Controlling Buoyancy When Towing, Lowering and Raising Submerged Structures

Abstract

A subsea structure comprises: an elongate articulated buoyancy string comprising a longitudinal series of buoyancy elements; and at least one elongate buoyancy holder that is fixed relative to the structure and is capable of housing the buoyancy string. The buoyancy of the structure may be altered by moving the buoyancy string along a transit path that extends along and within the buoyancy holder and through an end opening of the buoyancy holder. The buoyancy string passes through that end opening on being inserted into or withdrawn from the buoyancy holder.

Claims

1. A subsea structure in combination with: an elongate articulated buoyancy string comprising a longitudinal series of buoyancy elements; and at least one elongate buoyancy holder that is capable of housing the buoyancy string, the buoyancy holder being fixed relative to the subsea structure.

2. The structure of claim 2, wherein the buoyancy holder defines a transit path for movement of the buoyancy string along and within the buoyancy holder.

3. The structure of claim 2, wherein the transit path extends from at least one end opening of the buoyancy holder, through which end opening the buoyancy string can pass on withdrawal from the buoyancy holder or insertion into the buoyancy holder.

4. The structure of any preceding claim, wherein the buoyancy holder is tubular.

5. The structure of claim 4, wherein a major portion of the tubular buoyancy holder is substantially straight along a longitudinal axis and at least one minor end portion of the tubular buoyancy holder is curved away from the longitudinal axis.

6. The structure of claim 5, wherein the minor end portion of the tubular buoyancy holder is curved away from other elements of the structure.

7. The structure of any of claims 4 to 6, wherein the tubular buoyancy holder is continuous along its length.

8. The structure of any of claims 4 to 6, wherein the tubular buoyancy holder is divided longitudinally into segments with a gap between neighbouring segments.

9. The structure of any preceding claim, wherein a side wall of the buoyancy holder is penetrated by one or more lateral openings.

10. The structure of any preceding claim, comprising a plurality of said buoyancy holders.

11. The structure of claim 10, wherein the buoyancy holders of the plurality are arranged end-to-end.

12. The structure of any preceding claim, wherein the or each buoyancy holder is connected rigidly to the remainder of the subsea structure.

13. The structure of any preceding claim, wherein the or each buoyancy holder is integrated with the subsea structure.

14. The structure of any preceding claim, wherein the or each buoyancy holder is longer than at least one buoyancy element of the buoyancy string.

15. The structure of any preceding claim, wherein the buoyancy elements are rigid.

16. The structure of any preceding claim, wherein the buoyancy elements are solid blocks.

17. The structure of any of claims 1 to 15, wherein the buoyancy elements are hollow and are filled with a buoyant fluid or a mass of buoyant spheres.

18. The structure of any preceding claim, wherein the buoyancy elements have a friction-reducing coating.

19. The structure of any preceding claim, wherein the buoyancy elements are each connected to the or each neighbouring buoyancy element in the series by a link that permits relative angular movement between neighbouring buoyancy elements.

20. The structure of claim 19, wherein the link is a flexible member.

21. The structure of claim 19, wherein the link comprises parts attached by a pivotable joint.

22. The structure of any preceding claim, being a pipeline bundle along which the or each buoyancy holder extends.

23. A method of altering the buoyancy of a subsea structure, the method comprising moving an elongate articulated buoyancy string of buoyancy elements along a transit path extending along and within an elongate buoyancy holder that is fixed relative to the structure, wherein the transit path extends through an end opening of the buoyancy holder and the buoyancy string passes through that end opening.

24. The method of claim 23 when applied to reducing the buoyancy of the subsea structure, the method comprising withdrawing the buoyancy string from within the buoyancy holder through the end opening.

25. The method of claim 23 when applied to increasing the buoyancy of the subsea structure, the method comprising inserting the buoyancy string into the buoyancy holder through the end opening.

26. The method of any of claims 23 to 25, comprising pulling the buoyancy string through the end opening by applying tension to one end of the buoyancy string.

27. The method of claim 26, further comprising applying back-tension to the buoyancy string by applying tension to an opposite end of the buoyancy string.

28. The method of claim 27, wherein tension and back-tension are applied to respective ends of the buoyancy string by a single surface support vessel.

29. The method of any of claims 26 to 28, comprising applying tension to an end of the buoyancy string by pulling a tensile element that extends along the buoyancy holder.

30. The method of claim 29, wherein the tensile element extends along a full length of the buoyancy holder and through opposed end openings of the buoyancy holder.

31. The method of any of claims 23 to 30, comprising bending the buoyancy string along its length.

32. The method of claim 31, comprising bending the buoyancy string as the buoyancy string follows a curved transit path within the buoyancy holder.

33. The method of claim 31 or claim 32, comprising bending the buoyancy string outside the buoyancy holder between the end opening and the sea surface.

Description

[0062] Reference has already been made to FIG. 1 of the accompanying drawings, which is a side view of a pipeline bundle unit being towed to a subsea installation site using the controlled depth tow method known in the prior art. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which:

[0063] FIG. 2 is a side view of a pipeline bundle unit in accordance with the invention;

[0064] FIG. 3 is an enlarged detail perspective view of part of the pipeline bundle unit of FIG. 2;

[0065] FIG. 4 is a further enlarged detail side view of a buoyancy tube of the pipeline bundle unit of FIG. 2, shown in longitudinal section;;

[0066] FIG. 5 is an enlarged detail side view of the pipeline bundle unit of FIG. 2, shown in longitudinal section, with the buoyancy tube containing a series of interconnected buoyancy elements defining a buoyancy string;

[0067] FIG. 6 is an enlarged cross-sectional view of the pipeline bundle unit taken on line VI-VI of FIG. 5;

[0068] FIG. 7 is a schematic side view of a withdrawal operation in which a buoyancy string is being withdrawn from a buoyancy tube of a pipeline bundle;

[0069] FIG. 8 is a schematic side view of an insertion operation in which a buoyancy string is being inserted into a buoyancy tube of a pipeline bundle;

[0070] FIG. 9 is a schematic side view of a withdrawal or insertion operation in which a buoyancy string is being withdrawn from or inserted into a buoyancy tube of a pipeline bundle; and

[0071] FIG. 10 is a schematic side view of a pipeline bundle in a further variant of the invention.

[0072] Turning next, then, to FIGS. 2 to 6 of the drawings, FIG. 2 shows a pipeline bundle unit 30 of the invention and FIGS. 3 to 6 show details of a pipeline bundle 32 at the heart of the unit 30.

[0073] FIG. 2 shows the elongate bundle 32 in context within the pipeline bundle unit 30, extending between a leading towhead 34 and a trailing towhead 36 like the bundle 12 of the unit 10 shown in FIG. 1. The unit 30 is similarly apt to be towed for installation using the CDTM as described above with reference to FIG. 1. However by virtue of the invention, the unit 30 can be towed and installed in deeper water than can be achieved in the prior art.

[0074] As best appreciated in the perspective view of FIG. 3 and the cross-sectional view of FIG. 6, the bundle 32 of this example comprises a group of three substantially straight flowlines 38. The flowlines 38 extend parallel to each other and parallel to a central longitudinal plane 40 of the bundle 32 as shown in FIG. 6, which plane passes through the centre of a central one of the flowlines 38 of the group. In this example, the bundle 32 is not enclosed within a conventional external carrier pipe although it would be possible to add such a carrier pipe.

[0075] During installation, the flowlines 38 generally contain air. In service, after connection to the oil field, the flowlines 38 contain pressurised fluid such as crude oil flowing from a production well. Thus, each flowline 38 will typically be a substantially rigid pipe of steel or composite material, whose wall is strong enough to resist hydrostatic pressure expected at a subsea installation site.

[0076] Whilst rigid and nominally straight, the pipes of the flowlines 38 can bend elastically along their length during towing as exemplified by FIG. 1. This is consistent with the skilled person's understanding of rigid in the context of subsea pipes and is to be distinguished from the skilled person's understanding of flexible in that same context.

[0077] A longitudinal series of buoyancy tubes 42 also extends along the bundle 32, the buoyancy tubes 42 being disposed generally end-to-end with respect to each other. The buoyancy tubes 42 and the flowlines 38 are disposed symmetrically about the central longitudinal plane 40. Thus, the buoyancy tubes 42 all lie on the central longitudinal plane 40 above the group of flowlines 38. If the bundle 32 is enclosed within a single carrier pipe, as is optional but is not shown, at least one elongated buoyancy tube 42 can be mounted along that carrier pipe.

[0078] A major central portion 44 of each buoyancy tube 42 is substantially straight and generally parallel to the flowlines 38. Conversely, minor end portions 46 of each buoyancy tube 42 curve upwardly away from the group of flowlines 38, in this example by 90 degrees to the longitudinal axis of the major central portion 44. The end portions 46 define entry and exit openings 48 of the buoyancy tube 42 that communicate with the major central portion 44.

[0079] Longitudinally-spaced frames 50 connect each of the flowlines 38 directly to the buoyancy tubes 42, and indirectly via the buoyancy tubes 42 to each other. The cross-sectional view of FIG. 5 shows that each frame 50 comprises arms 52 that radiate with angular spacing from a buoyancy tube 42 to the respective flowlines 38. The arms 52 provide convenient mountings for other parts of the bundle 32 including control unit tubing 54 and other elongate elements 56, such as power and data highway cables or service fluid lines.

[0080] The buoyancy tubes 42 are penetrated by one or more lateral openings 58 that promote flooding of the buoyancy tubes 42 and hence enable pressure equalisation. A buoyancy tube 42 may therefore have a substantially thinner wall than a flowline 38 as it does not need to withstand hydrostatic pressure at depth. In this example, as best shown in the longitudinal sectional view of FIG. 4, the tubular wall of the buoyancy tube 42 is perforated with an array of many such openings 58 to ensure quick flooding and continuous equalisation of hydrostatic pressure.

[0081] As best appreciated in FIG. 5, each buoyancy tube 42 is capable of accommodating a respective buoyancy string 60 that can slide within the buoyancy tube 42 in telescopic fashion. The buoyancy string 60 comprises a plurality of interlinked buoyancy modules or elements 62 spaced end-to-end in longitudinal series like the links of a chain or the carriages of a train. Thus, the buoyancy string 60 is articulated and has flexibility to bend along its length, although the individual buoyancy elements 62 and the connections between them need not necessarily be flexible themselves.

[0082] Links 64 between adjacent buoyancy elements 62 allow relative angular movement between those buoyancy elements 62. Each link 64 may, for example, be a flexible link such as a rope of polymer or steel. The links 64 also transmit axial tensile loads between the adjacent buoyancy elements 62, which loads are transmitted along the buoyancy string 60 through the succession of buoyancy elements 62.

[0083] At least one end of each buoyancy string 60 terminates in a wire or rope 66 having a buoyant coupling formation 68 such as a monkey-fist at its free end. When the buoyancy string 60 is fully inserted into the buoyancy tube 42, the coupling formations 68 at the ends of the buoyancy string 60 float freely outside the respective entry and exit openings 48 of the buoyancy tube 42. The coupling formations 68 may then be grasped conveniently by an ROV to be coupled to a wire for withdrawing the buoyancy string 60, as will be explained with reference to later drawings.

[0084] Each buoyancy element 62 of the buoyancy string 60 preferably comprises a solid block of positively-buoyant material such as a syntactic foam. Such a block may be coated in a protective polymer layer and may also be coated with a surface layer of a low-friction material such as PTFE. The block also has rounded edges and smooth surfaces. The interior of the buoyancy tube 42 is similarly smooth and may also be coated in a low-friction material such as PTFE.

[0085] These various friction-reducing provisions ensure smooth movement of the buoyancy string 60 along the buoyancy tube 42 during insertion and withdrawal, without over-stressing the buoyancy string 60 due to blockages or jams. It will also be noted that the numerous lateral openings 58 allow water to flow readily into and out of the buoyancy tube 42 under the piston effect of a buoyancy string 60 moving along the buoyancy tube 42 during withdrawal and insertion. This also reduces resistance to motion of the buoyancy string 60 along the buoyancy tube 42.

[0086] FIGS. 7 to 9 show various ways in which a buoyancy string 60 may be removed or withdrawn from, and inserted into, a buoyancy tube 42 of a pipeline bundle 32 landed on the seabed 20 in accordance with the invention. These are simplified views that do not reflect the likely curvature of the buoyancy string 60 under its self-buoyancy when it is out of the confines of the buoyancy tube 42.

[0087] FIG. 7 shows a withdrawal operation, as may be performed when ballasting the bundle 32 to settle the bundle onto the seabed 20 during subsea installation. In this example, a support vessel 70 floating on the surface 18 has a winch 72 that pulls on a wire 74. The wire 74 extends down to a rope 66 at an end of the buoyancy string 60, which rope 66 protrudes from one end of the buoyancy tube 42. The coupling formation 68 at the end of the rope 66 has been grasped by a UUV such as an ROV 76, which has then connected the rope 66 to the wire 74. Divers could perform that connection operation instead of an ROV 76 if the water is shallow enough for divers to operate.

[0088] The buoyancy string 60 is shown in FIG. 7 having been pulled partially out of the buoyancy tube 42 by tension applied through the wire 74. Also, the vessel 70 is shown in FIG. 7 displaced on the surface 18 longitudinally with respect to the pipeline bundle unit 30 on the seabed 20. This allows space for the buoyancy string 60 to breach the surface 18 without hitting the vessel 70, bearing in mind that when it has been withdrawn sufficiently from the buoyancy tube 42, the buoyancy string 60 will rise quickly to the surface 18 under its own buoyancy.

[0089] If the buoyancy string 60 is long enough and the water is shallow enough, it would be possible for the ascent of the buoyancy string 60 to the surface 18 to be controlled by frictional engagement with the buoyancy tube 42. Specifically, control may be applied by virtue of an end portion of the buoyancy string 60 remaining frictionally engaged within the buoyancy tube 42 while the other end of the buoyancy string 60 reaches the surface 18. The vessel 70 can then pull the end portion of the buoyancy string 60 out of frictional engagement with the buoyancy tube 42 so that the remainder of the buoyancy string 60 can rise to the surface 18.

[0090] FIG. 8 shows an insertion operation, as may be performed when refloating the bundle 32 for relocation or decommissioning. In this example, a support vessel 70 floating on the surface 18 has a winch 72 that pulls on a wire 74 extending down to one end of the buoyancy tube 42. The wire 74 extends through the opening 48 at that end and along the buoyancy tube 42 to emerge from the opening 48 at the other end of the buoyancy tube 42. The wire 74 then extends upwardly to be connected via a rope 66 and a coupling formation 68 to a buoyancy string 60. The buoyancy string 60 is shown here being pulled down from the surface 18 under tension applied through the wire 74 and now partially inserted into the buoyancy tube 42. Eventually the buoyancy string 60 will be inserted fully within the buoyancy tube 42, whereupon an ROV 76 can disconnect the wire 74 from the rope 66 at an end of the buoyancy string 60.

[0091] FIG. 9 shows an arrangement that can be used for both withdrawal and insertion of a buoyancy string 60 from and into a buoyancy tube 42. In this arrangement, the buoyancy string 60 is tensioned between two wires, one being a first wire 74 tensioned by a first winch 72 and the other being a second wire 78 tensioned by a second winch 80. The buoyancy string 60 may have ropes 66 and coupling formations 68 at both ends to be connected by an ROV 76 to the respective wires 74, 78.

[0092] The wires 74, 78 and the winches 72, 80 operate in coordination to control the buoyancy string 60 and to apply back-tension to it. In this way, the buoyancy string 60 remains under control at all times when moving in either direction between the surface 18 and the buoyancy tube 42.

[0093] In the example shown in FIG. 9, both winches 72, 80 are supported by the same support vessel 70. However, it would be possible instead for two support vessels 70 to work in coordination, each having a respective one of the winches 72, 80. In effect, the result of using two support vessels 70 would be a combination of the arrangements shown in FIGS. 7 and 8.

[0094] The wire 74 extends down from the winch 72 to a sheave 82 at one end of the buoyancy tube 42. The wire 74 extends through the opening 48 at that end and along the buoyancy tube 42 to emerge from the opening 48 at the other end of the buoyancy tube 42, where it runs around another sheave 82. The wire 74 then extends upwardly to a buoyancy string 60, shown here under tension between downward force applied through the wire 74 and upward force applied by the winch 80 through the wire 78.

[0095] When the winch 72 pulls in the wire 74 and the winch 80 simultaneously pays out the wire 78, the wires 74, 78 and the buoyancy string 60 tensioned between the wires 74, 78 move clockwise relative to the buoyancy tube 42 as drawn. This movement inserts the buoyancy string 60 into the buoyancy tube 42. Of course, it would also be possible to insert the buoyancy string 60 from the other end of the buoyancy tube 42 by running the system in reverse, in an anticlockwise sense as drawn.

[0096] When the buoyancy string 60 has been inserted fully into the buoyancy tube 42, an ROV 76 can disconnect the wires 74, 78 from the ropes 66 at the ends of the buoyancy string 60.

[0097] It would also be possible to use the arrangement shown in FIG. 9 to withdraw the buoyancy string 60 from either end of the buoyancy tube 42. First, the wires 74, 78 are connected to the ropes 66 at the ends of the buoyancy string 60 protruding from the buoyancy tube 42. Next, the system is run in either direction, either by pulling in the wire 74 while paying out the wire 78 or by pulling in the wire 78 while paying out the wire 74.

[0098] Variations are possible within the inventive concept. For example, some pipeline bundles could have fewer flowlines or more flowlines than in the embodiments shown.

[0099] There could be just one buoyancy tube extending along substantially the full length of the bundle. This would help the buoyancy tube to contribute to bearing tensile stress in the bundle. Alternatively there could be more, shorter buoyancy tubes extending along the bundle. Correspondingly shorter buoyancy strings may be easier to manage during withdrawal and insertion than one longer string whose length is similar to their aggregate length.

[0100] FIG. 10 illustrates another possibility, having an end-to-end series of very short buoyancy tubes 84 spaced in longitudinal alignment along a flowline 38 of a bundle 86. Each individual buoyancy tube 84 is longer than at least one of the buoyancy elements 62. In this example, each of the buoyancy tubes 84 is long enough to accommodate only a few buoyancy elements 62, in this example just two buoyancy elements 62 per buoyancy tube 84. However, all of the buoyancy elements 62 of the combined buoyancy tubes 84 are interlinked into a single buoyancy string 88. The spacing of the buoyancy elements 62 varies along the buoyancy string 88, the buoyancy elements 62 being grouped in pairs in accordance with the relative positons of the successive buoyancy tubes 84.

[0101] In effect, therefore, the multiple buoyancy tubes 84 in FIG. 10 combine to form a single interrupted buoyancy tube, that tube being interrupted where the mutual longitudinal spacing between neighbouring buoyancy tubes defines transverse gaps 90. Each of those gaps 90 is narrower than the length of a buoyancy element 62 so that the buoyancy elements 62 remain confined within the row of buoyancy tubes 84. The gaps 90 facilitate effective flooding of the buoyancy tubes 84 without having to perforate the walls of those tubes 84 with multiple openings as in a preceding embodiment.

[0102] Many other variations are possible. For example, where present, minor end portions of each buoyancy tube may be flared and may turn away from the longitudinal axis by less than 90 degrees. Flared end portions of a buoyancy tube may even be aligned with the longitudinal axis.

[0103] Shorter buoyancy tubes may dispense with lateral openings for pressure equalisation, instead relying upon openings at the ends of the buoyancy tube to allow flooding with seawater.

[0104] A buoyancy tube preferably has two opposed ends that are open, or openable, to act as entry or exit openings for a buoyancy string. However it would be possible for only one of the ends of a buoyancy tube to be open, or openable. In that case, a buoyancy string may only be inserted into the buoyancy tube, or withdrawn from the buoyancy tube, through that one end.

[0105] The buoyancy elements could instead contain a positively-buoyant liquid such as kerosene encapsulated within a rigid or flexible shell, or a gas, or a mass of buoyant hollow macrospheres.

[0106] When a buoyancy string has been inserted into a buoyancy tube, the openings at the ends of the buoyancy tube could be capped with lids or membranes that block the entry of marine sediment. Such sediment could otherwise interfere with withdrawal of the buoyancy string from the buoyancy tube. Such lids or membranes could nevertheless have equalisation openings to allow water to flow through them into the buoyancy tube. Such lids or membranes could be removable or breakable on withdrawal of the buoyancy string from the buoyancy tube.

[0107] The links between adjacent buoyancy elements of a buoyancy string could instead be articulated links comprising cooperating rigid parts connected by pivot or hinge formations. The links could be separable so that buoyancy elements can be added to or removed from a buoyancy string.

[0108] The links between adjacent buoyancy elements of a buoyancy string could be part of a continuous tensile member that extends through the buoyancy elements. For example, a rope defining flexible links could extend through the buoyancy elements along the length of the buoyancy string.

[0109] The buoyancy tubes could be replaced by buoyancy holders or buoyancy housings having various other shapes or configurations. In particular, various cross-sectional shapes are possible provided that the holders or housings retain the interlinked buoyancy elements in the bundle and define a transit path serving as an insertion and/or withdrawal path for those buoyancy elements.

[0110] Winches are apt to pull the buoyancy string out of or into the buoyancy tubes. The examples described above employ winches aboard surface vessels. However, underwater winches could be used. For example, an underwater winch located on the seabed at some distance from the bundle can be connected to the buoyancy string, and be controlled remotely from the surface, or by an ROV, or by a diver.

[0111] The invention has particular benefits for controlling the buoyancy of elongate towable structures such as pipeline bundles. However, the invention could be used to control the buoyancy of other large submerged structures when towing them at any depth, when lowering them to the seabed and when raising them from the seabed.