Decommissioning Subsea Infrastructure

Abstract

A method of dewatering a subsea pipeline bundle assembly before lifting the assembly from the seabed comprises adding discrete buoyancy to an elongate carrier pipe of the assembly. The buoyancy elevates one or more portions of the carrier pipe disposed between drainage outlets of the assembly. Each elevated portion defines inclined falls that slope downwardly in opposed longitudinal directions toward the respective outlets. Water within the carrier pipe drains down the falls toward the outlets. Injecting a dewatering fluid into the carrier pipe promotes expulsion of water from the pipe through the outlets by downward displacement of the water. Drainage can be assisted by a venturi effect driven by accelerating the flow of the dewatering fluid at a location in line with an outlet.

Claims

1. A method of dewatering a subsea pipeline bundle assembly at a seabed location comprising: adding discrete buoyancy to an elongate carrier pipe of the assembly to elevate a portion of the carrier pipe disposed between drainage outlets of the assembly, the elevated portion defining inclined falls that slope downwardly in opposed longitudinal directions toward the respective outlets; draining water within the carrier pipe down the falls toward the outlets; and injecting a dewatering fluid into the carrier pipe to promote expulsion of the water from the pipe through the outlets.

2. The method of claim 1, wherein the assembly comprises at least one towhead at an end of the carrier pipe.

3. The method of claim 2, wherein at least one of the outlets is located at the at least one towhead.

4. The method of claim 2, further comprising a preliminary step of reducing apparent weight of the at least one towhead.

5. The method of claim 4, comprising dewatering the at least one towhead to reduce its apparent weight.

6. The method of claim 4, comprising maintaining negative buoyancy in the at least one towhead.

7. The method of claim 4, wherein at least one of the falls extends to and adjoins the at least one towhead.

8. The method of claim 7, comprising, after reducing its apparent weight, tilting the at least one towhead about a horizontal axis by applying a pivoting moment to the towhead from the elevated portion of the carrier pipe.

9. The method of claim 1, comprising extending the elevated portion along the carrier pipe by continuing injection of the dewatering fluid into the carrier pipe.

10. The method of claim 9, comprising migrating at least one of the falls along the carrier pipe.

11. The method of claim 1, comprising: expelling, through a first outlet, the water drained by one of the falls; closing the first outlet; and then continuing to inject the dewatering fluid into the carrier pipe to expel, through a second outlet, the water drained by the other of the falls.

12. The method of claim 11, comprising expelling the water through the first and second outlets simultaneously before closing the first outlet.

13. The method of claim 1, comprising adding the discrete buoyancy to the carrier pipe at a position offset longitudinally toward one end of the carrier pipe, substantially closer to one end of the carrier pipe than to an opposite end of the carrier pipe.

14. The method of claim 1, comprising restraining elevation of the carrier pipe at one or more locations along its length.

15. The method of claim 14, comprising placing at least one weight on the carrier pipe at the or each of said locations.

16. The method of claim 14, comprising restraining elevation of the carrier pipe at opposed ends of the carrier pipe by virtue of towheads at those ends.

17. The method of claim 14, comprising adding the discrete buoyancy to the carrier pipe at two or more positions spaced along the carrier pipe, the or each of said locations at which elevation of the carrier pipe is restrained being between said positions at which the discrete buoyancy is added.

18. The method of claim 14, wherein the outlets coincide with said locations at which elevation of the carrier pipe is restrained.

19. The method of claim 18, comprising expelling the water through those outlets in longitudinal succession.

20. The method of claim 18, comprising closing each outlet in longitudinal succession after the water stops being expelled through that outlet.

21. The method of claim 1, comprising injecting the dewatering fluid from one end of the assembly.

22. The method of claim 1, comprising conferring positive buoyancy on the carrier pipe.

23. The method of claim 1, further comprising releasing the added buoyancy from the carrier pipe after dewatering the assembly and before lifting the assembly from the seabed location.

24. The method of claim 1, comprising lifting the assembly from the seabed location suspended between lines that connect respective ends of the assembly to respective surface vessels.

25. The method of claim 1, comprising hot-tapping the outlets underwater.

26. The method of claim 1, comprising drawing the water toward at least one of the outlets by a pressure differential created by flow of the dewatering fluid within the assembly.

27. The method of claim 26, comprising creating the pressure differential in a venturi formation that is aligned with at least one of the outlets.

28. A subsea pipeline bundle assembly positioned at a seabed location comprises: an elongate carrier pipe having a portion elevated from the seabed by discrete buoyancy added to the carrier pipe, the elevated portion being disposed between longitudinally spaced drainage outlets and defining inclined falls that slope downwardly in opposed longitudinal directions toward the respective outlets; and an inlet for injection of a dewatering fluid into the carrier pipe to promote expulsion of water from the pipe through the outlets.

29. The assembly of claim 28, wherein the discrete buoyancy is at a position offset longitudinally toward one end of the carrier pipe, substantially closer to one end of the carrier pipe than to an opposite end of the carrier pipe.

30. The assembly of claim 28, wherein the discrete buoyancy is at two or more positions spaced along the carrier pipe and the carrier pipe is restrained against elevation from the seabed between those positions.

31. The assembly of claim 30, wherein the outlets coincide with locations at which elevation of the carrier pipe is restrained.

32. The assembly of claim 28, comprising at least one venturi formation that is aligned with at least one of the outlets and is in communication with the inlet to receive and accelerate a flow of the dewatering fluid.

33. A method of dewatering an elongate subsea structure comprises: directing a flow of dewatering fluid within the structure to a venturi formation; accelerating the flow through the venturi formation to generate a region of low fluid pressure within the structure; drawing water within the structure toward a drainage outlet positioned within the region of low fluid pressure; and expelling the water from the structure through the drainage outlet.

34. The method of claim 33, comprising drawing the water toward the drainage outlet by a combination of low fluid pressure and gravity.

35. The method of claim 33, comprising directing the flow of dewatering fluid through the venturi formation above a volume of water that is in alignment with the venturi formation and is in communication with the drainage outlet.

36. An elongate subsea structure comprises at least one venturi formation that is aligned with at least one drainage outlet and is in communication with a dewatering fluid inlet to receive and accelerate a flow of a dewatering fluid, wherein the accelerated flow gene a region of low fluid pressure within the structure that is capable of drawing water within the structure toward the drainage outlet.

37. The structure of claim 36, further comprising at least one internal fall that slopes downwardly toward the drainage outlet.

38. The structure of claim 37, wherein the or each fall is defined by at least one elevated portion of the structure.

39. The structure of claim 38, wherein the at least one elevated portion is supported by external buoyancy attached to the structure.

40. The structure of claim 38, further comprising weights that restrain elevation of the structure to define low points beside the or each elevated portion.

41. The structure of claim 40, wherein at least one drainage outlet is in longitudinal alignment with each of the low points.

42. The structure of claim 39, wherein the external buoyancy is at a position offset longitudinally toward one end of the structure, substantially closer to one end of the structure than to an opposite end of the structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1A is a sequence of schematic side view that shows a pipeline bundle being dewatered in accordance with a method of the invention, before being lifted from the seabed;

[0044] FIG. 16 is a sequence of schematic side view of the invention;

[0045] FIG. 1C is a sequence of schematic side view of the invention;

[0046] FIG. 1D is a sequence of schematic side view of the invention;

[0047] FIG. 1E is a sequence of schematic side view of the invention;

[0048] FIG. 1F is a sequence of schematic side view of the invention;

[0049] FIG. 1G is a sequence of schematic side view of the invention;

[0050] FIG. 1H is a sequence of schematic side view of the invention:

[0051] FIG. 1I is a sequence of schematic side view of the invention; and

[0052] FIG. 1J is a sequence of schematic side view of the invention.

[0053] FIG. 2A is a sequence of schematic side view that shows a pipeline bundle being dewatered in accordance with another method of the invention;

[0054] FIG. 2B is a sequence of schematic side view that shows a pipeline bundle of the invention;

[0055] FIG. 2C is a sequence of schematic side view that shows a pipeline bundle of the invention;

[0056] FIG. 2D is a sequence of schematic side view that shows a pipeline bundle of the invention;

[0057] FIG. 2E is a sequence of schematic side view that shows a pipeline bundle of the invention;

[0058] FIG. 2F is a sequence of schematic side view that shows a pipeline bundle of the invention;

[0059] FIG. 2G is a sequence of schematic side view that shows a pipeline bundle of the invention; and

[0060] FIG. 2H is a sequence of schematic side view that shows a pipeline bundle of the invention.

[0061] FIG. 3 is an enlarged schematic side view of part of a carrier pipe of a pipeline bundle in longitudinal section, showing an internal venturi profile in accordance with the invention, coincident with a drainage outlet of the carrier pipe.

DETAILED DESCRIPTION OF THE INVENTION

[0062] Referring firstly to FIGS. 1A to 1J, these drawings show a decommissioned pipeline bundle assembly 10 initially tying on the seabed 12 and then being lifted from the seabed 12 to be towed away. In this example, the bundle assembly 10 comprises a carrier pipe 14 that terminates in towheads 16, 18 at its respective ends.

[0063] Conventionally, the carrier pipe 14 will contain various parallel elongate elements extending between the towheads 16, 18, including flowlines, service fluid lines and cables, all typically supported by longitudinally-distributed spacers. Such elements and spacers have been omitted from the drawings for simplicity.

[0064] Initially, as shown in FIG. 1A, the carrier pipe 14 and the towheads 16, 18 contain residual seawater 20 remaining from installation and use. As explained above, the main purpose of flooding the carrier pipe 14 and the towheads 16, 18 with seawater 20 during installation was to confer negative buoyancy on the bundle assembly 10. At that stage, negative buoyancy distributed along the length of the assembly 10 was necessary to sink the assembly 10 to the seabed 12 after towing to the installation site and thereafter to stabilise the assembly 10 on the seabed 12.

[0065] Whilst the additional apparent weight of the assembly 10 conferred by ballasting with seawater 20 is an aid to installation and to use of the assembly 10, that weight is a hindrance to lifting and recovery of the assembly 10. Apart from its weight, even a relatively small amount of seawater 20 remaining within the carrier pipe 14 could move and gather longitudinally and hence destabilise the assembly 10 when the assembly 10 is raised from the seabed 12. Consequently, the invention proposes improved dewatering processes, one of which will now be described with reference to FIGS. 18 to 1I.

[0066] FIG. 18 shows that seawater 20 has been displaced from the towheads 16, 18 in a first, optional deballasting stage of the dewatering process. In particular, a dewatering fluid 22 injected into ballasting chambers of the towheads 16, 18, typically at above-ambient pressure, displaces the seawater 20 from the towheads 16. The dewatering fluid 22 may be a gas such as air or nitrogen or a liquid such as mineral oil or kerosene. Injection and drainage ports and related valves on the towheads 16, 18 have been omitted from FIGS. 1A and 1B for simplicity.

[0067] Being substantially less dense than the displaced seawater 20, the injected dewatering fluid 22 reduces the apparent weight of the towheads 16, 18. In principle, it could be possible to confer positive buoyancy on the towheads 16, 18 in this way. However, it is preferred for the towheads 16, 18 to remain slightly negatively buoyant, or at most of substantially neutral buoyancy, so as to maintain control of their position and orientation and to avoid inadvertently over-stressing the carrier pipe 14. In this example, the apparent weight of the towheads 16, 18 is reduced to about ten tonnes. As the remaining figures will show, reducing the apparent weight of the towheads 16, 18 in this way allows the towheads 16, 18 to tilt more readily about a transverse horizontal axis as the carrier pipe 14 is deballasted and as the assembly 10 is lifted from the seabed 12.

[0068] In FIG. 1C, a buoyancy module 24, which in this example exerts three tonnes of upthrust, has been attached to the carrier pipe 14 at an intermediate location between the towheads 16, 18. In principle, the buoyancy module 24 could be attached to the carrier pipe 14 at any point along its length but in this preferred example, the buoyancy module 24 is offset longitudinally toward one end of the carrier pipe 14. The buoyancy module 24 is therefore substantially closer to one of the towheads 16 than to the other towhead 18.

[0069] In consequence of the transverse point loading applied to the carrier pipe 14 by the buoyant upthrust of the buoyancy module 24, a portion 26 of the carrier pipe 14 beneath and adjacent to the buoyancy module 24 is lifted slightly from the seabed 12. The extent of uplift of the carrier pipe 14 is exaggerated in this schematic view. The elevated portion 26 of the carrier pipe 14 adopts upwardly-convex longitudinal curvature while the remainder of the carrier pipe 14 remains on and level with the seabed 12, hence remaining substantially horizontal.

[0070] Points of inflection in the longitudinal curvature of the carrier pipe 14 are thereby defined at the upper apex 28 of the elevated portion 26, directly beneath the buoyancy module 24, and at the ends of that portion 26 where the carrier pipe 14 returns to the level of the remainder of the carrier pipe 14 on the seabed 12.

[0071] It will be apparent from FIG. 1C that the elevated portion 28 of the carrier pipe 14 defines locally-Inclined or tilted sections or fails 30, 32 that extend from the apex 28 with downward inclination in opposite longitudinal directions. It will also be apparent that the deballasted towhead 16 closer to the buoyancy module 24 can tilt to complement the inclination of the adjoining fall 30. This mitigates stress in the carrier pipe 14 adjoining that towhead 16.

[0072] In consequence of the inclination of the falls 30, 32 in the carrier pipe 14 imparted by the upthrust of the buoyancy module 24, gravity causes seawater 20 in the elevated portion 26 of the carrier pipe 14 to drain away from the apex 28 in opposite longitudinal directions toward the towheads 16, 18.

[0073] In this respect, FIG. 1C also shows fluid ducts 34, 36, 38 that communicate with the interior of the carrier pipe 14. In this case, the ducts 34, 36, 38 are shown on the towheads 16, 18 and so communicate with the carrier pipe 14 through the towheads 16, 18 but corresponding ducts could instead be positioned directly on the carrier pipe 14. Specifically, the ducts 34, 36, 38 comprise an inlet duct 34 and a drainage duct 38 on the towhead 18 closer to the buoyancy module 24 and an additional drainage duct 38 on the towhead 18 further from the buoyancy module 24.

[0074] Fluid flow in the ducts 34, 38, 38 is controlled by respective valves 40, 42, 44. As is conventional, the valves 40, 42, 44 are represented in the drawings as white when open and as black when closed.

[0075] When its valve 40 is open as shown in FIG. 1C, the inlet duct 34 admits dewatering fluid 22 into the interior of the carrier pipe 14 to displace the seawater 20 and hence to increase the buoyancy of the carrier pipe 14. Potentially, deballasting can continue to the extent of imparting positive or substantially neutral buoyancy to the carrier pipe 14, as will be described.

[0076] When their valves 42, 44 are open as shown in FIG. 1C, the drainage ducts 36, 38 remove seawater 20 that drains away from the elevated portion 26 of the carrier pipe 14 under the influence of gravity and displacement by the dewatering fluid 22. Outward flow of seawater 20 through the drainage ducts 36, 38 could be assisted by external suction pumps but such pumps have been omitted from the drawings. Similarly, a pump for driving inward flow of dewatering fluid 22 through the inlet duct 34 has been omitted from the drawings.

[0077] Soon, substantially al of the water 20 in the fall 30 that extends from the apex 28 to the closer towhead 16 drains out of the carrier pipe 14 through the drainage duct 36 associated with that towhead 16. This is confirmed when dewatering fluid 22, still entering the carrier pipe 14 through the inlet duct 34, begins to emerge from that drainage duct 36. Then, the valve 42 of the drainage duct 36 is closed as shown in FIG. 10 to prevent further loss of the dewatering fluid 22. This preserves and increases the volume and pressure, and hence the displacing effect, of the dewatering fluid 22 on the seawater 20 remaining in the remainder of the carrier pipe 14, including the other fab 32 that extends from the apex 28 toward the further towhead 18.

[0078] As dewatering fluid 22 continues to flow into the carrier pipe 14 through the inlet duct 34 as shown in FIGS. 1E to 1G, seawater 20 continues to be displaced and expelled from the carrier pipe 14 through the drainage duct 38 of the opposite towhead 18. In this respect, it will be noted that the valve 44 of the drainage duct 38 remains open throughout the drainage process.

[0079] It will also be apparent that ongoing displacement of denser seawater 20 with less dense dewatering fluid 22 makes the carrier pipe 14 progressively more buoyant. Moreover, as low-density dewatering fluid 22 accumulates in the elevated portion 26 and the volume of the injected dewatering fluid 22 continues to increase, the elevated portion 26 will tend to grow and extend.

[0080] In this respect, FIGS. 1E to 1G show that the increasing buoyancy of the carrier pipe 14 is manifested by the elevated portion 26 lengthening progressively toward the towhead 18, as seawater 20 drains down the fall 32 and along the remaining horizontal part of the carrier pipe 14 toward the drainage duct 38 of the towhead 18. In effect, therefore, the fall 32 migrates along the carrier pipe 14 toward the towhead 18. Beneficially, therefore, longitudinally-successive portions of the carrier pipe 14 are inclined or tilted transiently as the fall 32 at the leading end of the lengthening elevated portion 26 migrates through them. This exploits gravity to promote drainage of water from all parts of the carrier pipe 14, without over-bending and over-stressing the carrier pipe 14 at any stage.

[0081] Eventually, as shown in FIG. 1G, the fall 32 migrates to adjoin the towhead 18. Beneficially, as with the other towhead 16 and the other fall 30, the deballasted towhead 18 can tilt readily to complement the inclination of the fall 32, which reduces stress in the carrier pipe 14 where it adjoins that towhead 18.

[0082] The valves 40, 44 in the inlet duct 34 of the towhead 16 and the drainage duct 38 of the other towhead 18 remain open until substantially all of the seawater 20 has drained out of the carrier pipe 14. At that point, the dewatering fluid 22 will begin to emerge from the drainage duct 38, whereupon the valves 40, 44 in the inlet duct 34 and the drainage duct 38 are closed as shown in FIG. 1H.

[0083] Next, as shown in FIG. 1I, the buoyancy module 24 is removed from the carrier pipe 14. This balances buoyancy forces along the length of the bundle assembly 10. The bundle assembly 10 can then be lifted from the seabed as shown in FIG. 1J, for example by tension in wires 48 attached to the towheads 18, 18 and extending to tugs (not shown) at the surface. The tugs can then tow the bundle assembly 10 between them at the surface or at a mid-water depth, for example using the controlled depth towing method (CDTM).

[0084] Turning next to FIGS. 2A to 2H, these drawings illustrate another dewatering method of the invention that may be particularly appropriate where the bundle assembly 10 is to be refurbished and reused. In that case, close control of the curvature of the carrier pipe 14 is important, as is substantially complete dewatering to ensure stability when the bundle assembly 10 is being towed away from the installation site. Like numerals are used for like features.

[0085] Again, a decommissioned bundle assembly 10 is shown initially lying on the seabed 12 before being lifted from the seabed 12 to be towed away. The bundle assembly 10 comprises a carrier pipe 14 that terminates in towheads 16, 18 at its respective ends. The towheads 16, 18 shown in FIG. 2A have already been deballasted in a manner skin to that shown in FIG. 18.

[0086] An inlet duct 34 and a drainage duct 36 are shown on the towhead 16 and an additional drainage duct 38 is shown on the opposite towhead 18. Fluid flow in the ducts 34, 36, 38 is controlled by respective valves 40, 42, 44.

[0087] Here, the carrier pipe 14 has at least one further drainage duct at an intermediate location along its length. The or each drainage duct may, for example, be formed underwater by hot-tapping an opening through the tubular wall of the carrier pipe 14. In this case, a series of drainage ducts 48, 50, 52 are distributed along the carrier pipe 14 in a longitudinally-spaced array. In this example, there is substantially equal longitudinal spacing between the ducts 48, 50, 52.

[0088] Outward flow of seawater 20 through the drainage ducts 48, 50, 52 is controlled by respective valves 54, 56, 58. Again, that flow could be assisted by suction pumps. It would also be possible to connect the ducts 48, 50, 52 to a common manifold, which could be connected to a single suction pump. However, such provisions have been omitted from the drawings.

[0089] Clump weights 60, such as ten-tonne chain saddles, are placed on the carrier pipe 14 at positions corresponding to the drainage ducts 48, 50, 52 as shown in FIG. 2A.

[0090] Next, as shown in FIG. 28, buoyancy modules 24 are attached to the carrier pipe 14 at locations midway between the clump weights 60 and midway between the outermost clump weights 60 and the towheads 16, 18. Thus, the buoyancy modules 24 alternate longitudinally with the clump weights 60 and the towheads 16, 18, which restrain elevation of the carrier pipe 14 between and outboard of the buoyancy modules 24.

[0091] In consequence of the buoyant upthrust of the buoyancy modules 24, portions 26 of the carrier pipe 14 beneath and adjacent to each buoyancy module 24 are lifted slightly from the seabed 12 as shown. Each elevated portion 26 of the carrier pipe 14 therefore adopts upwardly-convex longitudinal curvature and so defines falls 30 that extend from respective apices 28 with downward inclination in opposite longitudinal directions.

[0092] In consequence of the inclination of the falls 30 in the carrier pipe 14 imparted by the upthrust of the buoyancy modules 24, gravity causes seawater 20 in the elevated portions 26 of the carrier pipe 14 to drain away from the apices 28 in opposite longitudinal directions. This causes the seawater 20 to gather at the various low points of the carrier pipe 14. The bundle assembly 10 may be left in this state for a period sufficient for gravity to draw seawater 20 toward the low points of the carrier pipe 14 before drainage begins.

[0093] Low points at the ends of the carrier pipe 14 are defined where the carrier pipe 14 adjoins the towheads 16, 18. Beneficially, as before, the deballasted towheads 16, 18 can tilt to complement the inclination of the falls 30, which reduces stress in the carrier pipe 14 where it adjoins the towheads 16, 18. Additional intermediate low points of the carrier pipe 14 are defined by the clump weights 60 and so are coincident with the drainage ducts 48, 50, 52.

[0094] In this way, seawater 20 can be evacuated effectively from the carrier pipe 14 through the drainage ducts 36, 38 of the towheads 16, 18 and the drainage ducts 48, 50, 52 of the carrier pipe 14. In principle, drainage could be done simultaneously from two or more of those ducts 36, 38, 48, 50, 52, for example symmetrically about the longitudinal midpoint of the bundle assembly 10, or simultaneously from respective sides of that midpoint in inward or outward succession. However, FIGS. 2C to 2H show the option of sequential drainage from successive ducts in turn, in the order 36, 48, 50, 52, 38 along the length of the bundle assembly 10 from end to end.

[0095] In each of FIGS. 2C to 2G, the valve 40 of the inlet duct 34 of the towhead 16 remains open to admit dewatering fluid 22 into the bundle assembly 10 at elevated, slightly above-ambient pressure. FIG. 2C shows the valve 42 of the drainage duct 36 of the towhead 16 also open. Conversely, the valve 44 of the drainage duct 38 of the other towhead 18 and the valves 54, 56, 58 of the drainage ducts 48, 50, 52 of the carrier pipe 14 all remain closed. Dewatering fluid 22 is admitted into the carrier pipe 14 through the inlet duct 34 to promote drainage of the seawater 20 from the fail 30 adjacent to the towhead 16 and out through the drainage duct 36.

[0096] FIG. 2D shows the valve 54 of the drainage duct 48 of the carrier pipe 14 now open. The valves 42, 44 of the drainage ducts 36, 38 of the towheads 16, 18 and the valves 56, 58 of the other drainage ducts 50, 52 of the carrier pipe 14 are closed. Dewatering fluid 22 admitted into the carrier pipe 14 through the inlet duct 34 promotes drainage of the seawater 20 from the falls 30 that converge on the drainage duct 48 and then out of the carrier pipe 14 through that drainage duct 48.

[0097] FIG. 2E shows the valve 56 of the drainage duct 50 of the carrier pipe 14 now open. The valves 42, 44 of the drainage ducts 36, 38 of the towheads 16, 18 and the valves 54, 58 of the other drainage ducts 48, 52 of the carrier pipe 14 are closed. Dewatering fluid 22 admitted into the carrier pipe 14 through the inlet duct 34 promotes drainage of the seawater 20 from the falls 30 that converge on the drainage duct 50 and then out of the carrier pipe 14 through that drainage duct 50.

[0098] FIG. 2F shows the valve 58 of the drainage duct 52 of the carrier pipe 14 now open. The valves 42, 44 of the drainage ducts 36, 38 of the towheads 16, 18 and the valves 54, 58 of the other drainage ducts 48, 50 of the carrier pipe 14 are closed. Dewatering fluid 22 admitted into the carrier pipe 14 through the inlet duct 34 promotes drainage of the seawater 20 from the falls 30 that converge on the drainage duct 52 and then out of the carrier pipe 14 through that drainage duct 52.

[0099] FIG. 2G shows the valve 44 of the drainage duct 38 of the towhead 18 now open. Conversely, the valve 42 of the drainage duct 36 of the other towhead 16 and the valves 54, 56, 58 of the drainage ducts 48, 50, 52 of the carrier pipe 14 are all closed. Dewatering fluid 22 admitted into the carrier pipe 14 through the inlet duct 34 promotes drainage of the seawater 20 from the fall 32 adjacent to the towhead 18 and out through the drainage duct 38.

[0100] When substantially all of the seawater 20 has been expelled from the bundle assembly in this way, the buoyancy modules 24 are detached from the carrier pipe 14 as shown in FIG. 2H. The valves 40, 42, 44 of the towheads 16, 18 and the valves 54, 56, 58 of the carrier pipe 14 are now all closed.

[0101] At this stage, the clump weights 60 remain on the carrier pipe 14 but in a subsequent operation, not shown, the clump weights 60 are removed from the carrier pipe 14. For example, the clump weights 60 may be removed in longitudinal sequence or symmetrically about the longitudinal midpoint of the carrier pipe 14, or simultaneously from respective sides of that midpoint in inward or outward succession. This allows the carrier pipe 14 to adopt the upwardly-convex profile along its length shown in FIG. 1I and then for the bundle assembly 10 to be lifted from the seabed 12, for example as shown in FIG. 1J.

[0102] Turning finally to FIG. 3 of the drawings, this shows an optional venturi formation 62 of the invention that may be provided within the carrier pipe 14 at a longitudinal position coincident with any or all of the drainage ducts 48, 50, 52. One of those drainage ducts 48 is shown here. Again, internal features of the carrier pipe 14 such as flowlines, other pipes, cables and spacers have been omitted from this drawing.

[0103] In this example, the venturi formation 62 comprises a radially-inward bulge 64 on the inner wall of the carrier pipe 14 that defines a narrow throat 66. Dewatering fluid 22 is shown flowing along the carrier pipe 14 above a body of residual seawater 20. On encountering the throat 66, the longitudinal flow of dewatering fluid 22 accelerates. By virtue of Bernoulli's principle, the accelerated flow of dewatering fluid 22 generates a local reduction in fluid pressure in the throat 66. As shown, this locally-reduced fluid pressure draws seawater 20 toward the drainage duct 48 aligned with the throat 66, hence further promoting expulsion of the seawater 20 from the carrier pipe 14.

[0104] In the example shown, the bulge 64 that defines the venturi formation 62 is asymmetric, being present in an upper side of the carrier pipe 14 opposed to the drainage duct 48 but being absent in a lower side of the carrier pipe 14 to promote downward drainage. In other embodiments, the bulge 64 could extend around substantially the entire internal circumference of the carrier pipe 14. The bulge 64 could be defined by a local thickening of the wall of the carrier pipe 14, for example by incorporating a suitably-profiled insert piece into or within the carrier pipe 14.

[0105] In principle, it would be possible for a venturi formation 62 to coincide with a drainage outlet at any location in the bundle assembly 10, including the drainage ducts 36, 38 of the towheads 16, 18.

[0106] Many other variations are possible within the inventive concept. For example, the arrangement shown in FIGS. 1A to 1J could have one or more further drainage ducts at intermediate locations along its length, like those of FIGS. 2A to 2H.

[0107] In another variant, the apparent weight of the towheads could be reduced by attaching buoyancy modules to them, in addition to or instead of deballasting by expelling seawater from them.

[0108] The sequential order of valve opening, water draining and dewatering fluid injection steps expressed herein is preferred for operational reasons but it could be modified without departing from the inventive concept.