to any sub-sea
formation.
BACKGROUND OF THE INVENTION
[0002] Offshore vessels that deploy a fixed conduit to the ocean
floor, oftentimes require a motion compensation system which serves
to maintain the position of the conduit and also the levels of
compression and tension imposed on the conduit within acceptable
levels as the vessel follows the ocean movement up and down. In
most circumstances and conditions, the traditional motion
compensator is perfectly adequate. However, it is always possible
that, regardless of circumstances, a failure of the motion
compensator system can occur, in which instance the failed system
"locks" the vessel to the conduit. This "locking" of the vessel to
the conduit can occur at any part of the ocean heave cycle. When
the conduit is fixed to the seabed, as during drill stem testing or
completion operations and a "locking" of the vessel to the conduit
occurs during the upward motion of the vessel in the ocean heave
cycle or at the top thereof, a level of tension force may be
imparted to the conduit which exceeds an acceptable safe level,
and, conversely, a "locking" of the vessel to the conduit during
downward motion of the vessel in the ocean heave cycle or at the
bottom thereof can place the conduit under a compressive force
which exceeds an acceptably safe level. In either event, whenever
the force of compression or tension imparted to the conduit exceeds
a predetermind safe level as is normally maintained by the
traditional motion compensator, if this fails, catastrophic
consequences may be the result. The present invention is intended
to be used as the sole motion compensator to maintain the stress
within acceptable limits or to be used in series with a traditional
motion compensator as a safe-guard in the event of a failure of the
traditional motion compensator.
[0003] The invention is a safety device for compensating for motion
imparted to a floating vessel or barge by the ocean heaves when the
vessel or barge is connected to a conduit or drill string extending
to subsea formations and when the stress imposed on the conduit or
drill string exceeds predetermined levels. The system comprises at
least one vertically disposed piston and cylinder assembly, the
piston rod(s) of which connect to an upper support beam of the
device and each cylinder of which connects at its top to a
horizontal beam below and parallel to the upper support beam. The
beam which connects the top(s) of the cylinder(s) is connected by a
pair of vertical tension legs, each of which includes a gas
expansion chamber intermediate its ends. Pressurized gas is
supplied to the gas chambers and cylinder(s) by a conduit system
which includes a gas feed line with valve therein which connects
one of the chadders to a source of pressurized gas whereby a fixed
volume of gas and pressure is supplied to position the piston(s)
substantially mid-length of the cylinder(s). If more than one
piston and cylinder assent)y is provided, a gas line connects cacti
cylinder at the top, thereof with the gas expansion chambers.
[0004] The safety device is Counted in a frame or rig having a base
member, top beam and sides. The safety device includes a base which
connects with the bottom ends of the tension legs and is provided
with means for connecting the base of the safety device with the
conduit (or drill string) which extends through an opening in the
base of the frame or rig such that the safety device is coaxially
disposed with the conduit (or drill string). A second connecting
means connects the top beam of the safety device with a rigid
member of the vessel or barge or with another motion compensator
which may be used in series therewith.
[0005] Under conditions when the stress imposed on the conduit (or
drill string) is within acceptable levels, the length of the safety
device remains constant and the safety device acts as a rigid
connection between the watercraft and the conduit (or drill
string). However, should stress on the conduit (or drill string)
exceed acceptable levels with vessel pitch and heave, the device
responds by motion of the pistons inwardly of the cylinders to
shorten its length and thereby reduce compression stress to an
acceptable level or by motion of the pistons outwardly to increase
the length of the safety device and thereby reduce the tension
stress on the conduit (or drill string) and maintain it within an
acceptable limit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a schematic front view of a semi-submersible
vessel having a conventional lifting frame (or derrick) thereon and
a traditional motion compensator mounted on the frame and connected
to the safety device of the invention which serves as an interface
connection between the traditional motion compensator and a conduit
which passes from the vessel to the ocean floor;
[0007] FIG. 2 is a schematic front view similar to FIG. 1 but
showing an operative condition of the safety device of the
invention wherein the pistons of the safety cylinders of the safety
device have moved upward and thereby relieve tension on the conduit
after the traditional motion compensator has failed at the top of
an ocean heave cycle and placed the conduit under a tension force
which exceeds the predetermined limt of tension force normally
maintained by the traditional motion compensator; and
[0008] FIG. 3 is a schematic front view similar to FIG. 1 but
showing an operative condition of the safety device of the
invention wherein the pistons of the safety cylinders of the device
have moved downward into the cylinders and thereby relieve the
compression force on the conduit after the traditional motion
compensator has failed when the rig is at the bottom of the ocean
heave cycle and the compression force on the conduit has exceeded
the predetermined level of compression normally maintained by the
traditional motion compensator
DETAILED DESCRIPTION OF THE INVENTION
[0009] Referring more particularly to the drawings, FIG. 1 shows A
partly sectional view of an offshore vessel in the form of a semi
submersible rig 10 with a lifting frame 11 thereon and from which a
conduit 12 is suspended and and extends to the ocean floor (not
shown). The rig 10 includes a main deck or base 14 which is mounted
on floats 16a, 16b affixed to the bottom of the base 14 at opposite
sides thereof. The rig 10, however, might also be mounted on a
vessel or ship in lieu of the floats 16a, 16b.
[0010] The structure of the lifting frame 11 includes side members
18a, 18b which extend upwardly from opposite sides of the base 14
in a generally convergent direction and support at their tops a top
support beam 19 affixed thereto and mounted thereon in parallel
relation to the base 14.
[0011] The rig 10 with lift frame 11 have served in the past to
support and carry a state-of-the-art motion compensation system
which has been traditionally used to maintain the position of the
conduit which is fixed to the ocean floor and to maintain the
forces of tension and compression as may be imposed on the conduit
within acceptable limits. Such motion compensation systems for
seagoing vessels are commercially available from drilling companies
such as NATIONAL OILWELL.
[0012] The traditional motion compensation system 20 as shown in
FIGS. 1-3, is typically mounted atop the frame 11 on the horizontal
support beam 19 at the top of the frame and extends through an
opening 17 in the support beam 19 to provide a moveable travelling
block 22 from which a lower support member 24 is suspended on a
pair of leg supports 24a, 24b attached to the underside of the
travelling block 22. When the present safety device 30 of the
invention is not used, the support 24 at the low end of the motion
compensator 20 is normally fixed directly to the conduit 12 to the
ocean floor,which conduit extends through an opening 13 in the base
14 of the rig 10.
[0013] The safety device 30 of the present invention is
particularly designed for use on motion compensated floating
drilling vessels as are used in offshore drilling in the oil
industry The support 24, at the low end of the motion compensator
20 is connected directly to the upwardly extending connecting leg
31 of the safety device 30. The connecting leg 31 is designed with
an external cylindrical surface substantially identical to the
outer cylindrical surface of the conduit 12 and is connected as by
a clamp, welding or threads to the support beam 24 in the same
fashion as the conduit 12 would otherwise connect to the support 24
of the motion compensator 20 if the safety device 20 were not
used.
[0014] Near its lower end, the safety device 30 is provided with a
horizontal base 33 from which is suspended a pair of load bearing
leg members 34a,34b. A lower horizontal support beam 34 is fixed to
the lower ends of the legs 34a, 34b and constitute the low end of
the device 30. The legs 34a, 34b and support beam 34 are
substantially identical with both load bearing legs 24a, 24b and
lower support beam 24 from which suspends the travelling block 22
on the low end of the motion compensator 20.
[0015] The support beam 24 at the low end of the safety device 30
is connected to the upper end of the conduit 12 in the same fashion
the conduit 12 normally connects to the lower support beam 24 of
the motion compensator 20.
[0016] The safety device 30 includes an upper horizontal support
beam 35 which is vertically su per ed by the piston rods of the
pistons 40a, 41a of a pair of piston and cylinder assemblies 40,41.
The cylinders 40b, 41b of the respective assemblies 40, 41 are
fastened to and suspended from a horizontal support beam 42 which
is parallel to the support beam 35 and the base 33. The cylinders
40b, 41b receive their respective pistons 40a, 41a. The support
beam beam 42 is joined to the base member 33 of safety device 30 by
a pair of parallel vertically extending tension legs 51, 52. The
tension leg 51 includes an elongate gas expansion chamber 51a
located intermediate the ends of the leg 51 and configured in
coaxial relation therewith. The tension leg 52 similarly includes
an elongate cylindrical gas expansion chamber 52a located
intermediate the ends of the leg 52 and also configured in coaxial
relation therewith.
[0017] Pressurized gas is supplied to the gas chamber 52a by a gas
feed conduit 61 which the chamber 52a to a source of pressurized
gas (not shown). An additional gas transfer conduit 62 connects
with the upper end of the chamber 52a and extends across the
horizontal support beam 42 and downward to a fluid connection with
the expansion chamber 51a. The gas line conduit 62 is also provided
with fluid connection with each of the cylinders 41a, 41b at the
tops of the cylinders so as to pressurize the pistons 40b 41b in a
downward direction in their respective cylinders when pressurized
gas is in the chambers 51a, 52a. A valve 63 is installed in the gas
feed line 61 whereby the connection with the source of pressurized
gas or fluid can be opened to supply the chambers 52a, 51a. The
valve 63 may be closed when the gas is at a predetermined level of
pressure. At this pressure level, the pistons 41a, 40a are
positioned substantially at the mid-length points of their
respective cylinders 41b, 40b.
[0018] It is to be appreciated that so long as the traditional
motion compensator 20 maintains its normal operation, the safety
device 30 of the present invention is substantially inoperative in
the sense that the pistons 40a, 41a do not stroke but maintain
their positions in the safety cylinders 40b, 41b. In such normal
operation of the motion compensator 20, the length of the safety
device 30, that is, the distance between its connection with the
motion compensator 20 at the support 24 and its connection with the
conduit 12 at the support 34 remains constant and the safety device
30 acts as a rigid interface connection between the motion
compensator 20 and the conduit 12. However, should a failure of the
motion compensator 20 occur, as for example, when the vessel or rig
is at the trough of an ocean heave movement, undue stress of
compression on the conduit 12 is likely to produce catastrophic
results unless corrected. If such failure occurs, the safety device
30 responds when the tension of the conduit 12 exceeds the
predetermined level of tension at which it is normally maintained
by the motion compensator 20. The safety device 30 responds by
movement of the pistons 40a, 41a outward of their safety cylinders
as shown in FIG. 2 to, in effect, increase the length of the safety
device 30 and exert a pull on the conduit 12 to a degree which
relieves its tension stress to within a predetermined limit
controlled by the safety device 30. The predetermined limit of
tension maintained by the safety device 30 is incrementally higher,
for example, one percent higher than that maintained by the
traditional motion compensator 20.
[0019] On the other hand, if a failure of the motion compensator 20
occurs when the vessel or rig is at the top of an ocean heave
cycle, an undue level of compression on conduit 12 may also produce
catastrophic results unless corrected by the safety device 30. When
such a failure occurs to produce compressionstress on the conduit
12 above the level at which it is normally maintained by the motion
compensator 20, the safety device 30 responds by movement of the
pistons 40a, 41a to, in effect,shorten the length of the safety
device as the pistons move into the cylinders and thereby exert a
force to relieve the excessive compression on the conduit 12 and to
maintain the compression on the conduit 12 within an acceptable
predetermined limit which is incrementally greater, for example,
one percent greater than the limit of stress normally maintained by
the traditional motion compensator 20.
[0020] It is to be noted that as the pistons 40a, 41a move out of
the cylinders when relieving tension stress on the conduit 12,
cylinder gas is transferred to the gas expansion chambers 51a, 52a
as seen in FIG. 2. When the alternative occurs and the pistons move
into the safety cylinders as seen in FIG. 3, pressurized gas is
transfered from the expansion chambers 51a, 52a to the cylinders
40b, 41b.
[0021] It is also to be appreciated that a failure of the motion
compensator 20 might occur at any part of the ocean heave cycle,
but the safety device 30 responds as necessary to relieve excessive
tension on the conduit 12 within the incrementally higher
predetermined limit of tension as controlled by the safety device
30.
[0022] Furthermore, the safety device 30G has been shown and
described herein as being attached to a traditional motion
compensator 20 and to be responsive in the event of a failure of
the compensator 20 but, it may, however, be employed as the only
motion compensator provided on the vessel or barge for maintaing
the 35 levels of tension and compression imposed on the conduit 12
within acceptable limits. In such event, the safety device 30 is
connected to a rigid connector 20 part of the vessel in lieu of the
compensator 20
[0023] It is also to be appreciated that while the safety device 30
of the invention is heretofore described as having a pair of gas
expansion chambers 51a, 52a and a pair of piston and cylinder
assemblies 40, 41, it could be made with only one piston and
cylinder assembly located on the axis 50 of the safety device 30
and in coaxial alignment with the conduit 12 to the ocean floor. It
might also be made with more than a pair of such piston and
cylinder assemblies, preferably located symmetrically with respect
to the axis 50. Furthermore, the gas expansion chambers need not be
elongate in form and need not be in coaxial alignment with the
tension legs 51, 52, but could be located elsewhere. It is to be
understood therefore, that the fore-going description of a
preferred embodiment of the invention has been presented for
purposes of explanation and illustration and is not intended to
limit the invention to the precise form disclosed. Accordingly, it
is to be understood that various changes may be made to the safety
device 30 in structure and materials without departing from the
spirit of the invention.
* * * * *