WO2008053251A2

WO2008053251A2 - Insulated pipelines and methods of construction and installation thereof

Info

Publication number: WO2008053251A2
Application number: PCT/GB2007/050670
Authority: WO
Inventors: Brieuc Du Halgouet; Graham Hutt; Dominique Perinet
Original assignee: Acergy France Sa
Priority date: 2006-11-02
Filing date: 2007-11-01
Publication date: 2008-05-08
Also published as: WO2008053251A3; AU2007315926A1

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 outer pipe has a shorter length than the inner pipe and is also tapered inwardly by swaging at both ends, forming a part-conical shape (318b) and a straight cylindrical form (318a) that approaches and lies parallel to the surface of the inner pipe to enable welding. By doing this a low cost and weight field joint between pipe-in-pipe sections can be obtained.

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 VWm²X 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. In one aspect, the invention aims to provide a well-insulated field joint between pipe-in-pipe sections, at lower cost and/or with lower total weight than conventional solutions.

The invention in another, independent aims to provide pipe-in-pipe structures and methods of manufacturing the same in which a better weld quality can be obtained between the outer and inner pipes. The invention in another independent aspect aims to provide pipe-in-pipe structures manufactured so as to permit 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 so that the inner pipe can be located coaxially within the outer pipe to define an annular insulated space between them, the outer pipe section being shorter than the inner pipe section and being provided at each end with portions tapered inwardly so as to approach the surface of the inner pipe to permit welding of the outer pipe ends to the surface of the inner pipe near its ends, wherein extreme end portions of said outer pipe, beyond said tapered portions, have a straight cylindrical form so as to lie parallel to the surface of the inner pipe when assembled for welding.

This form of construction permits more conventional fillet welding, and also facilities automated inspection of the welds.

In one embodiment, said tapered portions and extreme end portions are formed by swaging the outer pipe on the inner pipe.

In another embodiment, the tapered portions and extreme end portions may be provided on a short pipe section butt-welded at each end of a main length of said outer pipe section. The short pipe section may have three distinct portions along its length: an extreme end portion comprising said straight cylindrical portion or fitting closely to the inner pipe section, a near end portion corresponding in diameter to said outer pipe section for welding thereto, and a tapered portion in between.

The straight cylindrical end portions preferably extend for a length of at least 3, preferably 4 to 5 times the wall thickness of the outer pipe parallel to the axis of the pipe section. Wall thickness vary widely depending on the intended installation depth, with resultant crushing pressure of the sub-sea environment.

The clearance between the inner pipe and said straight cylindrical section is preferably less than 1.5 mm, ideally less than 1 mm all round. This allows the concentric pipes to be joined by a standard fillet weld, of well-proven integrity.

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.

The inner pipe section may be provided with one or more shoulders near one end, suitable for supporting the pipe section in a J-lay operation. The shoulder(s) may be provided by a piece welded to the inner pipe section.

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 of their inner pipe sections end to end.

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.

The invention in a second 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 being fixedly located coaxially within the outer pipe to define an annular space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end, and a plurality of pre-fabricated insulated sleeves adapted to slide over said insulated pipe sections so as to cover the projecting ends of said inner pipe sections after welding, wherein 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 invention in the second aspect further provides an insulated pipeline installation 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 being fixedly located coaxially within the outer pipe to define an annular insulating space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end, a plurality of pre-fabricated insulated sleeves positioned coaxially over said insulated pipe sections so as to cover the projecting ends of said inner pipe sections after welding, each of said prefabricated insulated sleeves comprising a single structural pipe section and outer layers of insulation carried thereon a substantially incompressible material substantially filling an annular space between the interior of each sleeve and the walls of the welded pipe sections, The pipeline may be installed on the seabed. 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.

The invention in the second aspect further provides a method of installing an insulated pipeline, the method comprising:

(a) providing 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 being fixedly located coaxially within the outer pipe to define an annular space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end;

(b) providing a plurality of pre-fabricated insulated sleeves adapted to slide over said insulated pipe sections so as to cover the projecting ends of said inner pipe sections after welding, each of said prefabricated insulated sleeves comprising a single structural pipe section and outer layers of insulation carried thereon;

(c) arranging two of said double-walled pipe sections end-to-end in a pipeline fabrication apparatus;

(d) joining adjacent ends of the inner pipe sections of said first and second pipe sections by welding; (e) sliding one of said pre-fabricated insulated sleeves over the weld location; (f) filling an annular space between the structural pipe section and the welded pipe sections with a substantially incompressible solid material; (g) translating the joined pipe sections relative to said apparatus; (h) arranging a new one of said double-walled pipe sections end-to-end with the ones already joined; and repeating steps (d) to (h) to add successive pipe sections to form a continuous pipeline of a desired length.

In a preferred embodiment, a pre-fabricated insulated sleeve is located around a portion of each new pipe section, prior to supporting it above the joined ones in step (h). Sleeves may be pre-fitted around said plurality of double-walled pipe sections prior to commencing step (c).

In a sub-sea application, said fabrication apparatus may be arranged aboard a pipe laying vessel, step (g) comprising translating the joined pipe sections along a lay path leading overboard. In general, step (g) may involve translating the apparatus relative to the pipeline (for example for overland pipelines) or translating the pipeline relative to the apparatus (easier when laying at sea), or a combination of both to maximise lay rates.

In a J-lay embodiment, step (c) will comprise suspending said pipe sections in a substantially vertical orientation, step (g) comprising lowering the joined pipe sections along said lay path into water below the apparatus. Other arrangements, including S-lay and steep S-lay may be envisaged.

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 a novel 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 longitudinal cross-section (a) and radial cross-section of the insulating sleeve used in formation of the field joint shown in Figure 5; Figure 9 shows a longitudinal cross-section (a) and two radial cross-sectional details (b) and (c) in a double-walled insulated pipeline constructed according to a second embodiment of the present invention; and

Figure 10 is a longitudinal cross-section of a double-walled pipe section used in the fabrication of the pipeline of Figure 9.

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, 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 micro porous 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 "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-fabricated 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, particularly the outer pipe 306 is different, and the pipe-in-pipe insulated sleeve 402 is replaced by a novel lightweight sleeve 500, described in more detail below. 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 Figure 8 illustrates 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. As in the conventional pipe, the outer pipe 306 has been swaged, so that its ends closely surround the inner pipe 304, to which they are welded by fillet welds 320 and 322. 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, making it relatively more easily deformed than the inner pipe. 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.

An important distinction between the novel construction and that of Figure 4 is in the form of the swaged ends of the outer pipe 306. In order to illustrate this, the length of the end part of the outer pipe is divided into zones by transverse broken lines in Figure 5(a) and Figure 6. Instead of a single conical profile, leading from the main section of the outer pipe to the fillet weld 320/322, the swaging of the outer pipe is performed to define a straight cylindrical portion 316a and 318a at each end of the outer pipe. A part-conical (tapered cylindrical) portion 316b/318b extends between the main barrel of the outer pipe 306 and the straight end portions 316a/318a. These straight portions ensure that the conical portion, is spaced well back from the location of the critical fillet weld 320/322, which joins the outer pipe to the inner pipe. The length of each zone in the axial direction of the pipeline may be, for example, 100mm.

This modified form of end for the outer pipe brings immediate advantages, in that the welds 320, 322 become more conventional fillet welds. 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.

Again, these dimensions of 100mm are an example only. A minimum of 40mm is typically required, for clear access by the inspecting head. Beyond that, it is found that the length of the straight portion 316a/318a should be at least 3 and preferably 4 to 5 times the wall thickness of the outer pipe 306. Thus 50mm to 100mm will be typical for most applications. Of course, the length of these uninsulated sections should be minimised for thermal performance reasons, and to minimise the length and weight of the sleeve 500.

In a further modification, whereas the known pipe-in-pipe section is deliberately fabricated with a significant gap between the swaged ends of the outer pipe 306 and the inner pipe 304, the modified structure is preferably formed with ends swaged so as to minimise the gap which needs to be bridged by the welds 320/322. This may be achieved firstly by eliminating the shim which is inserted in the technique described in US 6446321 , mentioned above. Ideally the gap is substantially less than 2mm all round, typically on the order of 1mm or less. A further measure which may be applied to minimise this gap, or even eliminate it, is to compress the ends of the outer pipe onto the outside of the inner pipe with sufficient force to cause elastic (that is, non-permanent) deformation of the inner pipe, to a diameter less than its normal outside diameter. By doing this, when the swaged end material of the outer pipe springs back, as inevitably happens when the swaging pressure is released, the final gap between the inside of the outer pipe ends and the outside of the inner pipe will be less than the spring-back distance.

Details of swaging apparatus are not given herein. Swaging tools operated by water pressure are well-known in principle. Alternatives include forcing the pipes axially into a tapered former with the desired profile, or deforming it with rollers rotating around the pipe ends. A further alternative would be to adapt the apparatus described in US 6446321 , to form the modified end portions 316a/318a and 316b/318b, as shown in Figure 6.

An alternative method of forming similar ends without swaging will be described further below with reference to Figures 9 and 10.

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. 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 may be, for example, 300mm, allowing space for connection of the outer pipe with flat swaged portion, and further allowing the clear end 324 to project for 100mm or more, for handling during fabrication, transport and installation. In this illustration, the shoulder piece 308 has still to be added. 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.

The process of fabricating the pipe section of Figure 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 a suitable length of inner pipe 304, the first and second layers of insulating material 502 are applied around the pipe, and captured under the foil layer 506. Centralising rings or rib (not shown in the drawings) are included, if desired. • The 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.

• Each end of the outer pipe 306 is swaged into the special form illustrated in Figure 6. As already mentioned, this step optionally includes compression radially inward of the outer diameter of the inner pipe 304, to compensate for spring-back of the swaged portions and thus minimise the gap between inner and outer pipes at portion 316a/318a.

• The fillet welds 320, 322 are formed. • The fillet welds are inspected ultrasonically, to ensure high strength and uniformity around the circumference at each end of the pipeline.

• 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. • At some point in the process, which may be before or after mounting of the outer pipe 306, shoulder piece 308 is welded to one end of the inner pipe 304, to form joint 328.

• The pipe section 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.

Figure 8 shows in more detail the pre-fabricated insulating sleeve 500 used in the construction of the field joint in the novel insulated pipeline, as seen in Figure 5. The novel sleeve 500 has only one main structural element, namely a sleeve inner pipe 512. In a preferred embodiment, this is a steel pipe with outside diameter 470mm or so, and wall thickness 22mm. If preferred, the sleeve inner pipe could be fabricated of other material, such as composite, to make it even more lightweight, in comparison with the known sleeve construction of Figure 4. The length of the sleeve which overlaps the double-walled sections of the pipe either side of the field joint is typically in the range 0.5 to 2 times the outside diameter of the outer pipe.

On the outside of the sleeve inner pipe 512 a multi-layer foam insulation is provided, for example of the "5LPP" type. This type of insulation is commonly applied by manufacturing the steel pipe with a fusion bonded epoxy layer and then supplying it to a coating company. That company then applies primer followed by a layer of solid polypropylene extruded on top. An expanded (foam) polypropylene layer is added to make up the majority of the thickness as seen in the diagram, followed by a skin layer of solid polypropylene to provide protection against water ingress. This form of insulation is often referred to as "wet insulation" as it will be exposed to the sea water environment, in the finished installation. The overall thickness of the insulating structure is around 60mm, while the solid PP skin is around 4mm thick. As most layers are not visible on the scale of the drawings shown in Figure 5 and Figure 8, only the main foam layer 514 and the outer skin 516 are labelled. The outer skin layer 516 is bonded to the sleeve inner pipe 512 at each, fully enclosing the foam 514.

Alternative insulating structures are of course possible, for example syntactic resin packed with glass beads, for example GSPU (glass syntactic polyurethane). The insulation, including bead size and so on must of course be selected to withstand crushing by pressure at depth.

A screw 520 and eye 522 are also shown, for use as described later. In the preferred example, three screws 520 are arranged at 120-degree intervals around the circumference of the sleeve, and two eyes 522 are provided diametrically opposite one another.

Referring again now to Figure 5, it can be seen that the inner diameter of the sleeve inner pipe 512 leaves a significant gap around the outer pipe of the pipe sections, when fitted over two pipe sections at their field joint. This space is filled with a hard setting polyurethane resin material, indicated by 518 in the various cross-sectional drawings. The thickness of this material may be, for example, 5-1 Omm all around the outer pipes and much bigger in the central section around the shoulder piece 308 and the exposed ends of the inner pipes, 304, 304'. This resin layer 518 has primarily a structural function, and is selected to be incompressible, rather than requiring any tensile or shear strength. The resin is injected in liquid form, substantially filling the space inside the sleeve 500, via the steel or composite sleeve inner pipe 512, where it solidifies and hardens. As is well known in pipe-in- pipe constructions, this structural rigid joining of the outer pipes either side of the field joint is critical to avoid stress concentration at the field joint, where already inner pipes are joined. The resin layer 518 is not required to have high insulating properties thanks to the insulating structure on the outside of the sleeve 500. Nor is the resin required to have any particularly strong bonding function, as the sleeve 512 will not be subject to any great sliding forces. As indicated at 520 at one end of the sleeve, screws are commonly provided in threaded bores in the sleeve pipe 512, with sharp points impinging on the outside of the pipe sections 306, 306'. These ensure electrical conductivity between the sleeve metal and the pipe, but also assist to centre the sleeve prior to filling with resin 514.

It will be apparent that the pre-fabricated insulated sleeve 500 used in the novel arrangement can be of much lighter construction than the double steel pipe sleeve, used in the conventional installation method, as illustrated in Figure 4. As an example, where a pipeline of the dimensions indicated is to be laid in water 2000m deep, which is a requirement of some modern oilfields, the basic weight of the suspended pipeline may be of the order of 700 tonnes (1 tonne = 10³ kg) Allowing for contingencies such as flooding of the pipeline, such a load is at the limit of the capacity of even quite large pipelaying vessels. The weight saving achieved by the novel construction of Figure 5, compared with the conventional construction of Figure 4 is on the order of 70 tonnes, or 10% of the total, which may be a very significant saving, commercially. 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 8, and the J-lay apparatus illustrated in Figure 2. Basic steps are:

• A stock of 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 up- ended 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.

Figures 9 and 10 illustrate a similar but slightly modified pipeline structure, in which components corresponding to components of Figures 5 to 8 are given again the same reference numerals. Only the differences in the construction of this embodiment will be described, in the interest of conciseness. Figure 9(a) shows a longitudinal cross section through the field joint between two sections of double- walled pipeline, which again are surrounded by a sleeve 500 of the same general form as shown in Figures 5 and 8, with an annular space filled by hardened resin 518. Figure 9(b) shows a detail of the radial cross-section along line B-B, near to one end of the sleeve 500. Figure 9(c) shows a detail of the radial cross-section as a central portion of the field joint, along the line C-C.

Figure 10 shows in more detail one pipe section, used in fabrication of the pipeline Figure 9. Compared with the example construction illustrated in Figures 5 to 8, a first difference visible in Figure 9 is the provision of additional foam insulation around the projecting ends of the sleeve pipe 512. This comprises resin or foam 900, wrapped in a skin 902, applied after filling the field joint annulus with resin 518. In practice, the length of the sleeve 500 is likely to be such that conduction of heat along the sleeve pipe 512 to its exposed ends does not significantly degrade the overall thermal transfer properties of the insulated pipeline. As an alternative, however, if other considerations allow the length of the sleeve to be shortened so that its ends are not so far from the exposed portion of the inner pipes 304, 304', the additional insulation 900 may be warranted. Insulation 900 may be the same injected resin as is used at 518, or another suitable material wrapped or injected around the pipeline.

The second significant difference between the example Fig 9 and the examples of Figures 5 to 8, is best seen by reference to the enlarged view of the pipe section, shown in Figure 10. As explained already with reference to Figures 5 and 6, the outer pipe 306 is joined to inner pipe 304 by a specially formed tapered section, whose ends have a flat cylindrical shape, rather than tapering directly to the diameter of the inner pipe. Instead of being formed by swaging, however, these portions in this embodiment are followed by pre-shaped short pipe sections 916 and 918, which are butt-welded to respective ends of the outer pipe 306 at 920 and 922 respectively. Each piece 916/918 is pre-formed to define said straight cylindrical portion 916a/918a, a tapered cylindrical portion 916b/918b and finally a larger diameter straight cylindrical portion 916c/918c, corresponding to the end diameter to the outer pipe 306. The end pieces 916/918 may be machined pieces, avoiding the deformations and strains associated with swaging, and allowing potentially a tighter tolerance in fitting to the inner pipe 304, by the absence of the "spring-back effect", mentioned above. These machining steps, and the additional welds 920, 922 may add to the complexity of the fabrication process, and it is a matter of preference which method has the greater benefit on a given installation task. It should be noted, however, that the butt welds 920, 922 are conventional as to be well accepted in the industry, very quickly performed and tested. Moreover, the close fitting and un-distorted form of the end portions 916a and 918a permits very high quality fillet welds 924, 926 to be formed to join the outer pipe 306 to the inner pipe 304. Again, the closely-fitting straight cylindrical portions 916a and 918a allow automated ultrasonic or x-ray inspection of the welds 924, 926 to be performed, compared with the conventional swaged joint shown in Figure 4.

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 so that the inner pipe can be located coaxially within the outer pipe to define an annular insulating space between them, the outer pipe section being shorter than the inner pipe section and being provided at each end with portions tapered inwardly so as to approach the surface of the inner pipe to permit welding of the outer pipe ends to the surface of the inner pipe near its ends, wherein extreme end portions of said outer pipe, beyond said tapered portions, have a straight cylindrical form so as to lie parallel to the surface of the inner pipe when assembled for welding.

2. An assembly as claimed in claim 1 wherein said tapered portions and extreme end portions have been formed by swaging the outer pipe on said inner pipe.

3. An assembly as claimed in claim 1 wherein the tapered portions and extreme end portions are provided on a short pipe section butt-welded at each end of a main length of said outer pipe section.

4. An assembly as claimed in claim 3 wherein the short pipe section has three distinct portions along its length: an extreme end portion comprising said straight cylindrical portion or fitting closely to the inner pipe section, a near end portion corresponding in diameter to said outer pipe section for welding thereto, and a tapered portion in between.

5. An assembly as claimed in any preceding claim wherein said straight cylindrical end portions extend for a length of at least 4 times the wall thickness of the outer pipe parallel to the axis of the pipe section.

6. An assembly as claimed in any preceding claim wherein the clearance between the inner pipe and the inside of the outer pipe in said straight cylindrical section is less than 1.5 mm all round.

7. An assembly as claimed in any preceding claim wherein the assembly includes insulating material at least partially filling said annular space.

8. An assembly as claimed in any preceding claim wherein said annular space is maintained at a pressure lower than atmospheric pressure.

9. An assembly as claimed in any preceding claim wherein the inner pipe section is provided with one or more shoulders near one end, suitable for supporting the pipeline during installation.

10. A pipeline installation comprising a plurality of double walled pipe sections according to any preceding claim the invention as set forth above, joined by welding of their inner pipe sections end to end.

11. An installation as claimed in claim 10 wherein the pipe sections where joined together are surrounded by a reinforcing sleeve.

12. An installation as claimed in claim 10 or 11 wherein space within said sleeve is filled with an incompressible solid material.

13. An installation as claimed in claim 10 or 11 wherein the reinforcing sleeve is provided on its outside with an insulating coating.

14. A method of forming a double walled pipe assembly of the type claimed in claim 1 , the method comprising the following steps: (a) providing said inner pipe section and outer pipe sections;

(b) sliding said outer pipe section over said inner pipe section so that a portion of said inner pipe projects from the outer pipe section at each end; (c) forming said tapered portions at the ends of the outer pipe, including said straight cylindrical portions, this step being performed either before or after step (b); and

(d) welding the ends of said straight cylindrical portions to the outer surface of the inner pipe to define an enclosed annular space between said inner and outer pipe sections.

15. A method as claimed in claim 14, wherein step (c) is performed for at least one of said ends after step (b).

16. A method as claimed in claim 15 wherein step (a) further includes providing a blanket of insulating material around the inner pipe section, prior to step (b).

17. A method as claimed in any of claims 14 to 16 wherein the step (c) is performed by swaging the ends of the outer pipe section.

18. A method as claimed in claim 17 wherein said swaging is performed so as to include elastic deformation of the inner pipe, the inner diameter of the end of outer pipe section being temporarily lower than the natural outer diameter of the inner pipe section.

19. A method as claimed in any of claims 14 to 19 wherein the step (c) is performed by adding a short pipe section to ends of a plain outer pipe section, the short pipe section having pre-formed within it said tapered and straight cylindrical portions.

20. A method as claimed in any of claims 14 to 19 wherein said straight cylindrical end portions extend for a length of at least 4 times the wall thickness of the outer pipe parallel to the axis of the pipe section.

21. A method as claimed in any of claims 14 to 20 wherein the clearance between the inner pipe and the inside of the outer pipe in said straight cylindrical section prior to welding is less than 1.5 mm all round.

22. A method as claimed in any of claims 14 to 21 further comprising a step (e) of modifying the atmosphere within the annular space to increase its thermal insulation properties.

23. 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 being fixedly located coaxially within the outer pipe to define an annular insulating space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end, and a plurality of pre-fabricated insulated sleeves adapted to slide over said insulated pipe sections so as to cover the projecting ends of said inner pipe sections after welding, wherein each of said prefabricated insulated sleeves comprises a single structural pipe section and at least one outer layer of insulation carried thereon.

24. A set of parts as claimed in claim 23 wherein the structural pipe sections of said sleeves are of steel.

25. A set of parts as claimed in claim 23 or 24 wherein said at least one insulating layer comprises a layer of synthetic material.

26. A set of parts as claimed in claim 25 wherein said synthetic material is polyurethane.

27. A set of parts as claimed in claim 25 or 26 wherein said synthetic material is loaded with glass spheres to form a syntactic insulation material.

28. An insulated pipeline installation 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 being fixedly located coaxially within the outer pipe to define an annular insulating space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end, a plurality of pre-fabricated insulated sleeves positioned coaxially over said insulated pipe sections so as to cover the projecting ends of said inner pipe sections after welding, each of said prefabricated insulated sleeves comprising a single structural pipe section and outer layers of insulation carried thereon - a substantially incompressible material substantially filling an annular space between the interior of each sleeve and the walls of the welded pipe sections.

29. An installation as claimed in claim 28 wherein the structural pipe sections of said sleeves are of steel.

30. An installation as claimed in claim 28 or 29 wherein said at least one insulating layer comprises a layer of synthetic material.

31. An installation as claimed in claim 30 wherein said synthetic material is polyurethane.

32. An installation as claimed in claim 30 or 31 wherein said synthetic material is loaded with glass spheres to form a syntactic insulation material.

33. An installation as claimed in claim 28 wherein the pipeline is installed on the seabed.

34. An installation as claimed in claim 28 wherein the pipeline forms part of a riser structure for transferring fluids from the seabed to the sea surface.

35. An installation as claimed in claim 28 to 34 wherein each of said double walled pipe sections comprises an assembly as claimed in claim 1.

36. A method of installing an insulated sub-sea pipeline, the method comprising:

(a) providing 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 being fixedly located coaxially within the outer pipe to define an annular insulating space between them, the outer pipe section being shorter than the inner pipe section, such that end portions of the inner pipe section project sufficiently for welding together end to end;

(c) suspending two of said double-walled pipe sections in a substantially vertical orientation (d) joining the inner pipe sections of said first and second pipe sections end-to- end by welding;

(e) sliding one of said pre-fabricated insulated sleeves over the weld location;

(f) filling an annular space between the structural pipe section and the welded pipe sections with a substantially incompressible solid material; (g) lowering the suspended pipe sections toward the seabed;

(h) supporting a new one of said double-walled pipe sections above the ones already joined; and repeating steps (d) to (h) to add successive pipe sections to form a continuous pipeline of a desired length.

37. A method as claimed in claim 36 wherein a pre-fabricated insulated sleeve is located around a portion of each new pipe section, prior to supporting it above the joined ones in step (h).

38. A method as claimed in claim 37 wherein said sleeves are pre-fitted around said plurality of double-walled pipe sections prior to commencing step (c).

39. A method as claimed in claim 36, 37 or 38 wherein the structural pipe sections of said sleeves are of steel.

40. A method as claimed in any of claims 36 to 39 wherein said at least one insulating layer comprises a layer of synthetic material.

41. A method as claimed in any of claims 36 to 40 wherein said synthetic material is polyurethane.

42. A method as claimed in claim 40 or 41 wherein said synthetic material is loaded with glass spheres to form a syntactic insulation material.