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Title:
ASSEMBLY OF MULTI-CONDUIT PIPELINES
Document Type and Number:
WIPO Patent Application WO/2003/085312
Kind Code:
A1
Abstract:
There are disclosed methods, components and sub-assemblies for use in the construction of multi-conduit pipelines having multiple fluid conduits (160, 170) closely spaced to one another, and shielded by an outer conduit (200) for insulation. Known pipe-in-pipe constructions are not readily adaptable to include more than one conduit within the outer pipe. In the novel construction, a bulkhead sub-assembly (Fig.5(a)) is first provided, comprising a cast bulkhead unit 100, a primary pipe initiation segment and a secondary pipe initiation segment extending from the bulkhead unit alongside and shorter than the primary initiation segment (Fig. 3). The material and dimensions of the initiation segments are such as to permit temporary deflection of one away from the other sufficiently to permit orbital welding of a further secondary pipe segment to the end of the secondary pipe initiation segment. Both primary and secondary conduits (160, 170) and the outer conduit (200) can then be grown section-by- section using conventional welding equipment only.

Inventors:
BEATRIX ETIENNE (FR)
Application Number:
PCT/EP2003/004178
Publication Date:
October 16, 2003
Filing Date:
April 02, 2003
Export Citation:
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Assignee:
STOLT OFFSHORE SA (FR)
BEATRIX ETIENNE (FR)
International Classes:
F16L39/00; (IPC1-7): F16L39/00
Foreign References:
US2696835A1954-12-14
FR2341094A11977-09-09
GB1519722A1978-08-02
US4573714A1986-03-04
Attorney, Agent or Firm:
Fitzpatricks (Glasgow G2 4AD, GB)
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Claims:
CLAIMS
1. A bulkhead assembly for use in assembling a multiconduit pipeline, the assembly comprising: a bulkhead wall with apertures for at least a primary conduit and secondary conduit; at least one primary pipe initiation segment extending from the primary conduit aperture in the bulkhead wall; at least one secondary pipe initiation segment extending from the secondary conduit aperture in the bulkhead wall alongside and shorter than the primary pipe initiation segment, the material and dimensions of the initiation segments being such as to permit temporary deflection of one away from the other sufficiently to permit orbital welding of a further secondary pipe segment to the end of the secondary pipe initiation segment.
2. A bulkhead assembly as claimed in claim 1, wherein said permitted temporary deflection between said segments exceeds 50mm.
3. A bulkhead assembly as claimed in claims 1 or 2, wherein said sufficient deflection is accomplished by elastic deformation only of one or both initiation segments.
4. A bulkhead assembly as claimed in any of claims 1 to 3, wherein the secondary pipe initiation segment exceeds Im in length.
5. A bulkhead assembly as claimed in any preceding claim, wherein the bulkhead wall is of circular form, and is provided with chamfers around its circumference to facilitate subsequent the welding of initiating segments of an outer pipe to encase the primary and secondary pipes to form a pipeinpipe assembly.
6. A bulkhead assembly as claimed in any preceding claim, formed from a bulkhead unit with said primary and secondary initiation segments welded thereto.
7. A bulkhead assembly as claimed in any preceding claim, wherein at least one of said primary and secondary initiation segments are welded to the bulkhead unit by internal welding.
8. A bulkhead assembly as claimed in any of claims 1 to 6, wherein the primary and secondary initiation segments are welded to the bulkhead unit one by internal welding and the other by external welding.
9. A bulkhead assembly as claimed in any preceding claim, wherein the bulkhead unit comprises an integral preinitiation segment extending from a wall portion of the bulkhead unit prior to the location of said external welding.
10. A bulkhead assembly as claimed in any preceding claim, wherein the secondary pipe initiation segment is welded to a preinitiation segment formed integrally with the bulkhead unit, while the primary pipe initiation segment is welded internally, closer to the bulkhead wall.
11. A bulkhead assembly as claimed in any preceding claim, wherein the secondary conduit has a smaller diameter conduit than the primary conduit.
12. A bulkhead assembly as claimed in any preceding claim further comprising additional primary and secondary initiation segments extending in a direction opposite to the firstmentioned initiation segments.
13. A bulkhead assembly as claimed in any preceding claim, wherein clearance between the conduits at the bulkhead is within the range 10 to 30 mm.
14. A bulkhead unit for use in fabrication of a multiconduit pipeline, the bulkhead unit comprising a bulkhead wall with apertures for at least a primary conduit and a secondary conduit, the primary conduit aperture terminating in an internal welding chamfer for subsequent attachment of a primary pipe segment, the secondary conduit aperture being provided with an integral conduit portion extending away from the bulkhead wall and terminating in an external welding chamfer for attachment of a secondary pipe segment.
15. A bulkhead unit as claimed in claim 14 formed in one piece by forging and subsequent machining.
16. A bulkhead unit as claimed in claim 14 or 15, wherein the bulkhead wall has an outer circumference terminating in a welding chamfer for attachment of outer pipe segments to enclose a space around the primary and secondary pipe segments.
17. A bulkhead assembly comprising the bulkhead unit as claimed in any of claims 14, 15 or 16 and said primary and secondary pipe initiation segments welded thereto.
18. A method of fabricating a multiconduit pipeline, comprising providing at least a first bulkhead assembly according to the invention as set forth above and welding further primary and secondary pipe sections to said primary and secondary pipe initiation segments respectively to extend said primary and secondary conduits to a desired length, sufficiently distancing the free ends of said primary and secondary conduits by deflection to permit welding of said further secondary pipe section after welding of said further primary pipe section.
19. A method of fabricating as claimed in claim 18, wherein said further primary and secondary pipe sections are grown to the desired length by adding a succession of individual segments, each welded to a segment previously attached to the respective initiation segment.
20. A method of fabricating as claimed in claims 18 or 19, wherein an assembly of any desired length is grown by repeating the steps of : welding a yet further pipe section to the end of a first pipe section previously added to one of the primary and secondary initiation segments of the bulkhead assembly, the pipe section each time terminating beyond the end of a second pipe section previously added to the other of the primary and secondary initiation segments; distancing the ends of said first and second previously added pipe sections from one another; and welding a yet further pipe section to the end of said second previously added pipe section, the welded pipe section terminating short of the end of the first pipe section just added.
21. A method of fabricating as claimed in any of claim 18 to 20, further comprising the steps of providing at least a second bulkhead assembly having respective primary and secondary pipe segments extending toward the first bulkhead assembly and joining the respective primary and secondary pipe segments of the first and second bulkhead assemblies at their ends to complete a section of multiconduit pipeline between said bulkhead assemblies.
22. A method of fabricating as claimed in any of claims 18 to 21, further comprising welding an outer pipe section to the bulkhead assembly to encase the primary and secondary conduits in a pipeinpipe configuration.
23. A method of fabricating as claimed in any of claims 18 to 22, wherein the primary and secondary conduits are grown to the desired length by adding a succession of individual segments, each welded to a segment previously attached.
24. A method of fabricating as claimed in any of claims 18 to 23, wherein the outer pipe is grown by adding sections, the outer pipe being dimensioned to provide at its free end sufficient clearance to allow the free ends of said secondary and primary conduits to be sufficiently distanced to allow welding of further segments to said free ends.
25. A method of fabricating as claimed in any of claims 18 to 24, wherein a second bulkhead is added by welding a closing outer pipe section between the free end of the outer pipe of the first bulkhead assembly and said second bulkhead unit, thereby completing a closed pipeinpipe assembly.
26. A method of fabricating as claimed in any of claims 18 to 25, wherein a closing outer pipe section is added, comprising a single pipe dimensioned to slide over the bulkhead unit and outer pipe.
27. A method of fabricating as claimed in any of claims 18 to 25, wherein a closing outer pipe section is added, comprising at least two partcylindrical portions welded in place to complete the outer pipe.
28. A method of fabricating as claimed in any of claims 18 to 27, wherein any space remaining within the outer pipe has material added to it to provide thermal insulation.
29. A method of fabricating as claimed in any of claims 18 to 27, wherein any space remaining within the outer pipe has air removed from it to provide thermal insulation.
30. A method of fabricating as claimed in any of claims 18 to 29, wherein one or both of said conduits incorporates curvature to accommodate deflection for said welding operation and also deflection of the assembly in use.
31. A method of fabricating as claimed in any of claims 18 to 30, wherein the secondary conduit traverses the assembly helically.
32. A method of fabricating as claimed in any of claims 18 to 31, wherein said primary and secondary conduits display a helix shape within the carrier pipe.
33. A method of fabricating as claimed in any of claims 18 to 30, wherein the secondary conduit traverses the assembly in a series of shallow curves.
34. A section of multiconduit pipeline assembly fabricated by a method as claimed in any of claims 18 to 33. 35.
35. A multiconduit pipeline assembly incorporating a bulkhead assembly as claimed in any of claims 1 to 13.
36. A multiconduit pipeline assembly incorporating a bulkhead unit as claimed in any of claims 14 to 16.
37. A multiconduit pipeline comprising a plurality of multiconduit pipeline assemblies as claimed in claims 35 or 36.
38. A kit of parts comprising a plurality of bulkhead assemblies as claimed in any of claims 1 to 13.
39. A kit of parts as claimed in claim 38, further comprising a plurality of pipe sections adapted to form the primary and secondary conduits of the multiconduit pipeline.
Description:
ASSEMBLY OF MULTI-CONDUIT PIPELINES This invention relates to multi-conduit or"piggy-back"pipe assemblies, in particular, but not exclusively, to pipe-in-pipe assemblies. The invention further relates to methods and assemblies for use in the construction of multi-conduit pipelines.

The Offshore Industry makes use of multiple fluid conduits, often involving complex arrangements of pipes, arranged side-by-side and/or as pipes within pipes, to provide functions such as production fluid transfer, electrical transmission, gas-lift lines or fibre-optic communication. Due to the depths of waters involved, some pipes require insulation or active heating to keep the fluid they convey within a required temperature range. The use of insulation is clearly more desirable than active heating, due to the costs and engineering obstacles heating involves.

Pipe-in-pipe assemblies are used in particular to provide such insulation, or simple mechanical protection and support. These comprise an inner conduit surrounded by an outer conduit. Where insulation is the object, the annulus formed between inner and outer conduits may be held at a vacuum which minimises heat transfer between the primary pipe and the outer pipe, or it may be filled with foam or other insulating material. Whatever the purpose of adopting pipe-in-pipe construction, perfect water- tightness of the annulus is generally required, for example to maintain the integrity of the insulation or (as with microporous insulation) to boost its efficiency by depressurisation. Furthennore, segmentation of the annulus along the length of the pipeline is often required as well. This may be required by the laying method, or it may be the case simply to mitigate the consequences of flooding any given segment. One technique for terminating the annulus at each segment is firmly swaging the outer pipeline onto the inner pipeline, and then fillet-welding one to the other to ensure a good seal.

Sections of coaxial vacuum-insulated pipe-in-pipe assemblies are currently joined together during pipelay operation, such as J-lay (conventionally every 12 metres), S-lay or in the yard by pre-fabrication of pipe-in-pipe segments of around 250 metres. At

each joint only the inner pipes within the assembly require bonding together, such as by welding. The outer pipe is terminated near each end of the pipe section, and as a consequence does not require interconnection at each joint. An insulation sleeve is then fitted to each completed joint to protect it from its surrounding environment.

This method of joining pipes, however, does not facilitate including any further pipes within the outer pipe. This is partly because the swaging can only practicably be done onto a single inner pipe. Also, however, where the application requires welded joints, there will tend to be insufficient clearance between the pipes to permit welding by conventional orbital welding apparatus. To overcome these problems, spool pieces could be used that would circumvent the swaged sections, but this would lead to an undesirable increase in the pipe assembly cross-section, caused by the protruding spool piece. Screw-threaded connections could be made without requiring great clearance, but many applications require standard welding, for reasons of integrity and/or cost.

GB 2351301A describes a riser incorporating multiple inner conduits, but does not provide any teaching on how the multi-conduit riser is to be assembled.

Considering the foregoing matters, it is an object of the invention to provide a method of, and apparatus for, fabricating sections of multi-conduit pipeline, using conventional connection methods readily available in the offshore construction industry. Particular embodiments aim to provide a technique that will provide a high-integrity joint having multiple fluid conduits closely spaced to one another, and shielded by an outer conduit, all without requiring a substantial increase in assembly external diameter.

According to a first aspect of the present invention, there is provided a bulkhead assembly for use in assembling a multi-conduit pipeline, the assembly comprising : - a bulkhead wall with apertures for at least a primary conduit and secondary conduit; - at least one primary pipe initiation segment extending from the primary conduit aperture in the bulkhead wall; - at least one secondary pipe initiation segment extending from the secondary conduit aperture in the bulkhead wall alongside and shorter than the primary

pipe initiation segment, the material and dimensions of the initiation segments being such as to permit temporary deflection of one away from the other sufficiently to permit orbital welding of a further secondary pipe segment to the end of the secondary pipe initiation segment.

By starting with such an assembly, as described below, a complete multi-conduit pipeline can be assembled in stages with only conventional orbital welding equipment.

The clearance between the conduits at the bulkhead can be made as small as machining requirements will allow, less than 10 to 30 mm for example. At the same time, the deflection of the initiating segment (s) allows access for subsequent welding. Sufficient deflection in this context may be anything from 50mm or 60mm upwards. Current designs of welding equipment tend to require clearance of around 75mm around the pipe in order to operate, assuming an 8-inch diameter pipe of 12mm wall thickness.

Said sufficient deflection is preferably accomplished by elastic deformation only of one or both initiation segments. The secondary pipe initiation segment may be over Im long. For a typical 2-inch auxiliary steel pipe (diameter 50mm), a length of 2m should be sufficient to allow the end of the segment to be deflected the necessary distance.

The bulkhead wall may be of circular form, and may be provided with chamfers around its circumference to facilitate subsequent the welding of initiating segments of an outer pipe to encase the primary and secondary pipes to form a pipe-in-pipe assembly.

The assembly may be formed from a bulkhead unit with said primary and secondary initiation segments welded thereto.

At least one of the primary and secondary initiation segments may be welded to the bulkhead unit by internal welding. This operation, which may be performed in a workshop remote from the location of ultimate pipeline assembly, avoids the problem of access for welding equipment around the circumference of two pipe segments located closely side-by-side. Deflection of the secondary initiation segment thereafter

permits use of external welds at the yard or on a pipe laying vessel, without complex internal welding equipment.

The primary and secondary initiation segments may be welded to the bulkhead unit one by internal welding and the other by external welding.

In a preferred embodiment, the bulkhead unit comprises an integral pre-initiation segment extending from a wall portion of the bulkhead unit prior to the location of said external welding.

In a preferred embodiment, the secondary pipe initiation segment is welded to a pre- initiation segment formed integrally with the bulkhead unit, while the primary pipe initiation segment is welded internally, closer to the bulkhead wall.

Where the primary and secondary conduits are not of equal diameter, the secondary conduit may be the smaller diameter conduit.

The assembly may comprise further primary and secondary initiation segments extending in a direction opposite to the first-mentioned initiation segments. Such an assembly can be used in a middle section of the pipeline, which extends either side of the bulkhead assembly. A bulkhead assembly for terminating the pipeline need not be provided with identical initiation segments at both sides.

The invention further provides a bulkhead unit for use in fabrication of a multi-conduit pipeline, the bulkhead unit comprising a bulkhead wall with apertures for at least a primary conduit and a secondary conduit, the primary conduit aperture terminating in an internal welding chamfer for subsequent attachment of a primary pipe segment, the secondary conduit aperture being provided with an integral conduit portion extending away from the bulkhead wall and terminating in an external welding chamfer for attachment of a secondary pipe segment.

The bulkhead unit may be formed in one piece by forging and subsequent machining.

The bulkhead wall may have an outer circumference terminating in a welding chamfer for attachment of outer pipe segments to enclose a space around the primary and secondary pipe segments.

The invention yet further provides a bulkhead assembly comprising the bulkhead unit and said primary and secondary pipe initiation segments welded thereto.

The invention yet further provides a method of fabricating a multi-conduit pipeline, comprising providing at least a first bulkhead assembly according to the invention as set forth above and welding further primary and secondary pipe sections to said primary and secondary pipe initiation segments respectively to extend said primary and secondary conduits to a desired length, sufficiently distancing the free ends of said primary and secondary conduits by deflection to permit welding of said further secondary pipe section after welding of said further primary pipe section.

Said further primary and secondary pipe sections may be grown to the desired length by adding a succession of individual segments, each welded to a segment previously attached to the respective initiation segment.

In one such embodiment, an assembly of any desired length may be grown by repeating the steps of : - welding a yet further pipe section to the end of a first pipe section previously added to one of the primary and secondary initiation segments of the bulkhead assembly, the pipe section each time terminating beyond the end of a second pipe section previously added to the other of the primary and secondary initiation segments; - distancing the ends of said first and second previously added pipe sections from one another; and - welding a yet further pipe section to the end of said second previously added pipe section, the welded pipe section terminating short of the end of the first pipe section just added.

The method may further comprise providing at least a second bulkhead assembly having respective primary and secondary pipe segments extending toward the first bulkhead assembly and joining the respective primary and secondary pipe segments of the first and second bulkhead assemblies at their ends to complete a section of multi- conduit pipeline between said bulkhead assemblies.

The method may further comprise welding an outer pipe section to the bulkhead assembly to encase the primary and secondary conduits in a pipe-in-pipe configuration.

In embodiments where the primary and secondary conduits are grown to the desired length by adding a succession of individual segments, the outer pipe may also be grown by adding sections, the outer conduit being dimensioned to provide at its free end sufficient clearance to allow the free ends of said secondary and primary conduits to be sufficiently distanced to allow welding of further segments to said free ends.

In embodiments where a second bulkhead is added the method may further comprise welding a closing outer pipe section between the free end of the outer conduit of the first bulkhead assembly and said second bulkhead unit, thereby completing a closed pipe-in-pipe assembly.

The closing outer pipe section may comprise a single pipe dimensioned to slide over the bulkhead unit and outer conduit. Alternatively, said closing outer pipe section may comprise at least two part-cylindrical portions welded in place to complete the outer conduit. The latter technique allows the multi-conduit pipeline to maintain substantially the same external diameter throughout. The integrity of the outer pipe is not generally so critical as that of the inner conduits, which are typically seamless.

Fabricating pipe sections in this way allows fabrication of sections of pipe with multiple primary pipes, fully enclosed by an outer pipe. The space remaining within the outer pipe can have material added and/or air removed from it, to provide thermal insulation.

One or both of said conduits may incorporate curvature to accommodate deflection for said welding operation and also deflection of the assembly in use. The secondary conduit may traverse the assembly helically, or in a series of shallow curves. By providing some flexibility in length of the secondary pipe, the effects of differing expansion coefficients are overcome, and some flexibility is provided to allow manipulation of the secondary pipe with respect to the primary pipe, and to absorb any systematic errors during welding, which would otherwise result in a fixed curvature of the assembly. In a helical arrangement, both pipes might display a helix shape within the carrier pipe.

The method may be performed on land or, in an alternative embodiment, the method may be performed on a surface vessel at the time of laying the pipe beneath the sea surface, for example in a J-Lay operation.

The invention yet further provides a section of multi-conduit pipeline assembly fabricated by a method according to the invention as set forth above.

The invention yet further provides a multi-conduit pipeline assembly incorporating a bulkhead assembly according to the invention as set forth above.

The invention yet further provides a multi-conduit pipeline assembly incorporating a bulkhead unit according to the invention as set forth above.

The invention yet further provides a multi-conduit pipeline comprising a plurality of multi-conduit pipeline assemblies according to the invention as set forth above.

The invention yet further provides a kit of parts comprising a plurality of bulkhead assemblies according to the invention as set forth above.

The kit of parts may further comprise a plurality of pipe sections adapted to form the primary and secondary conduits of the multi-conduit pipeline.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 is a cross-section of a known field joint for pipe-in-pipe assemblies, with insulating jacket, over which has been superimposed an additional pipe, to demonstrate how difficult it would be to provide annular welding of the joint when two pipes are adjacently located; Figure 2 is a cross-section diagram of a bulkhead unit according to an embodiment of the present invention, illustrating an initial step in the fabrication of a bulkhead assembly; Figure 3 shows a further stage in the fabrication of the bulkhead assembly; Figure 4 illustrates a further stage in the fabrication of the bulkhead assembly according to the invention; Figures 5 (a) and 5 (b) show the complete bulkhead assembly in a mid-line version and a terminating version, respectively.

Figure 6 shows further stages in the assembly of a multi-conduit pipe-in-pipe assembly according to an embodiment of the invention; Figure 7 shows a further stage in the assembly of the multi-conduit pipe-in-pipe assembly; Figure 8 shows a closing stage in the fabrication of the multi-conduit pipe-in-pipe assembly in a first embodiment of the invention;

Figure 9 shows a closing stage in the fabrication of the multi-conduit pipe-in-pipe assembly in a first embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS As technology advances an increasing quantity of conduits are required between the seabed and surface to convey additional services, such as gas-lift lines in the riser section of a production line, or conductor steel tubes for fibre optics (temperature monitoring) hydraulic power and so forth. However, it is generally required for a number of reasons to locate auxiliary pipes within multi-conduit pipelines at as small a distance as possible from the primary pipe. For example, a gas lift line should be close to the corresponding flowline to benefit from the heating effect of the latter. Tubing for fibre optic temperature sensors obviously must be in close contact with the flowline whose temperature is to be sensed. As a consequence, conventional (orbital) welding of joints between sections cannot be performed to the whole annulus of adjacent pipes because each pipe requires a clearance which, if taken into account within the design, would be unacceptably detrimental to the diameter of the outer pipe.

To demonstrate the problem, Figure 1 is a cross-section diagram of a known insulated pipeline 10, comprising primary pipeline 12'and outer pipeline 14, and its field joint 20 and insulating jacket 30, over which has been superimposed an additional pipe 40, showing how difficult it would be to perform annular welding of the pipes 12,14 at the field joint 20, when the two pipes are located so closely. Without an additional pipe present it is a simple task to join sections of pipe together, involving only one annular weld per section. The void between primary 12 and outer 14 pipes contains the thermal insulation, such as low-pressure air, gas or micro-porous insulation.

There is now described a method of fabricating multi-conduit pipelines that overcomes the inability to weld all of the pipes at the joint between sections. It is particularly

suited towards pipe-in-pipe systems that will be installed in strings, either inserted in a riser tower, or reeled or towed to site.

The method involves fabricating initiating assemblies in a workshop, then some time later completing a section of multi-conduit pipeline by taking these bulkhead assemblies, adding sufficient primary and auxiliary pipe sections to provide two sections of the desired length, adding outer sections of pipe, joining the sections together then closing the outer annulus to complete the section. Further detail is provided in the following paragraphs.

Ignoring special assemblies that may be required for ends of the pipeline, branding points, pig trap and so forth all of the assemblies have the same form. Fabrication of the bulkhead assembly in the present embodiment uses the following components: - a bulkhead (forged or machined), two auxiliary pipe initiation segments for attachment to the bulkhead, - two primary pipe initiation segments for attachment to the bulkhead.

The weld points of the bulkhead are chamfered to support joint-fill welding. The weld points of the auxiliary pipe are tailored for external welding, whilst the primary pipe weld points are tailored for internal welding at the bulkhead, and external welding elsewhere. Since fabrication of the assemblies can be done in a workshop remote from the site where the pipeline is assembled, the need for special fabrication steps, internal welding equipment and so forth is not a great burden.

The lengths of the initiation segments are chosen such that, taking advantage of steel elasticity it is possible to separate the free ends of the auxiliary and primary pipes by a distance to allow welding of the next joint with conventional (orbital) welding, to assemble the complete pipeline in a yard close to the site, or on a pipe laying vessel.

A first embodiment is outlined here below, featuring one auxiliary pipe of diameter 5 cm in the annulus of a multi-conduit pipeline which has a primary conduit diameter of 20 cm and an outer diameter of 39.5 cm. In this example, strings are fabricated in 400-

metre sections separated by bulkheads. For pipe-in-pipe insulation, an outer pipe is added, to form a closed annular space between each pair of bulkheads.

Figure 2 is a cross-sectional diagram of a bulkhead unit 100 comprising a primary pipe portion 110, an adjacent auxiliary pipe portion 120 which is longer in both longitudinal directions than the primary pipe portion, and a bulkhead wall 130, through which the pipe portions pass. The auxiliary pipe portion is longer than the primary pipe portion by sufficient length, such as 5 to 10 cm on each side of the bulkhead, to allow orbital welding equipment access to make an auxiliary pipe joint. The bulkhead wall 130 also has a chamfered lip 140 to which the outer pipe can be welded. The primary and auxiliary pipes pass through the bulkhead off centre-axis to minimise assembly diameter.

The following steps, illustrated by Figures 3,4 and 5, describe workshop assembly of a bulkhead assembly:- 1) using external welding, weld onto one side of a bulkhead unit 100 an initiation segment 160 of the auxiliary pipe (typical length of 2 metres for a 6 cm external diameter ; 2) using internal welding, weld onto the bulkhead 100 an initiation segment 170 of the primary pipe (typical length of 5 cm longer than the initiation segment of the auxiliary pipe).

For a mid-line connection, these steps are repeated on the other side of the bulkhead unit loo. This completes the mid-line bulkhead assembly, as shown in Figure 5 (a). A sufficient number of these are fabricated in the workshop and shipped to the yard or lay site for final assembly.

As shown in Figure 5 (b), where a start or end assembly section is being fabricated, the auxiliary pipe initiation segment on the open side 172 of the bulkhead 100 is replaced by a bend 174, to provide separation of the primary and auxiliary conduits, thus allowing welding of the joints with conventional (orbital) welding. The primary pipe initiation segment 176 on the open side needs only to be sufficiently long to provide the

desired separation between conduit ends. As no outer pipe is being connected to the open side there is no need 178 for the open side of the bulkhead to be chamfered, or to have a lip.

Figures 6 to 8 illustrate the further steps in fabricating the multi-conduit pipe-in-pipe pipelines, performed in a location such as a yard, or laying ship: 3) using external welding, weld a further section 180 of primary pipe; 4) pull the free end 190 of the auxiliary pipe 160 away from the primary pipe by sufficient distance, such as 80 mm, to allow access by orbital external welding equipment, then weld the next joint of the auxiliary pipe; 5) using external welding, weld the first joint (standard length) of the outer pipe 200 to the bulkhead.

All three conduits (comprising primary pipe, auxiliary pipe and outer pipe) must be dimensioned so that it remains possible to perform annular welding between additional primary pipe sections, and to separate the free ends of the auxiliary pipe and the primary pipe to perform welding between additional auxiliary pipe sections according to steps 3 onwards.

Fabrication can then proceed by repeating steps 3-5 joint by joint: primary pipe, auxiliary pipe, outer pipe.

Note that only external welding equipment has been used in stages 3 onwards, allowing the use of less specialised welding equipment and allowing these stages to be performed in locations other than a specialised workshop, such as a yard or ship.

Conventional pipe sections with simple external chamfers can be used.

Once two half-sections 300, 310 have been fabricated (2 x 200 m for a total bulkhead to bulkhead length of 400 m), their"open"ends can be connected. Firstly the primary conduits 170 of both half-sections are welded together, then a spool piece 320 is welded between both half-sections of the auxiliary conduit 190, once again by pulling the free ends of the auxiliary conduit away from the primary conduit. The geometry of

the auxiliary conduit in this area is carefully controlled during this final connection.

Some flexibility in auxiliary conduit length is required to overcome the effects of differing expansion coefficients between conduit, to overcome building-in static errors during welding which would result in a fixed curvature of the assembly, and to provide some flexibility to allow manipulation of the auxiliary conduit with respect to the primary conduit for welding. This flexibility is achieved by having the auxiliary conduit traverse the assembly helically (space permitting), or building into the auxiliary conduit a series of shallow curves, both of which will provide the required movement in the longitudinal direction of the assembly. The features providing this flexibility are not illustrated in the accompanying figures.

Closing the annulus is achieved by welding half shells 330 between the ends of the outer pipes, thus providing the outer conduit its integrity.

Figure 9 illustrates completing a section of pipe-in-pipe using a different technique.

Rather than assembling two half-sections then welding them together in the middle, the assembly is fabricated starting at a bulkhead and adding sections of pipe until almost the desired length of a whole section is achieved, then attaching a closing bulkhead to complete the assembly.

The steps for this alternative embodiment are provided below: 1) repeat the steps of the first embodiment for producing a bulkhead assembly in a workshop.

2) using external welding, weld a further section 180 of primary pipe; 3) pull the free end 190 of the initiation segment 160 of the auxiliary pipe away from the primary pipe by sufficient distance to allow access by orbital welding equipment, such as 80 mm, then welding the next joint of the auxiliary pipe; 4) weld the first section of the outer pipe 200 to the bulkhead ; 5) weld further sections of pipe, the length of all sections dimensioned so that it remains possible to separate the free ends of the auxiliary pipe and the

primary pipe to perform the welding of the next joints according to steps 2 onwards; 6) to fit a closing bulkhead 400, separate the free ends of the auxiliary 190 and primary 170 pipes and weld the closing bulkhead 400 to the auxiliary pipe 190; 7) ensure the primary pipe joint is aligned, then through the opening of the primary conduit, weld the bulkhead to the primary pipe; 8) close the annulus by welding outer pipe half shells 330 between the free end of the outer pipe and the closing bulkhead.

A further alternative process, not illustrated in the drawings, involves a tubular closing member of a diameter sufficient for it to slide over the outer pipe, which is slid into place over the gap where the inner pipe sections have finally been joined. The ends of this closing section can be swaged into alignment with the outer pipe, and welded to complete the closure of the annulus. This is similar to the closing process described with reference to Figure 1, for the conventional single pipe-in-pipe construction.

However, the inner diameter of the closing tube will be only minimally larger than the outer diameter of the outer pipe.

For either embodiment, depressurisation of the annulus can be performed if required, concluding the fabrication of a complete reduced-pressure multi-conduit pipeline. The skilled person will appreciate that other forms of insulation can be used, such as microporous or inert-gas insulation.

The skilled person will further appreciate that the exact form of components and methods used can vary from the ones described herein without departing from the spirit and scope of invention.