Patent Description:
Pipelines are used underwater as 'tie-backs' to transport crude oil and/or natural gas from a subsea wellhead across the seabed on the way to the surface, typically via a riser. Specifically, on production of hydrocarbons from a subsea well, the produced fluid emerges from the wellhead at high temperature and pressure and then enters a subsea pipeline. During subsequent transportation along the pipeline, the temperature and pressure of the produced fluid have to be kept high enough to ensure a sufficient flow rate across the seabed and up the riser. Various measures are therefore taken to ensure that the internal temperature of the pipeline remains high despite thermal exchange with the much colder surrounding seawater.

PiP assemblies are often used as subsea pipelines in view of their thermally-insulating structure. A PiP pipeline comprises a fluid-carrying inner pipe positioned concentrically within an outer pipe. The inner and outer pipes are spaced from each other to define a thermally-insulating annulus between them. Typically, thermally insulating material is disposed in the annulus; it is also possible to draw down a partial vacuum in the annulus or to inject an insulating gas to reduce transmission of heat through the annulus.

Two main types of PiP assemblies are known, namely sliding and non-sliding arrangements.

In sliding PiP arrangements, the inner pipe is free to slide longitudinally within the outer pipe along most or substantially all of its length, eventually being restrained only at the ends of the pipeline. In that case, the inner pipe is supported within the outer pipe by longitudinally-distributed spacers or centralisers that extend radially across the annulus and that permit sliding movement of the inner pipe relative to the outer pipe.

Conversely, in non-sliding PiP arrangements, the inner pipe is restrained at much shorter longitudinal intervals between the ends of the pipeline. For example, the ends of the outer pipe may be swaged to close the annulus every two pipe joints, this being at <NUM> intervals given that a standard pipe joint is <NUM> in length.

Non-sliding PiP pipelines are relatively easy to manufacture because short sections of pipe can simply be welded together. However, they cannot easily accommodate variations of length caused by thermal expansion or bending during installation and in use. Sliding PiP pipelines are therefore preferred where thermal expansion of the inner pipe relative to the outer pipe cannot be neglected, for example in deep-water oil production.

An example of an S-lay vessel is shown in <CIT>. The firing line of such a vessel is narrow, congested and busy during pipelaying operations, noting that multiple pipeline assembly steps must be performed over a short distance within a limited time. Conventionally, the firing line comprises a longitudinal succession of workstations, for example for welding, testing and coating the pipeline. Different assembly steps are performed simultaneously at different workstations. The pipeline is advanced through the workstations in stepwise fashion as the respective assembly steps are completed.

Movement of the pipeline along the firing line is controlled by tensioners at the downstream end of the firing line, most commonly continuous-track tensioners as shown in <CIT>. The tensioners also support the weight of the pipeline catenary that is suspended between the vessel and the seabed.

The pipeline and its constituent pipe joints are typically supported on the firing line by roller boxes as shown in <CIT>, Support systems with mobile carriages are also known, for example as shown in <CIT>, but that option limits the number of workstations; static work stations are therefore preferred.

Assembling a PiP pipeline aboard an S-lay vessel can be very difficult in such a confined space. In this respect, the inner pipe has to be able to slide relative to the outer pipe in order to weld together adjoining inner pipes before welding together the surrounding outer pipes. This adds extra handling operations.

Roller boxes are used for displacing the inner pipe and the outer pipe during their passage through the successive workstations. An improved roller box is disclosed in <CIT>, in which clamps are added to guide the pipeline laterally. More generally, various clamps are known in the art. Spacer systems that double as retainers are also known in the art.

<CIT> discloses retaining means for a pipe stalk which prevents axial movement of an inner pipe with respect to an outer pipe, when in a first configuration, and enables the outer pipe to move freely axially with respect to the inner pipe in a second configuration. The retaining means comprises first and second abutment stops, provided on the inner surface of the outer pipe and the outer surface of the inner pipe, respectively. A rotatable key member is located between the first and second abutment stops; by rotating the key member such that it extends between the two abutment stops, axial movement between the inner and outer pipes is prevented.

<CIT> provides apparatus for supporting pipes as they are moved to prevent rolling, dragging or dropping of the pipes, which may damage any coating present on the pipes. Hydraulically driven pivotal lifters lift the pipe and deposit in on a rack. <CIT> discloses a welding complex in which pipe sections are supported and rotated to be brought into alignment with a pipeline so that a weld between the section and parent pipeline can be made.

<CIT> relates to a system for alignment and internal welding of pipe sections. An external alignment mechanism comprises rollers that allow longitudinal movement of the pipe sections. The rollers may also allow rotation of the pipes around the longitudinal axis and translational movement of the pipe sections. A welding mechanism may include a laser to track the profile of the weld.

<FIG> of the accompanying drawings illustrate a conventional method for assembling a sliding PiP pipeline <NUM> on the firing line <NUM> of an S-lay vessel. In the drawings, the pipeline <NUM> is assembled and launched from right to left as the vessel advances from left to right. Thus, a PiP pipe joint <NUM> shown on the right of the drawings is added to the upper, trailing end of the pipeline <NUM> and a catenary of the assembled pipeline <NUM> is gripped by, and suspended from, tensioners <NUM> shown on the left of the drawings.

Each pipe joint <NUM> comprises an inner pipe <NUM> disposed concentrically within an outer pipe <NUM>. The radial spacing between the inner and outer pipes <NUM>, <NUM> defines an annulus <NUM> between them. The inner pipe <NUM> is slidably supported within the outer pipe <NUM> by longitudinally-spaced spacers <NUM> that extend radially across, and circumferentially around, the annulus <NUM>.

Typically, the pipe joints <NUM> are single or double joints, hence being <NUM> or <NUM> in length, but they are shown greatly shortened in the drawings for ease of illustration. Similarly, thermal insulation that could be disposed in the annulus <NUM> is omitted from these simplified schematic views.

<FIG> shows a pipe joint <NUM> supported by a longitudinal array of roller boxes <NUM> and approaching the upper end of the pipeline <NUM>, which is also supported by roller boxes <NUM> of the array and is held stationary at an upstream workstation 28A by the tensioners <NUM>. It will be noted that the inner pipe <NUM> of the pipeline <NUM> has been displaced relative to the outer pipe <NUM> of the pipeline <NUM> in the upstream direction to protrude from the upper end of the outer pipe <NUM>. Thus, the upper ends of the inner and outer pipes <NUM>, <NUM> are staggered longitudinally to provide access to the upper end of the inner pipe <NUM> for welding.

Similarly, the inner and outer pipes <NUM>, <NUM> of the pipe joint <NUM> are also staggered longitudinally but in mirror relation. Thus, the inner pipe <NUM> has been displaced relative to the outer pipe <NUM> of the pipe joint <NUM> in the downstream direction to provide access to the downstream end of the inner pipe <NUM> for welding.

The inner and outer pipes <NUM>, <NUM> of the pipeline <NUM> and the pipe joint <NUM> are coaxially aligned with their counterparts in mutual opposition. In each case, the spacers <NUM> hold the inner pipes <NUM> in concentric relation with the outer pipes <NUM>. Thus, the outer pipes <NUM> are supported by the roller boxes <NUM> and the inner pipes <NUM> are supported by the outer pipes <NUM> via the spacers <NUM>.

When the inner pipes <NUM> of the pipeline <NUM> and the pipe joint <NUM> are brought together in mutual abutment at the upstream workstation 28A as shown in <FIG>, a welding apparatus <NUM> is fitted to one of the inner pipes <NUM> to form a circumferential butt weld <NUM> between them. Next, the welding apparatus <NUM> is removed from the inner pipe <NUM> and the tensioners <NUM> are activated to advance the pipeline <NUM> and the pipe joint <NUM> together until the trailing end of the outer pipe <NUM> of the pipeline <NUM> reaches a downstream workstation 28B. There, the outer pipe <NUM> of the pipe joint <NUM> is advanced in the downstream direction into mutual abutment with the outer pipe <NUM> of the pipeline <NUM> as shown in <FIG>. A welding apparatus <NUM> is shown fitted to one of the outer pipes <NUM> at the downstream workstation 28B to form a circumferential butt weld <NUM> between them.

With the pipe joint <NUM> thus added to the trailing end of the pipeline <NUM>, the tensioners <NUM> can then advance the pipeline <NUM> relative to the vessel and the firing line <NUM> in the downstream direction, from right to left as illustrated, as the vessel advances in the upstream direction, from left to right as illustrated. The process can then be repeated with an additional pipe joint 14A to extend the pipeline <NUM> in stepwise fashion.

It will be apparent that the welds <NUM>, <NUM> between the inner pipes <NUM> and the outer pipes <NUM> have to be performed in sequence at the workstations 28A, 28B. Thus, preparations for the weld <NUM> between the outer pipes <NUM> can only begin when the weld <NUM> between the inner pipes <NUM> has already been completed and the pipeline <NUM> has been advanced from the upstream workstation 28A to the downstream workstation 28B. This prolongs the fabrication process.

For simplicity, <FIG> focus upon the welding steps of pipeline fabrication but they omit the subsequent steps of testing the welds and coating the resulting field joints. The skilled reader will appreciate that performing these subsequent steps will further lengthen the fabrication process and will further complicate pipe handling.

Against this background, the invention resides in a method of assembling a pipe-in-pipe pipeline by adding a succession of inner and outer pipe sections to a trailing end of the partially-completed pipeline. The method is particularly apt to be performed aboard a pipelaying vessel during an S-lay operation. The method comprises: welding a leading end of a first inner pipe section to a trailing end of an inner pipe of the partially-completed pipeline; advancing a leading end of a second inner pipe section to a trailing end of the first inner pipe section; advancing an outer pipe section downstream from the second inner pipe section along and around the first inner pipe section and into abutment with a trailing end of an outer pipe of the partially-completed pipeline; and welding a leading end of the outer pipe section to the trailing end of the outer pipe. The outer pipe section may be advanced along at least a full length of the first inner pipe section, hence being advanced to a distance at least the full length of the outer pipe section.

Preferably, the leading end of the second inner pipe section is welded to the trailing end of the first inner pipe section before advancing the outer pipe section onto the first inner pipe section. Attachment of the first inner pipe section to the inner pipe of the partially-completed pipeline and to the second inner pipe section may be performed in simultaneous or chronologically overlapping welding operations. Conveniently, the outer pipe section and the second inner pipe section may be provided in concentric relation as a pipe-in-pipe pipe joint.

The outer pipe section may be welded to the outer pipe at a downstream workstation; and the second inner pipe section may be welded to the first inner pipe section at an upstream workstation. The first inner pipe section may be welded to the inner pipe at an intermediate workstation.

Advantageously, the advancing pipe sections may be supported centred on a longitudinal axis in common with that of the trailing end of the partially-completed pipeline. This may be achieved by deploying a secondary support system to support the first inner pipe section centred on that common axis while advancing and welding the first inner pipe section to the inner pipe. Thus, the first inner pipe section may be advanced when unsupported by the outer pipe section. The secondary support system may then be retracted before advancing the outer pipe section, supported by a primary support system, around the first inner pipe section.

The use of a secondary support system may also be expressed, within the inventive concept, as a method of assembling a pipe-in-pipe pipeline by adding inner and outer pipe sections to a trailing end of the partially-completed pipeline. That method comprises: using a primary support system to support an outer pipe section centred on a longitudinal axis in common with the trailing end of the partially-completed pipeline; deploying a secondary support system to support a first inner pipe section centred on that axis; while supported by the deployed secondary support system, advancing and welding the first inner pipe section to an inner pipe of the partially-completed pipeline; retracting the secondary support system; and while supported by the primary support system, advancing the outer pipe section around the first inner pipe section and welding the outer pipe section to an outer pipe of the partially-completed pipeline.

After the secondary support system has been retracted, the outer pipe section preferably supports the first inner pipe section centred on the common axis. For example, the outer pipe section may support the first inner pipe section via at least one spacer in an annulus between those pipe sections. The, or each, spacer may be mounted to the first inner pipe section before advancing the outer pipe section over the or each spacer. Alternatively, the outer pipe section may be advanced as the, or each, spacer, disposed within and moving with the outer pipe section, slides over the first inner pipe section.

The secondary support system may be redeployed behind the advanced outer pipe section to support a second inner pipe section on the deployed secondary support system, also centred on the common axis. While supported by the deployed secondary support system, the second inner pipe section may be advanced into abutment with the first inner pipe section. The outer pipe section may then be advanced from around the second inner pipe section and onto the first inner pipe section.

The primary support system may contact the outer pipe section at a first level and, when deployed, the secondary support system may contact the first inner pipe section at a second level above the first level.

Elegantly, retraction of the secondary support system away from the first inner pipe section may provide access for the outer pipe section to contact and be supported by the primary support system. For example, the secondary support system may comprise support formations such as rails that, during retraction, move apart from each other in opposed lateral directions to define a gap that accommodates the outer pipe section between them.

Advantageously, a longitudinal series of secondary support systems may be operated independently of each other for individual deployment and retraction in response to longitudinal movement of any of the pipe sections.

The inventive concept also embraces an apparatus for assembling a pipe-in-pipe pipeline, the apparatus comprising a primary support system for supporting an outer pipe section and a secondary support system for supporting an inner pipe section. The primary support system is arranged to support the outer pipe section centred on a longitudinal axis in common with a trailing end of a partially-completed pipeline. The secondary support system is deployable to support the inner pipe section also centred on that axis, to enable the inner pipe section to be advanced and welded to an inner pipe of the partially-completed pipeline. The secondary support system is retractable to advance the outer pipe section around the inner pipe section to enable the outer pipe section, when supported by the primary support system, to be welded to an outer pipe of the partially-completed pipeline.

Conveniently, the secondary support system may be mounted on the primary pipe support system, which may, for example, comprise at least one roller box or support table.

Preferably, the primary support system is configured to contact the outer pipe section at a first level and the secondary support system is configured, when deployed, to contact the first inner pipe section at a second level above the first level.

The secondary support system suitably comprises laterally-displaceable support formations such as longitudinally-extending parallel rails that, when deployed to respective inward positions, together define an axial path for guiding the advance of the inner pipe section. When the support formations are retracted in opposite lateral directions to respective outward positions, a gap between them may provide access for the outer pipe section to contact and be supported by the primary support system. The support formations may, for example, be pivotable about respective axes extending substantially parallel to the common longitudinal axis.

There could be a longitudinal series of secondary support systems, each of which is operable independently of the others for individual deployment and retraction. The primary support system may also be retractable away from the common axis to permit deployment of the secondary support system.

The inventive concept also embraces a pipelaying vessel configured for S-lay operation comprising apparatus of the invention or operating in accordance with methods of the invention.

By virtue of the invention, displaceable support bars or rails may be added to, or otherwise used in combination with, roller boxes or horizontal support tables. The roller boxes or support tables support the outer pipe of a PiP assembly for longitudinal movement along a desired central longitudinal axis. The support bars are arranged to support the inner pipe of the assembly for longitudinal movement along the same central longitudinal axis. As a result, the cycle time for adding a new pipe section or joint to the upper end of the catenary may be greatly reduced. Beneficially, two welding stations may be dedicated to inner pipe activities. With appropriate longitudinal spacing between the roller boxes or support tables and the workstations, inner pipe sections and/or outer pipe sections can be welded nearly simultaneously after the inner pipe of the first joint has been welded to the catenary.

In one practical embodiment of the invention, a PiP slide rail system allows a compact four-station welding spread to be used when fabricating and laying a PiP pipeline. This unique slide rail system overcomes problems arising from a differential in the level or height of the bottom of the pipe between the inner and outer pipes. Parallel rails support the inner pipe along the length of the main firing line with a higher centre bias, enabling the outer pipe to be sleeved telescopically over the inner pipe. This sleeving action is also unique to pipelaying and enables the aforementioned four-station welding spread. The rails and/or spacers or centralisers constantly support the inner pipe.

Embodiments of the invention provide a device for supporting the elements of a PiP assembly comprising an outer pipe and an inner pipe with smaller diameter during assembly. The device comprises: a primary pipe support system, such as a roller box or a support table, that is able to support the outer pipe; and a secondary pipe support system that is adjustable either to support the inner pipe or to be displaced outside the path of the outer pipe.

The secondary pipe support system may, for example, be mounted on the primary pipe support system. The secondary system may comprise a starboard support assembly and a port support assembly, each support assembly comprising at least one axially-oriented support rail and a mechanism for displacing the at least one support rail, so that the rails of those assemblies can together define an axial path in an inner pipe support mode.

The support rails may, for example, support the inner pipe by being pivoted inwards. The support rails may also be displaced by pivoting outwards beyond the outer pipe trajectory, allowing the outer pipe to be supported by the primary pipe support system in an outer pipe support mode.

Embodiments of the invention also provide equipment for supporting a PiP pipeline aboard a lay vessel, the equipment comprising: at least one roller box supporting the outer pipe; and at least one horizontal table axially aligned with the roller box; wherein the horizontal table comprises axially-oriented rails able to support the inner pipe where the inner pipe is not inside the outer pipe. The rails can be separated to allow the at least one roller box to support the outer pipe. The or each roller box may also comprise supporting rails.

Furthermore, embodiments of the invention implement a method for assembling a PiP pipeline aboard a pipelaying vessel performing an S-lay operation. The method comprises: (a) holding a catenary of the pipeline; (b) setting rails on support equipment for supporting and welding a first inner pipe section to the inner pipe of the pipeline catenary; (c) providing a pre-arranged PiP section comprising a second inner pipe section, an outer pipe section and spacers; (d) welding the second inner pipe section to the first inner pipe section; (e) removing the rails and (f) sliding the outer pipe section around the first inner pipe section while setting the rails for supporting the second inner pipe section; and advancing the catenary and the vessel. The method is then repeated from step (c) onwards to continue fabricating the pipeline. The inner pipe sections have a smaller diameter than the outer pipe section.

In summary, apparatus of the invention for assembling a pipe-in-pipe pipeline may comprise a primary support system, such as an array of rollers, for supporting an outer pipe section and a secondary support system, such as a set of rails, for supporting an inner pipe section. The primary support system supports the outer pipe section centred on a longitudinal axis in common with a trailing end of a partially-completed pipeline.

The secondary support system is deployed to support the inner pipe section also centred on that common axis, enabling the inner pipe section to be advanced and welded to an inner pipe of the partially-completed pipeline. The secondary support system is then retracted to provide clearance to advance the outer pipe section, supported by the primary support system, around the inner pipe section. This enables the outer pipe section to be welded to an outer pipe of the partially-completed pipeline.

To put the invention into context, reference has already been made to <FIG> of the accompanying drawings. As noted above, those figures are a sequence of schematic side views of the firing line of an S-lay vessel when fabricating a PiP pipeline in accordance with a conventional technique.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings, in which:.

Where appropriate, like numerals are used for like features in the description that follows.

Referring firstly to <FIG>, <FIG> shows a pipe joint <NUM> comprising concentric inner and outer pipes <NUM>, <NUM> of substantially the same length as each other. The inner and outer pipes <NUM>, <NUM> are coaxial, being rotationally symmetrical about a common central longitudinal axis <NUM>.

The inner pipe <NUM> has an external diameter that is substantially smaller than the internal diameter of the outer pipe <NUM>. Thus, the inner and outer pipes <NUM>, <NUM> are separated by an annulus <NUM> and, in particular, by a spacer <NUM> that is disposed in the annulus <NUM>.

<FIG> shows a primary support system comprising paired cylindrical rollers <NUM> that are arranged to support the outer pipe <NUM>. A longitudinal succession of such paired rollers <NUM> extends along and under the outer pipe <NUM>. The rollers <NUM> turn about respective axes of rotation <NUM> that lie in a plane orthogonal to the central longitudinal axis <NUM>, such that there is rolling contact between the rollers <NUM> and the outer pipe <NUM> in the longitudinal direction. As is conventional, the axes <NUM> of the rollers <NUM> converge downwardly so that the oppositely-inclined rollers <NUM> cradle the underside of the outer pipe <NUM>. This keeps the outer pipe <NUM> correctly aligned as it travels along the firing line parallel to the central longitudinal axis <NUM>.

<FIG> shows a secondary support system comprising paired rails <NUM> that are arranged to support the inner pipe <NUM> in accordance with the invention. The rails <NUM> extend parallel to each other and to the central longitudinal axis <NUM>, such that there is sliding contact between the rails <NUM> and the inner pipe <NUM>. The rails <NUM> are spaced apart laterally in a horizontal plane to cradle the underside of the inner pipe <NUM> in the gap between them. This keeps the inner pipe <NUM> correctly aligned as it travels along the firing line parallel to the central longitudinal axis <NUM>.

As a comparison between <FIG> makes clear, the rails <NUM> contact the inner pipe <NUM> at a level above the level at which the rollers <NUM> contact the outer pipe <NUM>. The difference in height between those levels is sufficient for the rails <NUM> to support the inner pipe <NUM> in concentric alignment with the outer pipe <NUM> on their common central longitudinal axis <NUM>, despite the much smaller external diameter of the inner pipe <NUM>. Put another way, the closest point of the rails <NUM> is nearer, radially and vertically, to the central longitudinal axis <NUM> than the closest point of the rollers <NUM>.

The rollers <NUM> may be part of a roller box or a support table. As will be explained, the rails <NUM> are apt to be fitted to a roller box or support table that is modified in accordance with the invention. The rails <NUM> could be replaced by a longitudinal array of rollers or could support such an array of rollers that effect rolling contact with, and axial alignment of, the inner pipe <NUM>. More generally, additional rollers, rails or other guides could bear against the outside of the inner pipe <NUM> and/or against the outside of the outer pipe <NUM> to keep those pipes <NUM>, <NUM> correctly aligned as they travel along the firing line parallel to the central longitudinal axis <NUM>.

<FIG> shows the rails <NUM> that exemplify the secondary support system deployed in a first operating mode in which the rails <NUM> are positioned to support the inner pipe <NUM>. However, it will be apparent that the rails <NUM> lie within the radius of the outer pipe <NUM> and so have to be moved laterally out of the way to allow clearance for longitudinal movement of the outer pipe <NUM>. In this respect, <FIG> shows the outer pipe <NUM> supported by the rollers <NUM> in a second operating mode as also shown in <FIG>. Here, the rails <NUM> have been retracted and displaced outwardly relative to the central longitudinal axis <NUM> to positions outside the external circumference or envelope of the outer pipe <NUM>. Optionally, but conveniently, the retracted rails <NUM> are also displaced upwardly and outwardly relative to their positions in the first operating mode shown in <FIG>.

In principle, it is possible to move the rails <NUM> of the secondary support system into the deployed positions of the first operating mode shown in <FIG> without moving the rollers <NUM> of the primary support system relative to the central longitudinal axis <NUM>. However, if there is insufficient clearance for this, provision can be made to retract the primary support system out of the way when deploying the secondary support system. In this respect, <FIG> show two ways in which the rollers <NUM> could be retracted to allow clearance for the rails <NUM> to move from the retracted position shown in <FIG> into the deployed position shown in <FIG>.

<FIG> shows the rollers <NUM> pivoted away in opposite directions from their positions in the second operating mode as shown in <FIG> and <FIG>. Conveniently, each roller <NUM> could be mounted with a respective one of the rails <NUM> on a common pivotable chassis, such that swinging the roller <NUM> out of contact with the outer pipe <NUM> in the second operating mode also swings the rail <NUM> into contact with the inner pipe <NUM> in the first operating mode. The rollers <NUM> and the rails <NUM> could, for example, pivot about axes parallel to the central longitudinal axis <NUM>.

<FIG> shows the rollers <NUM> lowered away from their positions in the second operating mode as shown in <FIG> and <FIG>, following a downward translational movement of both rollers <NUM> in unison.

Whilst <FIG> all show the rails <NUM> engaging the outer contour of the inner pipe <NUM>, the rails <NUM> may, in practice, bear against spacers or centralisers that are mounted on the inner pipe <NUM>. This keeps the rails <NUM> clear of fragile thermal insulation material that may be wrapped around the inner pipe <NUM>. If there is no risk of damage to thermal insulation, the rails <NUM> may bear directly against the inner pipe <NUM> when in the second operating mode. The rails <NUM> may also be adjustable to different radii relative to the central longitudinal axis <NUM> when in the second operating mode, also to cater for the presence of spacers or centralisers.

Turning next to <FIG>, these drawings illustrate a method of the invention for assembling a sliding PiP pipeline <NUM> on the firing line <NUM> of an S-lay vessel, using a secondary support system comprising rails <NUM> as illustrated in <FIG>. In this instance, the firing line <NUM> comprises, in downstream succession, a first workstation 28A, a second workstation 28B and a third workstation 28C. The workstations 28A, 28B, 28C are spaced longitudinally from each other by the length of a pipe joint <NUM>, this being either <NUM> or <NUM> depending upon whether the pipe joint <NUM> is a single or double joint.

Again, the pipeline <NUM> is assembled and launched in a downstream direction from right to left as the vessel advances from left to right. Thus, a PiP pipe joint <NUM> shown on the right of the drawings is added to the upper, trailing end of the pipeline <NUM> and a catenary of the assembled pipeline <NUM> is shown on the left of the drawings. The catenary is gripped by, and suspended from, tensioners downstream of the location shown in <FIG>. In this case, however, the tensioners have been omitted from these drawings.

In <FIG>, the rails <NUM> are shown in solid lines when lowered and deployed in the first operating mode as shown in <FIG>, <FIG> and in dashed lines when raised and retracted in the second operating mode as shown in <FIG>.

In this example, the secondary support system exemplified by the rails <NUM> and the primary support system exemplified by the rollers <NUM> of the roller boxes <NUM> are structurally connected. In particular, the rails <NUM> are supported by, and movable relative to, the roller boxes <NUM>. Also, when the rails <NUM> of neighbouring roller boxes <NUM> are deployed in the first operating mode, those rails <NUM> align and cooperate in series to form a substantially continuous track.

The rails <NUM> of neighbouring roller boxes <NUM> can be deployed and retracted independently as required. This allows deployed rails <NUM> of a roller box <NUM> to support part of an inner pipe <NUM> protruding from an outer pipe <NUM> while that outer pipe <NUM> is supported by the rollers <NUM> of a neighbouring roller box <NUM>, whose rails <NUM> are retracted. When an advancing outer pipe <NUM> supported by the rollers <NUM> of one roller box <NUM> approaches deployed rails <NUM> of a neighbouring roller box <NUM>, those rails <NUM> retract before collision to allow the outer pipe <NUM> to proceed unhindered.

<FIG> shows a discrete intermediate inner pipe 18A supported by deployed rails <NUM> and being advanced downstream while approaching the upper or upstream end of the pipeline <NUM>, which is supported by roller boxes <NUM> and held at the second intermediate workstation 28B. <FIG> also shows a PiP pipe joint <NUM> being advanced downstream while approaching the upstream end of the intermediate inner pipe 18A.

When the downstream or leading end of the intermediate pipe section 18A has been brought into abutment with the trailing end of the inner pipe <NUM> of the pipeline <NUM> at the second workstation 28B as shown in <FIG>, a welding apparatus <NUM> forms a circumferential butt weld <NUM> between them. In this respect, as before, the inner pipe <NUM> of the pipeline <NUM> is displaced relative to the outer pipe <NUM> of the pipeline <NUM> in the upstream direction to protrude from the upper end of the outer pipe <NUM>. By staggering the upper ends of the inner and outer pipes <NUM>, <NUM> in this way, the upper end of the inner pipe <NUM> is accessible for welding.

Next, with the intermediate inner pipe 18A still supported by the rails <NUM>, the pipe joint <NUM> is advanced to bring the downstream or leading end of its inner pipe <NUM> into abutment with the upstream or trailing end of the intermediate inner pipe 18A at the first workstation 28A. A welding apparatus <NUM> then forms a circumferential butt weld <NUM> between the intermediate inner pipe 18A and the inner pipe <NUM> of the pipe joint <NUM> as also shown in <FIG>.

In principle, it would be possible to form the welds <NUM> at the first and second workstations 28A, 28B simultaneously with each other and/or with the weld <NUM> at the third workstation 28C between successive outer pipe sections <NUM>.

<FIG> shows the trailing end of the pipeline <NUM> now advanced to the third workstation 28C, hence advancing the intermediate inner pipe 18A and the pipe joint <NUM> with it. The weld <NUM> between the trailing end of the intermediate inner pipe 18A and the leading end of the pipe joint <NUM> is therefore now at the second workstation 28B. An additional PiP pipe joint 14A is shown approaching the trailing end of the pipe joint <NUM>, which is now at the first workstation 28A.

<FIG> then shows the optional step of fixing spacers <NUM> around the intermediate pipe section 18A. Some of the rails <NUM> supporting the intermediate inner pipe 18A can be retracted, if required, to provide clearance for mounting the spacers <NUM>. The rails <NUM> can then be deployed radially inwardly to the extent required to bear against the spacers <NUM>. This step can be performed before the abovementioned steps if required; for example, the intermediate pipe section 18A could be provided with spacers <NUM> at the outset.

Next, as shown in <FIG>, the outer pipe <NUM> of the pipe joint <NUM> is advanced in the downstream direction along and around the intermediate inner pipe 18A. The outer pipe <NUM> of the pipe joint <NUM> slides over the spacers <NUM> of the intermediate inner pipe 18A as the rails <NUM> supporting the intermediate inner pipe 18A are retracted, allowing the rollers <NUM> of the roller boxes <NUM> to support the advancing outer pipe <NUM>. <FIG> shows the outer pipe <NUM> of the pipe joint <NUM> now advanced fully along the intermediate inner pipe 18A into mutual abutment with the outer pipe <NUM> of the pipeline <NUM> at the third workstation 28C.

<FIG> shows the additional pipe joint 14A brought into mutual abutment with the trailing end of the pipe joint <NUM> at the first workstation 28A. A first welding apparatus <NUM> is shown forming a circumferential butt weld <NUM> between the inner pipes <NUM> of the pipe joint <NUM> and of the additional pipe joint 14A. A second welding apparatus <NUM> shown at the third workstation 28C forms a circumferential butt weld <NUM> between the trailing end of the outer pipe of the pipeline <NUM> and the leading end of the outer pipe <NUM> of the pipe joint <NUM>. These two welding operations may be performed simultaneously at the workstations 28A, 28C as shown in <FIG>. However, in principle, either welding operation could be performed before the other, conveniently first at the third workstation 28C as soon as the outer pipe <NUM> of the pipe joint <NUM> has been advanced fully along the intermediate inner pipe 18A.

Thus extended, the pipeline <NUM> is advanced by the tensioners to bring the new trailing end of the pipeline <NUM> to the third workstation 28C as shown in <FIG>. The leading and trailing ends of the inner pipe <NUM> of the pipe joint <NUM> are now in line with the third and second workstations 28C, 28B respectively. The outer pipe <NUM> of the additional pipe joint 14A can then be advanced along the inner pipe <NUM> of the pipe joint <NUM>, as in <FIG>, to be welded to the outer pipe <NUM> at the new trailing end of the pipeline <NUM> as in <FIG>. Another PiP pipe joint 14B is shown in <FIG> ready for its inner pipe <NUM> to be welded at the first workstation 28A as in <FIG>. Similar steps are repeated until the pipeline <NUM> is complete.

<FIG> show another way of handling the spacers <NUM>. In this example, the spacers <NUM> move with the outer pipe <NUM> of the pipe joint <NUM>. Consequently, there is no need to place spacers <NUM> onto the intermediate inner pipe 18A before advancing the outer pipe <NUM> of the pipe joint <NUM> downstream. This arrangement could be adopted where there is no thermal insulation material preinstalled around the intermediate inner pipe 18A or the inner pipe <NUM> of the pipe joint.

In <FIG>, welds <NUM> have already been formed between the inner pipe <NUM> of the pipeline <NUM> and the intermediate inner pipe 18A and between the intermediate inner pipe 18A and the inner pipe <NUM> of the pipe joint <NUM>. The outer pipe <NUM> of the pipe joint <NUM> is shown being advanced downstream, together with its spacers <NUM>, along and around the intermediate inner pipe 18A. The spacers <NUM> within the outer pipe <NUM> of the pipe joint <NUM> slide over the intermediate inner pipe 18A as the rails <NUM> supporting the intermediate inner pipe 18A are retracted, allowing the rollers <NUM> of the roller boxes <NUM> to support the advancing outer pipe <NUM>. Conversely, the rails <NUM> of the upstream roller boxes <NUM> are deployed to support the inner pipe <NUM> of the pipe joint <NUM>.

<FIG> show a practical implementation of the invention. The S-lay firing line arrangement <NUM> of <FIG> comprises two axially-aligned groups of three independently-operable roller boxes <NUM> of the invention, alternating with workstations <NUM> between the groups and at their ends. <FIG> are enlarged views of one of the roller boxes <NUM> of that arrangement <NUM>.

<FIG>, <FIG> show the roller box <NUM> in a first operating mode in which rails <NUM> are deployed to support the inner pipe <NUM> of a PiP assembly as described above. Conversely, <FIG> show the roller box <NUM> in a second operating mode in which the rails <NUM> are retracted to allow rollers <NUM> beneath to support the outer pipe <NUM> of the PiP assembly. Driven by a tilt mechanism <NUM> and aided by counterweights <NUM>, the rails <NUM> retract by swinging upwardly and outwardly into the raised, separated position shown in <FIG>.

As best seen in <FIG>, <FIG>, arrays of guide rollers <NUM> cooperate with the rails <NUM> to define a path for the inner pipe <NUM>. The guide rollers <NUM> overhang the rails <NUM>, thus being positioned and oriented to prevent the inner pipe <NUM> from lifting away from the rails <NUM>. This keeps the inner pipe <NUM> aligned with the central longitudinal axis <NUM> as illustrated in <FIG>.

Claim 1:
A method of assembling a pipe-in-pipe pipeline (<NUM>) by adding a succession of inner and outer pipe sections to a trailing end of the partially-completed pipeline (<NUM>), the method comprising:
welding a leading end of a first inner pipe section (18A) to a trailing end of an inner pipe (<NUM>) of the partially-completed pipeline (<NUM>);
advancing a leading end of a second inner pipe section (<NUM>) to a trailing end of the first inner pipe section (18A);
advancing an outer pipe section (<NUM>) downstream from the second inner pipe section (<NUM>) along and around the first inner pipe section (18A) and into abutment with a trailing end of an outer pipe (<NUM>) of the partially-completed pipeline (<NUM>); and
welding a leading end of the outer pipe section (<NUM>) to the trailing end of the outer pipe (<NUM>).