Offloading hydrocarbons from subsea fields

Abstract

An offloading system for conveying hydrocarbons from a buoyancy-supported subsea riser to a surface tanker vessel comprises a flexible hose that hangs from the riser structure in a U-shape having first and second limbs. An upper end of the first limb communicates with the riser and an upper end of the second limb terminates in a pulling head for connecting the hose to the tanker. A clump weight acts on a lowermost bend of the hose between the limbs to maintain tension in the limbs. A subsurface holder fixed to the riser structure is arranged to hold the pulling head against the tension in the second limb of the hose when the system is in a standby state. The holder is offset laterally from a central longitudinal axis of the riser structure and a counterweight is positioned to a side of that axis opposed to the holder.

Claims

1. A subsea riser structure fix offloading hydrocarbons, the structure comprising: a riser column; a subsurface buoy that supports the riser column; and an offloading system for conveying hydrocarbons from the riser column to a surface tanker vessel, that system comprising: a flexible hose hanging from thy riser structure in a U-shape having first and second limbs, an upper end of the first limb communicating fluidly with the riser column and an upper end of the second limb terminating in a pulling head for connecting the hose to the tanker; at least one clump weight acting on a lowermost bend of the hose between the first and second limbs to maintain tension in the first and second limbs wherein the hose is movable along its length relative to the or each clump weight; and a subsurface holder fixed to the riser structure, the holder being arranged to engage with and support the pulling head so as to hold the pulling head against said tension in the second limb of the hose when the system is in a standby state, the holder being further arranged to guide the second limb of the hose when the pulling head is disengaged from the holder.

2. The riser structure of claim 1, wherein the holder is offset laterally from a central longitudinal axis of the riser column.

3. The riser structure of claim 2, wherein the holder is cantilevered away from a side of the riser structure.

4. The riser structure of claim 2, further comprising at least one counterweight positioned to a side of the central longitudinal axis opposed to the holder.

5. The riser structure of claim 1, wherein the holder slidingly surrounds the second limb of the hose when the pulling head is disengaged from the holder.

6. The riser structure of claim 1, wherein the first and second limbs of the hose lie in a substantially vertical plane.

7. The riser structure of claim 1, wherein the first and second limbs of the hose are substantially parallel to each other.

8. The riser structure of claim 1, wherein the first and second limbs of the hose are substantially parallel to the riser.

9. The riser structure of claim 1, wherein the first and second limbs of the hose are substantially coplanar with the riser.

10. The riser structure of claim 1, wherein the or each clump weight is supported by a cradle that embraces the lowermost bend of the hose, the cradle defining a path along which the hose may move relative to the cradle.

11. The riser structure of claim 1, wherein the or each clump weight is hung from the hose via one or more rollers that lie on the lowermost bend of the hose.

12. The riser structure of claim 1, wherein the clump weight acts on the hose via a bend restrictor that limits the bend radius of the lowermost bend of the hose.

13. The riser structure of claim 12, wherein the bend restrictor has a limiting radius defined by an army of rollers on an upper side of the lowermost bend of the hose.

14. The riser structure of claim 1, wherein the pulling head comprises a downwardly tapering engagement formation that complements a downwardly narrowing engagement formation of the holder.

15. The riser structure of claim 14, wherein the downwardly tapering engagement formation of the pulling head is a bend stiffener that surrounds the hose.

16. The riser structure of claim 1, wherein the pulling head comprises at least one buoyancy element that confers positive buoyancy on the pulling head.

17. The riser structure of claim 16, wherein negative buoyancy of the second limb of the hose and of the or each clump weight acting on the pulling head exceeds the positive buoyancy of the pulling head.

18. The riser structure of claim 1, wherein the subsurface buoy holds the riser upright and under tension.

19. The riser structure of claim 1, wherein the holder is attached to the buoy of the riser structure.

20. The riser structure of claim 1, wherein the holder is disposed at a level above a centre of buoyancy of the buoy.

21. The riser structure of claim 1, wherein the holder is disposed at a level below a centre of buoyancy of the buoy.

22. A method of offloading hydrocarbons to a surface tanker vessel from a buoyantly supported subsea riser structure, the method comprising: imparting tension in first and second limbs of a flexible hose that hangs from the riser structure in a U-shape, an upper end of the first limb fluidly communicating, with a riser column of the riser structure and an upper end of the second limb terminating in a pulling head for connecting the hose to the tanker; hanging at least one clump weight from a lowermost bend of the U-shaped hose to impart said tension in said limbs of the hose; engaging with and supporting the pulling head so as to hold the pulling head against said tension in the second limb of the hose, when the pulling head is subsurface in a standby state; guiding upwards movement of the second limb of the hose against said tension when the pulling head is being lifted toward the tanker; and moving the hose along its length relative to the or each clump weight.

23. The method of claim 22, comprising holding the pulling head offset laterally from a central longitudinal axis of the riser column.

24. The method of claim 23, comprising applying a counterbalancing moment to the riser structure to a side of the central longitudinal axis opposed to the pulling head.

25. The method of claim 22, comprising limiting a bend radius of the hose at a lowermost bend of the U-shape.

26. The method of claim 22, wherein buoyant upthrust of the pulling head is exceeded by said tension in the second limb of the hose.

27. The method of claim 22, comprising engaging the pulling head with a holder fixed relative to the riser, said engagement being promoted by said tension in the second limb of the hose.

28. The method of claim 22, comprising shortening the first limb of the hose and lengthening the second limb of the hose while lifting the pulling head toward the tanker.

29. The method of claim 22, comprising shortening a U-shape portion of the hose hanging from the riser structure while lifting the pulling head toward the tanker.

30. The method of claim 29, comprising maintaining a U-shaped portion of the hose hanging from the riser structure when the pulling head has been connected to the tanker.

31. The method of claim 30, comprising damping movements of the hose driven by movement of the tanker.

Description

(1) In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings, in which like numerals are used for like features. In those drawings:

(2) FIG. 1a shows an offloading system of the prior art.

(3) FIG. 1b shows another offloading system of the prior art.

(4) FIG. 1c shows yet another offloading system of the prior art.

(5) FIG. 2 is a side view of a shuttle tanker using an offloading system of the invention to offload oil processed and stored on the seabed in a subsea processing and storage installation;

(6) FIG. 3 is a perspective view of the tanker, offloading system and subsea processing and storage installation shown in FIG. 2;

(7) FIG. 4 is an enlarged detail perspective view showing a floating line with pick-up buoys on the surface beside the tanker;

(8) FIG. 5 is a side view of the offloading system of the invention, in a standby state not yet connected to the tanker;

(9) FIG. 6 is a perspective view of the offloading system as shown in FIG. 5;

(10) FIG. 7 is an enlarged perspective view corresponding to Detail VII of FIG. 6;

(11) FIG. 8 is an enlarged part-sectioned perspective view of a hose head of the system as shown in FIGS. 5 to 7;

(12) FIG. 9 is an enlarged perspective view of a bend restrictor and clump weight of the system as shown in FIGS. 5 and 6;

(13) FIG. 10 is a side view of the offloading system of the invention, in an operational state when connected to the tanker;

(14) FIG. 11 is a perspective view of the offloading system as shown in FIG. 10;

(15) FIG. 12 is a side view of another offloading system of the invention, also in a standby state like the system shown in FIG. 5; and

(16) FIG. 13 is an enlarged side view corresponding to Detail XIII of FIG. 12.

(17) Referring to FIGS. 2 and 3, which are not to scale, a shuttle tanker 10 equipped with a conventional bow loading system 26 is shown floating on the surface 12 above a riser column 16.

(18) The riser column 16 extends upwardly from the seabed 18 to a sub-surface buoy 20. The riser column 16 exemplified here comprises a flexible pipe that is kept upright and under tension by the buoy 20. In this example, the buoy 20 is at a depth of approximately 75 m below the surface 12. The depth from the surface 12 to the seabed 18 can be very much greater, in principle ranging from about 150 m to more than 3 km. Consequently, the riser column 16 may be extremely long but it is protected from damaging water dynamics near the surface 12.

(19) The riser column 16 may alternatively be made as a string of metallic rigid pipes or as a pipeline in composite materials.

(20) In this example, the riser column 16 is arranged to offload oil that is processed and stored on the seabed 18 in a subsea processing and storage installation 28. The benefits of the invention make it particularly apt for use when exploiting marginal fields for which a subsea processing and storage installation 28 may be helpful, including small, remote or inaccessible fields. However, the invention is not limited to such use and may find application with any subsea riser that terminates in a sub-surface support, especially where that riser is in deep water.

(21) The riser column 16 is adapted by the addition of an offloading system 30 in accordance with the invention, whose main components will now be described.

(22) The offloading system 30 comprises a loading riser hose 32 that hangs in parallel beside the riser column 16 in a U-shape below, and extending back up to, the buoy 20. Specifically, a first limb of the U-shaped hose 32 communicates with the riser column 16 at the buoy 20 and hangs from the buoy 20. A second parallel limb of the U-shaped hose 32 terminates at its free end in, and hangs from, a head 34 that is supported by the buoy 20.

(23) Typically the hose 32 is of bonded or unbonded flexible pipe. Bonded or unbonded flexible pipe has a multi-layered pipeline structure comprising elements that allow the pipe to be bent with a small radius of curvature without damage.

(24) FIGS. 2 to 4 show the tanker 10 picking up a floating line 36 at the surface 12. Conventionally, the tanker 10 may be guided to that location by a transponder system. The floating line 36 is supported at the surface 12 by a pair of pick-up buoys 38, as best shown in the enlarged view of FIG. 4.

(25) FIGS. 2 and 3 show that the floating line 36 is joined to a messenger line 40 that extends down below the surface 12 to the head 34 of the hose 32 supported by the buoy 20. Thus, when a winch on the tanker 10 picks up and pulls on the floating line 36, the messenger line 40, in turn, pulls on the head 34 and on the limb of the U-shaped hose 32 that hangs from the head 34. The U-shaped portion of the hose 32 shortens accordingly as the limb of the hose 32 hanging from the head 34 is pulled up past the buoy 20, which causes the limbs of the hose 32 progressively to become more unequal in length.

(26) The head 34 is thereby pulled to the surface 12 to couple the hose 32 and hence the riser column 16 to the tanker 10 via a manifold valve in the bow loading system 26 of the tanker 10. In the bow loading system 26, a ball joint in a loading manifold provides a substantially moment-free connection between the hose 32 and the tanker 10.

(27) It will be apparent that the design of the offloading system 30 greatly simplifies the pickup system comprising the messenger line 40 and makes it independent of the water depth. In particular, the messenger line 40 is very much shorter than the messenger line 24 shown in the prior art arrangement of FIGS. 1a to 1c. This is because the messenger line 40 need only reach from the buoy 20 to the floating line 34 at the surface 12. Thus, the messenger line 40 is less expensive and is easier to handle, and the tanker 10 requires less time to perform connection and disconnection operations. A further significant cost saving is achieved by eliminating the weighted line 22 of the prior art arrangement.

(28) Features of the riser column 16 and the offloading system 30 are shown in more detail in FIGS. 5 to 7 of the drawings, to which the following description refers.

(29) The sub-surface buoy 20 has a cylindrical buoyant body that may comprise one or more hollow chambers, may be formed of rigid buoyant material such as syntactic foam or may contain a mass of rigid buoyant macrospheres, depending upon the hydrostatic pressure expected at the operational depth.

(30) The buoy 20 and the riser column 16 are aligned with each other on a common central longitudinal axis 42.

(31) A guide and docking stab 44 extends laterally to one side of the buoy 20 from a structure 46 fixed at the lower end of the buoy 20. The stab 44 comprises a frusto-conical collar 48 that is cantilevered from the structure 46 and that is centred on an axis extending substantially parallel to the central longitudinal axis 42 of the riser column 16. The collar 48 is adapted to receive, support and locate the head 34 of the hose 32, in the manner of a socket receiving a plug.

(32) A counterweight 50 is also attached to the structure 46 at the lower end of the buoy 20, being cantilevered from a side of that structure 46 opposed to the guide and docking stab 44 about the central longitudinal axis 42. The counterweight 50 provides a counterbalancing effect for the offloading system 30, as will be explained.

(33) A downwardly-tapering bend stiffener 52 that surrounds the upper section of the riser column 16 is also fixed to the structure 46 at the lower end of the buoy 20.

(34) The hose 32 connects to the upper section of the riser column 16 immediately beneath the buoy 20. The hose 32 in connected to the riser column 16 to the same side of the central longitudinal axis 42 as the guide and docking stab 44 that extends from the structure 46 of the buoy 20 above. Viewed from above, the hose 32 is in co-planar angular alignment with the guide and docking stab 44.

(35) In this example, the hose 32 is connected to the riser column 16 through the side of a bend stiffener 52. The hose 32 hangs downwardly from that connection to extend parallel to the riser column 16 as the first limb 32A of the U-shape. The hose 32 is also fitted with a bend stiffener 54 around its end connected to the riser column 16.

(36) At the bottom of the U-shape, the hose 32 bends through 180° around a bend restrictor 56 and then extends upwardly into the second limb 32B of the U-shape. A clump weight 58 is attached to the bend restrictor 56 to maintain tension in both limbs 32A, 32B of the U-shaped hose 32.

(37) The second limb 32B extends substantially parallel to the first limb 32A and to the riser column 16 and terminates at its upper end in the head 34, which is shown in FIGS. 5 to 7 engaged with the collar 48 of the guide and docking stab 44.

(38) The lateral spacing between the first and second limbs 32A, 32B of the hose 32 is determined by the properties of the hose 32, in particular its Minimum Bend Radius or MBR. For example, an MBR of two metres may be appropriate for a flexible hose 32 having an internal diameter of twenty inches (50.8 cm).

(39) The features of the head 34 are shown in detail in FIG. 8 of the drawings. From bottom to top, the head 34 comprises:

(40) a frusto-conical bend stiffener 60 around the free end of the hose 32 that, in addition to protecting the hose 32, complements and engages in the frusto-conical collar 48 of the guide and docking stab 44; a buoyancy element 62 that partially offsets the weight in water of the head 34 and the proportion of the weight in water of the hose 32, the bend restrictor 56 and the clump weight 58 that is carried by the head 34; a hose end valve 64 that is cooperable with a manifold valve in the bow loading system 26 of the tanker 10; permanent rigging 66 that connects the head 34 to the messenger line 40; and a buoyancy element 68 that confers positive buoyancy on the permanent rigging 62 to hold the permanent rigging 62 above the head 34.

(41) The buoyancy elements 62, 68 are conveniently of syntactic foam but may instead comprise hollow chambers or contain a mass of rigid buoyant macrospheres.

(42) It will be apparent that the aggregate weight load of the hose 32, the bend restrictor 56 and the clump weight 58 is shared between the riser column 16 acting against tension in the first limb 32A of the hose 32 and the head 34 acting against tension in the second limb 32B of the hose 32.

(43) Together, the buoyancy elements 62, 68 confer positive buoyancy on the head 34. However, the resulting buoyant upthrust acting on the head 34 is slightly less than half of the aggregate weight in water of the hose 32, the bend restrictor 56 and the clump weight 58. Hence, the weight load carried by the head 34 is sufficient to overcome the positive buoyancy of the head 34. This makes the combination of the head 34 and of the components 30, 56, 58 suspended from the head 34 slightly negatively buoyant.

(44) Nevertheless, by reducing the apparent aggregate weight of the head 34 and of the components 30, 56, 58 suspended from the head 34, the buoyancy of the buoyancy elements 62, 68 reduces the pivoting moment that acts on the buoy 20 about a horizontal axis when the head 34 is engaged with the collar 48 of the guide and docking stab 44.

(45) The counterweight 50 that is opposed to the collar 48 of the guide and docking stab 44 about the central longitudinal axis 42 provides a counterbalancing moment. That counterbalancing moment substantially balances the moment exerted on the riser column 16 through the first limb 32A of the hose and the remaining moment exerted on the buoy 20 by the head 34 engaged with the collar 48. Thus, the net pivoting moment exerted on the riser column 16 and the buoy 20 by the offloading system 30 is negligible.

(46) If the MBR of the hose 32 requires the lateral spacing between the first and second limbs 32A, 32B to be increased, this requires the collar 48 of the guide and docking stab 44 to be spaced further from the central longitudinal axis 42. In that case, the mass of the counterweight 50 and/or its lateral offset from the central longitudinal axis 42 should also be increased.

(47) When the head 34 is disengaged from the collar 48 of the guide and docking stab 44, the buoyancy of the buoyancy elements 62, 68 also reduces the pull-in force that has to be exerted on the messenger line 40 by a winch on the tanker 10. This makes it easier and quicker to raise the head 34 to the surface 12.

(48) The features of the bend restrictor 56 and the clump weight 58 are shown in detail in FIG. 9 of the drawings. Here, it can be seen that the bend restrictor 56 comprises a U-shaped cradle 70 that embraces the 180° bend at the bottom of the U-shaped hose 32. The clump weight 58 hangs from the cradle 66 beneath the hose 32 on the outer side of the 180° bend.

(49) The cradle 70 supports a U-shaped array of rollers 72 that rest on top of the hose 32 on the inner side of the 180° bend. The rollers 72 have respective axes of rotation that are parallel to each other and to the axis of curvature of the 180° bend. The relative positions of the rollers 72 limits bending of the hose 32 and so determines the MBR at the 180° bend. This protects the hose 32 from permanent damage due to overbending.

(50) Turning next to FIGS. 10 and 11, these drawings show the offloading system 30 in an operational state with the head 34 of the hose 32 connected to the bow loading system 26 of the tanker 10.

(51) As the head 34 is lifted on the messenger line 40 toward the tanker 10, the second limb 32B of the hose 32 slides up through the collar 48 as the U-shaped portion of the hose 32 beneath the guide and docking stab 44 shortens accordingly. Thus, the bend restrictor 56 and the clump weight 58 are lifted toward the buoy 20 while remaining at the lowest point of the U-shaped portion of the hose 32. It will therefore be apparent that the hose 32 moves through the bend restrictor 56 as the rollers 72 turn about their respective axes of rotation. During that relative movement, the bend restrictor 56 continues to control the bend radius of the 180° bend in the hose 32.

(52) As the upthrust of its buoyancy is lost when the head 34 of the hose 32 clears the surface 12, the winch on the tanker 10 must briefly exert an increased pull-in force at that stage. The increased pull-in force then comprises the weight in air of the head 34, the weight in water of the second limb of the hose 32B and half of the weight in water of the bend restrictor 56 and the clump weight 58.

(53) When the head 34 of the hose 32 has been connected to the bow loading system 26 of the tanker 10, a remaining U-shaped portion of the hose 32 extends a few metres, for example eight metres, beneath the guide and docking stab 44. This slack portion of the hose 32 compensates for movements of the tanker 10 relative to the buoy 20 during offloading, such as surge and sway, and functions as a sprung damper with the aid of the ballast provided by the bend restrictor 56 and the clump weight 58.

(54) The skilled reader will appreciate that the hose 32 should not be exposed to contact with sharp edges or snagging points. In this respect, the lateral offset of the collar 46 of the guide and docking stab 44 and its vertical spacing from the top of the buoy 20 ensure that the tanker 10 can rotate 360° within a pick-up zone above the riser column 16 during offloading. The lateral offset of the collar 46 and the weight of the bend restrictor 56 and the clump weight 58 also minimise any risk of clashing between the U-shaped portion of the hose 32 and the parallel riser column 16.

(55) While the head 34 remains disengaged from the collar 48, the guide and docking stab 44 will no longer carry the apparent weight of the head 34 and of the components suspended from the head 34. Thus, the moment that continues to be exerted on the structure 46 of the buoy 20 by the counterweight 50 may cause the orientation of the buoy 20 to tilt slightly away from the vertical. However, with the aid of the bend stiffener 52 that is fixed to the structure 46 of the buoy 20, this small and temporary change in the angle of the buoy 20 will not have a materially adverse effect upon the capability or the working life of the riser column 16.

(56) The mass of the counterweight 50 and its lateral offset from the central longitudinal axis 42 should be chosen to minimise differences in the moments experienced by the riser column 16 and the buoy 20 between the standby and operational states.

(57) When offloading is complete, the head 34 of the hose 32 is disconnected from the bow loading system 26 of the tanker 10 and is lowered back into the water. The combined weights of the hose 32, the bend restrictor 56 and the clump weight 58 hanging from the head 34 exceed the buoyancy of the buoyancy elements 62, 68. Thus, the head 34 is ballasted to sink back into engagement with the collar 48 of the guide and docking stab 44. The collar 48 guides the second limb 32B of the hose 32 as it slides down through the collar 48. The U-shaped portion of the hose 32 beneath the guide and docking stab 44 lengthens accordingly.

(58) When the offloading system 30 of the invention has been returned to the standby state in this way, the head 34 is held in engagement with the collar 48 by the weight of the hose 32, the bend restrictor 56 and the clump weight 58 that hang from the head 34. That weight load and the resulting moment are transferred to the riser column 16 and the buoy 20 via the structure 46 of the buoy 20 and the bend stiffener 52 that is attached to the structure 46.

(59) The head 34 is thereby held against movement out of the collar 48 due to water dynamics, which in any event may be expected to be minimal at the typical depth of the buoy 20. The U-shaped loop of hose 32 hanging beneath the buoy 20 is even deeper in the water and therefore even less likely to be disturbed significantly by water dynamics that are prevalent nearer the surface 12.

(60) The messenger line 40 remains connected to the head 34 and to the floating line 36 that remains supported by the pair of pick-up buoys 38 at the surface 12, ready to be located and picked up by a tanker 10 again at the start of another offloading operation.

(61) Turning finally to FIGS. 12 and 13, these drawings show how the offloading system 30 of the invention may be used with a rigid riser column 16. Again, like numerals are used for like features. Here, the rigid riser column 16 is shown upstanding from a subsea processing and storage installation 28 that serves as a riser base.

(62) A large buoyancy tank 74 provides the increased uplift force that is required to impart the tension necessary to support a rigid riser column 16. Higher tension forces in the rigid riser column 16 do not have any negative effect on the offloading system 30.

(63) It will be noted that, in this example, the guide and docking stab 44 and the counterweight 50 are positioned near the top of the buoyancy tank 74, above its centre of buoyancy. This is in contrast to the stab 44 and the counterweight 50 being near the bottom of the buoy 20 that is used to support the flexible riser column 16 in the previous embodiment. The elevated position of the stab 44 and the counterweight 50 relative to the centre of buoyancy counteracts a tendency for any unbalanced moments to tilt the buoyancy tank 74 relative to the riser column 16.

(64) Many variations are possible within the inventive concept. For example, in principle, it would be possible for a guide and docking stab 44 and a counterweight 50 to be elevated above the bottom of a buoy 20 that is used to support a flexible riser column 16. It may also be possible to delete the counterweight 50 in some embodiments.

(65) In the standby state, the head 34 of the hose 32 may be held in engagement with the collar 48 by inter-engaging formations such as inwardly-facing fingers around the collar, in addition to the effect of the weight of the hose 32, the bend restrictor 56 and the clump weight 58 that hang from the head 34.