SUBSEA HYDROGEN STORAGE SYSTEM

Abstract

A subsea unit suitable for storing hydrogen gas underwater comprises a weighting base and an array of interconnecting storage tanks on the base. The base may be cast from concrete on a deck of a vessel from which the unit is subsequently launched into water. A protective structure fixed to the base covers the array of tanks. A restraint system, comprising a series of strap restraints curving around the top of each tank, secures the tanks to the base against buoyant upthrust. The restraints are attached to elongate tensile members extending upwardly from the base, disposed on opposite sides of the underlying tank. The arrangement transfers loads efficiently from each tank to the base on load paths that bypass the other tanks.

Claims

1.-37. (canceled)

38. A method of providing a subsea gas storage unit, the method comprising: manufacturing a weighting base; assembling on the base an array of interconnecting storage tanks and a restraint system that secures the tanks to the base against buoyant upthrust, wherein the restraint system defines load paths from each tank to the base that bypass all other tanks of the array; and fixing a protective cover structure to the base, the cover structure extending over the array of tanks, wherein the tanks have walls of a fibre reinforced polymer composite.

39. The method of claim 38, comprising building the unit on a deck of a barge, lowering the barge in water to flood the deck, floating the unit away from the deck, and separating the unit from the barge.

40. The method of claim 38, comprising manufacturing the base by casting the base from concrete poured into a mould, and assembling the array of tanks on the base at a location at which the base was cast.

41. The method of claim 40, comprising embedding anchor fixings of the restraint system into the concrete of the base during casting.

42. The method of claim 38, comprising urging the tanks toward the base by virtue of tension in the restraint system.

43. The method of claim 38, comprising resisting lateral movement of the tanks in directions parallel to the base by placing each tank upon at least one support that is contoured to complement an external contour of the tank.

44. The method of claim 43, comprising applying compression to the support by action of the restraint system on the tank.

45. The method of claim 43, comprising sandwiching at least one of the supports between tanks of successive layers of the array.

46. The method of claim 38, comprising: placing a set of upright members of the restraint system extending upwardly from the base, between and beside the tanks of the array; placing a series of restraints on top of each tank, each restraint being attached to upright members of the set disposed on opposed sides of the underlying tank; and by tension in the upright members, urging the restraints and the tanks toward the base.

47. The method of claim 46, comprising bending the restraints around an upper side of each tank and also placing the restraints under tension.

48. The method of claim 46, wherein the array of tanks comprises a stack of layers and the upright members extend upwardly beyond a lower layer of the stack to an upper layer of the stack, each upright member having at least two restraints attached at levels above the base corresponding to those layers.

49. The method of claim 38, wherein the cover structure is kept separate from the restraining system.

50. The method of claim 38, further comprising submerging at least the base and the tanks of the unit.

51. The method of claim 50, comprising conveying buoyant upthrust loads from each tank to the base through the restraint system on load paths that bypass all other tanks of the array.

52. The method of claim 50, comprising subsequently towing the unit through water in a surface towing operation or a mid-water towing operation before lowering the unit to a seabed location.

53. The method of claim 38, comprising connecting two or more of the units together in a modular arrangement.

54. The method of claim 53, comprising effecting fluid communication between the connected units.

55. The method of claim 53, comprising towing the connected units as a series before lowering the series of units to a seabed location.

56. The method of claim 38, followed by pumping hydrogen into the tanks of the array after the unit has been installed underwater.

57. A subsea gas storage unit, comprising: a weighting base; an array of interconnecting storage tanks positioned on the base; a restraint system that secures the tanks to the base against buoyant upthrust, wherein the restraint system defines load paths from each tank to the base that bypass all other tanks of the array; and a protective cover structure disposed over the array of tanks and fixed to the base, wherein the tanks have walls of a fibre reinforced polymer composite.

58. The unit of claim 57, wherein the base is a cast concrete slab.

59. The unit of claim 57, wherein the restraint system comprises anchor fixings fixed to an upper side of the base.

60. The unit of claim 57, wherein tension in the restraint system urges the tanks toward the base.

61. The unit of claim 57, further comprising supports under each tank, each support being contoured to complement an external contour of the tank to resist lateral movement of the tank in directions parallel to the base.

62. The unit of claim 61, wherein at least some of the supports are sandwiched between tanks of successive layers of the array.

63. The unit of claim 57, wherein the restraint system comprises: a set of upright tensile members extending upwardly from the base, between and beside the tanks of the array; and restraints on top of each tank, each restraint being attached to upright members of the set disposed on opposed sides of the underlying tank.

64. The unit of claim 63, wherein the restraints curve around an upper side of each tank.

65. The unit of claim 63, wherein the array of tanks comprises a stack of layers and the upright members extend upwardly beyond a lower layer of the stack to an upper layer of the stack, each upright member having at least two restraints attached at levels above the base corresponding to those layers.

66. The unit of claim 63, wherein the cover structure is spaced from the restraint system.

67. The unit of claim 63, having a centre of buoyancy disposed above a centre of gravity by virtue of positive buoyancy of the tanks opposed to negative buoyancy of the base.

68. A set of two or more of the units of claim 63, connected together in a modular arrangement.

69. The set of claim 68, wherein the connected units are in fluid communication with each other.

70. An offshore installation capable of generating electrical energy from a renewable source, the installation comprising an electrolyser powered by that energy and at least one set of two or more of the subsea gas storage unit of claim 57 connected together in a modular arrangement whose tanks are in fluid communication with the electrolyser.

71. The installation of claim 70, further comprising a fuel cell in fluid communication with the tanks of said at least one unit or set.

72. The installation of claim 70, situated at a surface location and in fluid communication with the tanks of said at least one unit or set via a riser.

73. An offshore installation capable of generating electrical energy from a renewable source, the installation comprising an electrolyser powered by that energy and at least one whose tanks are in fluid communication with the electrolyser.

Description

[0055] 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:

[0056] FIG. 1 is a perspective view of a semi-submersible barge for use in constructing and transporting a hydrogen storage unit of the invention, showing a mould assembled on a deck of the barge;

[0057] FIG. 2 is a top plan view of the barge of FIG. 1, showing a concrete base slab of the storage unit cast in the mould;

[0058] FIG. 3 is a perspective view of the barge of FIG. 1, showing an array of storage tanks of the storage unit being assembled on the base slab;

[0059] FIG. 4 is a perspective view of the completed array of storage tanks of the storage unit fully assembled on the base slab;

[0060] FIG. 5 is a perspective view of the completed storage unit with a protective cover assembled over the array of storage tanks of the unit;

[0061] FIG. 6 corresponds to FIG. 5 but shows the storage unit cut away in a longitudinal vertical sectional plane;

[0062] FIG. 7 is an enlarged detail perspective view of the storage unit showing a restraint system 28 for holding storage tanks relative to the slab;

[0063] FIG. 8 is an enlarged detail perspective view showing piping and coupling arrangements at an end of the storage unit;

[0064] FIG. 9 is an enlarged detail perspective view of the storage unit showing an alternative restraint system for holding storage tanks relative to the slab;

[0065] FIG. 10 is an enlarged detail perspective view of the storage unit showing the alternative restraint system of FIG. 9, with the frame of the restraint system removed for clarity;

[0066] FIG. 11 is another enlarged detail perspective view of the storage unit showing the restraint system of FIGS. 9 and 10;

[0067] FIGS. 12a, 12b, and 12c are a sequence of perspective views showing the storage unit being transported on, and then launched from, the barge shown in FIGS. 1 to 3;

[0068] FIG. 13 is a perspective view of a series of storage units of the invention coupled end-to-end;

[0069] FIG. 14 is a schematic side view of a storage unit of the invention in a surface tow operation;

[0070] FIG. 15 is a schematic side view of a storage unit of the invention in a mid-water tow operation;

[0071] FIG. 16 is a schematic side view of a storage unit of the invention being installed on the seabed; and

[0072] FIG. 17 is a schematic side view of a storage unit of the invention used in the context of an offshore hydrocarbon production or wind power installation.

[0073] FIGS. 1 to 3 show a semi-submersible barge 10 for use in constructing and transporting a hydrogen storage unit 12 of the invention. The barge 10 comprises a fully floodable working deck 14 surmounted by a superstructure 16 that floods partially when the barge 10 is ballasted to lower the working deck 14 beneath the surface.

[0074] FIG. 1 shows a generally rectangular mould 18 assembled on a deck 14 of the barge 10 into which concrete can be poured to cast a flat base slab 20 as shown in FIG. 2. The mould 18 is elongate in a fore-and-aft direction relative to the barge 10. Thus, the base slab 20 is similarly elongate and similarly oriented, comprising mutually parallel long sides 22 extending longitudinally joined by mutually parallel short sides, or ends 24, extending orthogonally relative to the long sides 22. By way of example, the base slab 20 may have a length of about 35 m, a width of about 9 m and a weight of about 700 tons.

[0075] FIG. 3 shows a storage system of the unit 12 being assembled from storage tanks 26 on the base slab 20 aboard the barge 10. Half of the set of storage tanks 26 of the system have now been installed onto one end of the base slab 20. In this example, there are three horizontal rows or layers of tanks 26 stacked onto the base slab 20. The other half of the set of storage tanks 26 remains to be installed onto the other end of the base slab 20, but an array of cradle supports 30 is shown here fixed to the slab 20 ready to receive the lowermost layer of those tanks 26.

[0076] FIG. 4 shows the fully assembled storage system atop the base slab 20. In this example, the storage system comprises a total of thirty elongate tanks 26 extending along axes parallel to the fore-and-aft direction and hence parallel to the long sides 22 of the base slab 20. The tanks 26 are grouped in three stacked horizontal rows that extend transversely across the base slab 20, each row comprising ten tanks 26 in five end-to-end pairs. The tanks 26 of each pair are aligned on respective common axes.

[0077] FIG. 4 also shows how the tanks 26 of the storage unit 12 are coupled to the base slab 20. The restraint system 28 provided for that purpose will be described in more detail with reference to FIG. 7.

[0078] FIGS. 5 and 6 show the storage unit 12 completed by the addition of a cover structure 32. The cover structure 32 has a generally flat top face 34 with radiused edges, the top face 34 being spaced apart from the base slab 20 and in a parallel plane. The top face 34 and the base slab 20 have the same width, whereas the top face 34 is shorter than the base slab 20 and is centred longitudinally with respect to the base slab 20. Thus, the cover structure 32 is shaped as a regular trapezium in longitudinal section or in side view. Downwardly tapering wedge-shaped end faces 36 extend from the ends of the top face 34 to the ends 24 of the base slab 20.

[0079] The cover structure 32 comprises panels that are apt to be moulded of fibre-reinforced composites. At least some of those panels may be movable or removable to provide access to the tanks 26 and other parts of the storage system for routine maintenance or other operations requiring subsea intervention, for example using an ROV. For the same reason, the vertical sides of the cover structure 32 may be left open as shown.

[0080] The cover structure 32 is arranged to protect the tanks 26 and other parts of the storage system against overtrawling and dropped objects when installed on the seabed. In particular, the storage unit 12 is overtrawlable by virtue of its wedge-shaped end faces 36, radiused edges and substantially flush-fitting panels.

[0081] The panels of the cover structure 32 may be supported by an underlying frame such as a lattice that can be assembled or fabricated from box-section members of composite material or steel. The frame need only be strong enough to support the panels and to resist overtrawling and dropped objects. This is because the base slab 20 provides the main structural strength of the storage unit 12, to the extent that the unit 12 could in theory be transported and installed without the cover structure 32 in place. However, it would be possible instead to integrate a frame of the cover structure 32 with the base slab 20 in such a way that the frame contributes substantially to the rigidity of the unit 12 as a whole. In any event, the unit 12 is of self-supporting strength sufficient for the unit 12 to be lifted, launched and/or towed while maintaining its structural integrity and protecting the tanks 26 within.

[0082] FIGS. 7 and 8 show details of how the tanks 26 of the storage unit 12 are coupled to the base slab 20, how the tanks 26 are coupled to each other, and how storage units 12 may be coupled together.

[0083] FIG. 7 shows a restraint system 28 that couples the tanks 26 of the storage unit 12 to the base slab 20. The restraint system 28 comprises longitudinal arrays of the aforementioned cradle supports 30 fixed to the upper side of the base slab 20. Each cradle support 30 extends transversely, parallel to the ends 24 of the base slab 20, and has upwardly concave curvature complementing the external convex curvature of the storage tank 26 supported above. Thus, each longitudinal array of cradle supports 30 defines a channel section that cradles the bottom of the tank 26 above and prevents lateral movement of the tank 26 relative to the base slab 20.

[0084] Similar longitudinal arrays of intermediate supports 38 are sandwiched between adjoining layers of tanks 26. The intermediate supports 38 have upwardly and downwardly concave curvature complementing the external curvature of the storage tanks 26 disposed above and below, thus preventing lateral movement of the second and third layers of tanks 26 relative to the base slab 20. Like the cradle supports 30, the intermediate supports 38 are apt to be made of moulded polymer or composite materials but could be made of metal. The cradle supports 30 and the intermediate supports 38 could be of resilient material or construction.

[0085] The stacked tanks 26 are held together row-to-row and the stack of tanks 26 is pressed down against the base slab 20, thus holding the tanks 26 in engagement with the cradle supports 30 and the intermediate supports 38. This compression applied to the stack is generated by tension in tensile rods 40 that extend upwardly in pairs from respective anchors 42 fixed in the base slab 20. For example, the anchors 42 could be cast into the base slab 20 when concrete of the base slab is poured around them. The pairs of tensile rods 40 are in longitudinal arrays that are themselves arranged in pairs straddling each of the tanks 26 of each row.

[0086] The rods 40 of each pair support, and are joined by, fixings 44 that are clamped to or otherwise engaged with the rods 40 so as not to move along the rods. Restraining straps 46 wrap circumferentially over the tanks 26 with part-circular shape and are kept under tension by being attached at their ends to corresponding fixings 44 of each pair of tensile rods 40 on opposite sides of each tank 26. For this purpose, each fixing 44 comprises a longitudinally-extending pin that engages with an end of a strap 46.

[0087] The restraining straps 46 are suitably made of composite materials, in which case fibre reinforcements may extend longitudinally along the length of each strap 46. The straps 46 may be flexible so as to bend along their length but are preferably substantially inextensible under the loads expected in use.

[0088] It will be noted that, advantageously, the restraint system 28 comprising the rods 40, fixings 44 and restraining straps 46 transfers buoyant upthrust loads from the tanks 26 directly to the base slab 20 on load paths that bypass any intermediate tanks 26. This minimises stress experienced by the tanks 26. In the example shown, this arrangement allows the cover structure 32 to be structurally independent and therefore simple and inexpensive. The cover structure 32 is therefore also light in weight, which contributes to the stability of the unit 10 when immersed in water.

[0089] FIG. 8 shows piping 48 and fittings including small-bore needle valves 50 that effect fluid communication between the tanks 26 of the storage system and that couple the tanks 26 to an inboard hub 52 located at a tie-in porch 54 on an end 24 of the base slab 20. The hub 52 may be connected to a riser directly or via a pipe, or via another storage unit 12 of the invention. Gas is thereby conveyed to and from the tanks 26 as required.

[0090] It will be apparent that FIG. 8 omits the cover structure 32 for clarity. It will also be apparent from FIGS. 5 and 6 that the cover structure 32 has an end aperture 56 providing access to the hub 52 and the tie-in porch 54.

[0091] FIG. 8 also shows brackets 58 protruding from an end of the base slab 20. Pivoting linkages may be attached to the brackets 58 to couple the storage unit 12 to towing or lifting wires or to another similar unit 12 joined in series end-to-end. In this respect, it is possible to combine two or more storage units 12 of the invention to make a larger storage facility, as exemplified in FIG. 10.

[0092] FIGS. 9, 10 and 11 show an alternative restraint system 28 for the tanks 26 which comprises a frame 138 and a plurality of ropes 146. The frame 138 is fixed to the base slab 20 and comprises a series of struts 138a extending transversely, and generally horizontally, across the array of tanks 26 between upright pillars 140. The struts 138a extend in between the rows of the array of tanks 26 so that each tank 26 rests on and is supported by a number of the struts 138a, except for those tanks 26 on the bottom row of the array, which rest on the base slab 20. There is a small vertical clearance between the bottom of the struts 138a and the tank 26 below. This helps mitigate negative effects of differential thermal expansion and contraction between the tanks 26 and the frame 138.

[0093] As in FIG. 7, a series of longitudinal arrays of cradle supports 30 are fixed to the base slab 20 and the struts 138a in order to cradle the bottom of the tank 26 above and prevent lateral movement thereof relative to the base slab 20. As before, the upwardly concave nature of each longitudinal array of cradle supports 30 defines a channel section within which the tank 26 is positioned. The cradle supports 30 that support tanks 26 in upper rows of the array are mounted to the struts 138a, and so only require one concave surface to support the tank 26 above, as opposed to the bi-concave intermediate supports 38 in the restraint system 28 of FIG. 7, which are positioned in between tanks 26 in adjacent layers. The pillars 140 may also comprise lateral supports 130, shown most clearly in FIG. 11, that engage with the sides of the tanks 26 to further aid in positioning the tanks 26 in the array.

[0094] The tanks 26 are secured against the cradle supports 30 by the ropes 146, which may be made of synthetic or natural fibres, or may take the form of steel cables. The ropes 146 are fixed to the base slab 20 by anchors 142, with one anchor 142 positioned on each side of the tank 26 with which the rope 146 is associated. Each rope 146 takes the form of a loop that is wrapped around a pin in each anchor 142 and extends part-circumferentially around the top of the tank 26 such that each tank 26 is wrapped by two parallel portions of the rope 146. Each anchor 142 may comprise multiple pins to define an anchoring point for a rope 146 associated with a tank 26 in each row of the array of tanks 26. Consequently, the anchors 142 in FIGS. 9 to 11 define an anchoring point for three ropes 146, to reflect the three rows in the array of tanks 26. The anchors 142 may define an array, extending transversely across the array of tanks 26 and longitudinally along each end-to-end pair of tanks 26 so that each tank 26 is secured against the cradle supports 30 by multiple ropes 146.

[0095] The ropes 146 are tensioned to hold the tanks 26 against the cradle supports 30 against the action of buoyant upthrust. As with the restraint system 28 of FIG. 7, the load path transferring buoyant upthrust from the tanks 26 passes directly through the struts 138a and the frame 138 and to the base slab 20 and so does not pass through intermediate tanks 26, thereby minimising stresses on those tanks 26 in lower rows of the array and enabling the cover structure 32 to be structurally independent.

[0096] FIG. 11 also shows an optional cover 148 for the piping 48 and valves 50 shown in FIG. 8. The hub 52 is still accessible through a hole in the cover to enable connection to a riser and transfer of the gas to and from the tanks, in a similar way to that described above with reference to FIG. 8.

[0097] FIG. 12a shows the completed storage unit 12 being transported on a barge 10 like that shown in FIGS. 1 to 3. The barge 10 is shown here in a transit mode in which the working deck 14 is raised clear of the water surface 60. In that mode, the barge 10 can convey the unit 12 all of the way to an installation site before launching the unit 12 into the water or part of the way to the installation site, for example to water that is deep enough for subsequently towing the unit 12 in mid-water the rest of the way. Alternatively, the barge 10 can remain in sheltered water near shore and launch the unit 12 from there, hence essentially serving only as a factory or dry dock for construction of the unit 12 without transportation duties.

[0098] FIG. 12b shows the barge 10 now ballasted into a launch mode in which the working deck 14 is lowered beneath the surface 60 and hence fully flooded. It will be noted that the waterline is now part of the way up the superstructure 16 and that the storage unit 12 is now partially submerged. Then, when the unit 12 is submerged to an extent that it becomes positively buoyant, the unit 12 can be released from the barge 10 to float clear of the deck 14. The unit 12 can then be towed clear of the superstructure 16 and into the surrounding water as shown in FIG. 12c. Alternatively, the unit 12 could simply be launched into water by moving the barge 10 away and sinking the unit 12 to rest on the seabed.

[0099] FIG. 13 shows a series of three storage units 12 coupled to each other end-to-end via linkages in the form of intermediate articulated connectors 62 such as chains. The units 12 could be launched from a barge 10 like that of FIGS. 1 to 3 individually and then connected in the water or could be launched as a pre-connected series if the barge 10 is large enough. The series of units 12 is apt to be towed and lowered together in towing and installation operations like those shown in FIGS. 14 to 16. For this purpose, further linkages on the units 12 at the ends 24 of the series are defined by pivoting A-frames 64 to which towing wires can be attached. Alternatively, the units 12 could be lowered individually to the seabed, for example by a crane, and then coupled together there.

[0100] FIGS. 14 to 16 show, schematically, a storage unit 12 of the invention being transported and installed by a wet towing method. This involves towing the unit 12 through water to an installation site and lowering it there to the seabed 66. In FIGS. 14 to 16, shading is used to show where ballast tanks 68 of the unit 12 contain mainly air (no shading, as in FIG. 14), a combination of water and air (half shading, as in FIG. 15) or mainly water (full shading, as in FIG. 16).

[0101] For surface towing as in FIG. 14, the ballast tanks 68 of the storage unit 12 contain mainly air to impart positive buoyancy to the unit 12. For mid-water towing as in FIG. 15, the ballast tanks 68 are partially filled with water and partially filled with air to impart near-neutral or slightly negative buoyancy to the unit 12. To settle the unit 12 on the seabed 66 as in FIG. 16, the ballast tanks 68 contain mainly water to impart strongly negative buoyancy to the unit 12.

[0102] By way of example, mid-water towing as in FIG. 15 could employ the Controlled Depth Tow Method or CDTM as described in EP 0069446, WO 2014/095942 and a technical paper OTC 6430 (OTC Conference, 1990). In the CDTM, the towed unit 12 is slightly negatively buoyant at a given water depth but it stabilises at that depth due to drag forces experienced during towing.

[0103] The CDTM principle involves transportation of the prefabricated and fully-tested storage unit 12 suspended on towing lines 70 between surface vessels 72 fore and aft. Unlike an installation barge 10, these vessels 72 may be relatively small and inexpensive vessels equipped with winches 74, such as tugs.

[0104] Drag chains could be used for ballasting and depth control but such chains are optional in the present invention if ballast tanks 68 are provided to control the depth and trim of the storage unit 12 during towing.

[0105] As the shading in FIG. 15 shows, the ballast tanks 68 of the storage unit 12 are partially flooded under the control of control systems on the storage unit 12 or on a surface vessel. This makes the storage unit 12 slightly negatively buoyant at a pre-determined mid-water towing depth, which is preferably at least fifty metres. Modest tension in the towing lines 70 under the drag forces of towing balances the slight negative buoyancy of the storage unit 12 to maintain the desired depth, assisted by ongoing control of the buoyancy of the ballast tanks 68.

[0106] At the desired towing depth, the storage unit 12 is held safely clear of the seabed 66 and beneath the influence of wave action near the surface 60. Even if the sea state deteriorates dramatically during the tow, the storage unit 12 can be lowered to the seabed 66 to await better weather conditions.

[0107] When the storage unit 12 reaches an installation site, it is lowered toward the seabed 66 by more fully flooding its ballast tanks 68 to increase its negative buoyancy as shown in FIG. 16. Meanwhile, the towing lines 70 are paid out from winches 74 of the surface vessels 72. The storage unit 12 then settles onto the seabed 66 as shown in FIG. 16.

[0108] By dark shading, FIG. 16 shows the ballast tanks 68 of the storage unit 12 having been flooded after landing on the seabed 66 to stabilise the unit 12. The static weight of the unit 12 after flooding provides sufficient inertia, friction and stability for the unit 12 to be anchored firmly to the seabed 66.

[0109] After installation on the seabed 66, the storage unit 12 can be coupled to a riser 76 that extends to a surface installation 78 as shown in FIG. 17. The installation 78 comprises consumer equipment 80 requiring a supply of electrical energy, such as a fluid processing system if the installation 78 is a hydrocarbon production facility.

[0110] Hydrogen can be pumped down the riser 76 from the installation 78 to the unit 12 when surplus electrical energy is available. For that purpose, the riser 76 is connected to an electrolyser 82 of the installation 78. Conversely, hydrogen can be drawn up the riser 76 from the unit 12 to the installation 78 whenever it is necessary to supplement electrical energy. For that purpose, the riser 76 is connected to a fuel cell 84 of the installation 78. The fuel cell is 84 connected, in turn, to the consumer equipment 80 of the installation 78 and to an export line 86 for use if the fuel cell 84 generates surplus electrical energy.

[0111] In this example, the installation 78 is a moored platform floating at the surface 60 but it could instead be an FPSO. Alternatively, the installation 78 could be a platform that stands on the seabed 66. A production riser 88 is shown here as a dashed line extending from the seabed 66 to the surface 60, indicating that the installation 78 could be a hydrocarbon production facility. The installation 78 could instead be a hub of a wind power installation, in which case the dashed line could instead represent a power export cable.

[0112] Similarly, FIG. 17 shows a moored wind turbine 90 floating at the surface 60 that, in shallow water of less than say 50 m, could instead stand on the seabed 66. A cable 92 conveys electrical energy from the wind turbine 90 to the installation 78, either to power the consumer equipment 80 for the purpose of hydrocarbon production or to contribute to power to be exported from the installation 78. There could be several such turbines 90 similarly connected to the installation 78, for example in a linear or circular array, especially if the main purpose of the installation 78 is to export electrical power.

[0113] A storage unit 12 of the invention can remain in service on the seabed 66 for many years but it is straightforward to recover the unit 12 from the seabed 66 to the surface 60 when it is no longer required. For this purpose, once the storage unit 12 has been disconnected from other subsea infrastructure, the ballast tanks 68 can be de-ballasted by displacing water with pressurised gas in a controlled manner. De-ballasting in this way reduces the apparent weight of the unit 12 for lifting by a crane or winch of a surface vessel.

[0114] If a storage unit 12 is to be scrapped and recycled after use, it could simply be raised to the surface and towed from there to a shore facility. Alternatively, if the unit 12 is to be refurbished and reused, a reverse CDTM process could be employed to avoid damage or fatigue caused by wave action. In that case, injection of de-ballasting gas is controlled to achieve slightly neutral buoyancy at a desired towing depth, whereupon CDTM towing takes place in the water column with controlled depth and buoyancy. Finally, the unit 12 is raised to the surface in shallower, sheltered water near shore to be refurbished for reuse. In essence, this is the reverse of the process shown in FIGS. 14 to 16. Alternatively, the unit 12 could simply be lifted onto an available barge and transported above the surface.

[0115] Many variations are possible within the inventive concept. For example, a storage unit of the invention could be constructed at a coastal yard and lifted or pulled from there into the water or onto a barge. Also, the unit could be transported to an installation site on a barge and lowered by a marine crane or winch to the seabed.