Production Riser
The present invention is directed to a system including a self supporting riser (SSR) which is connected to a well to produce fossil hydrocarbon reservoirs deep below the seafloor. The SSR is constructed of a plurality of joints comprising regular joints and specialty joints that define the SSR and are selected to optimize the SSR for a well in a specific location. A unique aspect of the SSR of the present invention is that while capable of connecting to the wellhead, or tree on the seafloor, it can also be secured to an anchor during operations. The invention is further directed to a small vessel moored to the SSR by a line such as a hawser, the riser providing an anchor to the vessel, and the SSR carrying fluids from the well to the vessel and from the vessel to the well. The vessel has provisions for processing the fluids from the wellhead.
This application is a continuation-in-part application of Ser. No. 12/714,919 filed Mar. 1, 2010, titled “Riser Technology”, which claims the benefit of U.S. Provisional Application Ser. Nos. 61/351,374 filed Jun. 4, 2010, 61/225,601 filed Jul. 15, 2009; 61/232,551 filed Aug. 10, 2009; 61/252,815 filed Oct. 19, 2009; 61/253,230 filed Oct. 20, 2009; and 61/253,200 filed Oct. 20, 2009, all of which are incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTNot applicable
FIELD OF INVENTIONThe present invention is directed to a riser for the production of hydrocarbons from fossil hydrocarbon reservoirs deep below the seafloor. Further, the present invention is directed to the interfacing of the riser to a vessel subject to high vessel motions of pitch and roll. The small vessel employs a unique stabilization system for the separation processing equipment on deck.
BACKGROUND OF THE INVENTIONIt has been the practice for the recovery of hydrocarbons from fossil hydrocarbon reservoirs deep below the Gulf of Mexico and other offshore areas to build platforms of various designs upon which the separation equipment for separating the products from the wells; namely, the liquid hydrocarbons (oil), from water and gas, are supported. These platform structures cost millions of dollars and can not be cost justified unless they service more than one well and the indications have been determined there is sufficient oil/gas from the wells to put these structures in place. The production risers of the present invention may be employed on a newly drilled well even before the extent of the field or hydrocarbon reservoirs are fully developed. Using a low cost production structure, namely a SSR, for the first drilled well permits evaluation of the reservoir without the drilling of additional wells. The SSR must be capable of handling the unexpected as well as the expected.
SUMMARY OF THE INVENTIONThe present invention is directed to a system including a self supporting riser (SSR) which is connected to a well to provide fluid communication to fossil hydrocarbon reservoirs deep below the seafloor. The SSR is constructed of a plurality of joints comprising regular joints and specialty joints that define the SSR and are selected to optimize the SSR for a well in a specific location. A unique aspect of the SSR of the present invention is that while capable of connecting to the wellhead, or tree on the seafloor, it can also be secured to an anchor during operations. The invention is further directed to a small vessel subject to high vessel motions moored to the SSR by a line such as a hawser, the riser providing an anchor to the vessel, and the SSR carrying fluids from the well to the vessel and from the vessel to the well.
The present invention is directed to a riser system including a self supporting riser (SSR) which is in fluid communication with a well to test and produce fossil hydrocarbon reservoirs deep below the seafloor. Still further the present invention is directed to a self supporting riser (SSR) which is in fluid communication with a well and preferably includes a small vessel with processing equipment on a stable frame on the deck moored to the (SSR) carrying fluids from the subsea well to the processing equipment on the vessel.
To substantially lower cost over the prior art, the present invention uses a small vessel to facilitate operation of processing equipment on board rather than a multi-million dollar platform or a large vessel having large day-rates. It is preferred that the vessel is moored to the SSR so that the small vessel does not require using dynamic positioning to maintain vessel position, further lowing cost.
The present invention uses a SSR to provide fluid communication from a well or seafloor production equipment to the small vessel moored to the SSR rather than a riser fixed to a platform or a large vessel.
The methods and techniques for the SSR design and assembly and placement on an element of a subsea infrastructure are fully set forth in U.S. patent application Ser. No. 12/714,919.
Referring to
A specific design of the SSR 10 is only illustrated in
Referring to
Riser 10 may be attached to an element of a seafloor infrastructure, such as a wellhead or tree 20 (
Attachment of the SSR 10 to the seafloor anchor is preferably by a flexible connector 25, such as two half links of chain 25′ which permit inclination in any direction but prevent axial rotation of riser 10. Possible alternatives include a section of flexible pipe which bends without buckling or a flexible joint such as is commonly used as a hanger for steel catenary risers. A mechanical connection such as two half links of chain allows the SSR to freely incline from vertical at any compass bearing, thus avoiding bending moment in the SSR near the seafloor. Configuring the mechanical connection between the SSR and the anchor to prevent the SSR from rotating about its axis prevents excessive loads on the flexible pipe 26 shown connected to the well or production equipment. A mechanical connection can be simple, or can be more sophisticated and may include provisions for functions such as connection and release by ROV or other remote means.
A swivel can be placed at any location in the SSR 10 to allow the vessel to weather van freely without causing excessive torsion in the riser. The production riser 10 preferably has a swivel 24 mounted above the near-surface buoyancy module 19. Placing a swivel high in the SSR but below the buoyancy would require a swivel that functions under high tension. Placing the swivel near the seafloor locates it where SSR tension is low, but subjects the swivel to high ambient pressure and places it in a relatively inaccessible location. The swivel 24 (or swivels) is preferably located in the SSR above the load path to the buoyancy as illustrated to avoid both high tension and the complications associated with placing a swivel near the seafloor. A single swivel 24 for flow of fluid between riser 10 and vessel 30, controls (umbilical 12), and connecting the mooring line 36 can be placed as shown, or separate swivels, with or without provisions to avoid mooring line tension on the fluid swivel can be located in the position as shown. The torque required to operate the swivel(s) must be less than the torque rating of the SSR and must be less than the torque required to break out any threaded connections in the SSR. Wind direction current frequently shift gradually from east to south to west to north, or vice versa. Wind shifts such as this can drive a moored vessel multiple times around the mooring point, either clockwise or counter clockwise. By use of one or more swivels, the potentially damaging axial torsion to the SSR and associated flexible pipe can be avoided.
Mooring of a small vessel 30 can be as shown in
A small vessel 30, which is subject to relatively high pitch and roll motions due to its size, is used in the production system of the present invention to avoid overloading the SSR 10 as a larger vessel might. When not subject to vessel mooring loads the SSR 10 stands upright, subject only to self weight and drag due to ocean currents (as illustrated in
Wind, current, or other forces on the moored vessel 30 pull on the hawser line 38 and move the surface buoy 37 until both line 36 and line 38 are taut. The vessel then continues to move and pull on the hawser line 38 until an adequate restoring force is created by pulling the riser off vertical and pulling the surface buoy deeper into the water. The vessel becomes essentially stationary when the restoring force is equal to the force acting to move the vessel. An increase or decrease in forces acting to displace the vessel will cause this geometry to adjust until vessel position is again stable.
The horizontal force from a moored vessel 30 pulls the SSR 10 off vertical and consequently causes the top of the SSR to move down to a greater depth below the surface. Because line 36 is not horizontal, tension in this line includes a vertical component which pulls the surface buoy deeper into the water and increases the tension in the riser. The horizontal restoring force from the buoyancy module 19 is proportional to the total upward force at the top of the riser times the sine of the angle of inclination off vertical of the SSR and this inclination increases as the SSR is pulled further off vertical. Therefore the restoring force increases as the top of the SSR is pulled further from its vertical position. The surface buoy 37 is sufficiently large to prevent it from being pulled completely underwater and the length of line 36 is chosen to achieve the desired relationship between riser tension and riser inclination. Line 36 is always long enough to allow the surface buoy to float with freeboard. Beyond this, making line 36 longer results in greater maximum inclination of the riser and reduced maximum tension in the riser.
The total horizontal force at the top of the riser must be reacted by a horizontal force component at the seafloor. The use of a flexible connection 25, the preferred embodiment of which is two half links of chain, allows the horizontal component of riser tension to be transmitted to the anchor 22 without a bending moment in the riser.
The depth of the top of the SSR increases as the SSR is pulled off vertical. If the buoyancy module 19 is a sealed gas can, the pressure differential across its hull will increase, and must not be allowed to exceed the rating of the hull. If the buoyancy module 19 is a vented gas can, the gas in it will compress so buoyancy will decrease, and buoyancy must not be allowed to decrease below the required value. In either case these difficulties can be avoided by using umbilical 12 from the vessel to trim the gas fill of the buoyancy module 19. The umbilical 12 is preferably dressed with the flexible pipe 27, but can be a separate line from the vessel.
When hydrocarbon fluids are brought onboard vessel 30 they must be processed prior to transportation. The equipment 29 for this processing typically must be held reasonably steady to prevent sloshing of the fluids in tanks and to allow the liquid oil to separate from water. A small vessel 30, such as can reasonably be moored to an SSR 10 as described above, exhibits relatively large pitch and roll motions for any given sea state. It is not practical to operate a production system only when the sea is relatively calm. Therefore practical use of a small production vessel moored as above requires a stabilized support on which to mount the fluid processing equipment 29. Heave (vertical) motions have little effect on the process equipment. Surge and sway motions are typically quite small, but pitch and roll motions require stabilization. With minor adaptation, the pitch/roll stabilization system described in U.S. Ser. No. 12/714,919 can be used here to support a frame upon which to install process equipment that is sensitive to pitch and roll. In the processing system of the present invention; however, the embodiment that has the stable frame above the cylinders is preferred.
Referring now to
As shown in
As shown in
A feedback signal from an inclinometer is subtracted from a reference signal and the resulting error signal is used to control the direction and speed of the pump, inclinometer 58 controlling pump 57 and inclinometer 59 controlling pump 55. The pump thus speeds up as the frame tips along the axis between the pair of cylinders and the pump slows down and stops as this axis on the frame becomes level. It is apparent that the inclinometers (58′ and 59′) could alternately be fixed with respect to the deck 31 of the vessel and used to drive the frame 40 in the direction opposite the deck's direction of motion. It is also apparent that accelerometers with appropriate signal conditioning could be used as an alternate or complement to inclinometers, and further that a combination of sensors on the deck and on the frame could be used.
The frame could be mounted on three cylinders (or any odd number of cylinders) if used with some method of apportioning flow between them to keep the frame level. Pairs of cylinders are preferred for simplicity of the control system. Two pair of cylinders are adequate for the desired performance. If there are 3 or more pairs the system will continue to function after any failure, so long as two pair remain functional and the other pair(s) are not locked in place. Therefore, using 3 or more pairs allows operation to continue normally following a failure and allows one pair to be removed from service for maintenance while the remaining pairs continue to keep the frame level.
It is desirable but not necessary to locate the center of gravity of the frame and its load at the midpoint between the cylinders. However, centering the center of gravity is frequently not practical and the load may shift during operations. For the system as described here, quiescence with an offset center of gravity requires a higher pressure in the cylinder(s) closer to the center of gravity and a correspondingly lower pressure in the cylinders further from the center of gravity in order to hold the frame level. This difference (the bias pressure) may be different for each pair of cylinders. Since each pump is controlled by a signal generated from inclinometers or accelerometers they automatically generate the required bias pressure. When transferring fluid from a more heavily loaded cylinder chamber to a less heavily loaded cylinder chamber it may be necessary for the pump to operate as a motor and deliver energy from the load to the power supply. This can be accomplished by, for instance, using hydraulic gear pumps which operate as motors if the pressure of the fluid flowing into the gear motor/pump exceeds the pressure of the fluid flowing out of the device.
Claims
1. A riser system comprising:
- a single self supporting riser (SSR) which comprises a plurality of joints comprising regular joints and specialty joints that define said SSR and selected to optimize said SSR for a particular location; at least one specialty joint comprising a buoyancy module, said one is the uppermost buoyancy module near but below the sea surface and another specialty joint having provisions at the lower end of said SSR to prevent excessive bending moment in the riser; said riser providing fluid communication between said SSR and a well having a wellhead on the seafloor; and
- a specialty joint in the SSR providing fluid communication between said SSR and a vessel subject to high vessel motions of heave, pitch and roll.
2. A riser system according to claim 1 wherein said specialty joint in the SSR providing fluid communication between said SSR and a vessel includes a swivel.
3. A riser system according to claim 1 wherein said SSR is secured to an element of a subsea infrastructure.
4. A riser system according to claim 1 wherein said specialty joint having provisions at the lower end of said SSR to prevent excessive bending moment in the riser is a flexible connector.
5. A riser system according to claim 4 wherein said flexible connector is connected to an anchor on the seafloor.
6. A riser system according to claim 4 wherein said flexible connector is a section of flexible pipe.
7. A riser system according to claim 4 wherein said flexible connector is links of chain.
8. A riser system according to claim 2 wherein said swivel is above said buoyancy module.
9. A production system comprising:
- a self supporting riser (SSR) which comprises a plurality of joints comprising regular joints and specialty joints that define said SSR and selected to optimize said SSR for a particular location; at least one specialty joint comprising a buoyancy module, said one is the uppermost buoyancy module near but below the sea surface; said riser in fluid communication with a hydrocarbon producing well having a wellhead on the seafloor; and
- a vessel subject to high vessel motions of heave, pitch and roll; said riser providing an anchor to said vessel and providing fluid communication between said well and said vessel.
10. A small sea vessel subject to high vessel motion of heave, pitch and roll having on deck a hydrocarbon processing system comprising:
- a stabilized frame on which said hydrocarbon processing equipment is held; and
- a stabilization system consisting of paired cylinders to maintain said frame essentially level.
11. A small sea vessel according to claim 10 wherein each cylinder is mounted on said deck and each cylinder rod extends upward to a compliant connection to support said frame.
12. A small sea vessel according to claim 10 wherein there are at least two pair of cylinders.
13. A small sea vessel according to claim 10 wherein each pair of cylinders is controlled by accelerometers or inclinometers.
14. A small sea vessel according to claim 10 wherein each pair of cylinders has a hydraulic control system consisting of a fluid line between the load bearing chambers and a second line between the non-load bearing chambers; and
- a reversible pump in one of said lines.
15. A small sea vessel according to claim 14 wherein said pump is in said non-load bearing line.
16. A small sea vessel according to claim 14 wherein said pump is controlled by accelerometers or inclinometers.
17. A small sea vessel according to claim 10 including mooring lines to a self supporting riser having a flexible connection at the lower end permitting said SSR to be pulled off vertical without introducing a high bending moment to said SSR.
18. A small sea vessel according to claim 10 including:
- lines and a surface buoy securing and mooring said vessel to a self supporting riser through which said hydrocarbons are carried from the seafloor to said hydrocarbon processing equipment.
19. A self supporting riser (SSR) comprising:
- a plurality of joints comprising regular joints and specialty joints that define said SSR and selected to optimize said SSR for a particular location;
- at least one specialty joint comprising a buoyancy module, said one is the uppermost buoyancy module near but below the sea surface; and
- a lowermost specialty joint that is a flexible connector.
20. A self supporting riser (SSR) according to claim 19 further including:
- a specialty joint that is a seafloor shutoff device above said connector; and
- a specialty joint that has blow-out preventer functions below said buoyancy module.
21. A self supporting riser (SSR) according to claim 20 wherein said riser is multiple tubular.
22. A self supporting riser (SSR) according to claim 19 wherein said flexible connector is connected to a subsea infrastructure.
23. A self supporting riser (SSR) according to claim 22 wherein said subsea infrastructure is an anchor.
24. A self supporting riser (SSR) according to claim 19 wherein said flexible connector is a section of flexible pipe.
25. A self supporting riser (SSR) according to claim 19 wherein said flexible connector is links of chain.
26. A self supporting riser (SSR) according to claim 19 further comprising:
- a swivel located at the top end of the SSR for connection of a line to carry hydrocarbons from said SSR to processing equipment on a stabilized platform on a vessel.
27. A self supporting riser (SSR) according to claim 26 wherein said swivel is above said buoyancy module.
28. A hydrocarbon production system comprising:
- a self supporting riser (SSR) which comprises a plurality of joints comprising regular joints and specialty joints that define said SSR and selected to optimize said SSR for a particular location; at least one specialty joint comprising a buoyancy module, said one is the uppermost buoyancy module near but below the sea surface; said riser in fluid communication with a hydrocarbon producing well having a wellhead on the seafloor, and
- a vessel moored to said SSR;
- said riser having a specialty joint at the lower end of said SSR to prevent excessive bending moment in the riser to provide an anchor to said vessel and providing fluid communication between said well to said vessel.
29. A hydrocarbon production system according to claim 28 wherein said SSR further includes:
- a specialty joint that is a seafloor shutoff device near said specialty joint at the lower end to prevent excessive bending moment in the riser; and
- a specialty joint that has blow-out preventer functions below said buoyancy module.
30. A hydrocarbon production system according to claim 28 wherein said riser is multiple tubular.
Type: Application
Filed: Jul 14, 2010
Publication Date: May 31, 2012
Inventor: Charles R. Yemington (Arlington, TX)
Application Number: 13/384,490
International Classification: E21B 17/01 (20060101); B63B 35/44 (20060101);