WO1999034141A1

WO1999034141A1 - Double-walled pipeline with pretensioned inner pipe

Info

Publication number: WO1999034141A1
Application number: PCT/GB1998/003915
Authority: WO
Inventors: James Daniel Davidson; William Duncan Watt; Christopher Allan Scott
Original assignee: Rockwater Ltd
Current assignee: Rockwater Ltd
Priority date: 1997-12-24
Filing date: 1998-12-24
Publication date: 1999-07-08
Anticipated expiration: 2000-06-24
Also published as: GB9727307D0; AU2063299A

Abstract

Pipeline comprises an inner pipe (10, 10', 10'') fixedly secured to an outer pipe (12, 12'), the arrangement being such that, when the temperatures of the inner and the outer pipes are the same, the inner pipe (10, 10', 10'') is in a state of longitudinal tension and the outer pipe (12, 12') is in a state of longitudianl compression. A method and apparatus for forming the pipeline are also described.

Description

DOUBLE-WALLED PIPELINE WITH PRETENSIONED INNER PIPE

This invention relates to pipeline. The invention also relates to a method and apparatus for making pipeline. High temperature oil field pipelines typically carry untreated oil or gas from one or more oil wells to a processing facility some kilometres distant. The unprocessed oil must be kept warm to prevent deposition of waxes or formation of hydrates. There is therefore a requirement that the pipelines be insulated. Many of the more efficient insulating materials are of low density, and they do not perform as well as desired in subsea applications, where the environment is seawater at high pressures. A common solution to the insulation requirement is a 'pipe-in-pipe' system, where the inner pipe contains the fluids being transported, where the outer pipe is a barrier to seawater, and where the annulus between the two pipes is a dry environment at low or atmospheric pressure in which the insulation can perform efficiently. The weight of a long length of pipeline is considerable, and the weight acting on the seabed gives rise to friction which tends to restrain longitudinal expansion of the pipeline. In a pipe-in-pipe system the weight .and longitudinal stiffness of the outer pipe are additional sources of restraint, which in a high temperature pipeline can lead to high axial compressive stresses. These axial stresses .are a major factor in the design of a high temperature pipeline, and they govern the choice of pipeline material and effectively limit the pipeline operating temperature. We have now found a way to overcome these problems by prestressing the inner pipe, during manufacture of the pipeline, to a state of longitudinal tension. Thus, at the time of manufacture of the pipeline, the inner pipe is in a state of longitudinal tension and the outer pipe is in a state of longitudinal compression. This has the effect that the longitudinal stresses at the operating temperature, which is higher than the manufacturing temperature, are reduced by the amount of the prestress. Such a prestressed pipeline permits use of material of lower strength, or of lower wall thickness, or permits operation at a higher temperatures, or some combination of these.

According to one aspect of the invention there is provided pipeline comprising .an inner pipe fixedly secured to an outer pipe, the inner pipe being in a state of longitudinal tension and the outer pipe being in a state of longitudinal compression, wherein the longitudinal tension and compression were induced by a pretreatment carried out prior to securing the inner pipe to the outer pipe.

The pretreatment may, comprise, for example, extending the inner pipe, shortening the outer pipe, or both. The inner pipe may be extended by the application of a force, by heating or both. The outer pipe may be shortened by the application of a force, by cooling or both. Thus at the time of manufacture of the pipeline, i.e. before the pipeline is in use for the transport of fluids, the inner pipe is in a state of longitudinal tension and the outer pipe is in a state of longitudinal compression. It will be apparent from the foregoing that when the inner and outer pipes are at the same temperature, the inner pipe will be in a state of longitudinal tension and the outer pipe will be in a state of longitudinal compression. The inner and outer pipes may be secured to one mother by any means that permits loads on the inner pipe to be transferred to the outer pipe, and vice versa. Typically the inner pipe and the outer pipe are secured to one another by means of a weld.

Preferably, at least one of the inner and outer pipes is adapted to be subjected to a tensile or compressive force. This may be achieved by means of formations on the inner and or outer pipe, which can be engaged by cooperating formations on a force application device.

In one embodiment, the inner and outer pipes are fixed to one another by means of an intermediate member, the intermediate member being fixedly attached to both the inner and outer pipes. The intermediate member is preferably disposed at one or both ends of the inner and outer pipes. The pipeline preferably comprises a plurality of inner pipes arranged end-to-end and fixed to one another, and a plurality of corresponding outer pipes arranged end-to-end and fixed to one another, each inner pipe being fixedly secured to a respective one of the outer pipes.

Each inner pipe may be formed of a plurality of inner pipe segments arranged end-to-end, and secured to one another. Similarly, each outer pipe may be formed of a plurality of outer pipe segments arranged end-to-end, and secured to one another. Typically the inner pipe would comprise 15 to 50, preferably 15 to 20, inner pipe segments and the outer pipe would comprise

15 to 50, preferably 15 to 20, outer pipe segments. Each inner and outer pipe segment would typically have a length of 12 m.

The inner and outer pipes are typically met^s, such as steel or a corrosion resistant alloy. An insulation material, such as polyurethane, melamine, mineral wool or glass wool, may be disposed between the inner and outer pipes by conventional means. According to another aspect of the invention there is provided a method of making pipeline, comprising arranging an inner pipe at least partially within an outer pipe, contracting the outer pipe and/or expanding the inner pipe, and fixedly securing the inner pipe to the outer pipe while the inner .and outer pipes are still at least partially in their expanded and/or contracted states.

The expansion of the inner pipe may be carried out by the application of a tensile force to the inner pipe, by heating the inner pipe, or both. The contraction of the outer pipe may be carried out by the application of a compressive force to the outer pipe, by cooling the outer pipe, or both. Thus, in the method according to the invention the inner pipe is extended axially prior to being fixed to the outer pipe, and/or the outer pipe is shortened axially prior to being fixed to the inner pipe.

When both the inner pipe is expanded and the outer pipe is contracted it is possible for the expansion/contraction of one pipe to be carried out by the application of a force while the expansion/contraction of the other pipe is carried out by a change in temperature.

In one embodiment, a compressive force is applied to the outer pipe, without any expansion of the inner pipe, then the inner and outer pipes are fixedly secured to one another. In this embodiment, the outer pipe may be provided with at least one formation that can be engaged by a compression means, so that the compression me.ans can compress the outer pipe. The inner pipe is then fixedly secured to the outer pipe, then the force caused by the compression means is released, leaving the inner pipe in tension and the outer pipe in compression.

In another embodiment, a tensile force may be applied to the inner pipe, and a compressive force may be applied to the outer pipe, then the inner and outer pipes are fixedly secured to one another. In this embodiment, the inner and outer pipes may be jacked one against the other so that a tensile load is applied to the inner pipe, and a compressive load is applied to the outer pipe. The inner pipe is then fixedly secured to the outer pipe, then the jacks are released, leaving the inner pipe in tension and the outer pipe in compression.

In another embodiment, the inner pipe is expanded by heating without any contraction of the outer pipe. In this embodiment, a heating means may be inserted into the inner pipe to expand it. The inner pipe is then fixedly secured to the outer pipe. The heating means is then removed, whereupon the inner pipe and outer pipe both contract, leaving the inner pipe in tension and the outer pipe in compression.

Further inner and outer pipes can be secured to the first inner and outer pipes, and the expansion and/or contraction can be repeated. This produces a pipeline comprising a plurality of inner and outer pipes secured end to end, in which each of the inner pipes is in a state of tension and each of the outer pipes is in a state of compression.

According to another aspect of the invention there is provided an apparatus for m-aking a pipeline, comprising: expansion means for expanding an inner pipe and/or contraction means for contracting an outer pipe; and securing means to secure the inner pipe to the outer pipe while the inner .and outer pipes .are still at least partially in their expanded and/or contracted states. The expansion means may comprise means to apply a tensile force to the inner pipe and/or means to heat the inner pipe. The contraction means may comprise means to apply a compressive force to the outer pipe and/or means to cool the outer pipe.

Means is advantageously provided for connecting further inner .and outer pipes to the first inner and outer pipes. In certain applications, for example the construction of bundled pipelines, the distance between the points where outer pipe .and inner pipe .are connected together may be such that the compressed length of outer pipe would tend to buckle unless steps were taken to prevent buckling.

One way of preventing bucking is to arrange for a number of longitudinal tension members to run parallel to the outer pipe, configured so that their centroid is approximately concentric with the outer pipe, with the total tension load in the longitudinal tension members being substantially equal in magnitude to the compression load in the outer pipe. The apparatus is preferably provided with guide means at intervals preventing relative lateral displacement of the outer pipe and longitudinal tension members. In this way the tendency of the compression member to displace away from a condition of straightness is balanced by the tendency of the tension members to return to a condition of straightness, and the pipes remains straight.

The guide means may comprise a plurality of rollers disposed in contact with the outer surface of the outer pipe. The rollers preferably permit axial movement of the outer pipe, but prevent transverse movement of the outer pipe. The rollers .are preferably provided with a surface for engaging the outer pipe, which has substantially the same curvature as the outer surface of the outer pipe. A support frame may be provided for supporting the guide means and the longitudinal tension members.

According to another aspect of the invention there is provided a method of making incrementally prestressed pipeline comprising:

(a) arranging a first inner pipe at least partially within a first outer pipe;

(b) contracting the first outer pipe and/or expanding the first inner pipe;

(c) fixedly securing the first inner pipe to the first outer pipe while the first inner .and first outer pipes are still at least partially in their expanded and/or contracted states;

(d) securing a further inner pipe to the first inner pipe and securing a farther outer pipe to the first outer pipe;

(e) contracting the further outer pipe and/or expanding the further inner pipe;

(f) fixedly securing the further inner pipe to the further outer pipe while the further inner and outer pipes are still at least parti-aHy in their expanded and/or contracted states; and

(g) repeating steps (d) to (f) until the pipeline reaches a desired length.

In accordance with the present invention it is possible to produce long lengths of pipeline, comprising smaller sections of inner and outer pipe, in which each of the smaller sections of inner and outer pipe h.as been individually stressed .axially. The invention makes it possible to construct a pipeline by taking a first inner and outer pipe, prestressing the pipes, connecting the pipes together, then adding further inner and outer pipes, prestressing the pipes, and connecting the pipes together. In this way, long lengths of pipeline can be constructed which are resistant to high temperature fluids flowing through the inner pipe. Reference is now made to the accompanying drawings in which:

Figures la to le show several stages in an embodiment of a method of making a pipeline according to the invention;

Figure 2 is a view on an enlarged scale of part of the pipeline shown in Figures la to le; Figures 3a to 3d show several stages in another embodiment of a method of making a pipeline according to the invention;

Figure 4 is a view on an enlarged scale of part of the pipeline shown in Figures 3 a to 3d; Figure 5 is an alternative embodiment of the part of the pipeline shown in Figure

4;

Figure 6 is an alternative embodiment of the part of the pipeline shown in Figure 4;

Figure 7 is a cross-sectional view of an apparatus for making a pipeline in accordance 5 with the invention;

Figures 8a to 8d show several stages in yet another embodiment of making a pipeline according to the invention; and

Figure 9 is a view on an enlarged scale of part of the pipeline shown in figures 8a to 8d;

Figures la to le show the .assembly sequence of a pipeline in which the inner pipe to outer 10 pipe connection incorporates a cylinder made up from two half shells.

In Figure la, an inner pipe 10, and an outer pipe 12 (which is also known as a sleeve pipe), form part of an already completed and prestressed pipeline 14. An inner pipe 10' and a further outer pipe 12' have been welded on to the completed pipeline 14 but have not yet been prestressed. A further inner pipe 10" is connected by a weld 16 to the inner pipe 10'. A fitting 15 18 is provided on die inner pipe 10", and a fitting 20 is provided on the outer pipe 12'. The outer pipe 12' is adapted to have a compressive force applied thereto by means of formations 22 arranged on the outer surface of the outer pipe 12' at one end thereof. The formations 22 may be grooves, indentations or profiles in the surface of the outer pipe 12' which facilitate transfer of load into the outer pipe. 20 In Figure lb, force application devices 24 and 26 are connected to the outer pipe 12', and are prevented from shding with respect to the pipe 12' by the formations 22 (in fact, the members 26 engage the formations 22 at one end of the outer member 12) . A force is applied to the devices 24 and 26 in the direction of arrows 28 and 30 respectively. This force causes the outer pipe 12' to contract axially. An intermediate cylindrical member comprises two half shells 32 25 of pipe sized to fit in the annulus between the inner and outer pipes 10' and 12'. The half shells 32 are connected to one another and to the fittings 18 and 20 by welds 34. The half shells 32 provide a load carrying connection between the inner .and outer pipes 10" and 12'.

Figure lc shows the situation after the release of the compressive force, and the removal of the devices 24 and 26. The outer pipe 12' expands on release of the compressive force, but 30 being connected by the half shells 32 to the inner pipe 10', the outer pipe 12' is left in compression, while the inner pipe 10' is left in tension. In Figure Id, a further length of outer pipe 12" is added and connected by weld 36 to the already completed pipeline. Although shown as one length of pipe, the outer pipe 12" could be assembled in situ over the inner pipe 10" from a number of plain pipes, with only the last one added incorporating the detail to connect inner and outer pipes. For example, each of the pipes 10, 10', 10", 10'", 12, 12' and 12" typically comprises fifteen lengths of pipe segments, each pipe segment having a length of about 12 m. Thus, the distance between the connections between the inner and outer pipes is about 180 m.

In Figure le, a further length of inner pipe 10'" is added and connected by a weld 38 to the already completed pipeline. Although shown as one length of pipe, the inner pipe 10'" could be assembled in situ from a number of plain pipes, with only the first one added incorporating the detail to connect inner and outer pipes.

Connection of the inner pipe 10'" in Figure le, is the same as the connection of the inner pipe 10" in Figure la. The sequence Figure la to figure le describes one prestressing and assembly cycle. This sequence can be repeated as often as is necessary to achieve the required length in construction of a complete pipeline.

Figure 2 shows an embodiment of a fully assembled joint detail consistent with the assembly sequence described in Figures la to le. The in-line fitting 20 is provided in the outer pipe 12', and terminates in a prepared face for a butt weld 42. A weld 44 is made in preassembly of the outer pipe 12'. The in-line fitting 18 is connected to d e inner pipe 10" by a weld 48 made in preassembly of the inner pipe 10". The fitting 18 terminates in a prepared face for a butt weld 50. The weld 16 is made in final assembly of the pipeline 14. The half shells 32 each comprise a short length of pipe sized to fit in me annulus between the inner and outer pipes 10' and 12'. The welds 34 connect the two half shells to one another .and to fittings 18 and 20. The weld 36 connects the outer pipe 12" to the fitting 20. Figures 3a to 3d show the assembly sequence of a pipeline in which the inner pipe to sleeve pipe connection detail incorporates a slip-over connector.

In Figure 3 a, a length of inner pipe 100 is to be connected to an already completed pipeline 102 by a weld 104.

In Figure 3b, a length of an outer pipe 106 has been moved into position over the inner pipe 100. It is connected to the already completed pipeline 102 by a weld 138. As in the embodiment of Figure 1, the outer pipe 106 is provided with formations 110 which enable a compressive stress to be applied to the outer pipe 106.

In Figure 3c, force application devices 112 and 1 14 are connected to the outer pipe 106, and are prevented from sliding with respect to the pipe 106 by the formations 110. A force is applied to the devices 112 and 114 in the direction of arrows 116 and 118 respectively. This force

5 causes the outer pipe 106 to contract axially. A weld 136 is made while the compressive force is applied. The weld 136 fixedly connects the pipes 100 and 106.

In Figure 3d, the compressive force has been released. The outer pipe 106 expands axially on release of the compressive force. Owing to the weld 136, the outer pipe 106 is left in compression, while inner pipe 100 is left in tension. A further inner pipe 100' is placed ready to

10 be welded to the inner pipe 100. The connection of the inner pipe 100' in Figure 3d is the same as the connection of inner pipe 100 in Figure 3a. The sequence Figure 3a to Figure 3d describes one assembly and prestressing cycle. The cycle can be repeated as each length of pipe is added.

Figure 4 shows .an embodiment of a fully assembled joint detail consistent with the assembly sequence described in Figures 3a to 3d. The pipe 106 is provided with an in-line fitting

15 122 which is connected to a short length of pipe 124, which is sized to fit in the annulus between pipes 106 and 100. The inner pipe 100 is provided with an in-line fitting 126 which is of larger outside diameter th.an the pipe 100 itself. Welds 128, 130 and 132 are made during pre-assembly. Weld 134 fixes the inner pipe 100' to the inner pipe 100. A fillet weld 136 is made during prestressing which connects the inner pipe 100 to the outer pipe 106. A weld 138 connect the

20 outer pipe 106 to a further length of outer pipe 106'.

Figure 5 shows another embodiment of a fully assembled joint detail consistent with the assembly sequence described in Figures 3a to 3d. This joint is generally similar to that shown in Figure 4, except that the short length of pipe 124 is replaced with a thicker pipe 124' which permits a machined preparation 140 to be made, which allows a full penetration weld 136' to

25 replace the weld 136.

Figure 6 shows .another embodiment of a fully assembled joint detail consistent with the assembly sequence described in Figures 3a to 3d. Many of the parts shown in Figure 6 are similar to those shown in Figure 5, and like parts have been designated with like reference numerals. A short length of pipe 124" is sized to fit in the annulus between the pipes 106 and 100. One end

30 of the pipe 124" is swaged out to form a cone such that a suitable welding gap is formed at the end adjacent to an in-line fitting 122' provided on the end of the outer pipe 106. At the other end, the pipe 124" is swaged inwards to form a cone such that a suitable welding gap is formed at the end adjacent to an in-line fitting 126' attached to the inner pipe 100, and which is of larger outside diameter than the pipe 100 itself. Welds 128, 132 and 130' are welds made curing preassembly. The weld 136' is a butt weld made during prestressing which connects the inner pipe 5 100 to the outer pipe 106. Weld 134 is made in final assembly of the inner pipe 100'.

Figure 7 shows an apparatus suitable for making the pipeline shown in Figs. 1 to 6, which prevents buckling of the pipes during assembly. In Figure 7 the inner pipe 10' of Figure 1 is shown within the outer pipe 12'. The outer pipe 12' is in compression. Two longitudinal tension members 200 are provided at diametrically opposed positions near the outer surface the outer 0 pipe 12'. Four rollers 202 are provided in contact with the outer surface of the outer pipe 12 ' , and are equi-spaced about the outer surface of the outer pipe 12'. The rollers 202 prevent transverse movement of the outer pipe 12' relative to the tension members 200, but allow relative longitudinal movement. A support frame 204 supports the rollers 202 relative to the longitudinal tension members 200. A person skilled in the art will appreciate that this arrangement requires 5 fixtures (not shown) to transfer compression loads into the outer pipe 12' .and tension loads into the longitudinal tension members 200.

Figures 8a to 8d show the assembly sequence of a pipeline in which me inner pipe to outer pipe connection incorporates a cylinder made up from two half shells.

In Figure 8a, an inner pipe 310 and an outer pipe 312 form part of an already completed 0 and prestressed pipeline 314. An inner pipe 310' and a further outer pipe 312' have been welded on to the completed pipeline 314 but have not yet been prestressed. A further inner pipe 310" is connected by weld 316 to the inner pipe 310' . A fitting 318 is provided on the inner pipe 310" and a fitting 320 is provided on the outer pipe 312'. The fittings 318 .and 320 are adapted to have forces applied to them by means of formations 322 and 324 on their outer surfaces. The 5 formations 322 and 324 may be grooves, indentations or profiles in the outer surface which facilitate transfer of load into pipes.

In Figure 8b, a force application device 326 is connected to the inner pipe 3 Wand is prevented from sliding with reject to the pipe by the formations 322. A force application device 328 is connected to the outer pipe 312' .and is prevented from sliding with respect to the pipe by 0 formations 324. Forces are apphed to devices 326 and 328 in the direction of the .arrows 330 and 332 respectively. These forces cause outer pipe 312' to contract axially and cause inner pipe 310' to extend axially. An intermediate member comprises two half shells 334 of pipe sized to fit in the annular gap between the inner and outer pipes 310' and 312'. The half shells 334 are connected to one another and to the fittings 318 and 320 by welds 336. The half shells 334 provide a load carrying connection between the inner and outer pipes 310' and 312'. After completion of welds 336, the devices 326 and 328 are removed, leaving outer pipe

312' in compression, while the inner pipe 310' is left in tension.

In Figure 8c, a further length of outer pipe 312" is added and connected by weld 338 to the already completed pipeline. Although shown as one length of pipe, the outer pipe 312" could be .assembled in situ over the inner pipe 310" from a number of plain pipes, with only the last one added incorporating the detail to connect inner .and outer pipes. For example, each of the pipes 310, 310', 310", 310'", 312, 312' and 312" typically comprises fifteen lengths of pipe segments, each pipe segment having a length of about 12m. Thus the distance between the connections between the inner and outer pipes is about 180m.

In Figure 8d, a further length of pipe 310'" is added and connected by a weld 340 to the already complete pipeline. Although shown as one length of pipe, the inner pipe 310'" could be assembled in situ from a number of plain pipes, with only the first one added incorporating the detail to connect inner and outer pipes.

Connection of the inner pipe 310'" in Figure 8d is the same as the connection of the inner pipe 310" in Figure 8a. The sequence 8a to 8d describes one prestressing .and assembly cycle. This sequence can be repeated as often as is necessary to achieve the required length in construction of a complete pipeline.

Figure 9 shows .an embodiment of a fully .assembled joint detail consistent with the assembly sequence described in Figures 8a to 8d. The in-line fitting 320 is provided in the outer pipe 312' and terminates in a prepared face for a butt weld 342. A weld 344 is made in preassembly of the outer pipe 312'. The in-line fitting 318 is connected to the inner pipe 310" by a weld 348, made in preassembly of the inner pipe 310". The fitting 318 terminates in a prepared face for a butt weld 350. The weld 316 is made in final assembly of the pipeline 314. The half shells 334 each comprise a short length of pipe sized to fit in d e annulus between the inner and outer pipes 310' and 312'. The welds 336 connect two half shells to one another and to fittings 318 and 320. The weld 338 connects the outer pipe 312" to the fitting 320.

It will be appreciated tiiat the magnitude of the compressive and tensile forces will depend, inter alia, upon the size of the inner pipes, their material of manufacture, and the conditions to which they will be exposed. In one example, the outer pipe has a 470 mm outer diameter and a wall thickness of 9.5 mm, and the inner pipe has a 324 mm outer diameter and a wall thickness of 20.5 mm. The inner pipes comprises 15 segments of 12 m pipe, which are 5 secured end to end to create a single inner pipe of length 180 m. Similarly, the outer pipes comprises 15 segments of 12 m pipe, which are secured end to end to create a single outer pipe of length 180 m. The pipeline is to be capable of handling fluids at a temperature of 160 ° C and a pressure of 150 bar. The outer sleeve is compressed to 2525 KN to an initial compressive stress of 184 MPa. The compressive force is released to leave a compressive stress in the outer pipe of 10 108 MPa, and to generate a tensile stress in the inner pipe of 76 MPa.

It will be appreciated that the invention described above may be modified.

Claims

1. Pipeline comprising an inner pipe fixedly secured to an outer pipe, the inner pipe being in a state of longitudinal tension and the outer pipe being in a state of longitudinal compression, wherein the longitudinal tension and compression were induced by a pretreatment carried out prior to securing the inner pipe to the outer pipe.

2. Pipeline according to claim 1, wherein at least one of the inner and outer pipes is provided with formations which are adapted to be engaged by cooperating formations on a force application device, whereby the inner pipe can be stressed relative to the outer pipe by the force application device.

3. Pipeline according to claim 1 or 2, wherein the inner and outer pipes are fixed to one another by means of an intermediate member, the intermediate member being fixedly attached to both the inner and outer pipes.

4. Pipeline according to claim 1, 2 or 3, further comprising a plurality of inner pipes arranged end-to-end and fixed to one another, and a plurality of corresponding outer pipes arranged end-to-end and fixed to one another, each inner pipe being fixedly secured to a respective one of me outer pipes.

5. A method of making pipeline, comprising arranging an inner pipe at least partially within Γêæm outer pipe, contracting the outer pipe and/or expanding the inner pipe, and fixedly securing the inner pipe to the outer pipe while the inner and outer pipes are still at least partially in their expanded and/or contracted states.

6. A method according to claim 5, wherein the exp.ansion of the inner pipe is carried out by the application of a tensile force to die inner pipe.

7. A method according to claim 5 or 6, wherein the expansion pipe of the inner pipe is carried out by heating the inner pipe.

8. A method according to claim 5, 6 or 7, wherein the contraction of the outer pipe is carried out by the application of a compressive force to the outer pipe.

9. A method according to any of claims 5 to 8, wherein the contraction of the outer pipe is 5 carried out by cooling the outer pipe.

10. Apparatus for making a pipeline comprising inner and outer pipes, said apparatus comprising: expansion means for expanding the inner pipe and/or contraction means for contracting the outer pipe; and securing means to secure the inner pipe to the outer pipe while

10 the inner and outer pipes are still at least partially in their expanded and/or contracted states.

11. Apparatus according to claim 10, wherein the expansion means comprises means to apply a tensile force to the inner pipe.

15 12. Apparatus according to claim 10 or 11 , wherein the expansion means comprises means to heat the inner pipe.

13. Apparatus according to claim 10, 11, or 12, wherein the contraction means comprises means to apply a compressive force to the outer pipe.

20

14. Apparatus according to any one of claims 10 to 13, wherein the contraction means comprises means to cool the outer pipe.

15. Apparatus according to any one of claims 11 to 14, further comprising means for 25 connecting further inner and outer pipes to die first inner .and outer pipes.

16. Apparatus according to any one of claims 11 to 15, further comprising a plurality of longitudinal tension members arranged to run parallel to the outer pipe and so that their centroid is approximately concentric with the outer pipe, and wherein the total tension load in the

30 longitudinΓêæil tension members is substanti-ally equal in magmtude to the compression load in the outer pipe.

17. Apparatus according to claim 16, comprising guide means arranged at intervals for preventing relative lateral displacement of the outer pipe and longitudinal tension members.

18. Apparatus according to claim 17, wherein me guide means comprises a plurality of rollers disposed in contact with the outer surface of the outer pipe, which permit axial movement of the outer pipe, but prevent transverse movement of the outer pipe.

19. Apparatus according to claim 18, wherein the rollers have a surface for engaging outer pipe which has substantially the same curvature as the outer surface of the outer pipe.

20. A method of making incrementally prestressed pipeline comprising:

(a) arranging a first inner pipe at least partially within a first outer pipe;

(b) contracting me first outer pipe and/or expanding the first inner pipe; (c) fixedly securing die first inner pipe to the first outer pipe while the first inner and first outer pipes are still at least partially in their expanded and/or contracted states; (d) securing a further inner pipe to die first inner pipe and securing a further outer pipe to the first outer pipe; (e) contracting the further outer pipe and/or expanding d e further inner pipe;

(f) fixedly securing the further inner pipe to the further outer pipe while the further inner and outer pipes are still at least partially in their exp.anded and/or contracted states; and

(g) repeating steps (d) to (f) until the pipeline reaches a desired lengtii.