Offloading hydrocarbons from subsea fields

Abstract

A subsea hydrocarbon export system includes a riser tower having a riser column extending from a seabed location to a sub-surface buoy that supports the riser column in an upright orientation. A subsea connector is operable underwater to couple the riser column temporarily to a hose suspended from a surface shuttle tanker vessel for an export operation and to release the hose after the export operation.

Claims

1. A subsea hydrocarbon export system, comprising: a riser tower having a column extending from a seabed location to a sub-surface buoy that supports the column in an upright orientation, wherein the column communicates with a subsea tank for storing hydrocarbon fluids and with a subsea processing system for processing hydrocarbon fluids; and a subsea connector that is operable underwater to couple the column temporarily to a hose suspended from a surface shuttle tanker vessel for an export operation and to release the hose after the export operation; wherein the subsea connector comprises an upwardly facing socket at an upper extremity of the column for receiving a plug connector of the hose; and wherein the buoy comprises one or more external tubes or sockets that open upwardly to receive at least one clump weight.

2. The system of claim 1, wherein the subsea tank serves as a foundation for the riser tower.

3. The system of claim 1, further comprising a subsea pump for pumping hydrocarbon fluids up the column to the subsea connector.

4. The system of claim 1, wherein the column extends through the buoy.

5. The system of claim 1, wherein the buoy surrounds an upper end portion of the column.

6. The system of claim 5, wherein the buoy comprises shell elements assembled together around the upper end portion of the column.

7. The system of claim 1, wherein the column comprises a connection between a major lower section and a minor upper section, the buoy being attached to the upper section of the column.

8. The system of claim 1, comprising at least one laterally projecting male formation on the column, wherein the male formation is engaged with a female interlocking formation of the buoy.

9. The system of claim 8, wherein the male formation surrounds the column.

10. The system of claim 8, wherein the male formation is formed integrally with the column.

11. The system of claim 1, wherein the buoy comprises a sleeve fixed to and surrounding the column.

12. The system of claim 11, further comprising an upper cross-member extending laterally from the sleeve, wherein the cross-member supports one or more lifting points.

13. The system of claim 12, wherein the upper cross-member also supports one or more attachment points for the attachment of at least one clump weight.

14. The system of claim 12, wherein one or more buoyant elements of the buoy bear against an underside of the upper cross-member to apply buoyant upthrust to the upper cross-member in use.

15. The system of claim 11, further comprising a lower cross-member extending laterally from the sleeve.

16. The system of claim 15, wherein the lower cross-member supports one or more attachment points for the attachment of at least one clump weight.

17. The system of claim 15, wherein one or more buoyant elements of the buoy rest upon the lower cross-member.

18. The system of claim 1, wherein a bend restrictor is attached to the buoy and extends along and around the column under the buoy.

19. The system of claim 1, further comprising at least one clump weight releasably attached to the buoy.

20. The system of claim 19, wherein the or each clump weight comprises a chain.

21. The system of claim 19, wherein the or each clump weight is a rigid structure attachable to the buoy.

22. The system of claim 1, wherein the buoy comprises non-floodable buoyancy.

23. The system of claim 22, wherein the buoy comprises rigid buoyant foam or macrospheres.

24. The system of claim 1, wherein the hose is a bonded polymer composite hose.

25. The system of claim 1, wherein the hose is longitudinally flexible and comprises a rigid guide structure at a distal end of the hose.

26. The system of claim 1, wherein the column is of pipe that can be wound onto a reel or carousel onboard an installation vessel, without substantial plastic deformation of the pipe.

27. The system of claim 1, wherein the column is of bonded polymer composite pipe.

28. The system of claim 1, wherein the column is of material that is substantially neutrally buoyant in sea water.

29. The system of claim 1, wherein the hose is connected to the column at a depth of between 30 m and 200 m underwater.

30. A method of exporting hydrocarbon fluids from a seabed location, the method comprising: sailing a shuttle tanker vessel to a surface export location above a column that extends from the seabed location and communicates with a subsea tank for storing the hydrocarbon fluids to a sub-surface buoy, wherein the buoy supports the column in an upright orientation, and wherein the buoy comprises one or more external tubes or sockets that open upwardly to receive at least one clump weight; suspending a hose from the vessel to reach the column; operating a subsea connector underwater to couple the hose temporarily to the column for an export operation by inserting a plug connector of the hose into an upwardly facing socket at an upper extremity of the column; during the export operation, causing hydrocarbon fluids to flow from the seabed location up the column and along the coupled hose to the vessel; and on completion of the export operation, releasing the hose from the column by removing the plug connector of the hose from the upwardly facing socket, lifting the hose to the vessel and sailing the vessel away from the surface export location.

31. The method of claim 30, comprising storing the hydrocarbon fluids at the seabed location before the export operation.

32. The method of claim 30, comprising processing the hydrocarbon fluids at the seabed location before or during the export operation.

33. The method of claim 30, comprising pumping the hydrocarbon fluids at the seabed location during the export operation to flow up the column.

34. A method of installing a subsea hydrocarbon export system, the method comprising: lowering a major lower section of a column into water beneath an installation vessel; suspending the lower section from the installation vessel; positioning a buoy and a minor upper section of the column over the suspended lower section; joining the upper section to the lower section to complete the column; adding ballast to the buoy by attaching one or more clump weights to the buoy, wherein attaching one or more clump weights to the buoy comprises inserting at least part of a clump weight into an upwardly extending external tube of socket on the buoy; lowering the buoy and the completed column into the water beneath the installation vessel to anchor a lower end of the column to a subsea facility comprising a subsea tank for storing hydrocarbon fluids at a seabed location, the buoy then being at a sub-surface location; and removing the added ballast from the buoy after anchoring the lower end of the column at the seabed location.

35. The method of claim 34, comprising unwinding the lower section of the column from shipboard storage while launching the lower section into the water.

36. The method of claim 34, comprising raising the buoy and the upper section from a stowed position on the installation vessel into an upright orientation when positioning the buoy and the upper section over the suspended lower section.

37. The method of claim 34, comprising attaching the or each clump weight to the buoy at a level beneath a mid-point of the buoy.

38. The method of claim 37, comprising attaching the or each clump weight to a lower end region of the buoy.

39. The method of claim 34, comprising attaching the or each clump weight to an upper end region of the buoy.

Description

(1) In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

(2) FIG. 1 is a schematic side view of an offloading system in accordance with the invention, showing a tanker connected to the system to offload production fluids from subsea storage;

(3) FIG. 2 is a perspective view of an embodiment of an offloading system in accordance with FIG. 1;

(4) FIG. 3 is an enlarged perspective view of a buoy being part of the offloading system of FIG. 2, now coupled to a hose suspended from the tanker;

(5) FIG. 4 is a side view of the buoy of FIG. 3;

(6) FIG. 5 is a sectional side view on line A-A of FIG. 4;

(7) FIG. 6 is an exploded perspective view of the buoy of FIG. 3 and a riser column to which the buoy is attached;

(8) FIG. 7 corresponds to FIG. 6 but shows the buoy partially assembled around the riser column;

(9) FIG. 8 is a detail perspective view of a lower cross-member of the buoy;

(10) FIG. 9 corresponds to FIG. 8 but shows a bend restrictor attached to the lower cross-member;

(11) FIG. 10 is a perspective view that corresponds to FIG. 1 but shows a variant of an offloading system of the invention;

(12) FIG. 11 is a side view that corresponds to FIG. 4 but shows the buoy in the variant of FIG. 10;

(13) FIG. 12 is a sectional side view on line A-A of FIG. 11;

(14) FIG. 13 is an exploded perspective view of the buoy of FIG. 11 and a riser column to which the buoy is attached;

(15) FIG. 14 is a perspective view of the buoy of FIG. 11 from beneath, showing how clump weight chains may be attached to the buoy;

(16) FIG. 15 is a perspective view corresponding to FIG. 14 but from above;

(17) FIG. 16 is a perspective view of the buoy of FIG. 11, showing an alternative hose arrangement that may also be used in the embodiment of FIGS. 1 to 9;

(18) FIG. 17 is a perspective view of an installation vessel that is configured to install the riser column and to attach the buoy to the riser column to form a riser tower;

(19) FIG. 18 corresponds to FIG. 17 but shows the installation vessel from the other side;

(20) FIGS. 19a to 19e are a series of schematic side views of the installation vessel of FIGS. 17 and 18 when assembling and installing a riser tower;

(21) FIG. 20 is a perspective view of a variant of the buoy of FIG. 13 showing an alternative clump weight support;

(22) FIGS. 21 and 22 are perspective views that correspond to FIG. 20 but show clump weight arrangements held by the clump weight support;

(23) FIGS. 23 and 24 are perspective views of a variant of the buoy of FIG. 13 showing alternative clump weight arrangements;

(24) FIG. 25 is a perspective view of another variant of the buoy of FIG. 13 showing a further alternative clump weight support; and

(25) FIGS. 26 to 28 are perspective views showing how various clump weights can be engaged with and disengaged from the clump weight support of FIG. 25.

(26) Referring firstly to FIG. 1 of the drawings, an offloading system 10 of the invention comprises a subsea processing and/or storage facility 12 that lies on the seabed 14. The facility 12 optionally processes and temporarily stores crude oil before periodically offloading the oil to a visiting shuttle tanker 16 that floats on the surface 18 above the facility 12.

(27) For this purpose, a riser tower 20 extends upwardly from the subsea facility 12 to an upper end beneath the surface 18. The riser tower 20 comprises a composite riser pipe 22 that is kept upright and under tension by a buoy 24 at or near to its upper end.

(28) Conveniently, in this example, the riser tower 20 is anchored by the weight of the subsea facility 12. However, other well-known foundation arrangements such as weights or piles could be used to anchor the riser tower 20 to the seabed 14 instead.

(29) The upper end of the riser pipe 22 includes interface features for mating with, and fluid connection to, a flexible hose 26 that hangs under the surface 18 from the tanker 16. When the hose 26 is engaged with the riser pipe 22 in this way, fluid communication is effected between the subsea facility 12 and the tanker 16 via the riser pipe 22 and the hose 26.

(30) The interface features shown here at the upper end of the riser pipe 22 comprise an upwardly-facing socket 28 that receives a plug connector 30 on the free end of the hose 26. However, it would be possible to have alternative interface features, such as a plug connector on the upper end of the riser pipe 22 that mates with a socket at the end of the hose 26.

(31) In this example, the buoy 24 surrounds a short upper section 22U of the riser pipe 22 that implements the interface features at the upper end of the riser pipe 22. A flange connection 32 joins the upper section 22U end-to-end to a much longer lower section 22L of the riser pipe 22. The lower section 22L may extend from the connection 32 all the way down through the water column to the subsea facility 12.

(32) Within the buoy 24, the upper section 22U of the riser pipe 22 is surrounded by a fixed tubular sleeve 34 in concentric, telescopic relation. The buoy 24 further comprises a tubular buoyant body 36 that surrounds the sleeve 34. The buoyant body 36 may comprise one or more hollow chambers, may be formed of rigid buoyant material such as syntactic foam or may comprise a mass of rigid buoyant macrospheres, depending upon the hydrostatic pressure expected at the operational depth

(33) The buoyant body 36 bears against an upper cross-member 38 that is fixed to the sleeve 34 above the buoy 24. Consequently, buoyant upthrust of the buoyant body 36 exerted via the upper cross-member 38 and the sleeve 34 imparts tension in the riser pipe 22. The sleeve 34 and the upper cross-member 38 are suitably of steel and so are apt to be welded together.

(34) In this example, the buoyant body 36 comprises shells 40 of part-circular cross-section that are brought together and fixed together as an annulus, for example by clamping under tension applied to external straps, closely to encircle the sleeve 34 and the upper section 22U of the riser pipe 22 within the sleeve 34. In this example, there are two sets of shells 40 stacked one above the other. There could be only one such set of shells 40 or more than two such sets of shells 40.

(35) In addition to the upper cross-member 36, the shells 40 are located against axial movement along the riser pipe 22 by engagement of locating formations on an inner side of each shell 40 with complementary locating formations on an outer side of the sleeve 34. The locating formations are exemplified here by male formations on the sleeve 34 that engage with female formations of the shells 40. Specifically, axially-spaced collars 42 encircle the sleeve 34 to engage with grooves on an inner side of each shell 40.

(36) The collars 42 may be attached to the sleeve 34 by welding, clamping and/or by bonding. Alternatively the sleeve 34 could be omitted. In that case, the collars 42 may be clamped or bonded directly to the riser pipe 22 or could be formed integrally with the riser pipe 22 by locally thickening the wall of the riser pipe 22 to increase its external diameter.

(37) An optional bend restrictor 44 surrounds the upper section 22U of the riser pipe 22 immediately beneath the buoy 24. Conveniently, the bend restrictor 44 is attached to the underside of the buoy 24 and tapers downwardly as shown here. However, as will be explained, other bend restrictor arrangements are possible.

(38) With reference to FIG. 1, exemplary dimensions are set out below for ease of understanding. These dimensions are provided only to put the invention into context and are not intended to be limiting. h.sub.1—the depth of the top of the riser tower 20 beneath the surface 18—75 m; h.sub.2—the height of the buoy 24—7.5 m; h.sub.3—the height of the riser tower 20 from the seabed 18 to the bottom of the buoy 24—50 m to >2000 m h.sub.4—the length of the bend restrictor 44—5 m; and h.sub.5—the protruding length of the upper section 22U between the bottom of the buoy 24 and the connection 32—10 m.

(39) Reference is now made to FIGS. 2 to 9 to describe a specific embodiment of the offloading system 10 in more detail.

(40) FIG. 2 shows that the subsea facility 12 includes a feed pump 46, which during offloading pumps crude oil from the facility 12 up the riser pipe 22 and the hose 26 to the tanker 16.

(41) The upwardly-facing socket 28 at the upper end of the riser pipe 22 is shown in FIG. 2 ready for engagement with a plug connector 30 of a hose 26 suspended from a tanker 16. FIG. 3 shows the plug connector 30 of the hose 26 now engaged with the socket 28.

(42) The enlarged view of FIG. 3 shows further details of the top of the riser tower 20, namely upwardly-protruding lifting padeyes 48 for attachment of lifting lines during installation, and straps 50 that tightly encircle the shells 40 of the buoyant body 36. The straps 48 are received in respective circumferential grooves 52 that are best appreciated in the similarity-enlarged side view of FIG. 4.

(43) The further enlarged sectional view of FIG. 5 shows details of the interface between the riser pipe 22 and the hose 26, effected via the socket 28 and the complementary plug connector 30.

(44) The socket 28 is defined by a tubular steel funnel 54 fixed to the top of the sleeve 34. The funnel 54 is stiffened by radial webs 56 and is surrounded by a tubular upper housing 58. The funnel 54 and the plug connector 30 have complementary frusto-conical mating surfaces that guide those parts into mutual alignment as the plug connector 30 moves downwardly.

(45) The upper section 22U of the riser pipe 22 is shown here extending concentrically within the sleeve 34 and protruding from the sleeve 34 into the funnel 54. The protruding end of the riser pipe 22 is surrounded by a steel collar 60. When the plug connector 30 engages within the funnel 54, the collar 60 is received in a complementary recess 62 in a distal end face of the plug connector 30.

(46) FIGS. 6 and 7 are exploded views that between them show: the upper section 22U of the riser pipe 22; the flange connection 32; the sleeve 34; the upper cross-member 38 fixed to the sleeve 34; the shells 40 of the buoyant body 36; the collars 42 that encircle the sleeve 34; the bend restrictor 44; and the lifting padeyes 48.

(47) FIGS. 6 and 7 also show other details. For example, it will be apparent that the lifting padeyes 48 are fixed to the upper cross-member 38 and protrude through respective slots 64 in the upper housing 58 that surrounds the funnel 54. It will also be apparent that there is a lower cross-member 66 fixed to a lower end of the sleeve 34, which provides further axial location for the shells 40 of the buoyant body 36. Also, the upper and lower cross-members 38, 66 are apt to support sacrificial anodes 68 that protect the steel parts from corrosion.

(48) The lower cross-member 66 provides a connection point for the attachment of clump weights as will be explained later with reference to FIGS. 16 to 18. To this end, the underside of the lower cross-member 66 supports hanging padeyes 70 that protrude through respective slots 72 in a lower housing 74 surrounding the lower cross-member 66.

(49) The lower cross-member 66 may also have another function, namely to provide an attachment point for the bend restrictor 44. In this respect, FIGS. 8 and 9 show that the lower cross-member 66 has a circular flange 76 that lies in a plane orthogonal to the common central longitudinal axis of the riser pipe 22 and the sleeve 34. The flange 76 is penetrated by an array of circumferentially-spaced holes 78 that have counterpart holes 80 in a parallel upper face of the bend restrictor 44. This allows the bend restrictor 44 to be bolted securely to the flange 76 of the lower cross-member 66 as shown in FIG. 9.

(50) Turning next to FIGS. 10 to 16, these show a second embodiment of the invention. Many features are in common with the first embodiment shown in FIGS. 1 to 9 and so will not be repeated here; also, like numerals are used for like features. FIGS. 11 and 12 best show the main differences between the first and second embodiments.

(51) FIG. 11 shows that the second embodiment has a longer and narrower bend restrictor 44. In this case, the bend restrictor 44 is parallel-sided along most of its length and tapers only near its lower end down to the diameter of the riser pipe 22 disposed concentrically within.

(52) FIG. 12 shows an alternative arrangement for the interface between the riser pipe 22 and the hose 26. In this case, the socket 28 is recessed into the top face of an upper housing 58. Again, the upper housing 58 surrounds an upper cross-member 38 atop the uppermost shells 40 of the buoyant body 36. Also, the plug connector 30 has a straight-sided cylindrical body 82 in this example and the socket 28 has a complementary straight-sided recess 84. However, the recess 84 is surmounted by a frusto-conical guide surface 86 to guide the body 82 into alignment and engagement with the recess 84.

(53) FIGS. 14 and 15 show clump weights in the form of chains 88 hung from hanging padeyes 70 supported by the lower cross-member 66. The weight of the chains 88 is necessary to overcome the buoyant upthrust of the buoy 24 so as to sink the riser tower 20 to the required depth upon installation. Locating the clump weights at the bottom of the buoy improves stability by lowering the centre of gravity or centre of buoyancy and by decreasing rotational moments.

(54) Once the riser tower 20 has been anchored to the subsea facility 12 or to another foundation on the seabed 14, the chains 88 are removed so that the buoyant upthrust of the buoy 24 can apply the necessary tension to the riser pipe 22. Depending upon the water depth, divers or an ROV may be used to attach the chains 88 to suitable lifting lines and to release the chains 88 at the appropriate time for recovery to the surface. The lower end of the chain 88 is lifted and then the upper end of the chain 88 is disconnected from the hanging padeyes 70 below the buoy 24.

(55) FIG. 16 shows an alternative hose arrangement. This is shown in relation to the second embodiment but it may also be used in the first embodiment shown in FIGS. 1 to 9. Here, the lower end of the hose 26 is defined by a rigid tubular dog-leg structure 90 that offsets the plug connector 30 laterally from the generally downward axis of the hose 26. The structure 90 is surmounted by a fixing point 92 to which a control wire 94 may be attached to control the position of the plug connector 30 for alignment with the socket 28.

(56) Moving on now to FIGS. 17 and 18, these drawings exemplify how an installation vessel 96 may be adapted to install a riser tower 20 of the invention. Whilst FIGS. 17 and 18 depict elements of the second embodiment shown in FIGS. 10 to 16, it will be evident that the same principles can be applied to installation of the first embodiment shown in FIGS. 1 to 9.

(57) The installation vessel 96 has a working deck 98 that supports a carousel 100 on which the major lower section 22L of the riser pipe 22 can be wound or spooled. In this respect, it will be noted that the composite riser pipe 22 has some flexibility to be bent elastically along its length if a sufficiently large minimum bend radius is observed. In principle, a reel with a horizontal axis could be used instead of a carousel to carry the lower section 22L of the riser pipe 22.

(58) The lower section 22L of the riser pipe 22 is unspooled from the carousel 100 through a spooler 102 on the working deck 98 beside the carousel 100 and then is overboarded into the sea along a chute 104. At this stage, a tensioner 106 upstream of the chute 104 carries the weight load of the launched portion of the lower section 22L.

(59) Once the lower section 22L of the riser pipe 22 has been fully unspooled from the carousel 100 and lowered into the sea, its weight load is transferred to a crane 108 on the working deck 98. As best shown in FIG. 18, a top flange part 110 of the lower section 22L is then engaged with a hang-off structure 112 outboard of the working deck 98. This leaves the remainder of the lower section 22L hanging in the water column beneath the installation vessel 96.

(60) The working deck 98 supports a frame 114 that in turn supports the upper section 22U of the riser pipe 22 surrounded by the buoy 24. The frame 114 is shown here in a horizontal stowed position but can be pivoted about a horizontal axis into a vertical installation position. This pivoting movement upends the upper section 22U and the buoy 24 and brings them into alignment with the vertical axis of the lower section 22L hung off from the hang-off structure 112 below. It also brings a bottom flange part 116 of the upper section 22U into alignment with the top flange part 110 of the lower section 22L. The top and bottom flange parts 110, 116 can then be bolted together.

(61) When united in this way, the top flange part 110 of the lower section 22L and the bottom flange part 116 of the upper section 22U together form the aforementioned flange connection 32 between the upper and lower sections 22U, 22L. This completes the full length of the riser pipe 22.

(62) The crane 108 can now take the load of the riser tower 20 comprising the riser pipe 22 and the buoy 24 by attaching lifting lines to the lifting padeyes 48 shown in preceding figures. Clump weights are attached to the buoy 24 using the hanging padeyes 70 also shown in the preceding figures. This added ballast overcomes the buoyancy of the buoy 24 and allows the crane 108 to lower the riser tower 20 to the required depth. When the bottom end of the riser pipe 22 has been anchored to the subsea facility 12 or other subsea foundation, the clump weights can be removed from the buoy 24 and recovered to the surface by the crane 108.

(63) Reference is now made to FIGS. 19a to 19e, which show the installation vessel 96 performing the abovementioned installation process in simplified, schematic form.

(64) FIG. 19a shows the lower section 22L of the riser pipe 22 being lowered by the crane 108 for engagement with the hang-off structure 112. At this stage, the frame 114 that supports the buoy 24 and the upper section 22U of the riser pipe 22 is in the horizontal stowed position.

(65) In FIG. 19b, the crane 108 has transferred the load of the lower section 22L to the hang-off structure 112. With the crane 108 now disengaged from the lower section 22L, the frame 114 has been pivoted into the vertical installation position. The buoy 24 and the upper section 22U have thereby been upended and brought into vertical alignment with the lower section 22L suspended from the hang-off structure 112.

(66) FIG. 19c shows the flange connection 32 now made between the upper and lower sections 22U, 22L to complete the full length of the riser pipe 22. The crane 108 has now taken the load of the riser tower 20 comprising the riser pipe 22 and the buoy 24.

(67) Also, clump weights exemplified here by chains 88 have been attached to the lower end of the buoy 24.

(68) The added ballast of the chains 88 overcomes the buoyancy of the buoy 24 and allows the crane 108 to lower the riser tower 20 to the required depth in the water as shown in FIG. 19d. At that depth, the bottom end of the riser pipe 22 can be anchored to the subsea facility 12 as shown. Finally, as shown in FIG. 19e, the chains 88 can be removed from the buoy 24 and recovered to the surface by the crane 108. An ROV 118 is shown in attendance in FIGS. 19d and 19e to assist with the necessary connection, disconnection and recovery operations.

(69) FIGS. 20 to 28 show various alternative arrangements for supporting clump weights.

(70) In FIG. 20, a buoy 24 supports an array of parallel upright tubes 120 that are equi-angularly spaced around the central vertical axis of the buoy 24. In this example, there are three tubes 120; there could instead be two such tubes or four or more such tubes.

(71) FIG. 21 shows how the tubes 120 may be used to support removable solid clump weights 122, such as beams, rods or bars, inserted into the open upper ends of the tubes 120. For removal, the clump weights 122 are lifted by a crane to pull them out of the tubes 120.

(72) Whilst the solid clump weights 122 allow for the addition of ample ballast, FIG. 22 shows how the clump weights 122 could be supplemented by stud link chains 88 that may hang inside and/or outside the tubes 120. FIG. 22 also shows an offshore worker 124 beside a chain 88 to illustrate scale.

(73) The chains 88 shown in FIG. 22 may be attached to or separate from the solid clump weights 122. Where the chains 88 are attached to the clump weights 122, the chains 88 make it easier to handle and grab the weights under water, for example by attaching a hook or a shackle to an upper link of a chain 88. Alternatively, chains 88 may be used alone, instead of the solid clump weights 122.

(74) FIGS. 23 and 24 show solutions that enable clump weight chains 88 to be attached to the top of a buoy 24. In each case, an upper cross-member 38 supports hooks 126 on its outboard ends that enable chains 88 to be hooked in place. The chains 88 then hang beside and outside the buoy 24. This positioning has the advantage that the chains 88 are easily accessible for removal and retrieval.

(75) Finally, FIGS. 25 to 28 show a further clump weight arrangement, in which solid clump weights 128 are supported on the exterior of a buoy 24.

(76) A frame 130 at the bottom of the buoy 24 supports an array of upwardly-opening buckets or sockets 132 that are equi-angulary spaced around the central vertical axis of the buoy 24.

(77) Part-circular solid clump weights 128 are assembled together as a lifting ring or flange that encircles the buoy 24. In this case, there are two semi-circular clump weights 128.

(78) Each clump weight 128 has attachment points 134 on its upper side to allow lifting lines 136 to be attached. Each clump weight 128 also has angularity-spaced pins 138 on its underside that are spaced to align with and engage into the sockets 132. In this example, there are four sockets 132 and therefore each of the two clump weights 128 has two pins 138.

(79) On each clump weight 128, one pin 138 is preferably longer than the other pin 138 as shown. This allows the longer pin 138 to be engaged with its socket 132 first and then to serve as a pivot that helps to guide the shorter pin 138 into an adjacent socket 132.

(80) The clump weights 128 can be installed onto the frame 130 of the buoy 24 or removed from the frame 130 together as shown in FIGS. 26 and 27 or separately as shown in FIG. 28. In principle it would be possible for a single clump weight 128 to encircle the buoy 24 rather than being divided into parts.

(81) As in some preceding embodiments, locating the clump weights 128 near the bottom end of the buoy 24 improves stability by lowering the centre of gravity or centre of buoyancy and by decreasing rotational moments.