Subsea Buoyancy Systems

Abstract

A method of providing buoyancy to a subsea structure such as a pipeline bundle comprises attaching to the structure a rigid elongate buoyancy tube that defines a floodable envelope. The envelope is arranged to contain a mass of buoyant macrospheres. Multiple openings penetrate a tubular wall of the buoyancy tube, in fluid communication with a void that extends between the macrospheres inside the tube. The void floods via the openings when the buoyancy tube is submerged, whereupon the macrospheres apply buoyant upthrust to the surrounding buoyancy tube and hence to the subsea structure to which the tube is rigidly attached.

Claims

1. A buoyant element, comprising a rigid elongate buoyancy tube defining a floodable envelope that holds a mass of buoyant macrospheres within, the buoyancy tube having at least one opening in fluid communication with a void that extends between the macrospheres inside the buoyancy tube.

2. The buoyant element of claim 1, wherein multiple openings penetrate a tubular wall of the buoyancy tube, in fluid communication with the void that extends between the macrospheres inside the tube.

3. The buoyant element of claim 1 or claim 2, wherein the macrospheres are wider than the or each opening of the buoyancy tube.

4. The buoyant element of any preceding claim, further comprising connection structures or fixings for attaching the buoyancy tube to a subsea structure.

5. The buoyant element of any preceding claim, wherein the buoyancy tube is of polymer or polymer composite material.

6. The buoyant element of any preceding claim, wherein the buoyancy tube is foraminous along its length.

7. The buoyant element of any preceding claim, wherein the buoyancy tube has one or more closed ends.

8. The buoyant element of claim 7, wherein one or more of the closed ends is defined by a movable or removable closure affording access to the interior of the buoyancy tube for loading or removing the macrospheres.

9. The buoyant element of any preceding claim, wherein the macrospheres are contained within one or more floodable auxiliary containers within the buoyancy tube.

10. The buoyant element of claim 9, wherein the or each auxiliary container is a foraminous bag defining holes that are smaller than the macrospheres contained within.

11. The buoyant element of claim 9 or claim 10, wherein the auxiliary containers within the buoyancy tube are linked in longitudinal series.

12. The buoyant element of any preceding claim, wherein the or each opening is no larger than 10 mm in diameter.

13. The buoyant element of any preceding claim, wherein the macrospheres are between 15 mm and 35 mm in diameter.

14. In combination, a subsea structure and a buoyant element as defined in any of claims 1 to 13, attached to the subsea structure.

15. The combination of claim 14, wherein the subsea structure is elongate and the buoyancy tube extends along and substantially parallel to the subsea structure.

16. The combination of claim 15, wherein the subsea structure is a pipeline bundle.

17. The combination of claim 16, wherein the buoyancy tube is attached externally to the pipeline bundle.

18. The combination of claim 16, wherein the buoyancy tube is located within an external carrier pipe of the pipeline bundle.

19. The combination of any of claims 16 to 18, wherein the buoyancy tube is secured to transversely-extending, longitudinally-spaced guide frames of the pipeline bundle.

20. A method of providing buoyancy to a subsea structure, the method comprising attaching to the structure a rigid elongate buoyancy tube defining a floodable envelope for containing a mass of buoyant macrospheres.

21. The method of claim 20, comprising placing the mass of macrospheres into the buoyancy tube after attaching the buoyancy tube to the structure.

22. The method of claim 20 or claim 21, comprising placing the mass of macrospheres into the buoyancy tube through at least one open end of the buoyancy tube, and then closing the or each open end.

23. The method of claim 21 or claim 22, comprising placing the mass of macrospheres into the buoyancy tube in discrete, individually contained portions.

24. The method of any of claims 20 to 23, comprising immersing the buoyancy tube containing the mass of buoyant macrospheres while flooding a void that surrounds the macrospheres within the buoyancy tube.

25. The method of claim 24, comprising subsequently detaching the buoyancy tube from the subsea structure.

26. The method of claim 24, comprising subsequently removing at least some of the mass of macrospheres from the buoyancy tube while the buoyancy tube remains attached to the subsea structure.

27. The method of claim 26, comprising removing macrospheres from the buoyancy tube while the macrospheres are contained in a bag or other auxiliary container.

28. The method of claim 26 or claim 27, comprising removing one or more discrete portions of the mass of macrospheres from the buoyancy tube while leaving one or more other discrete portions of that mass in the buoyancy tube.

29. The method of claim 24, comprising subsequently destroying at least some of the mass of macrospheres while the macrospheres are in the buoyancy tube and the buoyancy tube remains attached to the subsea structure.

Description

[0047] 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:

[0048] FIG. 1 is a schematic cross-sectional view of a pipeline bundle comprising a buoyancy tube of the invention filled with buoyant macrospheres and flooded with seawater in voids between the macrospheres;

[0049] FIG. 2 is a schematic side view of a buoyancy tube like that shown in FIG. 1, filled with closely-packed but loose macrospheres;

[0050] FIG. 3 corresponds to FIG. 2 but shows the closely-packed macrospheres contained within porous bags in the buoyancy tube;

[0051] FIG. 4 is a variant of the embodiments shown in FIGS. 2 and 3, in which ends of the buoyancy tube have openings through which the interior of the tube can be flooded;

[0052] FIG. 5 shows a buoyancy tube of the invention attached to the outside of a pipeline bundle; and

[0053] FIG. 6 shows a buoyancy tube of the invention supported within a carrier pipe of a pipeline bundle.

[0054] Referring firstly to FIG. 1 of the drawings, which is much-simplified and not to scale, a pipeline bundle 10 for use in deep-water applications integrates heating, power and control systems. The bundle 10 comprises a buoyant element that, in this example, takes the form of a closed-ended buoyancy tube 12 positioned toward an upper side of the bundle 10. The bundle 10 further comprises power cables 14, data lines 16 and various flowlines 18 for production fluids and for the injection of water, gas or chemicals, all extending generally parallel to the buoyancy tube 12 and to each other.

[0055] One of several longitudinally-spaced transverse guide frames 20 is also shown in FIG. 1. The guide frames 20 hold the various elongate components of the bundle 10 relative to each other with appropriate mutual transverse spacing. In practice, the shape of the guide frames 20 will be more complex than is represented schematically here.

[0056] The buoyancy tube 12 has a substantially rigid tubular wall and is suitably of a polymer or of fibre-reinforced polymer composites. The use of such lightweight materials is enabled by holes 22 that penetrate the tubular wall of the buoyancy tube 12. The holes 22 allow seawater 24 to flood the interior of the buoyancy tube 12 to preclude its collapse under hydrostatic pressure. The holes 22 are shown here distributed angularly around the circumference of the buoyancy tube 12.

[0057] The buoyancy tube 12 is packed with a mass of gas-filled macrospheres 26 to provide the buoyancy required to tow the bundle 10 from an assembly yard to a deep-water installation site. Even when closely packed, the macrospheres 26 leave a void in the buoyancy tube 12 that extends between and around the macrospheres 26. The void communicates with the holes 22 to allow free circulation of seawater 24 into the buoyancy tube 12 and between and around the macrospheres 26. This ensures effective pressure equalisation between the interior and the exterior of the buoyancy tube 12.

[0058] FIG. 2 shows the buoyancy tube 12 filled with closely-packed but loose macrospheres 26. It will be apparent from FIG. 2 that the holes 22 are distributed longitudinally along the length of the buoyancy tube 12. It will also be apparent that the holes 22 are of smaller diameter than the macrospheres 26 so that the macrospheres 26 cannot spill out of the buoyancy tube 26 through the holes 22.

[0059] FIG. 3 corresponds to FIG. 2 but shows the closely-packed macrospheres 26 instead contained within bags 28 in the buoyancy tube 12. The bags 28 are porous, perforated or of mesh, with pores or other openings that are narrower than the diameter of the macrospheres 26 so as to retain the macrospheres 26 in the bags 28.

[0060] The bags 28 are conveniently shaped to fit closely into the interior of the buoyancy tube 12. By dividing the mass of macrospheres 26 into smaller and more easily handled portions, the bags 28 may help with the operation of packing the macrospheres 26 into the buoyancy tube 12. The bags 28 may be flexible or rigid envelopes or containers.

[0061] The use of bags 28 may also help to achieve greater packing density of the macrospheres 26. In this respect, it will be noted that such small envelopes as bags 28 may be more easily packed with macrospheres 26 than the long buoyancy tube 12. For example, it would be more practical to shake a bag 28 to improve the packing density of macrospheres 26 within than it would be to shake the entire buoyancy tube 12.

[0062] When the macrospheres 26 are packed in bags 28, the holes 22 that penetrate the tubular wall of the buoyancy tube 12 could, in principle, be slightly larger than the macrospheres 26. This is because the bags 28 will resist spillage of the macrospheres 26 from the buoyancy tube 12.

[0063] As the bags 28 will retain the macrospheres 26 packed within, it would be possible in principle to pull one or more of the bags 28 out of an open end of the buoyancy pipe 26 when it is desired to reduce buoyancy. If removed from the buoyancy tube 12 underwater, the bags 28 will float to the surface while preventing the macrospheres 26 from spilling into the sea. The bags 28 could be connected in longitudinal series, for example by a wire, so that a chain of multiple bags 28 can be removed conveniently from the buoyancy tube 12 in a single pulling action.

[0064] FIGS. 2 and 3 also show that one or both ends of the buoyancy tube 12 may be closed with a movable or removable closure 30 after macrospheres 26 have been poured or packed into the interior of the buoyancy tube 12 through a previously open end.

[0065] FIG. 4 is a variant of the embodiments shown in FIGS. 2 and 3, in which one or both ends of the buoyancy tube 12 have one or more openings through which the interior of the tube can be flooded upon immersion. In this example, both ends of the tube have perforated end covers 32 penetrated by an array of holes 34.

[0066] Flooding of the buoyancy tube 12 may be effected primarily through the holes 34 in the end covers 32. However, to aid flooding and purging of air without pressure waves damaging the macrospheres 26 within the tube 12, some holes 22 may optionally also be provided in the wall of the tube 12 as in the preceding embodiments. Nevertheless, as shown, fewer holes 22 may be necessary than in the preceding embodiments.

[0067] FIG. 4 illustrates two approaches to retain the macrospheres 26 within the buoyancy tube 12. In the end cover 32 on the left in FIG. 4, the holes 34 are relatively small in diameter, being smaller than the diameter of the macrospheres 26. The loose macrospheres 26 at this end of the buoyancy tube 12 are therefore contained by the end cover 32. Conversely, at the other end of the buoyancy tube 12 shown on the right in FIG. 4, the macrospheres 26 are contained in porous bags 28 whose pores or other openings are narrower than the diameter of the macrospheres 26. This retains the macrospheres 26 in the bags 28 and so allows the holes 34 in the end cover 32 on the right in FIG. 4 to be, optionally, larger than the diameter of the macrospheres 26.

[0068] Finally, FIGS. 5 and 6 show the buoyancy tube 12 of the invention applied to different pipeline bundles, along which the buoyancy tube 12 extends in parallel relation.

[0069] The pipeline bundle 36 represented schematically in FIG. 5 has no external carrier pipe. In this example, the buoyancy tube 12 is conveniently attached to the outside of the bundle 36 by longitudinally-spaced fixings 38. The fixings 38 are exemplified here by straps that extend around both the bundle 36 and the buoyancy tube 12. The straps are tensioned to pull together the bundle 36 and the buoyancy tube 12.

[0070] In contrast, the pipeline bundle 40 represented schematically in FIG. 6 is contained within an external carrier pipe 42. Here, the buoyancy tube 12 is conveniently secured inside the carrier pipe 42, in this example by being clamped or otherwise attached to longitudinally-spaced guide frames 44 that also support one or more pipelines 46 of the bundle 40.

[0071] In principle, it would be possible to secure the buoyancy tube 12 to the outside of the carrier pipe 42 of the bundle 40 in FIG. 6, in a similar manner to the arrangement shown in FIG. 5.

[0072] Other variations are possible within the inventive concept. At one extreme, it would be possible to rely entirely upon a bag to contain the macrospheres and so for the buoyancy tube to have wholly open ends without end covers. It would also be possible for the end covers to be nets or meshes with appropriate mesh sizes, depending upon whether or not the end covers are relied upon to retain the macrospheres within the buoyancy tube.

[0073] In principle, the buoyancy tube could itself be a rigid hollow elongate body of mesh with a mesh size appropriate to retain loose macrospheres within or to hold floodable bags that, in turn, retain the macrospheres.