WO2009109950A2

WO2009109950A2 - Insulated pipelines and methods of construction and installation thereof

Info

Publication number: WO2009109950A2
Application number: PCT/IB2009/052728
Authority: WO
Inventors: Brieuc Du Halgouet; Daniel Bertaso
Original assignee: Acergy France Sa
Priority date: 2008-02-28
Filing date: 2009-03-02
Publication date: 2009-09-11
Also published as: WO2009109950A3; GB0803664D0

Abstract

A double walled pipe assembly comprises an inner component pipe (304) and an outer component pipe(306) and an annular insulating space (314) between them. The ends of the pipes are joined by circumferential welds to close the annular space. At least one of said component pipes is provided at at least one of its ends with a thickened portion, the annular space at said portion being reduced substantially to zero. The outer pipe is welded in said portion to the inner pipe by a circumferential lap weld (508). This construction accommodates a relatively wide tolerance in length and parallelism between the inner and outer pipe ends prior to welding. The thickened portion may be provided by deformation of the material of the pipe section, or on a separately fabricated end-piece butt welded to a plain- ended section of the inner or outer pipe section.

Description

INSULATED PIPELINES AND METHODS OF CONSTRUCTION AND

INSTALLATION THEREOF

The invention relates to insulated pipeline structures, particularly offshore pipelines of double-walled construction, and to methods of fabrication and installation of such pipelines.

Pipe-in-pipe (PiP) structures have been proposed and used in recent years to provide a high level of thermal insulation to pipelines for the transportation of hydrocarbons along the seabed and from the seabed to the surface. These pipelines are double-walled, with both walls typically of steel. They are made of two concentric pipes, and can therefore incorporate dry insulation in the annulus. This makes it possible to reach low conductivity values. The overall heat transfer coefficient (often referred to as the U-value) is usually below 2 VWm². K and can reach values of 0.5 or even lower.

Like normal single-walled pipelines (putting aside reeled pipelines), these double wall pipelines are constructed and installed from short sections, which are then welded together on a pipelaying vessel using variations of either J-lay or S-lay techniques. The area around each offshore weld is called the "field joint". According to field joint design, two kinds of pipe-in-pipe may be distinguished:

• The first kind has a welded link between two consecutive inner pipes and another welded link between two consecutive outer pipes. Therefore the field joint and the length between field joints have a similar profile. In the installation process, the outer pipe is slid over the inner pipe after the inner pipe connection has been performed. This technique may be referred to as the 'sliding pipe-in- pipe'. This technique requires two offshore welds, which is detrimental to laying rate in J-lay.

• Another type of double walled pipeline, and the one with which this document is primarily concerned, is made by pre-assembling double walled pipe sections so that they require only one offshore weld. Each section comprises an inner pipe that is slightly longer at each end, and the outer pipe ends are welded to the inner pipe to close the annulus. Special measures are required in making each field joint to provide a joint with both mechanical rigidity and thermal insulation properties compatible with those of the pipe between joints. However, the need for only one offshore weld per field joint makes it faster to install than the sliding pipe-in-pipe.

Aside from being faster to install (lay), the latter design is also interesting for the ability to draw a vacuum in the annulus during the prefabrication step. This enhances insulation properties of commonly used porous materials, like aerogels. For this reason, and because of the speed advantage, the latter method is preferred for present purposes.

The conventional approach to joining this double walled pipe is to provide a steel sleeve that slides over the joint when the inner pipes have been joined, and is filled with some solid material such as polyurethane resin. In order to attain sufficient insulation properties, however, these sleeves have frequently been themselves made in the form of double walled insulated pipes. This contributes greatly to the cost and weight of the overall pipeline. Any additional weight implies not only cost increases, but severe limitations on the capacity of existing pipe-laying vessels to handle such a product in very deep waters where new production takes place today. The weight of these sleeves is potentially 10% of the entire weight of the pipeline.

Patent US 6446321 (Marchal/ITP) describes in detail the manufacturing of such a pipe-in-pipe section, using a special machine for swaging (deforming by radial compression) the outer pipe ends into a part-conical shape approaching the inner pipe, to which they are then welded. The patent proposes that, to facilitate the welding, the end of the outer pipe be deformed into a part-conical form, and that a gap be engineered between the conical end of the outer pipe and the outer surface of the inner pipe. This gap arises partly because the swaged material springs back from its point of maximum deflection, and partly because an incompressible, ring- shaped shim is deliberately introduced around the outer pipe to limit its approach during swaging, the shim being removed prior to welding. The gap in practice is designed to be at least 2-3mm.

The inventors have determined that the large gap between the pipes in fact limits the manufacturer's ability to perform a good, uniform fillet weld. The quality of this weld in a pipe-in-pipe system is most critical, however, because of the extreme temperature differential, and temperature differential cycling, between the inner and outer pipes, and hence very high stresses caused by differential thermal expansion. For flowlines (laid on the seabed), lateral buckling of the line due to its expansion may also emphasize the level of stress. This stress translates also to fatigue as the pipeline is cycled in and out of operation. In case of hanging vertical pipelines (risers), the motion of the line induces a high level of cycles, and fatigue issues are one major obstacle to the qualification of this type of PiP for application as risers. This stress and associated weld critical ity is exacerbated by the angled approach of the outer pipe wall to the inner pipe.

Even if a good weld is performed, the conical approach of the outer pipe wall, with distortions (wrinkling) of the wall as a result of the swaging operation, makes it difficult or impossible to apply best practice non-destructive testing technique, which is automated ultrasonic testing (AUT), to the weld to verify its integrity and uniformity all around the circumference of the pipe. The AUT probe may be placed on the inner pipe internal surface to inspect the side of the weld towards the inner pipe, this is a compromise and not a proper way to inspect the other side of the weld.

In another construction known from US 4560188 (Snam), machined joining pieces with a Y or double-Y cross-section (in the radial direction) are used, these closing the annulus by being butt welded to the ends of the inner and outer pipes. These joining pieces form the ends of the prefabricated pipe-in-pipe sections. A drawback with such constructions is that the pipe sections must match very closely in length and parallelism of faces for the four butt welds to be made easily and with high integrity. Another drawback is that one of the welds to the inner pipe has to be performed with access from the inside of the pipe, which is a challenge given the reduced size of typical production flowlines.

To achieve access from the outside, a variant has been developed and disclosed by L Delebecque et al in "High Performance Pipe-in-pipe Solutions for Deepwater Field Developments", presented at DOT 2007, Stavanger, Norway, November 2007. After welding a Y piece on one side, this technique involves simultaneous high tensioning of the inner pipe and high compression of the outer one. The level of forces is especially high when considering shorter prefabricated PiP sections (for example, a fixed axial length of the opening requires double strains and stresses with double joints instead of quadruple joints). Therefore this technique has limitations because, while it solves partially the problems of access and tolerances, it raises new manufacturing challenges to ensure safety and pipe integrity.

The invention aims generally to enable the manufacture and installation of insulated pipelines having a double walled structure. The inventors have identified a desire for pipe-in-pipe structures and methods of manufacturing the same in which a better weld quality can be assured between the outer and inner pipes, without the onerous dimensional specifications and challenging access management of the known butt welded technique.

The invention in a first aspect provides a double walled pipe assembly comprising an inner component pipe and an outer component pipe having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe being located coaxially within the outer pipe to define an annular insulating space between them, the ends of the inner and outer pipes being joined by circumferential welds to close the annular space, wherein at least one of said component pipes is provided at at least one of its ends with a thickened portion, the annular space at said portion being reduced substantially to zero, and wherein the outer pipe is welded in said portion to the inner pipe by a circumferential lap weld.

By this construction, a relatively wide tolerance in length and parallelism can be accommodated between the inner and outer pipe ends prior to welding, compared with that required by butt welding, for example. While a lap weld would normally be regarded as of lower integrity than a butt weld, the lap weld can be made stronger on the thickened portion than on the normal pipe, while also being amenable to testing.

The thickened portion may be provided by deformation of the material of the pipe section, such as is provided by a hot-formed upsetting operation, optionally followed by machining.

Alternatively, the thickened portion may be provided on an end-piece butt welded to a plain-ended section of the inner or outer pipe section. The designer thus has a choice whether to modify the end of a length of pipe or fabricate separately a short end-piece machined from a forged cylinder of metal or a short length of thicker pipe.

The end-piece may be provided with at least one exterior projection to serve for supporting the assembly during J-lay operations. The end-piece in a preferred embodiment includes two circumferential shoulder projections, spaced in an axial direction of the assembly. Although the provision of such end-pieces implies additional components formed by machining or the like, the integration of one of those pieces with J-lay collar formation means that the increase in cost and delay is minimised.

The end-piece may be provided on the inner pipe section, or alternatively on the outer pipe section.

The internal diameter of the inner pipe section will not be increased significantly, preferably not at all, at the thickened end(s). This thickening is therefore to be distinguished from widening the pipe by flaring or hydroforming, for example, which would effectively be an inverse form of swaging with attendant drawbacks as discussed above.

Thickened portions may be provided at both ends of the inner pipe and extend to the very ends of the assembly with matching thickness, whereby the assembly and similar assemblies may be joined by welding end to end. By providing thickened ends for welding such assemblies together, the stiffness in the region of the field joint can be made comparable to that of the double walled assemble as a whole, reducing or eliminating the need for a heavy reinforcing sleeve.

The assembly may be provided at its ends with complementary threaded portions, whereby a number of such assemblies can be screwed together to form an insulated pipeline. Thickened end portions may be machined to provide threaded portions, either before or after the double -walled pipe assembly is made.

In a first group of embodiments, the inner pipe section has a constant inner diameter and is provided at both ends with thickened portions having increased outer diameter, to which the outer pipe ends are welded. The outer pipe may be provided with thickened portions having reduced inner diameter at one or both ends.

In some such embodiments the thickened end portions of the inner pipe are formed by end-pieces welded onto a plain-ended pipe section. One of said end-pieces may be provided with projections for supporting the assembly during J-lay operations.

In a second group of embodiments, the inner pipe section has a constant inner diameter and is provided at only one end with a thickened portion having increased outer diameter, to which the corresponding outer pipe end is welded, while the outer pipe is provided with a thickened portion having reduced inner diameter at the opposite end of the assembly. In such an embodiment, the thickened portion of the inner pipe will typically be set back from the end of the pipe, whereby the thicknesses of the ends match for joining assemblies together directly by butt welding. The thickened portion may be provided in the form of an end-piece welded to a plain pipe section.

The assembly may include insulating material partially filling said annular space. The space may be maintained at lower than atmospheric pressure, or substituted with a low conductivity gas.

Each of the inner and outer pipes may be formed from two or more sections of pipe welded end-to-end. For example, many J-lay systems work most efficiently with a stalk length of approximately 24m, known as a "double joint", while the original pipe material is supplied in standard 12m (40 foot) lengths.

The invention in the first aspect further provides a pipeline installation comprising a plurality of double walled pipe sections according to the invention as set forth above, joined end to end. The section ends may be joined by welding, as one example. They may alternatively be joined by mechanical connection, for example by screwing, provided they have been previously threaded.

The pipe sections where joined together may be surrounded by an insulating coating. The insulating coating may be formed at least partially by a prefabricated insulating sleeve.

The pipe sections where joined together may be surrounded by a reinforcing sleeve. Space within said sleeve may be filled with an incompressible solidifying material. The reinforcing sleeve may be provided on its outside with an insulating coating.

The invention in the first aspect further provides a method of manufacturing the double walled pipe sections as set forth above, and a method of installing a pipeline comprising a plurality of said sections. Examples of such methods are described below and in the appended claims.

The invention in the first aspect provides a set of parts for use in installation of an insulated pipeline, the parts comprising: a plurality of double walled pipe sections of the type set forth above, and a plurality of pre-fabhcated insulated sleeves adapted to slide over said insulated pipe sections so as to cover joints made between them.

In a preferred embodiment, each of said prefabricated insulated sleeves comprises a cylindrical substrate and outer layers of insulation carried thereon.

The substrates of said sleeves may be of steel. Polymer or composite materials are a lighter alternative, however. In embodiments where the inner pipe is thickened at both ends and the thickened portions are joined directly to one another, the sleeves need not perform any reinforcing function and can be made of light weight and low cost polymer material.

The insulating layers may be organic based material optionally glass syntactic, such as polyurethane or polypropylene . In a preferred embodiment, the insulating layer is glass syntactic polyurethane (GSPU). A multi-layer polypropylene structure such as 5LPP is an alternative.

The pipeline may form part of a riser structure for transferring fluids from the seabed to the sea surface. Insulated pipelines are also applicable in overland applications, for example in transmitting LNG or LPG at cryogenic temperatures. 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 schematic drawing of a process for laying pipeline using the J-lay system, suitable for laying insulated pipe in accordance with the present invention;

Figure 2 is a more detailed illustration of one specific J-lay apparatus, available for example on the vessel Acergy Polaris, used in one embodiment of the present invention;

Figure 3 is a longitudinal cross-section of one section of double-walled pipe, fabricated according to a known swaging process and including an optional J-lay collar;

Figure 4 is a longitudinal cross-section (a) and radial section detail (b) of a known form of field joint, formed when joining two of the pipe sections shown in Figure 3;

Figure 5 shows a longitudinal cross-section (a) and longitudinal section end detail (b) of a double-walled pipe assembly according to a first embodiment of the invention;

Figure 6 is a longitudinal section end detail of a variant of the first embodiment;

Figure 7 shows a longitudinal cross-section (a) and two radial section details (b) and (c) of a completed field joint formed between two assemblies of the type shown in Figure 5;

Figure 8 is a longitudinal cross-section of a double-walled pipe assembly according to a second embodiment of the invention; Figure 9 is a longitudinal cross-section of a double-walled pipe assembly according to a third embodiment of the invention;

Figure 10 shows longitudinal section details at a first end (a) and a second end (b) of a double-walled pipe assembly according to a fourth embodiment of the invention

Figure 11 is a longitudinal cross-section detail of a completed field joint formed between two assemblies of the type shown in Figure 10;

Figure 12 shows longitudinal section detail of a double-walled pipe assembly according to a fifth embodiment of the invention; and

Figure 13 shows longitudinal section detail of a double-walled pipe assembly according to a sixth embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Background

Figure 1 shows in schematic form the basic components of a J-lay system, suitable for fabricating and installing insulated pipelines of pipe-in-pipe type according to the present invention. The system is mounted on a sea-going vessel 100, which might be a large semi-submersible for the largest systems. The suspended pipeline 102 is held by a fixed clamp arrangement in a working table 104 above the point where it descends below the sea surface 106. A new section of pipe 108, for example a double- or quad-joint (24m or 48m typical length) is supported in a vertical tower structure 110, to be welded to the top of the suspended pipeline just above the table 104. Each new section is supported at its upper end by a travelling block 112. Once the welding is complete, the hold-off clamp at the table 104 is opened and the travelling block 112 takes the weight of the entire pipeline 102, lowering it until the top of the added section is gripped again at the level of working table 104. A further new pipe section 114 is then elevated into the tower and this process repeated to add any number of sections to the pipe.

As the entire arrangement remains vertical (or at whatever angle is necessary for the tension in the pipe to be aligned with the departure axis), it is a relatively simple matter to add Tees, terminations and so forth. On the other hand, the overall lay rate achievable is heavily dependent on the amount of time that paying out has to be interrupted for the complete duration of making and testing each joint. Where the pipeline is an insulated pipeline formed of pipe-in-pipe sections, each joining operation includes not only welding the inner pipe, but also fitting the sleeve for insulating the joint.

Figure 2 illustrates in more detail an actual J-Lay system, of the type present on the applicant's vessel, Acergy Polaris. Reference signs 200-214 indicate like components to those labelled 100-114 in Figure 1. This example is adapted for the fabrication of pipe from double-joints, rather than quad-joints. Below the working table 204, a guide or stinger 216 extends. Below that, a second workstation 218 is provided, which can be deployed, above the waterline, and 24m below the level of the first workstation represented by working table 204. This permits simultaneous execution of tasks, to increase the overall lay rate. In particular, while one joint is being welded at the upper work station (table 204), testing and coating of the previous joint can be performed at the lower workstation. In order to provide the necessary height for this arrangement, the entire apparatus is mounted on a framework 220, above deck level.

The invention is not limited to J-lay applications, however, and the examples to be described include variants with and without collars, for example, which can be selected and/or adapted to a range of J-Lay and S-Lay systems, for example.

Figure 3 shows a double-walled pipe section of the type commonly used in constructing insulated pipelines of the pipe-in-pipe type. The pipe is shown in a horizontal orientation, as it will be fabricated, stored and transported to the laying location. The length of the pipe section between ends 300 and 302 will typically be 24 m (double-joint) or 48 m (quad-joint) for J-Lay, though single joint and other lengths are possible. Only the end portions of the pipe section are shown in the diagram. Along the majority of its length, the double-walled construction is provided by inner pipe 304 and outer pipe 306, both of steel. For J-lay operations, unless a friction clamp is used, to pay out the pipeline, the inner pipe 304 is extended by a shoulder piece 308, formed with shoulders 310 and 312 suitable to be engaged respectively by the travelling table and working table of the J-lay apparatus. Thus, when being assembled into a pipeline for laying either J-lay systems illustrated in Figures 1 and 2, each pipe section will first be suspended from the travelling table 112/212 using the shoulders 310, and with end 302 uppermost. The lower end 300 is then butt-welded to the upper end of the suspended pipeline 102/202, hanging from the first workstation 104/204. Once that joint is completed, the travelling table moves down, and releases the pipe section, to be held on the fixed working table 104/204 on the second shoulder 312. This process can be repeated as many times as necessary to fabricate and install a required length of pipeline. In the double- walled section, an annular space 314 between the outer surface of the inner pipe 304 and the inner surface of the outer pipe 306 is filled partly by insulation, generally of loose micro-porous material within a plastic bag, to form a blanket-like wrapping. Further space, filled with air or other gas at atmospheric or reduced pressure lies between the microporous blanket and the inside of the outer pipe 306. The ends of the annular space 314 are closed by swaging end portions 316 and 318 of outer pipe 306 into a conical form, nearly to the diameter of the inner pipe. The ends of these conical portions are then joined and sealed to the inner pipe 304 by fillet welds 320 and 322 respectively. The outer pipe is deliberately made shorter than the inner pipe, to leave clear portions 324 and 326 for handling and fabrication of the field joint.

In practice, the fabrication of this pipe section as illustrated in Figure 3 is completed on-shore, including the wrapping of the inner pipe with insulation, installation of the outer pipe 306, formation of the conical end portions 316 and 318, the welds 320 and 322 and the weld 328 which joins the shoulder piece 308 to the inner pipe main section 304. A stock of these pipe sections is then provided on the pipelaying vessel 100/200, to be used in the J-lay process as described. The pipe sections may be pre-fitted within insulating sleeves to slide over the field joints, as described further below.

Referring now also to Figure 4, it has been explained that the field joint, where the end 300 of a new pipe section is joined to the end 302 of the pipe section already joined to the pipeline and suspended from the lay ship, must be both strengthened and insulated, before laying to the sea. Figure 4 illustrates the completed field joint in (a) longitudinal cross-section and (b) radial cross-section detail. In this diagram, reference numbers 304 etc. are used for the lower pipe section, and similar reference signs with primes (304' etc.) are used for corresponding parts of the "upper" pipe section, which is added to the suspended pipeline at this field joint. Reference 400 represents the welded joint between pipe sections, in this case between the end 302 of the lower section and the end 300' of the upper section. The field joint is completed by the fitting of a pre-fabhcated insulated sleeve 402, which itself is of pipe-in-pipe construction. That is to say, an inner sleeve pipe 404 and outer sleeve 406 have been coupled together with swaged end sections and fillet welds in a miniature version of the pipe section shown in Figure 3, having an inner diameter sufficient to slide over the joined pipe sections and cover the exposed parts of the inner pipeline, around the field weld 400. Again, between the inner and outer sleeve pipes 404 and 406 a layer of insulation and then space is provided. Also at the field joint inside the sleeve 402, a hardening resin has been injected to fill space 408, providing rigidity to the entire joint region, and also a measure of insulation. Shading to represent this resin is only partially shown in Figure 4(a), for reasons of space and clarity. The resin filling extends to the ends of the sleeve 402. The same comment applies to later figures, described below.

Figure 4(b) shows in detail the complete cross-section along one radius on the line B-B shown in Figure 4(a). Starting from the centre of the pipeline, we see first the wall of the inner pipe 304, or more precisely the shoulder piece 308 at this point.

Next we find the space filled with hardening resin 408, then the inner sleeve pipe

404, insulation 410, space 412 and finally the sleeve outer pipe 406. As explained in the introduction, using the steel pipe-in-pipe structure for the field joint sleeve 402 contributes greatly to the cost and weight of the entire installation, and in fact limits the range of installations which can be undertaken by a pipelaying vessel having a finite capacity.

Novel Pipe Assemblies

Figure 5(a) shows in cross-section a novel insulated pipe section 500. As in the earlier drawings, this has ends 300 and 302, and for the vast majority of its length (not shown to scale) has a double-walled construction, comprising inner pipe 304 and outer pipe 306. Between pipes 304 and 306 is an annular space 314, which may be evacuated and/or filled with insulating material as previously described. Figure 5(b) is a more detailed cross-section in the vicinity of end 302. As can be seen in this detailed view, each pipe 304, 306 has a thickened or "upset" end portion. Insulating material 502 can be seen in the annulus, and spacing rings or windings may also be included to aid construction, or corrugated metal sheets and other measures known in the art. The upset end 504 has a greater outer diameter (OD) while the inner diameter (ID) remains substantially constant from end 300 to end 302. The upset portion 506 of outer pipe 306 has a substantially constant OD, but reduced ID, as shown. These thickened portions 504, 506 are welded together by a lap weld 508, closing the annulus 314 at its end nearest to end 302 of the assembly. At the opposite end of the pipe (300) there are similar upset portions on both pipes 304, 306, with a lap weld closing the annulus 314 in the vicinity of end 300.

Compared with the fillet welds in Figures 3 and 4, a strong weld can be achieved, and furthermore a weld which can be inspected reliably using automated ultrasonic testing. Because of the increased thicknesses, and particularly the increased thickness of the inner pipe 304 in the upset portion 504, fatigue resistance of the weld can be improved. One benefit is that mechanical stresses can be spread on larger thicknesses and axial lengths. Another benefit is that the heat affected zone (known to the skilled reader as the zone where the metal properties may be affected by the weld) is offset away from the inner diameter, so that the inner pipe

(fluid carrier pipe) is affected only in the extra thickness of the upset end.

Various processes are possible to create the upset portions 504, 506. In later embodiments, it will be seen how an end-piece can be welded to plain-ended pipe to provide a thickened portion at one or both ends. For the example of Figure 5, however, a hot forming process has been used, forming the upset portions from the metal of the thinner pipe itself. Manufacturers supplying the pipe with upset ends of this type include Vallourec & Mannesmann V&M (www.vmtybes.com) or Tenaris (wwwjenaris^com) . Note that each pipe 304, 306 may be formed itself from two or more units of pipe, for example a "double joint" length 24m from two 12m pipes. In that case, the individual pipe units will be provided with an upset at only one end, their "normal" ends being welded together to form a double joint length with upset portions at opposite ends.

Note that tolerance management is relatively easy in this design, particularly in relation to line up and length tolerances, because the ends of outer pipe need only land somewhere on the upset portion 504 of inner pipe 304. This is in contrast to the known designs, in which the annulus is closed at both ends by Y-section pup pieces, and where the lengths, parallelism and ovality of the inner and outer pipes must consequently be very tightly controlled.

Figure 6 shows a variation on the lap weld, using corrosion resistant alloy (CRA) such as lnconel ®. lnconel welds can provide an insulated pipe section with high fatigue resistance and high thermal performance, and may be particularly useful in applications such as steel catenary risers, where the performance of the weld needs to be significantly greater than general flow line applications.

Fabricating the lnconel weld is very similar to that of Figure 5 but with lnconel as added material. The weld may be performed as in Figure 6 with the addition of lnconel clad layers 610 and 612 and burr machining at the toes, prior to final machining and welding.

The burr grinding helps to eliminate microscopic cracks prior to welding which could in the finished article serve as initiation sites for larger cracks. The drawing also shows the grinding of additional grooves 614-618, further to reduce the risks of crack initiation. All the steps to fabricate the assemblies of Figures 5 and 6 can be made in a well-controlled factory environment onshore, and transported to an other location, typically offshore, for final assembly and installation as a pipeline. The main lap weld 608 will be completed also using lnconel. Of course, lnconel ® is mentioned as only one example of such an alloy and other CRA materials are known. Figure 7 illustrates a field joint in which two such assemblies, labelled 500 and 500' in Figure 7(a), are joined end to end and protected in the area of this field joint by an insulating sleeve 700. 702 indicates the offshore (field) weld which is a traditional butt weld performed between the upset portions of the inner pipes 304 and 304'. High performance orbital welding apparatus and inspection tools can be used for this weld. Because the inner pipe 304/304' in the region of the field joint is thicker than hitherto, the bending stiffness in this region between the actual double- walled pipe sections can be made comparable to or exceeding the stiffness of the double-walled sections themselves. In such a situation, the sleeve 700 can be constructed to perform only an insulating function, rather than requiring to provide also a reinforcing structural function. A polymer-based sleeve may be sufficient. Even if the sleeve 700 is still formed on a metal substrate, this can be of reduced thickness, and hence reduced weight and cost.

Figure 7(b) and (c) show different radial cross-sections along the lines b/b' and c/c' respectively in Figure 7(a). In the cross-section of Figure 7(b), a prefabricated insulating sleeve 700 can be seen surrounding outer pipe 306. Sleeve 700 comprises a cylindrical metal or polymer substrate 712, a flow insulation layer 714, and a protective layer 716. Spaces between the sleeve 700 and outer pipe 306 are filled with a hard setting material 718, which may be a resin or a thermoplastic polymer.

Referring to cross-section of Figure 7(c), the region of the field joint can be seen, including the thickened (upset) portion of inner pipe 304, referred to as 504 in Figures 5 and 6. The remaining space within the insulating sleeve 700 is filled with the same material 718 as in Figure 7(b).

In an example embodiment, the inner pipe 304 is steel of outer diameter 323.9 mm (known in the art as API 12-inch pipe) and wall thickness 15 mm. The outer pipe 306 has an outer diameter of 417.9 mm with a wall thickness 15 mm. The gap between the upset parts of inner and outer pipes (near the weld) is 2 mm. As seen best in the cross-section of Figure 7(b), the annular space 314 of approximately 32 mm between the inner and outer pipes is occupied half by an insulating layer 702 and half by a space 704 occupied by air or other gas, preferably at reduced pressure (vacuum).

The dimensions given above are strictly examples only. Diameter and wall thickness are determined case-by-case, to balance weight and cost against performance and strength requirements. For example, the thickness of the outer pipe wall may vary from 10mm to 25mm or more, depending on the crushing pressure exerted by the depth of water at which it is to be installed. The inner pipe may be the thicker of the two, or it may be the outer pipe.

Figure 8 shows an example in which double-walled pipe section 800 incorporates a J-lay collar (similar to the one shown in Figures 3 and 4). Specifically, collar 802 is provided at end 302 of the pipe section 800, and includes upper and lower shoulders 310 and 312, respectively. In this example, J-lay collar 308 forms part of the outer pipe 306, while end 302 is formed by thickened portions 504 of inner pipe 304. Again, towards end 300, both pipes 304 and 306 are provided with upset portions, and the annulus is closed by a lap weld 804. At end 302, inner pipe 304 again has an upset portion 504, and thus has a form very similar to that shown in Figures 5-7. A thickened portion of outer pipe 306 in this example is not made by hot forming the end of pipe 306, but rather as a thicker portion of the machined J- lay collar 802. J-lay collar 802 thus forms a pup piece with thickened cross-section, joined to the pipe 306 by a butt weld 806.

All welds shown in Figure 8 are performed in factory conditions on shore and amenable to automated ultrasonic testing. The upset portions of 304 form thickened ends 300 and 302 for forming field joints. Again, annulus 314 may be part-filled with the insulating material, not shown in Figure 8. Again, fabrication of pipe section 800 is facilitated by the lap-welds which do not require exact dimensioning of the pipe lengths and parallelism of their ends (related to pipe straightness). Again the thickened portions 504 of inner pipe 304 provide a longer weld and thicker material for the formation of the lap-welds 804 and 808, providing a high integrity pipe section at reduced weight and cost.

Figure 9 shows a further alternative pipe section 900, again incorporating a J-lay collar pup piece 902, albeit of a slightly different form from J-lay collar 802 in Figure 8. In this example, inner pipe 304 has its end 300 formed by a second pup piece 904, pup pieces 902 and 904 being butt welded to pipes 306 and 304 respectively, prior to assembly of the inner and outer pipes together. It will be seen that inner pipe 304 is in fact of plain diameter, apart from in the pup piece 904 which has a thickened portion 906. The thickened portion does not extend all the way to end 300 of the assembly, so that the inner pipe at end 300 has only its normal thickness, rather than the thickness of the upset portion 906. In this way, the thickness at end 300 matches the thickness of the plain pipe 304 at end 302 of another pup section 900, to which it will be welded off-shore.

J-lay collar 902 has for most of its length the same wall thickness as outer pipe 306, but terminates in a thickened portion, suitable for forming a lap weld 908 against the exterior of inner pipe 304

In both Figures 8 and 9, the J-lay collar 802/902 is provided in the outer pipe 306 as a matter of design choice only. In general, it will be preferred to put the load- supporting shoulders 310, 312 on the thicker of the two pipes 304, 306. Whether this is the inner or the outer pipe depends on various design choices in light of the expected operating conditions.

As mentioned above, the choice whether to upset the ends of the pipe section directly, or to weld on a pre-formed end-piece is a matter of convenience. Where the pipe sections 304 and 306 are seam welded, for example, it will typically be preferable to apply end-pieces than attempt upsetting by heat forming.

Figure 10 illustrates an alternative pipe section 1000, in which a J-lay collar 1002 is again provided but this time as part of the inner pipe 304. The whole pipe section is not shown in Figure 10, but rather a cross-section detail towards end 302 in Figure 10(a) and a corresponding detail towards end 300 in Figure 10(b).

Within pipe section 1000, outer pipe 306 is provided with an upset portion 506, of increased wall thickness and reduced ID. J-lay collar 1002 is a pup-piece welded at 1003 to the end of inner pipe 304, so as to provide a thicker portion 504 having the same ID as pipe 304, but increased OD. Outer pipe 306 is welded to the outside of this upset portion by a lap weld 1010, just as in the earlier examples. The thickened portion 504 of inner pipe 304 continues throughout the J-lay collar 1002 and right up to end 302 of pipe section 1000. Similarly, at the opposite end of the pipe section 1000, seen in Figure 10(b), inner pipe 304 is provided with an upset portion 1012, and outer pipe 306 is provided with an inwardly upset portion 1014, these being joined by a lap-weld 1016 to close the annulus 314. Inner pipe 304 may be provided with the upset portion 1012 by the addition of a pup piece welded at 1018 to the end of a plain pipe 304, as shown, or by hot forming possibly followed by machining. J-Lay collars and pup-pieces, in contrast, are likely to be machined from thick pipe.

Figure 11 shows a detail of the completed field joint including pipe sections 1000 and 1000', including the off-shore weld 1004 and insulated sleeve 1100. As mentioned previously, the increased thickness of the inner pipe 304 in the region of the field joint means that the sleeve 1100 can be reduced to a purely insulating function, and need not provide structural reinforcement. Consequently it may be based on a polymer base rather than a steel pipe, or it may be based on a steel pipe of much reduced weight and cost. The field joint may even be insulated by the addition of coatings or wrappings not prefabricated into a sleeve 1100. The provision of the prefabricated insulating sleeve 1100 is however beneficial in terms of productivity (pipe laying rate), as described in our earlier application

PCT/GB2007/050670 (Acergy ref 113), not published at the present priority date.

More detail of the insulation within the annular space is also provided in our earlier application PCT/GB2007/050670 (Acergy ref 113). As seen in the present drawings, end portions of the inner pipe 304 protrude from either end of an insulating blanket, for handling during fabrication, transport and installation. These end portions can be shorter than that required in swaged constructions. In embodiments using end-pieces, one or both of these may be welded to the inner pipe 304 prior to wrapping. Onto this are wrapped two separate layers of insulating material, preferably in a self-sustaining blanket form. In an particularly preferred embodiment, these layers are made of ASPEN Aerogel™ sheets 6 mm thick. Plastic bag layers are provided, to facilitate handling of the Aerogel sheets, which can release dust particles otherwise. These plastic bag sheets 508 are pierced in places, to allow air to escape as the blankets are handled and compressed (and in case air is partly evacuated). Layers may be preassembled in a stepped way so that there is an overlapping of joining opposite sides of the blanket. Outside these layers of blanket material, there may be provided a protective wrapping of metal film. This will typically have a thickness such as 0.2mm, and may be backed with a self-adhesive coating, and wound with an overlap. The pitch of the helical winding of the foil layer may be around 1.5m. In the case of a double-joint pipe section, the overall length of the inner pipe 304 will be roughly 24m. An optional feature not shown in the drawings, is to include centralising rings regularly spaced along the inner pipe, say at 2m to 10m intervals, or a helical centralising rib. These can prevent mechanical loads being applied to the insulation, either during fabrication or as a result of bending of the complete assembly. Since the insulating blanket is interrupted by these rings, they are preferably made in a high strength, low conductivity material such as a high performance polymer.

Figure 12 shows a further variation of a pipe-in-pipe assembly 1200 including J-Lay collar. This is similar to the example of Figure 8, and like reference signs have been used. The difference is that the portion of the J-Lay collar pup-piece 802 which carries the shoulders 310 and 312 is of reduced diameter, so that the shoulders do not project beyond the outer diameter of the outer pipe section 306. This allows a smaller diameter insulating sleeve to be used, and a thinner mother pipe, if the pup piece is machined from a thick pipe. Figure 13 shows a further assembly 1300 which is a variation on the assembly 900 shown in Figure 9. Like reference numerals are used for similar features. The difference in this case is at the right-hand (upper) end 302 of assembly 1300, where the machined piece 1302 which includes the shoulders 310, 312 and end 302, also provides a "Y-section" piece for closing that end of the annulus 314 by butt welding to both the inner and outer pipes 304, 306. Piece 1302 includes inner and outer joining portions 1304, 1306 and a closing wall 1308. Portion 1304 is dimensioned to weld directly to inner pipe 304, which is consequently plain pipe with no upset portion. This shows the compatibility of this design with pipes that would not accept upsetting, like seam welded pipes. Outer joining portion 1306 is similarly dimensioned to weld directly to plain outer pipe section 306. The end of inner section 1304 projects clear of outer section 1306, so that the inner weld 1312 can be completed before the outer weld 1314 (see description of assembly process below).

Fabrication of Novel Pipe Assemblies

The process of fabricating the pipe section of Figures 5 and 6 will now be summarised, based on the description of the structure given above. Steps in the fabrication of the pipe section are as follows:

• Starting with two 12m unit lengths of inner pipe 304, outwardly upset end portions 504 are formed by application of heat and pressure at one end.

• Pairs of these are welded to form a double-joint length with upsets 504 at both ends (triple and quad joint lengths can of course be made just as easily).

• Insulation 502 is applied around the inner pipe 304, and captured under the foil layer 506. Centralising rings and/or a helical rib are included.

• One 12m unit length of outer pipe 306 (with an upset end) is fed on each side of the insulated inner pipe, during which process the metal foil 506 protects the insulating blanket from being damaged or displaced. The two outer pipes are welded in the middle to form the outer pipe 306. The weld may be performed with backing rings. A special insulation blanket may be placed beneath the rings to protect the underlying layers of insulation and metal sheet from the heat. An external clamp may be used to achieve the line up. (Alternatively the double-length outer pipe can be made before feeding onto the inner pipe and blanket.)

• The end of outer pipe 306 is lap welded at one end to outer face of upset portion 504, thereby closing one end of the annular space 314.

• The opposite end of the outer pipe 306 is lap welded to outer face of inner pipe 304 at the opposite end, to close the annulus completely. • A small port is drilled if required, and pumped to form a vacuum within the annulus. The port is closed securely, for example by a steel plug driven and welded in place.

• The insulated pipe section 600 is put to storage for loading onto the vessel and use in fabrication of the pipeline.

This fabrication will typically be conducted on-shore, although, in principle, it could be conducted on a lay vessel having a sufficiently large working deck. Once the pipe-in-pipe sections have the outer pipe 306 on them, they are sufficiently robust for storage and handling as any normal pipeline. The welding process for external welds could be a single manual, semi-automatic or automatic process or a combination (SMAW, FCAW, SAW, GTAW, auto-GTAW auto-GMAW). The illustrated examples do not require internal welds, but these can be done if required by a single automatic process or a combination (auto-FCAW, SAW or/and auto- GTAW). Each weld can be fully inspected by automated ultrasonic testing (AUT) prior to completing/using the assembly.

The skilled reader will readily appreciate how to adapt the above process to the fabrication of the examples shown in Figures 8 to 10, and further variations not shown in the drawings.

Note : in of the examples where a J-Lay collar is integrated as a pup piece to inner pipe (as in Fig 10), the following sequence may be considered:

• Make double joint of inner pipe and weld 1012 to 304 • Apply insulation (except in the vicinity of weld 1003).

• Feed one outer joint 12m with upset part 506 onto 304 and store temporarily beyond its final place, so that weld 1003 may be performed.

• Perform weld 1003 and apply insulation (with reduced thickness, or no insulation at all) in the vicinity of 1003.

• Slide outer pipe 506 back to its final place

• Feed second half of outer pipe 506 with upset end 1014 onto insulated pipe

• Weld outer joints together (may be with backing rings)

• Make both lap joints 1010, 1016 and draw vacuum.

In the example of Figure 13 where a "Y-section" pup-piece includes the J-Lay collar, the welds can be performed in the order 1316, 1312, 1314 and finally the lap weld 1318. Outer pipe 306 can be fed onto inner pipe 304 after completion of weld 1316, and stored temporarily beyond its final position (slightly passing over 906) so that weld 1312 can be achieved. After weld 1312 is completed, the outer pipe can be slid completely to meet outer joining portion 1306 of pup-piece 1302, leaving the sliding fit around thickened portion 906 to accommodate any variation in parallelism and length of the different pipes. In examples with two lap welds on the outer pipe (as in Figures 8 to11 ), it is a matter of convenience which one is made last.

Fabrication and Installation of Insulated Pipeline

The complete process of fabricating the novel pipeline, of which representative sections are shown in Figures 8 to 11 , will now be described with reference also to the J-lay apparatus illustrated in Figures 1 and 2. Basic steps are:

• A stock of insulated pipe sections 800, 900 or 1000 as described above are provided on board the lay vessel, ready to be lifted, one-by-one, into the J- lay apparatus of Figure 2. An insulating sleeve 700/1100 may be preinstalled on the joint and fixed with removable devices. • Ignoring for simplicity any end terminations and special steps that may be required for initiation of the pipelaying process, a first section of double walled pipe is placed in an erector 214, elevated into the tower 210, gripped around the upper shoulder 310 by the travelling table 212, and lowered though the working table 204, whereupon it is supported on the lower shoulder 312.

• A second section of double walled pipe is loaded into the erector 214, with the prefabricated insulated sleeve 700 or 1100 already threaded partway along its length. The sleeve is held in place to prevent it sliding off the pipe section when up-ended into the tower 210. This may be by wedges of wood or polymer, for example. Whether the sleeves are threaded onto the pipe sections onshore, or in the course of operations on the vessel 200 is a matter of choice for the operator.

• As in conventional J-lay, the lower end 300' of the new pipe section is brought down by the travelling table and aligned by the tower equipment with the upper end 302 of the first pipe section, and the two are welded together.

• The collar at working table 204 is opened, and the travelling table, gripping the shoulder 310' on the upper pipe section lowers the complete assembly through working table 204, until the new pipe section is held by its lower shoulder 312 by the working table 204, now closed.

• At this point, the tower is ready to receive and fit a further pipe section by repetition of the steps just described. Meanwhile, below working table 204, the section of pipe just added extends down through stinger 216 to the lower workstation 218.

• Workers at the lower work station 218 connect eyes on the sleeve to a convenient hoist. They then remove the wedges holding the sleeve, and lower it into position, surrounding the welded joint and shoulder piece 802/902/1002.

• The lower end of the annular space between the sleeve 700 and the outer pipe 306 below the field joint is blocked off to prevent running out of the filling resin 718, which is injected in liquid form. As mentioned already, this gap may be 5-10mm. The lower end of the sleeve is centred using three screws. • Resin 718 is poured through the open, upper end of the annular space, that is between the sleeve 700 and the outer pipe 306' of the upper pipe section. The liquid resin flows easily through gaps of the size mentioned, filling the annular space progressively from its lower end, at the side of the outer pipe 306, through the central space around the shoulder piece 308, finally filling the upper annular space between sleeve 700 and outer pipe 306' of the upper section. The resin is allowed to harden, the material being chosen if possible to harden within a matter of minutes, so that the entire operation is complete by the time the next welded joint has been completed at the upper work station (working table 204). • With the complete field joint now finished, the pipe section located in the tower 210 is gripped by travelling table 212, and the entire pipeline assembly lowered ready for the next joint to be formed. The joint just welded at the upper workstation is thus located at the lower workstation 218, ready for the application of sleeve and resin to be repeated.

Variations on the above sequence can be envisaged, to maximise efficiency using the exact facilities available for forming joints, inspection of welds and so forth at each workstation, and depending on the duration of different steps using the equipment provided. In particular, a J lay system may not need a collar and only use friction clamps or tensioners.

More detail of the insulated sleeve and its construction can be found in our earlier application (Acergy 113) mentioned above. On the other hand, the present invention is in no way limited to include the use of this sleeve and other means for insulating and/or reinforcing the field joint may be used, whether known already to yet to be invented. As already discussed, the pipe in the region of the field joint may in the above examples be thick enough that sleeve 700 or 1100 is not required to perform the same reinforcing function as in the earlier application. The hard resin 718 may be replaced by a foam with less strength but better insulating properties.

Although the examples above are implemented in a J-Lay system, the novel construction does not exclude installation by S-Lay or Steep S-Lay systems if desired. In that case, the fabrication of the pipeline is performed in a horizontal layout, and the pipeline diverted onto the lay path subsequently. The tensioning mechanism may use shoulders, as illustrated for J-Lay. Alternatively, the shoulder pieces may be omitted and friction clamps or track tensioners used in known manner. In that case, consideration has to be given to how the insulated sleeve and completed field joints can pass through the tensioning apparatus and diverter (stinger) without damage. In general, the solution will be to provide redundancy in the tensioning mechanism, so that one tensioning device can be opened for passage of the insulated sleeve, while the lay tension is taken by another device or devices at other points along the pipeline.

Those skilled in the art will readily appreciate that other modifications and variations that are possible within the spirit and scope of the invention in each of the aspects defined above and in the appended claims.

Claims

1. A double walled pipe assembly comprising an inner component pipe and an outer component pipe having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe being located coaxially within the outer pipe to define an annular insulating space between them, the ends of the inner and outer pipes being joined by circumferential welds to close the annular space, wherein at least one of said component pipes is provided at at least one of its ends with a thickened portion, the annular space at said portion being reduced substantially to zero, and wherein the outer pipe is welded in said portion to the inner pipe by a circumferential lap weld.

2. An assembly as claimed in claim 1 wherein the thickened portion is provided by deformation of the material of the pipe.

3. An assembly as claimed in claim 1 wherein the thickened portion is provided on an end-piece butt welded to a plain-ended section of the inner or outer pipe.

4. An assembly as claimed in claim 3 wherein the end-piece is provided with at least one exterior projection to serve for supporting the assembly during J-lay operations.

5. An assembly as claimed in claim 4 wherein the end-piece includes two circumferential shoulder projections, spaced in an axial direction of the assembly.

6. An assembly as claimed in claims 3, 4 or 5 wherein the end-piece is provided on the inner pipe.

7. An assembly as claimed in claims 3, 4 or 5 wherein the end-piece is provided on the outer pipe,.

8. An assembly as claimed in any preceding claim wherein thickened portions are provided at both ends of the inner pipe and extend to the very ends of the assembly with matching thickness, whereby the assembly and similar assemblies may be joined by welding end to end.

9. An assembly as claimed in any preceding claim wherein the inner pipe has a constant inner diameter and is provided at both ends with thickened portions having increased outer diameter, to which the outer pipe ends are welded.

10. An assembly as claimed in claim 9 wherein the outer pipe is provided with thickened portions having reduced inner diameter at one or both ends.

11. An assembly as claimed in claim 9 or 10 wherein the thickened end portions of the inner pipe are formed by end-pieces welded onto a plain-ended pipe.

12. An assembly as claimed in claim 11 wherein one of said end-pieces is provided with projections for supporting the assembly during J-lay operations.

13. An assembly as claimed in any of claims 1 to 7 wherein the inner pipe has a constant inner diameter and is provided at only one end with a thickened portion having increased outer diameter, to which the corresponding outer pipe end is welded, while the outer pipe is provided with a thickened portion having reduced inner diameter at the opposite end of the assembly.

14. An assembly as claimed in claim 13 wherein the thickened portion of the inner pipe is set back from the end of the pipe, whereby the thicknesses of the ends match for joining assemblies together directly by butt welding.

15. An assembly as claimed in any preceding claim including insulating material partially filling said annular space.

16. A pipeline installation comprising a plurality of double walled pipe sections according to any preceding claim above, joined end to end.

17. An installation as claimed in claim 16 wherein the pipe sections where joined together are surrounded by an insulating coating.

18. An installation as claimed in claim 17 wherein said insulating coating is at least partially formed by a prefabricated insulating sleeve.

19. A method of manufacturing a double walled pipe section as claimed in any preceding claim, the method comprising the steps:

(a) providing inner and outer component pipes, the outer component pipe having an inside diameter larger than an outside diameter of the inner pipe,

(b) locating the inner pipe being located coaxially within the outer pipe to define an annular insulating space between them, and

(c) joining the ends of the inner and outer pipes by circumferential welds to close the annular space, wherein at least one of said component pipes is provided at at least one of its ends with a thickened portion, wherein the annular space at said portion is reduced substantially to zero, and wherein said joining step (c) concludes by welding the outer pipe in said portion to the inner pipe by a circumferential lap weld.

20. A method as claimed in claim 19 wherein at least one thickened portion is provided by deformation of the material of the pipe carrying that thickened portion.

21. A method as claimed in claim 19 or 20 wherein at least one thickened portion is provided on a pre-fabricated end-piece and butt welded to a plain-ended section of the pipe carrying that thickened portion.

22. A method as claimed in claim 21 wherein the end-piece is provided with at least one exterior projection to serve for supporting the assembly during J-lay operations.

23. A method as claimed in claim 21 or 22 wherein the end-piece is welded onto the inner pipe prior to assembly with the outer pipe.

24. A method as claimed in claim 21 or 22 wherein the end-piece is provided on the outer pipe prior to assembly with the inner pipe.

25. A method as claimed in any of claims 19 to 24 wherein the inner pipe has a constant inner diameter and is provided at both ends with thickened portions having increased outer diameter, to which the outer pipe ends are welded.

26. A method as claimed in any of claims 19 to 24 wherein the inner pipe has a constant inner diameter and is provided at only one end with a thickened portion having increased outer diameter, to which the corresponding outer pipe end is welded, while the outer pipe is provided with a thickened portion having reduced inner diameter at the opposite end of the assembly.

27. A method as claimed in any of claims 19 to 26 wherein all of said welding operations are performed from the exterior of the pipes.

28. A method of installing an insulated pipeline comprising a plurality of insulated pipe assemblies as claimed in any of claims 1 to 15, the method comprising adding further assemblies by: suspending an upper end of said pipeline at a first workstation; - adding a further such assembly to the pipeline by making a field joint between assemblies; transferring a load comprising the suspended pipeline to the added assembly; and paying out the suspended load lowering the added assembly to said first workstation ready to repeat the above steps for addition of a yet further assembly.

29. A method as claimed in claim 28 wherein in said lowering step the field joints of adjacent assemblies are lowered to a second workstation, the method including applying insulation and/or reinforcement to field joint concurrently with making a next field joint.

30. A method as claimed in claim 28 or 29 wherein said insulation includes a prefabricated insulated sleeve which is fitted onto each further assembly prior to making said field joint, said sleeve being slid to cover the field joint after transferring said load to the added assembly.

31. A method as claimed in claim 28, 29 or 30 wherein each assembly is provided at its upper end with one or more projections for use in supporting the suspended pipeline.

32. A set of parts for use in installation of an insulated pipeline, the parts comprising: a plurality of double walled pipe sections, each comprising an assembly as claimed in any of claims 1 to 15, and a plurality of pre-fabhcated insulated sleeves adapted to slide over said insulated pipe sections so as to cover joints made between them.

33. A set of parts as claimed in claim 21 wherein each of said prefabricated insulated sleeves comprises a cylindrical substrate and outer layers of insulation carried thereon.

34. A set of parts as claimed in claim 33 wherein said insulated sleeves are pre- fitted around said assemblies.