WO2009083937A2 - Insulated pipelines and methods of construction and installation thereof - Google Patents

Abstract

Disclosed is a double walled pipeline assembly, an associated set of parts and a pipeline installation and a method for forming and installing the same. The double walled pipeline assembly consists of an inner pipe section (304) and an outer pipe section (306) having an internal diameter larger than the external diameter of the inner pipe, enabling the inner pipe to be located coaxially within the outer pipe to leave an insulating gap (314) between them. The gap is sealed at both ends by welding to first and second joining pieces (308, 610) which are formed by machining from a thicker section of pipe or cast material. The first joining piece (308) is formed integrally with shoulders (310, 312) for supporting the pipe sections and welded pipeline during laying, so that the machined pieces add little to the cost, while allowing more conventional welds to be used. Alternatively the second end may be sealed by a circumferential lap weld (1318) formed on a thickened portion (1322) of one of the pipes.

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 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 W/nrf.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 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.

This stress translates also to fatigue as the pipeline is cycled in and out of operation. This stress and associated weld criticality 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 outside of the weld .

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 and to facilitate automated inspection of the welds between inner and outer pipes.

The invention in a first aspect provides a double walled pipe assembly comprising an inner pipe section and an outer pipe section having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe section being located coaxially within the outer pipe to define an annular insulated space between them, ends of the inner and outer pipes being joined at at least a first end of the assembly by circumferential welds to respective inner and outer proximal portions of a joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the other end of the assembly, whereby the assembly and similar assemblies may be joined by welding end to end, the joining piece incorporating at least one projection between the closing wall and said distal portion to serve for supporting the assembly during J-lay operations.

This form of construction permits conventional orbital penetration welding to be used in fabrication of the sections. This makes for high production speed and uses proven technology, reducing qualification time of a given design. It also facilitates automated inspection of the welds. Although the provision of such a joining piece 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 joining piece in a preferred embodiment includes two circumferential shoulder projections.

The assembly may further include a second joining piece, whereby ends of the inner and outer pipes are joined at a second end of the assembly by circumferential welds to respective inner and outer proximal portions of a second joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the distal end of the first joining piece.

In an alternative embodiment, ends of the inner and outer pipes are joined at a second end of the assembly by a circumferential lap weld, one of said pipes being provided with a thickened portion, the annular space at said portion being reduced substantially to zero. 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 relevant 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. In a preferred embodiment of this type, the thickened portion is provided on the inner pipe section, the matching ends of the assembly comprising substantially continuations of said inner pipe section diameter. Alternative embodiments are possible, however, in which the ends of the assembly continue substantially the diameter of the outer pipe section.

The formation of pipe-in-pipe assemblies using lap welds based on thickened portions of one or both pipes is the subject of our co-pending patent application GB 0803664.2 (Acergy ref 121 ), not published at the present filing date, the contents of which are hereby incorporated by reference.

In the preferred embodiments, the (first) joining piece has a substantially constant inner diameter continuous with that of the inner pipe section. The thickness of the material in said distal portion may be the same as or thicker than that of the inner pipe section to which it is joined. 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.

In one embodiment, the inner pipe section is formed by a plurality of inner pipe sections welded end to end, while the outer pipe section is formed by a plurality of outer pipe sections and completed by a pair of half shells.

In another embodiment, the inner pipe section is joined to one of said joining pieces by an internal circumferential weld. This is preferably the shorter of the two joining pieces, where they have different lengths. The second joining piece can be shorter than the first joining piece, as it does not need to carry the shoulder projections.

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 by welding the distal portion of the (first) joining piece on one section to the second end (optionally the second joining piece) of another. In the present description, 'distal' refers to the part of a joining piece furthest from the inner and outer pipe sections within the assembly, while 'proximal' refers to the parts closest to the pipe sections within the assembly.

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. A novel lightweight sleeve 500, described in more detail in our earlier application WO2008053251 (Acergy ref 113), not published at the present priority date. 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, each comprising an inner pipe section and an outer pipe section having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe section being located coaxially within the outer pipe to define an annular insulated space between them, ends of the inner and outer pipes being joined at at least a first end of the assembly by circumferential welds to respective inner and outer proximal portions of a joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the other end of the assembly, whereby the assembly and similar assemblies may be joined by welding end to end, the joining piece incorporating at least one projection between the closing wall and said distal portion to serve for supporting the assembly during J-lay operations, and a plurality of pre-fabhcated insulated sleeves adapted to slide over said insulated pipe sections so as to cover the joining piece after welding.

In a preferred embodiment, each of said prefabricated insulated sleeves comprises a single structural pipe section and outer layers of insulation carried thereon.

By providing a single structural sleeve pipe, rather than a double walled steel sleeve, significant weight savings are possible (especially considering submerged weight), without loss of mechanical strength. Thermal insulation performance may also be better. The inner structural pipe sections of said sleeves may be of steel. Composite materials are an alternative, however.

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 Seaway 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;

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 two radial section details (b) and (c) of the field joint formed according to a first embodiment of the invention, using novel form of double-walled pipe sections, and an insulation sleeve;

Figure 6 is a longitudinal cross-section of one double-walled pipe section used in the fabrication of the pipeline of Figure 5;

Figure 7 shows an external schematic view (a) and a radial cross-section detail (b) of the insulated inner pipe, prior to fabrication of the pipe section of Figure 6;

Figure 8 shows a radial cross-section detail of a first joining piece/shoulder piece used in formation of the field joint shown in Figure 5; Figure 9 shows a radial cross-section detail of a first joining piece/shoulder piece used in formation of the field joint shown in Figure 5;

Figure 10 shows a final stage in the assembly of an insulated pipe section according to a second embodiment of the invention; and

Figure 11 is a longitudinal cross-section of one double-walled pipe section according to a third 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 applicants vessel, Seaway 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.

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). 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, 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 inner pipe is deliberately made shorter than the outer 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.

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'lippef¹ 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.

Figure 5 shows a novel pipeline construction, focusing on the area of the field joint in a manner similar to Figure 4. The drawing comprises (a) longitudinal cross- section of the field joint, (b) radial cross-section detail the long line B-B and (c) radial cross-section detail along line C-C. Like reference numerals will be used for the parts of the novel construction which correspond to parts in the known example (Figure 4). However the form of some of these parts, and the manner of joining the inner and outer pipes is different. In this example, the pipe-in-pipe insulated sleeve 402 is replaced by a novel lightweight sleeve 500, described in more detail in our earlier application WO2008053251 (Acergy ref 113), not published at the present priority date.

While the finished pipeline construction is illustrated in Figure 5, Figure 6 illustrates one pipe section 600 representing a modified form of the pipe section shown in Figure 3. Figure 7 illustrates an intermediate stage in the construction of the insulated pipe section of Figure 6, while the earlier PCT application includes more detail of the construction of the field joint insulated sleeve 500, prior to assembly with the pipeline. It will be understood that these sub-assemblies are fabricated on- shore, while the complete field joint illustrated in Figure 5 is finished off-shore at the time of laying the pipeline. The complete fabrication and installation process will be described in more detail below. The structure of the novel pipeline will be described first in detail.

Referring now to Figure 6, while having the construction of Figure 5 in mind, we see a longitudinal cross-section of one pipe section having a main portion of its length of double-walled construction. For consistency with the description of Figure 4, the inner pipe is again labelled 304, and the outer pipe 306. Again, a shoulder piece 308 has been welded to one end of the main inner pipe section, at a joint 328. Ends 300 and 302 of the pipe section are again available for connecting to the pipeline at the time of installation (these ends will be bevelled immediately prior to laying). Again, space 314 between the inner and outer pipes is provided with insulation, as will be described in more detail with reference to Figure 7.

In an example embodiment, the inner pipe is steel of outer diameter 323.9 mm (known in the art as 12-inch pipe) and wall thickness 27 mm. The outer pipe 306 has an outer diameter of 384 mm with a wall thickness 17.7 mm. As seen best in the cross-section Figure 5(b), the annular space of approximately 12 mm between the inner and outer pipes is occupied approximately half by an insulating layer 502 and half by a space 504 occupied by air or other gas preferably at reduced pressure.

The dimensions given above are strictly examples only. Diameter and wall thickness are determined 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.

In contrast to the known construction, the outer pipe 306 has not been swaged and fillet welded to the inner pipe 304. Rather, the shoulder piece 308 is extended with a radial wall 602 to form a joining piece for welding to outer pipe 306. This weld 604 is a Standard, orbital penetration weld just like weld 328. At the opposite end of the insulated pipe section 600, a joining piece 610 is welded to the ends of the inner and outer pipe sections 304, 306 to close the annular space and to provide the end 300 for welding to end 302 of the preceding pipe in the pipeline construction process. Like the proximal end of the shoulder piece 308, this has effectively a radial cross-section in the form of a ¹Y, with inner and outer annular portions of dimensions matched for welding to the ends of the inner and outer pipes 304, 306 respectively. The main body of the joining piece 610 is substantially a cylinder continuing the bore of inner pipe section 304 and terminating at its distal end 300 with dimensions matched for welding to the distal end 302 of the next pipe section.

The welds 612 and 614 between joining piece 610 and inner and outer pipe sections 304, 306 are again standard penetration welds. The outer weld 614 is performed from the outside, but the inner weld 614 is one made from inside the bore of the pipe, in order to permit the assembly process as described below.

This modified form of joining the inner and outer pipes brings immediate advantages, in that the welds become more conventional penetration welds rather than the fillet welding to a tapered, swaged en of the outer pipe around the inner pipe. Furthermore, the simple cylindrical form of the metal on either side of the weld location facilitates automated ultrasonic inspection to verify the quality of these critical welds.

Figure 7 illustrates the inner pipe of the pipe section shown in Figure 6, prior to addition of the outer pipe and the welding of the two pipes together sith joining pieces. Figure 7(a) is a side view of the inner pipe section 304 with ends projecting from a blanket of insulating material 502.

A portion 700, 702 of the inner pipe projects at one end of the insulating blanket has a length which can be less than that required in swaged constructions, for handling during fabrication, transport and installation. In this illustration, the shoulder piece 308 and joining piece 610 have still to be added, though in a preferred embodiment one or other of these is welded to the inner pipe 304 prior to wrapping.

Around the insulating blanket 502 in this example there is provided a protective wrapping of metal film 506. The construction is illustrated in more detail in the partial radial cross-section Figure 7(b). Firstly we see the thickness of the inner pipe 304. Onto this are wrapped two separate layers 502a and 502b of insulating material, preferably in a self-sustaining blanket form. In an particularly preferred embodiment, these layers are made of ASPEN Aerogel™ sheets 5mm thick. Plastic bag layers, of which one is illustrated at 508 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. Outside these two layers of blanket material, the metal foil 506 is wound. This will typically have a thickness such as 0.2mm, and may be backed with a self-adhesive coating, and wound with an overlap, as illustrated at 510. 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 in Figure 7(a) will be 24m minus the length of the shoulder piece 308, still to be added. 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.

Figures 8 and 9 illustrate in more detail the forms of the joining pieces 308 and 610 respectively. Each piece is formed by machining from a solid cylinder of metal, shown in radial cross-section at 800 and 900, respectively. These starting pieces can be either a forging or a section of seamless pipe, having a wall thickness of perhaps 75-80mm. Though the starting pieces 800 and 900 are shown as separate, it is possible alternatively to machine the pieces 308 and 610 from a single cylinder of metal. Although machining is in principle a costly step, it should be appreciated that the shoulder piece 308 already needs to be machined for the J- lay process, and altering its form adds little to the overall cost, compared with the saving made by avoiding the work and the additional testing inherent in the swaging and fillet welding processes previously required.

Referring to Figure 8, the shoulder piece 308 is seen with its proximal end at left and its distal end at right (opposite orientation to Figures 5 and 6). At the proximal end, an inner annular face 802 is joined to an outer annular face 804 by closing wall 602, these being welded to the ends of the inner and outer pipe sections 304, 306 to close the annular space of the pipe-in-pipe section 600. Note that the closing wall and outer face 804 are spaced axially away from the inner face, providing clearance for access by welding apparatus such as an automated orbital welding 'bug'. The piece extends through shoulders 310, 312 to its distal end face 806, which in the finished assembly section forms end 302 of the insulated pipe section 600. Chamfers are shown at each face, which are part of the conventional preparation for butt-welding two pipes. These chamfers may be formed in the machining process or as a preparatory step in the welding stage.

Referring to Figure 9, the joining piece 610 is seen with its proximal end at right and its distal end at left (opposite orientation to Figures 5 and 6). At the proximal end, an inner annular face 902 is joined to an outer annular face 904 by closing wall 906, these being welded to the ends of the inner and outer pipe sections 304, 306 to close the other end of annular space of the pipe-in-pipe section 600. Note that the inner annular face is provided substantially at the foot of the closing wall 906, so that the corresponding leg of the Υ form in this example has effectively zero length. Inner face 902 and outer face 904 are spaced axially from one another by an amount which may match the spacing of the corresponding faces in the shoulder piece 308, so that the inner and outer pipe sections 304 and 306 can be formed with equal lengths, but this is not essential. The piece extends with the diameter of inner pipe section 304 to its distal end face 908, which in the finished assembly section forms end 300 of the insulated pipe section 600. Note that the chamfer of the inner face is adapted for welding from the inside of the bore. The same could be applied additionally or instead to the inner face of the shoulder piece 308, but it is preferred to minimise the number of interior welding operations, and also the shorter axial length of the piece 610 makes the interior weld that is necessary somewhat easier than if it were at the proximal end of the shoulder piece 308. These chamfers may be formed in the machining process or as a preparatory step in the welding stage. In a second embodiment to be described all welds are performed externally, in which case the chamfers would all be formed for accordingly.

The process of fabricating the pipe section of Figure 6 according to the first embodiment 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 a suitable length of inner pipe 304, the inner face 802 of the first joining piece formed by modified shoulder piece 308 is welded to the end of inner pipe 304 (this can be performed after the next step if preferred). An internal pipe alignment tool may assist in aligning the surfaces to be welded. • The first and second layers of insulating material 502 are applied around the pipe, and captured under the foil layer 506. Centralising rings and/or a helical rib (not shown in the drawings) are included.

• Outer pipe 306 is fed along the insulated inner pipe, during which process the metal foil 506 protects the insulating blanket from being damaged or displaced.

• The end of outer pipe 306 is welded to outer face 804 of shoulder piece 308, closing one end of the annular space.

• The second joining piece 610 is presented to the opposite ends of the inner and outer pipes 304 and 306 and these are welded to its faces 902, 904 respectively. The interior weld may be performed first, or the exterior weld, depending on convenience and the type of alignment tools available. • 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 welding process for internal weld could be 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.

Figure 10 shows a final stage in the assembly of the insulated pipe section in second embodiment of the invention. In the first embodiment, the inner and outer pipes 304, 306 are prefabricated at double-joint length by joining pairs of 12m lengths. These are then treated as the inner and outer pipes of the assembly and process described above. In the second embodiment, the inner and outer pipes are formed of shorter lengths which are not joined together until the last steps in forming the pipe-in-pipe assembly. This allows all welds to be performed from the exterior.

The assembly process of the second embodiment follows the sequence:

• Starting with first and second 12m lengths of inner pipe 304a and 304b, the inner face 802 of the first joining piece formed by modified shoulder piece 308 is welded to the end of inner pipe 304a (this can be performed after the next step if preferred, but in that case care must be taken to keep the insulation clear of the weld location). • The first and second layers of insulating material 502 are applied around the inner pipe 304, and captured under the foil layer 506. Centralising rings 1002 and/or a helical rib (not shown) are included at intervals along the pipe. The insulation 502 is omitted in Figure 10 for clarity. • The resulting subassembly 304a, 308 is then mirrored by assembling a second 12m length 304b of inner pipe to the second joining piece 610 and applying insulation to pipe length 304b.

• A first length 306a of outer pipe 306 is fed along the insulated inner pipe304a and welded to outer face 804 of shoulder piece 308, closing one end of the annular space.

• A second length 306b of outer pipe is fed along the insulated pipe length 304b and presented to the joining piece 610 for welding to outer face 904. The lengths of the outer pipe sections 306a and 306b are somewhat shorter than the inner pipes, leaving 25cm or so of each inner pipe 304a, 304b exposed at the ends opposite the joining pieces.

• The two sub-assemblies are then presented to one another and the inner pipe lengths 304a, 304b welded together at 1002 to complete the inner pipe 304. At this point, which is the state of the process illustrated in Figure 10, both ends of the annular space between the pipes 304, 306 are closed, but leaving a gap of 50cm or so in the middle of the outer pipe 306.

• Insulation is added around the exposed portion of the inner pipe, and centralising rings as appropriate.

• The outer pipe 306 is completed and closed by the addition of a pair of half- shells 1004, 1006, welded orbitally to the pipe sections and longitudinally to one another in a well-known operation. The pipe-in-pipe assembly is complete.

• 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. In the above sequence, variations are possible, such as reversing the roles of the first and second joining pieces. Hybrids of the first and second embodiments are also possible, for example starting with the inner pipe 304 in double joint length before welding to the joining pieces, but keeping the outer pipe in two sections so as to arrive again at the position shown in Figure 10.

The complete process of fabricating the novel pipeline, of which a representative section is shown in Figure 5, will now be described with reference also to Figures 6 to 9, and the J-lay apparatus illustrated in Figure 2. Basic steps are: • A stock of insulated pipe sections as generally shown in Figure 6 are provided on board the lay vessel, ready to be lifted, one-by-one, into the J- lay apparatus of Figure 2.

• Ignoring for simplicity any end terminations and special steps that may be required for initiation of the pipelaying process, a first section of 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 pipe is loaded into the erector 214, with the prefabricated insulated sleeve 500 already threaded partway along its length. The sleeve is held in place to prevent it sliding off the pipe section when upended into the tower 210. This may be by wedges of wood or polymer, for example. Whether the sleeves 500 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 the eyes 522 of the sleeve to a convenient hoist. They then remove the wedges holding the sleeve 500, and lower it into position, surrounding the welded joint and shoulder piece 308.

• The lower end of the annular space between the sleeve 500 and the outer pipe 306 below the field joint is blocked off to prevent running out of the filling resin 518, which is injected in liquid form. As mentioned already, this gap may be 5-10mm. The lower end of the sleeve is centred using the three screws 520.

• Resin 518 is poured through the open, upper end of the annular space, that is between the sleeve 500 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 500 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, in the state illustrated in Fig 5, the pipe section located in the tower 210 is grouped 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.

Figure 11 shows a further assembly 1300 which is a variation on the assemblies 500, 600 shown in Figures 5, and 8 to 10. Like reference numerals are used for similar features. The orientation of the assembly is reversed in this drawing, so that the first or upper end 302 of assembly 1300 is at the right-hand side. Here, a machined piece 1302 includes the shoulders 310, 312 and end 302, also provides a Υ-sectiorf 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. 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 above).

The major difference between the Figure 11 assembly and those of other drawings is at the second end, where there is not a second joining piece of Y-cross section. Rather, in this example, a lap weld is formed between the outer pipe 306 and inner pipe 304. To facilitate this, inner pipe 304 has its second end 300 formed by a pup piece 1320, butt welded to pipe 304, prior to assembly of the inner and outer pipes together. Whereas the inner pipe 304 is generally of plain diameter, in portion formed by the pup piece 1320 it has a thickened or 'upsef portion 1322. At the thickened portion 1322, the annular gap between the inner and outer pipes 304, 306 reduces substantially to zero to allow the lap weld 1318 against the exterior of inner pipe 304. 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 portion 1322. In this way, the thickness at end 300 matches the thickness of the first end 302 of another 1300, to which it will be welded off-shore. The formation of pipe-in-pipe assemblies using lap welds based on thickened portions of one or both pipes is the subject of our co-pending patent application GB 0803664.2, in which this Figure 11 is one embodiment. 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. On the other hand, 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 1322 of inner pipe 304. This is in contrast to the earlier examples, in which the annulus is closed at both ends by Y-section joining pieces, and where the lengths, parallelism and ovality of the inner and outer pipes must consequently be very tightly controlled.

Because of the increased thicknesses, and particularly the increased thickness of the inner pipe 304 in the thickened portion 1322, 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 thickened or 'upsef portions 1322. In Figure 11 it is seen how an end piece 1320 can be welded to plain-ended pipe to provide a thickened portion at one end. In other embodiments, 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.-.y.IBlMbes.com) or Tenaris (www.tenaris.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. 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.

A variation on the lap weld uses corrosion resistant alloy (CRA) such as Inconel®. 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. The weld 1318 may be performed with the addition of Inconel clad layers 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 grinding of additional grooves may further reduce the risks of crack initiation. All the steps to fabricate the assemblies of Figure 11 can be made in a well-controlled factory environment onshore, and transported to another location, typically offshore, for final assembly and installation as a pipeline. The other welds can be completed also using Inconel. Of course, lnconel®is mentioned as only one example of such an alloy and other CRA materials are known.

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.

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 pipe section and an outer pipe section having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe section being located coaxially within the outer pipe to define an annular insulated space between them, ends of the inner and outer pipes being joined at at least a first end of the assembly by circumferential welds to respective inner and outer proximal portions of a joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the other end of the assembly, whereby the assembly and similar assemblies may be joined by welding end to end, the joining piece incorporating at least one projection between the closing wall and said distal portion to serve for supporting the assembly during J-lay operations.

2. An assembly as claimed in claim 1 wherein the joining piece includes two circumferential shoulder projections.

3. An assembly as claimed in claim 1 or 2 further including a second joining piece, whereby ends of the inner and outer pipes are joined at a second end of the assembly by circumferential welds to respective inner and outer proximal portions of a second joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the distal end of the first joining piece.

4. An assembly as claimed in claim 1 or 2 wherein ends of the inner and outer pipes are joined at a second end of the assembly by a circumferential lap weld, one of said pipes being provided with a thickened portion, the annular space at said portion being reduced substantially to zero.

5. An assembly as claimed in claim 4 wherein the thickened portion is provided on an end-piece butt welded to a plain-ended section of the inner or outer pipe section.

6. An assembly as claimed in any preceding claim wherein each end of the assembly has a substantially constant inner diameter continuous with that of the inner pipe section.

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

8. An assembly as claimed in any preceding claim wherein each of the inner and outer pipes is formed from two or more sections of pipe welded end-to-end, giving a total length of the assembly greater than 20m.

9. An assembly as claimed in claim 8 wherein the inner pipe section is formed by a plurality of inner pipe sections welded end to end, while the outer pipe section is formed by a plurality of outer pipe sections and completed by a pair of half shells.

10. An assembly as claimed in any of claims 1 to 8 wherein the inner pipe section is joined to one of said joining pieces by an internal circumferential weld.

11. A pipeline installation comprising a plurality of double walled pipe sections each comprising an assembly as claimed in any of claims 1 to 10 joined by welding the distal portion of the (first) joining piece on one section to the second end of another.

12. An installation as claimed in claim 11 where the pipe sections where joined together are surrounded by a reinforcing sleeve.

13. An installation as claimed in claim 11 wherein the reinforcing sleeve is provided on its outside with an insulating coating.

14. A method of manufacturing a double walled pipe section as claimed in any of claims 1 to 11 , comprising: providing inner and outer pipe sections; providing said first joining piece incorporating supporting projections; providing either said second joining piece or said thickened portion; assembling said pipe sections and joining piece(s) to complete the assembly.

15. A method as claimed in claim 14 wherein said outer pipe section is formed by welding together two or more separate lengths of pipe.

16. A method as claimed in claim 15 wherein said outer pipe section formed by said lengths of pipe is shorter than said inner pipe section and is completed by the addition of a plurality of part-cylindrical shells.

17. A method as claimed in claim 14, 15 or 16 wherein said inner pipe section is formed by welding together two or more separate lengths of pipe.

18. A method as claimed in claim 17 wherein at least one of said inner pipe lengths is welded to at least one of said joining piece prior to welding said lengths to form the inner pipe section.

19. A method as claimed in claim 14, 15, 16, 17 or 18 wherein all of said welding operations are performed from the exterior of the pipe sections and joining pieces.

20. A method as claimed in claim 14, 15, 16, 17 or 18 wherein first and second joining pieces are provided so as to form an assembly as claimed in claim 3, and one of said joining pieces is welded to the inner pipe section working from the interior of the pipe section.

21. A method of installing an insulated pipeline comprising a plurality of insulated pipe assemblies as claimed in any of claims 1 to 10, the method comprising adding further assemblies by: suspending an upper end of said pipeline at a first workstation using a projection on the uppermost first joining piece in the pipeline; adding a further such assembly by welding the second joining piece of the further assembly to said uppermost first joining piece; transferring a load comprising the suspended pipeline to a projection on the first joining piece of the added assembly; and - paying out the suspended load lowering the first joining piece of the added assembly to said first workstation ready to repeat the above steps for addition of a yet further assembly.

22. A method as claimed in claim 21 wherein in said lowering step the welded first and second joining pieces of adjacent assemblies are lowered to a second workstation, the method including applying insulation and/or reinforcement to the welded pieces concurrently with a next welding operation.

23. A method as claimed in claim 21 or 22 wherein a sleeve is applied around each further assembly prior to welding, said sleeve being slid to cover the welded joining pieces after transferring said load to the first joining piece of the added assembly.

24. 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 inner pipe section and an outer pipe section having an inside diameter larger than an outside diameter of the inner pipe, the inner pipe section being located coaxially within the outer pipe to define an annular insulated space between them, ends of the inner and outer pipes being joined at at least a first end of the assembly by circumferential welds to respective inner and outer proximal portions of a joining piece, the inner and outer proximal portions of said joining piece being joined by a closing wall, the joining piece having a distal portion matching in size the other end of the assembly, whereby the assembly and similar assemblies may be joined by welding end to end, the joining piece incorporating at least one projection between the closing wall and said distal portion to serve for supporting the assembly during J-lay operations, and a plurality of pre-fabhcated insulated sleeves adapted to slide over said insulated pipe sections so as to cover the joining pieces after welding.

25. A set of parts as claimed in claim 24 wherein each of said prefabricated insulated sleeves comprises a single structural pipe section and outer layers of insulation carried thereon.