Mitigation of Buckling in Subsea Pipe-in-Pipe Systems

Abstract

A subsea pipeline (10) of pipe-in-pipe configuration comprises an inner pipe (14), an outer pipe (16) spaced radially from the inner pipe and an annulus (20) defined by the radial spacing between the inner and outer pipes. A series of longitudinally-spaced outward projections (22) extend radially outwardly into the annulus from the inner pipe and are movable longitudinally relative to the outer pipe. A corresponding series of longitudinally-spaced inward projections (24) extend radially inwardly into the annulus from the outer pipe and are movable longitudinally relative to the inner pipe. When the inner pipe is subject to thermal elongation or contraction in use of the pipeline, the inner pipe is movable longitudinally relative to the outer pipe, hence moving the outward projections between and relative to the inward projections. The pipeline may be buried to restrain the outer pipe. The annulus may be flooded, in which case the inner pipe is covered with wet insulation.

Claims

1. A subsea pipeline of pipe-in-pipe configuration, comprising: an inner pipe; an outer pipe spaced radially from the inner pipe; an annulus defined by the radial spacing between the inner and outer pipes; a series of longitudinally-spaced outward projections that extend radially outwardly into the annulus from the inner pipe and that are movable longitudinally relative to the outer pipe; and a series of longitudinally-spaced inward projections that extend radially inwardly into the annulus from the outer pipe and that are movable longitudinally relative to the inner pipe; wherein, in use of the pipeline, the inner pipe is movable longitudinally relative to the outer pipe, hence moving the outward projections between and relative to the inward projections; wherein the longitudinal spacing between the inward and outward projections defines, by interlocking abutment of the inward projections with the outward projections, a range of said longitudinal movement of the inner pipe relative to the outer pipe, said range of relative longitudinal movement being delimited by a first position in which the inward projections bear against first faces of the outward projections and a second position in which the inward projections bear against second faces of the outward projections opposed longitudinally to the first faces.

2. The pipeline of any preceding claim, wherein the inward and/or outward projections substantially centralise the inner pipe within the outer pipe.

3. The pipeline of any preceding claim, further comprising longitudinally-spaced centralisers that extend radially across the annulus between the inner and outer pipes.

4. The pipeline of any preceding claim, further comprising a low-friction coating on the inward and/or outward projections.

5. The pipeline of any preceding claim, further comprising rollers mounted on the inward and/or outward projections.

6. The pipeline of any preceding claim, further comprising at least one layer of thermal insulation on the inner pipe.

7. The pipeline of claim 6, wherein the thermal insulation is wet insulation and the annulus is filled with water.

8. The pipeline of claim 7, wherein the annulus is in fluid communication with a body of water surrounding the pipeline.

9. The pipeline of any of claims 6 to 8, wherein the outward projections are integral with the at least one layer of thermal insulation material.

10. The pipeline of any preceding claim, wherein the pipeline is covered to fix the outer pipe relative to the seabed.

11. The pipeline of claim 10, wherein the pipeline is buried to its full diameter in a seabed trench.

12. A method of operating a subsea pipeline of pipe-in-pipe configuration comprising an outer pipe spaced radially from an inner pipe by an annulus, the method comprising: causing thermal expansion or contraction of the inner pipe relative to the outer pipe; permitting said relative expansion or contraction of the inner pipe within a range of relative longitudinal movement; and limiting said range of relative longitudinal movement by bringing interlocking projections of the inner and outer pipes into mutual abutment in the annulus.

13. The method of claim 12, comprising limiting said range of relative longitudinal movement between a first position in which inward projections of the outer pipe bear against first faces of outward projections of the inner pipe and a second position in which the inward projections bear against second faces of the outward projections, opposed longitudinally to the first faces.

14. The method of claim 12 or claim 13, comprising restraining corresponding expansion or contraction of the outer pipe.

15. The method of claim 14, comprising covering the pipeline to fix the outer pipe relative to the seabed.

16. The method of claim 15, comprising burying the full diameter of the pipeline in a seabed trench.

17. The method of any of claims 12 to 16, comprising thermally insulating the inner pipe with water that fills the annulus.

18. The method of any of claims 12 to 17, comprising thermally insulating the inner pipe with wet insulation.

19. The method of any of claims 12 to 18, comprising using at least some of the interlocking projections to centralise the inner pipe within the outer pipe.

20. A method of fabricating a subsea pipeline of pipe-in-pipe configuration comprising an outer pipe spaced radially from an inner pipe by an annulus, the method comprising: supporting the outer pipe by a hang-off system of an installation vessel; supporting the inner pipe by mutual engagement between interlocking projections of the inner and outer pipes within the annulus; pulling an end portion of the inner pipe out from an end of the outer pipe, hence disengaging the interlocking projections; and welding a new inner pipe joint to the end portion of the inner pipe.

21. The method of claim 20, further comprising welding a new outer pipe joint to the end of the outer pipe.

22. The method of claim 20 or claim 21, further comprising returning the end portion of the inner pipe into the end of the outer pipe after welding the new inner pipe joint to the end portion of the inner pipe, hence reengaging the interlocking projections.

Description

[0041] 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:

[0042] FIGS. 1a, 1b and 1c are a sequence of a side views of a PiP pipeline of the invention in longitudinal section, when buried in a subsea trench and responding to thermal stress in use;

[0043] FIG. 2 is an enlarged sectional side view of a variant of the pipeline of FIG. 1;

[0044] FIG. 3 is an enlarged sectional side view of another variant of the pipeline of FIG. 1;

[0045] FIG. 4 is a detail view of another variant of the pipeline of FIG. 1, in longitudinal section;

[0046] FIG. 5 is a detail view of another variant of the pipeline of FIG. 1, in longitudinal section; and

[0047] FIGS. 6a, 6b and 6c are a sequence of a side views of a PiP pipeline of the invention in longitudinal section, being fabricated during a J-lay operation.

[0048] The drawings are schematic and are not to scale. In particular, the pipelines shown in the drawings have been shortened longitudinally to illustrate the principles of the invention more clearly.

[0049] Referring firstly to FIGS. 1a to 1c, a PiP pipeline 10 of the invention is shown here covered to constrain the pipeline 10 against buckling. In this example, the pipeline 10 has been covered by being buried in soil of the seabed 12, for example in a backfilled trench.

[0050] As is conventional, the pipeline 10 comprises an inner pipe 14 and an outer pipe 16 that are in concentric relation about a common central longitudinal axis 18. The inner pipe 14 serves as a flowline for hot fluids such as hydrocarbon production fluids. The inner pipe 14 is therefore held spaced apart from the outer pipe 16 to define a thermally-insulating annulus 20 between them. The annulus 20 may contain a thermally-insulating material wrapped around or layered onto the inner pipe 14, although such a material has been omitted from these drawings for clarity.

[0051] The inner and outer pipes 14, 16 are both conventionally fabricated of steel, although either or both of them could instead be made of a fibre-reinforced polymer composite material such as a thermoplastic composite pipe. Both steel pipes and composite pipes are regarded in the art as nominally rigid pipes, albeit that they are routinely bent along their length during installation and in use. Rigid pipes are distinguished in the art from flexible pipes such as unbonded flexible pipelines that have a layered wall structure of steel reinforcements alternating with flexible impermeable membranes. Relative to rigid pipes, flexible pipes have a much smaller minimum bending radius and they experience minimal elastic recovery once they are bent along their length.

[0052] The inner pipe 14 has a series of longitudinally-spaced external rings or outward projections 22 that project radially outwardly from its outer face into the annulus 20. The outward projections 22 are equi-spaced along the length of the inner pipe 14. Each outward projection 22 extends circumferentially around the inner pipe 14 and preferably extends continuously around the inner pipe 14 like an external flange or collar.

[0053] Correspondingly, the outer pipe 16 has a series of longitudinally-spaced internal rings or inward projections 24 that project radially inwardly from its inner face into the annulus 20. The inward projections 24 are equi-spaced along the length of the outer pipe 16, with spacing corresponding to the spacing between the outward projections 22 of the inner pipe 14. Each inward projection 24 extends circumferentially, and preferably continuously, within the outer pipe 16 like an internal flange or collar.

[0054] The outward and inward projections 22, 24 may be integral with the respective pipes 14, 16, especially in the case of composite pipes, or may be attached to the respective pipes 14, 16, for example by clamping, overmoulding, adhesive bonding or welding. The projections 22, 24 may be of steel, polymer or a reinforced polymer.

[0055] The outward and inward projections 22, 24 project far enough into the annulus 20 in their respective radial directions that there is an interlocking radial overlap between their confronting facing surfaces. Thus, the radially outermost point of each outward projection 22 lies on a greater circumference than the radially innermost point of each inward projection 24. In other words, the radially outermost point of the outward projection 22 is radially outboard of the radially innermost point of the inward projection 24.

[0056] Neither projection 22, 24 needs to extend across the full radial depth of the annulus 20. Thus, the radially outermost point of the outward projection 22 may be spaced from the inner face of the outer pipe 16 and/or the radially innermost point of the inward projection 24 may be spaced from the outer face of the inner pipe 14. The clearance is sufficient to ensure a sliding fit between the projections 22, 24 and the opposed surfaces of the pipes 14, 16, while still ensuring substantial concentricity between the pipes 14, 16.

[0057] To give clearance for their interlocking overlap, the outward and inward projections 22, 24 are offset longitudinally from each other along the central longitudinal axis 18. Thus, the outward and inward projections 22, 24 alternate with each other moving lengthwise along the pipeline 10. The radially-overlapping facing surfaces of the projections 22, 24 confront each other within the annulus 20.

[0058] By virtue of the invention, the inner pipe 14 can move longitudinally within and relative to the outer pipe 16. In this respect, when the pipeline 10 is covered or buried as shown in these drawings, the outer pipe 16 is substantially fixed or restrained relative to the seabed 12 and hence the inward projections 24 remain substantially stationary. To avoid massive compressive stresses in the inner pipe 14 due to thermal expansion, the inner pipe 14 is free to move within a range of longitudinal movement that is illustrated in FIGS. 1a to 1c.

[0059] Specifically: FIG. 1a shows the outward projections 22 abutting the corresponding inward projections 24; FIG. 1b shows the outward projections 22 in an intermediate longitudinal position, carried away from the inward projections 24 by thermal elongation of the inner pipe 14 and hence now positioned between successive inward projections 24 of the series; and FIG. 1c shows the outward projections 22 abutting the next inward projections 24 of the series but on the longitudinally-opposite side.

[0060] It will be apparent that the positions of abutting contact between the projections 22, 24 shown in FIGS. 1a and 1c define the extremes of movement of the inner pipe 14 relative to the outer pipe 16. Thus, the positions of abutting contact between the projections 22, 24 define the threshold at which further thermal elongation or contraction of the inner pipe 14 will be resisted or prevented by transferring longitudinal loads from the inner pipe 14 to the outer pipe 16.

[0061] In the arrangement exemplified here with a fully-restrained outer pipe 16, the inward projections 24 of the outer pipe 16 therefore serve as stoppers to limit longitudinal movement of the outward projections 22 of the inner pipe 14. However in other arrangements, either type of projection 22, 24 could serve as a stopper for the other type of projection 22, 24.

[0062] The outward and inward projections 22, 24 serve as spacers or centralisers to define the annulus 20 and to maintain concentricity between the pipes 14, 16. However, additional conventional centralisers or spacers 26 may be provided on the inner pipe 14 between successive projections 22, 24 as shown in FIG. 2.

[0063] Moving on to FIG. 3, this illustrates a variant in which the inner pipe 14 is coated with a thermally-insulating coating 28 of, for example, polypropylene. In this example, the outward projections 22 are moulded integrally with, or overmoulded onto, the coating 28.

[0064] Those skilled in the art will know that a polymeric coating 28 such as that shown in FIG. 3 is apt to be used as wet insulation, hence being in direct contact with seawater. Typically in the art, wet insulation is used on the exterior of single-wall pipelines. In this instance, however, the coating 28 can serve as wet insulation on the exterior of the inner pipe 14 of a PiP system. Thus, the annulus 20 can be flooded with water and there is therefore no need for bulkheads, or at least there is no need to seal the annulus between a pair of bulkheads. Advantageously, water trapped in the annulus 20 within the outer pipe 16 will add a thermally insulating effect to the wet insulation provided by the coating 28.

[0065] FIG. 4 shows that the outward projection 22 may present a low-friction surface 30, for example of PTFE, to the inner face of the outer pipe 16. Here, the inward projection 24 serves only as a stop formation for the outward projection 22, which extends across substantially the full radial depth of the annulus 20. It would be possible to reverse the arrangement shown in FIG. 4 by providing the low-friction surface 30 instead on an inward projection 24 that extends across substantially the full radial depth of the annulus 20 to slide along the outer face of the inner pipe 14.

[0066] FIG. 5 shows that rolling contact is also possible to reduce friction between a projection 22, 24 and the opposed pipe 14, 16. In this example, the outward projection 22 is provided with wheels or rollers 32 that roll along the inner face of the outer pipe 16. Conversely, it would also be possible for the inward projection 24 to have similar provisions so as to roll along the outer face of the inner pipe 14.

[0067] Turning finally to FIGS. 6a to 6c, these drawings show how a pipeline 10 of the invention may be fabricated from a succession of pipe joints offshore. In this example, the pipeline 10 is being fabricated aboard an installation vessel by a J-lay method on an upright axis 34 that may be substantially vertical as shown. Previous welds 36 between successive pipe joints of the inner and outer pipes 14, 16 are shown.

[0068] As is conventional, the weight of the pipeline 10 suspended between the vessel and the seabed is supported by a hold-back system 38 of the vessel comprising a clamp and/or tensioners as exemplified here. Such a hold-back system 38 can only act on the outer pipe 16; the weight of the inner pipe 14 must be supported by other means.

[0069] Conveniently, in accordance with the invention, the weight of the inner pipe 14 is supported by an interlocking action between the outward projections 22 of the inner pipe 14 and the underlying inward projections 24 of the outer pipe 16. In effect, the outward projections 22, and hence the inner pipe 14, are hung on the inward projections.

[0070] In FIGS. 6a to 6c, the inner and outer pipes 14, 16 are longitudinally aligned and therefore are coterminous, hence with their upper ends at the same horizontal level, when the outward projections 22 rest on the inward projections 24 as shown in FIGS. 6a and 6c.

[0071] FIG. 6b shows a new inner pipe joint 40 being added to the top of the inner pipe 14. The inner pipe joint 40 contains an internal line-up clamp 42. The line-up clamp 42 is temporarily fixed to the inner pipe joint 40 by upper clamp shoes 44 that are extended radially outwardly against the inner face of the inner pipe joint 40.

[0072] A portion of the line-up clamp 42 protruding downwardly from the inner pipe joint 40 is inserted into the open top of the inner pipe 14. Once fully inserted into the top of the inner pipe 14, lower clamp shoes 46 of the line-up clamp 42 are extended radially outwardly against the inner face of the inner pipe 14. In this way, the line-up clamp 42 aligns the inner pipe joint 40 with the top of the inner pipe 14 and holds them together for circumferential butt-welding around their mutual interface.

[0073] FIG. 6b shows the line-up clamp 42 raised to lift the inner pipe joint 40 and the inner pipe 14 relative to the outer pipe 16. The top of the inner pipe 14 then protrudes above the top of the outer pipe 16 to allow access to a welding head 48 for forming a weld at the interface between the inner pipe joint 40 and the top of the inner pipe 14.

[0074] After the weld 50 between the inner pipe joint 40 and the top of the inner pipe 14 has been completed, tested and coated, the line-up clamp 42 is lowered to transfer the weight of the thus-extended inner pipe 14 back onto the interlocking projections. The line-up clamp 42 can then be removed. A new outer pipe joint 52 is then lowered onto the top of the outer pipe 16 as shown in FIG. 6c, which also shows a welding head 48 forming a weld around their mutual interface.

[0075] When the weld between the outer pipe joint 52 and the outer pipe 16 is completed, the hold-back system 38 lowers the thus-extended pipeline 10 to allow the process to be repeated with the next new inner and outer pipe joints 40, 52.