Production Riser

Abstract

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.

Description

FIELD OF INVENTION

The 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 INVENTION

It 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 INVENTION

The 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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view a self supporting riser (SSR) connected to a well for testing and producing hydrocarbons from a fossil hydrocarbon reservoir deep below the seafloor;

FIG. 2 is a schematic cross-sectional view of one embodiment of a self supporting riser (SSR) illustrating multiple tubulars side by side;

FIG. 3 is a schematic cross-sectional view of another embodiment of a self supporting riser (SSR) illustrating multiple tubulars in a concentric configuration;

FIG. 4 is a schematic view a self supporting riser (SSR) connected to a well for producing hydrocarbons from a fossil hydrocarbon reservoir deep below the seafloor; and a vessel subject to high vessel motions moored to the SSR with the riser providing an anchor to the vessel;

FIG. 5 is a schematic top view of a novel production vessel having processing equipment on a stabilized frame on the vessel;

FIG. 6 is a schematic side view of a novel production vessel;

FIG. 7 is a top isometric view of a vessel configuration;

FIG. 8 is an isometric view of a stabilized frame on the vessel; and

FIG. 8A is a schematic diagram illustrating a hydraulic system for stabilizing the frame on the vessel.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

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 FIG. 1, a Self Supporting Riser (SSR) 10 is illustrated on an element of a subsea infrastructure, such as a wellhead 20. When placed on the wellhead 20 the SSR 10 provides fluid communication with a well beneath the seafloor. Riser 10 has one or more buoyancy modules 15 and 19. The uppermost buoyancy module 19 is referred to herein as the near-surface buoyancy module and any module below module 19 is referred to herein as a mid-water buoyancy module. At the lower end of the SSR 10 is a connector 25 suited to the target wellhead or tree or another element of seafloor infrastructure. Preferably, a Seafloor Shutoff Device 11 (when used) may be directly above the connector 25. A specialty joint 18 with functions of a Blow Out Preventer (BOP) is preferably below the near-surface buoyancy module 19. Functions of a Blow Out Preventer (BOP) 18 may be to shear tubing or pipe if necessary and provide a seal to fluids in the riser 10. When there is no tubing or pipe passing through the BOP device this SSD11 can consist of simply activating one or more valves.

A specific design of the SSR 10 is only illustrated in FIG. 1; however, the present invention as explained in Ser. No. 12/714,919, also includes provisions to assemble an SSR for a particular purpose, water depth, current conditions, or location from an inventory of standardized joints and to recover the joints and assemble some or all of them into a different SSR configuration for a different application. Having placed the SSR 10 on a wellhead or tree 20, the pre-production of the well is accomplished by opening the well to flow and permitting fluid communication to the SSR. One aspect of the present invention is that the system of the present invention may allow fluids from the well to a moored vessel over a period of time to determine the material parameters of the reservoir from which the fluids are flowing. While pressure and temperatures are indicated in the drilling procedures, the sustainability of pressure in the reservoir or the amounts of gas and water over time flowing from a specific reservoir are preferably measured by allowing the flow of fluids from the reservoir over time.

Referring to FIGS. 2 and 3, these Figures illustrate that a riser 10 may be of multiple tubular construction; FIG. 2 illustrating one or more side by side tubulars 10.sup.1 and 10.sup.2 with the remaining area either empty or filled with insulation and FIG. 3 illustrating one or more concentric tubulars 10.sup.1 and 10.sup.2 separated by spacers 10.sup.3 surrounded by insulation. During the drilling procedures, indication of high pressures in the reservoirs or mixtures of gas and water that are prone to form hydrates may indicate that a special configuration of the riser is desired. For example, side by side tubular in the riser may be used when the well has more than one tubular in the well casing such as one tubular extending to a reservoir at one depth and another tubular extending to a reservoir at a lower depth, possibly for the purpose of gas reinjection. For example a concentric tubular riser may be used when double containment is required or when heated water may be used to heat the fluid flowing in the inner tubular where hydrates may form.

Riser 10 may be attached to an element of a seafloor infrastructure, such as a wellhead or tree 20 (FIG. 1), throughout production or may be attached to a seafloor anchor 22 such as a pile; or a gravity or embedment anchor (FIG. 4). The lower end of the SSR 10 has provision for a flexible jumper line 26 to connect to the tree 20 for the flow of hydrocarbons from the well and up through the riser 10. There are also provisions for the control umbilical 12 to extend to the 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 FIG. 4, where a mooring line(s) 36 extends from the top of the SSR 10 to a mooring buoy 37 that floats on the sea surface, and mooring line(s) 38 extend to the vessel 30 to secure the vessel 30 to the upper part of the SSR 10. The mooring line 36 attached to the buoy 37 may be installed as part of assembling the production riser 10. A flexible pipe 27 is shown connected from the upper part of the SSR, preferably through the swivel 24, to the vessel 30 to provide continuation of the flow path for produced fluids from the well or production equipment below. Buoyancy 28 may be used to support and tend the flexible pipe 27.

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 FIG. 1). When a vessel 30 is moored to the SSR 10 (illustrated in FIG. 4), wind and surface current pull the vessel and the top of the SSR away from the otherwise upright position of the SSR until the resulting offset angle and upward force of the buoyancy create a restoring force to balance the forces on the vessel to secure and moor the vessel. When the force of the vessel is relaxed or released the SSR restores itself to the nearly vertical attitude where it can survive untended and be ready for subsequent use.

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 FIGS. 5, 6 and 7, illustrate the stable frame 40 on the deck 31 of vessel 30 that supports the processing equipment 29. The processing equipment 29 is schematically shown without all the connecting lines as a number of combinations are possible. For example, processing equipment may comprise separator tanks 42 and 43 that separate the gas, the liquid hydrocarbons, and water. The gas is removed from the top of the tanks 42 and 43 and is compressed and transferred to a gas tank 44. Water settles to the bottom of tanks 42 and 43 and is removed, purified, and discharged. The liquid hydrocarbons are removed from above the water of tanks 42 and 43 and transferred to an oil tank 48 which may be below the deck. The combination of the arrangement of the equipment 29 includes using one, two, or more tanks 42 and 43 as per existing industry practice with flow from line 27 into one or more separator tanks 42 and 43. The separated oil may be held in tank 48 on vessel 30 or may be transferred to a second vessel or barge (not shown) to be taken to shore. The water may be purified in device 46, or in a centrifuge 49; or treated in a series of centrifuges 49; or be treated on another vessel before being returned to the sea. This is not intended to recite all the possible equipment combinations as the specific hydrocarbon and water mixture brought up riser 10 will determine the most suitable combination of equipment.

As shown in FIGS. 8 and 8A frame 40 can be mounted on and supported by 2 (two) or more pairs of vertically mounted hydraulic cylinders. It is preferred that each cylinder be attached to the vessel deck 31 with the cylinder rods upward, and the rods attached by compliant joints 51 to the frame 40 to accommodate the changes of alignment as the vessel pitches and rolls. Each pair is connected by relatively large diameter hydraulic line connected to the bottom of each pair and a smaller diameter hydraulic line is connected to the top of each pair of cylinders. In FIG. 8, for simplicity of explanation, one pair of cylinders is shown fore (52F) and aft (52A) and the other pair is shown port (53P) to starboard (53S). When the vessel pitches upward in front (see FIG. 8A) inclination of the frame can be avoided by transferring fluid from the fore cylinder 52F to the aft cylinder 52A to make the fore cylinder shorter and the aft cylinder longer. A force is necessary to accelerate the frame, so any acceleration of the frame results in higher force on the cylinder where the vessel is rising (52F), and consequently the pressure in that cylinder goes up causing the fluid to flow to the cylinder located where the vessel is dropping (52A). In a frictionless system inertia would thus keep the frame level, so long as the center of gravity is centered between the cylinders. Active control is required to overcome friction and supply the associated energy and to compensate for offset center of gravity.

As shown in FIGS. 8 and 8A, a reversible pump is mounted between each pair of cylinders, pump 55 between cylinders 52F and 52A and pump 57 between cylinders 53P and 53S, the pumps are preferably between the non-load bearing chambers of the cylinders. This facilitates pumping at a lower pressure and, with the cylinders mounted with the rod ends up, pumping smaller quantities of fluid thus reducing energy consumption and improving reliability.

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.