Connecting Multi-Bore Structures in Water

Abstract

A method for connecting sections of a multi-bore structure in water comprises connecting the sections to bring corresponding bores of the sections into mutual alignment while those bores are each closed by a plug that excludes the water from the bores. Then, with the sections connected and the corresponding bores sealed together in fluid communication with each other, the plugs are flushed away in a flushing fluid that flows along the communicating bores.

Claims

1. A method for connecting sections of a multi-bore structure in seawater, the method comprising: connecting the sections to bring corresponding bores of the sections into mutual alignment while those bores are closed by respective plugs that exclude seawater from the bores; and with the sections connected and the corresponding bores sealed together in fluid communication with each other, flushing away the plugs in a flushing fluid that flows along the communicating bores.

2. The method of claim 1, comprising dissolving and/or fragmenting the plugs in the flushing fluid.

3. The method of claim 1, comprising initially shielding the plugs from seawater before connecting the sections.

4. The method of claim 3, comprising subsequently exposing the plugs to seawater before connecting the sections.

5. The method of claim 4, comprising removing one or more water-tight caps from the bores to expose the plugs to seawater.

6. The method of claim 1, comprising connecting the sections when the bores are substantially submerged in seawater.

7. The method of claim 1, comprising allowing gas to escape from at least one of the bores around a plug situated in that bore.

8. The method of claim 7, comprising deforming said plug under fluid pressure of the gas.

9. The method of claim 1, wherein the flushing fluid contains water.

10. The method of claim 9, wherein the flushing fluid is substantially fresh water.

11. The method of claim 1, wherein the flushing fluid contains a glycol.

12. The method of claim 1, comprising exposing at least one of the plugs to a differential fluid pressure of at least two bars before connecting the sections.

13. The method of claim 1, preceded by force-fitting the plugs into the bores with an interference fit.

14. The method of claim 1, preceded by bonding the plugs into the bores.

15. The method of claim 1, comprising exposing the plugs to seawater for at least one hour before connecting the sections, while keeping the bores closed by the plugs.

16. A plug for temporarily isolating a flowline bore from water, the plug comprising a body of substantially circular cross-section surrounded by a slide-resistant bore interface, the body being made of a liquid-degradable flushable material that is capable of substantially maintaining its structural integrity for at least one hour of exposure to seawater.

17. The plug of claim 16, wherein the interface comprises a water-soluble adhesive.

18. The plug of claim 16, wherein the interface comprises an alternating series of circumferential ridges and grooves.

19. The plug of claim 16, wherein the flushable material comprises a soluble material that is soluble in fresh water or in a glycol.

20. The plug of claim 19, wherein the soluble material is substantially more soluble in fresh water or in a glycol than in seawater.

21. The plug of claim 16, wherein the flushable material comprises an organic composite.

22. The plug of claim 16, wherein the flushable material comprises paper.

23. The plug of claim 16, wherein the flushable material comprises a non-Newtonian gel.

24. The plug of claim 16, further comprising a barrier layer supported on the body that is more resistant than the material of the body to degradation in seawater.

25. The plug of claim 24, wherein the barrier layer relies for its integrity on the support of the body.

26. A section of a multi-bore structure, fitted with at least one plug as defined in claim 16 to define an interface between the plug and a bore of the section.

27. The section of claim 26, wherein the plug is an interference fit in a bore of the section.

28. The section of claim 26, wherein the plug is bonded to a bore of the section.

29. The section of claim 26, wherein the interface is capable of withstanding a pressure differential of at least 2 bars without the plug sliding in the bore.

30. The section of claim 26, wherein the bore contains gas at an overpressure of at least 2 bars relative to ambient pressure.

Description

[0046] 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:

[0047] FIG. 1 is a longitudinal sectional view of a pipeline bundle section comprising flowlines or other fluid conduits that contain plugs in accordance with the invention, protected by a removable end cap;

[0048] FIG. 2 corresponds to FIG. 1 but shows valves opened to equalise pressure across the end cap;

[0049] FIG. 3 corresponds to FIG. 2 but shows the pipeline bundle section with the end cap removed;

[0050] FIG. 4 shows two of the pipeline bundle sections of FIG. 3 joined end to end to bring their aligned fluid conduits into fluid communication;

[0051] FIG. 5 corresponds to FIG. 4 but shows a flow of a solvent in the fluid conduits dissolving the plugs;

[0052] FIG. 6 corresponds to FIG. 5 but shows the dissolved plugs being flushed away along the fluid conduits in the flow of solvent;

[0053] FIG. 7 is an enlarged detail view showing a variant of the plugs shown in FIGS. 1 to 4; and

[0054] FIG. 8 is an enlarged detail view showing a further variant of the plugs shown in FIGS. 1 to 4.

[0055] Referring firstly to FIGS. 1 and 2 of the drawings, a pipeline bundle portion or section 10 is shown floating in seawater 12. The bundle section 10 is shown here fully submerged beneath the surface 14 but it could instead be partially submerged and hence may protrude partially above the surface 14.

[0056] The bundle section 10 comprises a rigid hollow carrier pipe 16 that surrounds parallel fluid conduits, exemplified here by rigid flowlines 18. The carrier pipe 16 and the flowlines 18 are of steel in this example but any of them could be of composite materials.

[0057] The flowlines 18 are shown here as being of single-wall construction although one or more of them could be of PiP construction instead. The carrier pipe 16 will also contain spacers and may contain other elongate elements such as cables, but these conventional details have been omitted for simplicity.

[0058] The carrier pipe 16 is closed by transverse bulkheads 20 at its opposed ends, only one of which is shown. This defines a sealed chamber 22 within the carrier pipe 16. The chamber 22 may be packed or charged with a substantially inert gas such as nitrogen that surrounds the flowlines 18.

[0059] Optionally, the chamber 22 within the carrier pipe 16 may be pressurised to an elevated pressure to counteract hydrostatic pressure at the water depth anticipated during towing or installation. In some applications, it would also be possible for the chamber 22 to be flooded in a controlled manner for buoyancy control or to settle the bundle onto the seabed.

[0060] The flowlines 18 may also be pressurised to an elevated pressure to counteract hydrostatic pressure at the water depth anticipated during towing or installation. For example, the flowlines 18 may be packed or charged with a substantially inert gas such as nitrogen at a gauge pressure that exceeds the expected ambient water pressure by say 2 bars. This excess pressure helps to prevent seawater 12 entering the flowlines 18.

[0061] The bulkhead 20 is a disc-shaped machined steel forging that extends in a plane orthogonal to a central longitudinal axis 24 of the bundle section 10. The bulkhead 20 has an inner face 26 that faces axially inwardly toward the inside of the associated carrier pipe 16 and an outer face 28 that faces axially outwardly away from the associated carrier pipe 16. The inner face 26 of the bulkhead 20 is welded around its periphery to an end of the carrier pipe 16.

[0062] A circumferential flange 30 protrudes radially from the bulkhead 20. The flange 30 is penetrated by a circumferential array of axially-extending holes 32.

[0063] The bulkhead 20 is penetrated by axially-extending openings 34 whose positions correspond to the angular and radial positions of the flowlines 18 within the carrier pipe 16 about the central longitudinal axis 24. The flowlines 18 are welded to the inner face 26 of the bulkhead 20 around the peripheries of the respective openings 34 in a leak-tight manner, such that the openings 34 are in fluid communication with the interior of the flowlines 18.

[0064] Each opening 34 is surrounded by a resilient annular gasket or seal 36 on the outer face 28 of the bulkhead 20. A disc-shaped end cap 38 is held parallel to the outer face 28 of the bulkhead 20 by clamps 40 that act axially between the flange 30 of the bulkhead 20 and a similar radially-protruding circumferential flange 42 on the end cap 38. The clamps 40 force the end cap 38 axially toward the bulkhead 20 to compress the seals 36. The end cap 38 thereby closes the openings 34 of the bulkhead 20 and hence isolates the interior of the flowlines 18 from the surrounding seawater 12, in addition to protecting the seals 36.

[0065] In accordance with the invention, the flowlines 18 are also sealed by plugs 44. In this example, the plugs 44 are cylindrical blocks that extend from the flowlines 18 into the openings 34 in the bulkhead 20. The plugs 44 could instead be positioned wholly in the flowlines 18 or wholly in the openings 34.

[0066] There is an interference fit between each plug 44 and the surrounding wall defined by the interior of the flowline 18 and the opening 34 in the bulkhead 20. Thus, each plug 44 may be force-fitted into the bore defined by that surrounding wall.

[0067] The plugs 44 are made from a rigid soluble material such as a paper or an organic composite comprising a soluble adhesive or matrix. The plugs 44 may instead, or additionally, comprise a non-Newtonian gel material such as agar or another gel.

[0068] Whilst they are rigid to the extent of being self-supporting and pressure-resistant, the plugs 44 may have some flexibility or resilience to conform to, and to seal against, the surrounding wall defined by the interior of the flowline 18 and the opening 34 in the bulkhead 20.

[0069] The end cap 38 must be removed from the bundle section 10 to enable end-to-end coupling with another bundle section 10 as shown in FIG. 4. For this purpose, equalising tubes 46 communicate with the small cavities within the seals 36 between the bulkhead 20 and the end cap 38. Fluid flow along each equalising tube 46 is controlled by a respective valve 48, for example a needle valve.

[0070] The valves 48 are normally closed, as shown in black in FIG. 1. When the end cap 38 is to be removed from the bundle section 10, the valves 48 are opened as shown in white in FIG. 2. This equalises the pressure in the cavities within the seals 36 with the ambient pressure of the surrounding seawater 12. Equalising the pressure in this way enables the end cap 38 to be removed from the bulkhead 20, as shown in FIG. 3, after releasing and removing the clamps 40.

[0071] When the end cap 38 has been removed as shown in FIG. 3, the plugs 44 continue to isolate the interior of the flowlines 18 from the surrounding seawater 12, at least for long enough to enable end-to-end coupling with another floating bundle section 10 as shown in FIG. 4. That coupling operation may be expected to take at least an hour.

[0072] Referring now to FIG. 4 in detail, this shows two bundle sections 10 coupled together end-to-end and in mutual alignment along their common central longitudinal axis 24. The outer faces 28 of their bulkheads 20 face each other across their mutual interface. The bulkheads 20 transmit forces between, and provide for leak-tight fluid communication between, the adjoining bundle sections 10. Two or more bundle sections 10 may be joined in this way to make a longer pipeline bundle assembly 50.

[0073] When the bundle sections 10 are brought together end-to-end to form a bundle assembly 50 as shown, the openings 34 of each bulkhead 20 align with their counterparts in the opposing, facing bulkhead 20. Hence, when the correctly-aligned bulkheads 20 are brought together in an axial or longitudinal direction parallel to the central longitudinal axis 24, the seals 36 around the opposed openings 34 cooperate and seal together.

[0074] The cooperating openings 34 together form respective longitudinal passages that extend parallel to the central longitudinal axis 24. The seals 36 act in compression between the outer faces 28 of the coupled bulkheads 20 to maintain leak-tightness in those passages. The passages thereby enable leak-proof fluid communication along the bundle assembly 50 from the flowlines 18 of one bundle section 10 through the openings 34 to the flowlines 18 of the next bundle section 10.

[0075] The bulkheads 20 are pressed together mechanically and held together in a state of mutual axial compression by a ring of bolts 52 that act in axial tension. The bolts 52 are received in respective aligned holes 32 in the parallel circumferential flanges 30 of the bulkheads 20. The bolts 52 therefore encircle the bundle assembly 22 and extend parallel to the central longitudinal axis 24.

[0076] The arrows in the flowlines 18 in FIG. 5 show a solvent liquid such as fresh water or MEG now introduced into the flowlines 18 and starting to dissolve the plugs 44. The remnants of the plugs 44 are thereby entrained in the flow of solvent liquid as shown in FIG. 6 and flushed away, hence being removed without requiring pigging of the flowlines 18.

[0077] Turning finally to FIGS. 7 and 8, these drawings show variants of the plug 44.

[0078] FIG. 7 shows a first variant 44A of the plug 44. The plug 44A has an optional high-grip layer or coating 54 on the cylindrically-curved radially-outermost surface of its body 56. The high-grip coating 54, which may be of adhesive or a resilient material, supplements the frictional engagement between the plug 44A and the surrounding wall defined by the interior of the flowline 18 and the opening 34 in the bulkhead 20. The high-grip coating 54 is preferably soluble in the same solvent that can dissolve the body 56 of the plug 44A.

[0079] The plug 44A shown in FIG. 7 also has an optional barrier layer 58 at an exposed end of the body 56. The barrier layer 58 may be more resistant than the body 56 of the plug 44A to dissolution in seawater. In this way, the barrier layer 58 protects the body 56 from premature dissolution when the end cap 38 is removed.

[0080] Optionally, the barrier layer 58 and/or the high-grip coating 54 is mechanically weak relative to the supporting body 56 of the plug 44A and relies for its integrity on the greater mechanical strength of the body 56. Thus, when the body 56 eventually dissolves in a flow of solvent in the flowline 18, the barrier layer 58 and/or the high-grip coating 54 will fragment readily into particles that are entrained in the flow and flushed away.

[0081] FIG. 8 shows a second variant 448 of the plug 44. Here, the plug 448 is encircled by circumferential grooves 60 that define radially-extending circumferential ridges or fins 62. The grooves 60 allow the fins 62 to deform longitudinally under differential pressure. Under sufficient differential pressure, deformation of the fins 62 can create a narrow peripheral passageway between the plug 44B and the surrounding wall defined by the interior of the flowline 18 and the opening 34 in the bulkhead 20. Such a passageway is apt to vent any excessive overpressure of gas in the flowline 18, without that overpressure forcing the plug 448 out of the opening 34.

[0082] The circumferentially-ridged outer profile of the fins 62 also enhances frictional engagement with the surrounding wall defined by the interior of the flowline 18 and the opening 34 in the bulkhead 20.

[0083] The fins 62 of the plug 448 could have longitudinal asymmetry to respond to differential pressure in an asymmetric manner depending upon the direction from which fluid pressure is exerted on the plug 448. Thus, the fins 62 could deflect in a longitudinally-outward direction more readily than they deflect in a longitudinally-inward direction, hence allowing gas egress from the flowline 18 to release overpressure while resisting water ingress into the flowline 18 in the opposite direction. For example, the fins 62 could have convex curvature on an inner side and concave curvature on an outer side.

[0084] Many other variations are possible within the inventive concept. For example, one or more features of the variant plug 44A of FIG. 7 may be combined with one or more features of the variant plug 44B of FIG. 8. Also, instead of using clamps 40, the end cap 38 could be held on the bulkhead 20 by bolts 52 like those used to couple the bundle sections 10 of a bundle assembly 50.