GB2531601A - Assembling pipe-in-pipe systems and pipeline bundles - Google Patents

Abstract

A pipeline assembly such as a pipe-in-pipe system or a pipeline bundle comprising a plurality of elongate elements comprises an outer pipe string 56, an inner section 22 arranged for telescopic insertion into the outer pipe string 56 and at least one spacer 10 mounted on and surrounding the inner section 22 to lie between the inner section 22 and the outer pipe string 56. The spacer 10 comprises contact points for contacting the outer pipe string 56, each contact point being defined by a respective bearing 42 such as a ball supported for rotation by the spacer 10. The bearings 42 are angularly spaced around the spacer 10. Each bearing 42 is adapted to minimise friction on movement relative to the outer pipe string 56 in at least two directions: both longitudinally, as during insertion of the inner section 22 into the outer pipe string 56; and circumferentially, as during twisting of the inner section 22 relative to the outer pipe string 56. The spacer can comprise opposed segments with each segment supporting at least two of the bearings 42. The spacer can be integral with the inner section 22 or frictionally engaged.

Description

Assembling pipe-in-pipe systems and pipeline bundles This invention relates to the assembly of pipe-in-pipe (PiP) systems and pipeline bundles that are suitable for use in subsea oil and gas installations.

In subsea oil and gas production, multi-phase production fluid comprising crude oil and/or natural gas flows along underwater pipelines. The pipelines comprise flowlines extending across the seabed from wellheads or from other subsea structures. To reduce heat loss from the production fluid to the much colder surrounding sea, flowlines may be coated with thermal insulation, shrouded by an external carrier pipe and/or defined by the inner pipe of a PiP system.

PiP systems position an inner pipe within an outer pipe to leave an insulating annulus between them. Similarly, in relation to pipeline bundles, the invention is particularly concerned with bundles of pipes and other elongate elements that are positioned within a surrounding carrier pipe. Thus, in general terms, the invention addresses the problems of inserting an inner section into an outer pipe, where the inner section is either an inner pipe or a bundle of pipes and/or other elongate elements.

The inner and outer pipes of a PiP system have to be held spaced apart to maintain the intermediate annulus. In short PiP sections, namely pipe joints that are typically a standard 12m in length, the inner and outer pipes are usually held apart only by connecting end walls. A succession of such PiP sections may be welded together endto-end to form a pipeline of any desired length, in which the annulus is interrupted between abutting sections.

Longer PiP sections form pipe stalks that may be many hundreds of metres in length. Such pipe stalks may comprise several successive pipe sections fabricated into inner and outer pipe strings, in which case the annulus may extend continuously between abutting pipe sections without interruption. The absence of end walls between the successive pipe sections means that longer PiP sections require spacing supports between the inner pipe string and the outer pipe string.

The spacing supports may comprise a series of spacers or centralisers, or a solid layer of thermal insulation arranged in the annulus between the inner and the outer pipe strings. It is also possible to have a combination of spacers or centralisers with an insulation layer, where the spacers or centralisers leave between them annular spaces in which the insulation layer is located.

Like an insulation layer, spacers or centralisers in PiP systems that carry production fluid must be designed to reduce heat transfer between the inner pipe string and the outer pipe string. This is because a cold spot caused by thermal conduction through a spacer or centraliser from the warmer inner pipe string to the colder outer pipe string could promote the precipitation of a solid plug of wax, asphaltene or gas hydrate from the production fluid.

Riser pipes extend from seabed flowlines to the surface, where the production fluid typically undergoes treatment and temporary storage at a surface facility. Reciprocally, other fluids must be conveyed from the surface facility to the wellheads. An example is pressurised water for injection into subterranean formations to enhance the recovery of crude oil. Such fluids require additional pipes that follow paths generally parallel to the main flowlines. Other elongate elements of a subsea production installation follow similar generally parallel paths, such as: cables for supplying electrical power and for carrying data; umbilicals; and service fluid tubing.

It is well known to simplify the subsea installation of multiple elongate elements by grouping them together as bundles in which they are transported and installed together. For example, flowline pipes, cables and umbilicals may be bundled together by a series of transverse frames or other spacing supports distributed along the length of the bundle and enclosed within a carrier pipe of steel or other material. Frames that hold together the bundle suitably serve as spacers or centralisers that locate the bundle arrangement at the desired position with respect to the inner surface of the carrier pipe.

The carrier pipe of a bundle contributes structural stiffness and isolates the bundled pipes from the surrounding seawater, thereby providing insulation against heat loss from the production fluid and adding protection against corrosion and against external hazards such as trawl board impacts. Additionally, bundles advantageously minimise the pipeline corridor width and reduce congestion on the seabed, allowing more space for future developments and easing the placement of anchors.

Conventionally, bundles are transported to an installation site by towing them behind a vessel such as a tug. In that case, the carrier pipe contributes buoyancy and structural integrity that enables the bundle to be towed to the installation site while absorbing the loads that the bundle will experience during towing and installation.

A favoured towing method is known in the art as the 'controlled-depth tow method' or CDTM, as described in US 4363566. In CDTM, forward and rearward end structures or towheads of a pipeline bundle are tethered respectively to leading and trailing tugs. The bundle is made neutrally buoyant at a mid-water depth. On arrival at the field, the bundle is lowered to the seabed and the carrier pipe and towheads are flooded to stabilise the bundle in its final position.

Integrating bundles and towheads allows a bundle system to be prefabricated, assembled and tested onshore or in sheltered water before towing to the field for installation. This improves the reliability of the system as compared with connecting equivalent units at a subsea location and performing tests there instead.

Conventional methods for fabricating a PiP system or a pipeline bundle start with preparing an inner section, that inner section being either an inner pipe or a bundle of pipes and/or other elongate elements connected by spacing supports. Then, the inner section is placed in an outer pipe string. Commonly, several successive sections are tied-in by welding together successive inner sections followed by successive outer pipes.

The welding or tie-in operations of conventional fabrication methods can be difficult because there may be little space for welding the inner section, the interior of the outer pipe string or the spacing supports that connect the inner section with the outer pipe string. The lack of space and difficulty of access also makes it challenging to inspect the quality of internal welds. In addition, welding heat can damage parts in the annulus between an inner pipe string and the outer pipe string or in the space between a bundle and the outer pipe string.

If composite materials are used instead of traditional steel for the carrier pipe or for other components of the bundle, the problem is exacerbated because tying-in composite pipes is more complex than welding steel pipes.

It is known to fabricate an outer shell to close a gap in the outer pipe wall after welding together the inner and outer pipes. A drawback of this approach is that it requires additional welding on the outer pipe. Additional welds are time-consuming to produce and to test; they also increase the risk of failure.

In view of the above drawbacks, it has been proposed to pre-fabricate a long inner section and a similarly long outer pipe string or 'pipe stalk' before the inner section, complete with spacing supports, is inserted telescopically into the outer pipe string.

The length of the inner section and the outer pipe string is a key issue. Subsea flowlines often extend over long distances between wellheads and surface facilities.

Indeed, there is a trend toward even longer flowlines as oil and gas production extends into deeper and more challenging waters. Similarly, subsea flowlines may extend over long distances between two distant subsea structures or between two distant surface facilities via pipeline sections laid on the seabed.

By way of example, the Applicant owns a bundle fabrication site in Scotland, UK that is more than 7km long. In principle, therefore, an inner section and an outer pipe string of more than 3.5km each could be pre-fabricated and held end-to-end at that site before the inner section is inserted telescopically into the outer pipe string. In practice, however, the problems of the prior art -which increase as pipe length increases -precluded this before the present invention was made.

Aside from the land space that is available for a fabrication site, the main limitation on the length of a pipe or bundle that can be produced by inserting an inner section telescopically into an outer pipe string is the force that must be applied to the respective components. The force must be sufficient to overcome friction between the inner section and the outer pipe string, which friction will tend to increase during the insertion operation due to the increasing contact area as a greater length of the inner section is inserted into the outer pipe string.

If the force applied to overcome friction between the inner section and the outer pipe string is excessive, over-compressing or over-tensioning may damage the inner section. It is also possible for spacing supports to be displaced or damaged and to become ineffective.

The risk of damage is exacerbated by the possibility of jamming, for example if a spacing support of the inner section catches against an inwardly-protruding weld of the outer pipe string. In addition, very long pipe strings present the possibility that some local twisting of the inner section may occur about the central longitudinal axis of the pipe string.

US 8365775 discloses a basic spacer in the form of an annular centraliser for use between concentric pipes. Each centraliser is secured around an inner pipe to maintain proper spacing between the inner pipe and an outer pipe. Radially-projecting and radially-recessed portions alternate around the circumference of the centraliser to minimise the contact surface area. Whilst this arrangement restricts heat transfer and maintains adequate structural support between the two pipes, the coefficient of friction remains too high to enable a very long inner section to be slid into a similarly long outer pipe string. Also, jamming remains a possibility.

WO 2007/057695 discloses a sliding spacer of cast nylon to be clamped around an inner pipe string. Again, the spacer does not reduce friction sufficiently to be used with pipe strings that are long enough fully to exploit very large manufacturing facilities, such as the Applicant's aforementioned bundle fabrication site in Scotland. Also, a risk of jamming remains.

KR 2013-0122178 discloses a spacer for the concentric assembly of an inner pipe within an outer pipe. The spacer comprises a collar mounted on the inner pipe. US 4941773 also discloses a spacer, but in this case for a pipeline bundle that fits within an outer pipe. In each case, the spacers support rollers that turn about generally circumferential axes lying in planes that are orthogonal to the central longitudinal axis of the outer pipe. Thus, the rollers are adapted to roll in a longitudinal direction within the outer pipe.

In principle, the rollers of KR 2013-0122178 and US 4941773 reduce friction in relation to the sliding arrangements disclosed in US 8365775 and WO 2007/057695. However, the roller-equipped spacers disclosed in KR 2013-0122178 and US 4941773 have drawbacks that make them unsuitable for the purposes of the invention, particularly where the inner section is very long. As noted above, a very long inner section may twist during insertion into a similarly-long outer pipe string. The roller arrangements of KR 2013-0122178 and US 4941773 are not designed to allow for twisting. There remains a risk of jamming or of incorrect positioning of the inner section within the outer pipe string.

Another challenge is that the relative positions of the inner section and the outer pipe string of a bundle or a PiP system will change longitudinally, radially and circumferentially during installation and in service. In service, the inner section will contract and expand due to fluctuations in the temperature of the production fluid; also, pipes of the inner section will ovalise due to hoop stress caused by internal fluid pressure. The outer pipe string may also ovalise due to external hydrostatic pressure in service or loads experienced during installation. The spacers of the prior art are not apt to accommodate such changes while maintaining the inner section at the desired position with respect to the inner surface of the outer pipe string.

In preferred embodiments, the present invention provides a spacer with rolling elements that reduce friction in the longitudinal direction to enable a very long inner section to be inserted into a similarly long outer pipe string. Unlike the prior art, the spacer of the invention provides rolling elements that roll with a circumferential component of movement. This facilitates an additional degree of freedom by turning about the longitudinal axis as the inner section twists within the outer pipe string.

More generally, the invention resides in a pipeline assembly, comprising: an outer pipe string; an inner section arranged for telescopic insertion into the outer pipe string; and at least one spacer mounted on and surrounding the inner section to lie between the inner section and the outer pipe string when the inner section is inserted into the outer pipe string. The spacer comprises contact points for contacting the outer pipe string, each contact point being defined by a respective friction-reducing element supported by the spacer and angularly spaced or distributed around the spacer. In accordance with the invention, each friction-reducing element is adapted to minimise resistance to movement of the inner section relative to the outer pipe string both longitudinally, as during insertion of the inner section into the outer pipe string, and circumferentially, as during twisting of the inner section relative to the outer pipe string.

Typically, the inner section comprises at least one pipe string arranged to carry hot production fluid. The inner section may be a pipe string in a PiP system or a bundle comprising a plurality of elongate elements such as pipe strings and other elements.

The friction-reducing elements are advantageously spaced around the circumference of the spacer and protrude radially outwardly from the spacer toward the outer pipe string. In that case, the friction-reducing elements suitably protrude to an extent that maintains radial clearance between the outer pipe string and a circumferential portion of the spacer that lies between the friction-reducing elements.

Preferably, circumferential portions of the spacer that lie between the protruding friction-reducing elements together extend around a majority of the circumference of the spacer. Radial clearance between the outer pipe string and the circumferential portions of the spacer between the protruding friction-reducing elements minimises frictional resistance and reduces the risk of jamming, However, it is advantageous for that radial clearance to be small, preferably less than 5mm, as this improves mechanical contact between the spacer and the outer pipe during deformation or relative movement of the components in service.

The spacer suitably comprises opposed segments, in which case each segment preferably supports at least two friction-reducing elements.

Advantageously, at least one friction-reducing element is a rolling element mounted for rotation with respect to the spacer. Such a rolling element is suitably supported for rotation about an axis whose orientation relative to the spacer can vary or float. For example, the rolling element may be a ball of a ball bearing, which ball protrudes radially outwardly from the spacer. To minimise the risk of jamming, the ball is preferably surrounded by a frusto-conical collar extending between the ball and the spacer.

The spacer may be integral with the inner section. Alternatively, the spacer may be separate from and engaged with the inner section, for example by frictional engagement. In that case, the coefficient of friction between the spacer and the inner section preferably exceeds the coefficient of friction at the contact points between the spacer and the outer pipe string. The coefficient of friction between the friction-reducing element and the outer pipe string may, for example, be below 0.1.

The invention enables the assembly to have great length. For example, the assembly may have a length of at least 500m.

The invention also resides in method of assembling a pipeline. That method comprises: telescopically inserting an inner section into an outer pipe string; supporting the inner section within the outer pipe string by at least one spacer that lies between the inner section and the outer pipe string and defines contact points between the spacer and the outer pipe string; and, during insertion of the inner section into the outer pipe string, minimising frictional resistance at each contact point to movement of the inner section relative to the outer pipe string both longitudinally and circumferentially.

Rolling contact is preferably effected between the spacer and the outer pipe string at the contact points. More preferably, a rolling element is allowed to roll while also allowing the axis of rotation of that element to vary or to float in orientation relative to the spacer.

In summary, the invention provides a pipeline assembly comprising: at least one inner pipe string; an outer pipe string; at least one spacer mounted on the inner pipe string, surrounding said pipe string, and contacting said outer pipe string at at least one point, the at least one spacer preventing contact between the inner pipe string and the outer pipe string. The interface between the spacer and the outer pipe string is ensured by at least one low-friction part inserted in the spacer and integral with the spacer. The inner pipe string can move freely inside the outer pipe string with at least two degrees of freedom at the location of the at least one spacer.

In preferred embodiments, said interface is ensured by at least one ball bearing with a low friction coefficient between the ball and the cage of the bearing. Each ball bearing may comprise one ball. In another approach, said interface could be a PTFE pad.

The space between spacer wall and the outer pipe inner wall may be less than 5 mm The spacer may be slidably mounted on the inner pipe string, in which case the coefficient of friction at the interface with the inner pipe string is suitably higher than the coefficient of friction at the interface with the outer pipe string.

The invention may also be summarised as a method to assemble an inner pipe string with an outer pipe string, the method comprising: mounting at least one spacer on the inner pipe string; and inserting the inner pipe string with the at least one spacer into the outer pipe string. The sliding or rolling interface between the at least one spacer and the outer pipe string is ensured by a part with a low coefficient of friction integral with the spacer. The inner pipe string can move freely inside the outer pipe string with at least two degrees of freedom at the location of the at least one spacer.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a part-sectioned front view of a pair of segments of a spacer in accordance with the invention; Figure 2 is a side view of one of the segments shown in Figure 1; Figure 3 is a side view of a bearing unit that is engageable with one of the segments shown in Figures 1 and 2; Figure 4 is a part-sectioned in-situ front view of a spacer comprising an assembled pair of segments that corresponds to Figure 1 but shows each segment supporting a respective pair of the bearing units shown in Figure 3; Figure 5 is an in-situ side view of the spacer shown in Figure 4; and Figure 6 is a perspective view of the spacer shown in Figures 4 and 5.

As shown in Figures 1, 2 and 4 to 6 of the drawings, a spacer 10 comprises two substantially-identical C-shaped segments 12. The segments 12 are suitably moulded of a plastics material such as nylon for thermal insulation and for ease of series manufacture.

Each segment 12 has a generally part-cylindrical concave inner face 14 and a generally part-cylindrical convex outer face 16. Flat end faces 18, which lie in the same plane as each other, extend between the inner and outer faces 14, 16.

The inner face 14 of each segment 12 is of part-circular cross-section apart from lead-in chamfers 20 at the junctions with the end faces 18. The radius of curvature of the inner face 14 substantially matches that of the outer face of an inner pipe 22 of a PiP system as shown in Figures 4 and 5. Thus, the inner face 14 and the outer face of the inner pipe 22 share the same axis of curvature 24 when the segment 12 is seated against the inner pipe 22. The chamfers 20 ease engagement of the segments 12 with the inner pipe 22.

The outer face 16 of each segment 12 is also of part-circular cross-section apart from two flats 26. Each flat 26 surrounds a respective circular-section cylindrical socket 28 that faces radially outwardly. Each socket 28 has central axis 30 that is orthogonal to the plane of the associated flat 26. A radially-inward projection of that axis 30 intersects the axis of curvature 24. In this example, the central axes 30 of the sockets 28 are mutually orthogonal about the axis of curvature 24.

A countersunk cavity 32 is disposed between each socket 28 and the adjacent end face 18 of each segment 12. A through-hole 34 extends from the base of each cavity 32 to the adjacent end face 18 and is oriented orthogonally to that end face 18. As shown in Figures 1 and 4, the through-holes 34 of the segments 12 align in mutually-opposed pairs when the segments 12 are held in face-to-face relation with their end faces 18 opposed to their counterparts on the other segment 12.

As best shown in Figure 4, fasteners such as bolts 36 are inserted through the aligned pairs of through-holes 34 to couple the segments 12 to form the spacer 10. Tightening nuts 38 on the bolts 36 draws the segments 12 together to clamp tightly around the inner pipe 22. Frictional engagement between the inner faces 14 of the segments 12 and the inner pipe 22 locates the spacer 10 against longitudinal and circumferential movement relative to the inner pipe 22. If needs be, frictional engagement may be enhanced by interposing a high-grip surface, formation or element between the inner faces 14 of the segments 12 and the inner pipe 22.

Figures 1 and 4 show that each segment 12 is nearly semi-circular, extending around slightly less than 180° of arc between the end faces 18. Thus, when the segments 12 are assembled in face-to-face relation around, and seated against, the inner pipe 22 to form the spacer 10 as shown in Figure 4, gaps 40 remain between the end faces 18.

The gaps 40 provide clearance for the segments 12 to deform resiliently under loads imparted through the bolts 36, hence applying radially-inward clamping forces to the inner pipe 22.

Figure 3 of the drawings show one of four ball bearing units 42 used with the spacer 10. The ball bearing unit 42 comprises a cylindrical cage 44 that is rotationally symmetrical about a central longitudinal axis 46, and a ball 48 that is centred on the longitudinal axis 46. The ball 48 is free to turn about any axis with respect to the cage 44.

The ball 48 of the ball bearing unit 42 is surrounded and retained in the cage 44 by a collar 50. The collar 50 surmounts the cage 44 and extends radially outwardly with respect to the central longitudinal axis 46 beyond the diameter of the cage 44 to define an undercut shoulder 52. On its other side, the collar 50 defines a frusto-conical ramp surface 54 that rises radially inwardly with respect to the longitudinal axis 46 to a circular opening around the ball 48. The diameter of that opening is smaller than the diameter of the ball 48, leaving an exposed portion of the ball 48 protruding from the collar 50 by a distance smaller than the radius of the ball 48.

The cage 44 and the collar 50 of the ball bearing unit 42 may be of metal such as steel or of plastics such as nylon. The ball 48 is typically of steel. A low-friction insert such as PTFE may be interposed between the ball 48 and the cage 44 and/or the collar 50.

Figures 4, 5 and 6 show how each segment 12 carries a pair of the ball bearing units 42 shown in Figure 3, making a total of four ball bearing units 42 per spacer 10. Specifically, the cylindrical cage 44 of each ball bearing unit 42 is pushed into a respective socket 28 as an interference fit until the shoulder 52 bears against the surrounding flat 26 of the segment 12. The central longitudinal axis 46 of the ball bearing unit 42 aligns with the central axis 30 of the socket 28.

Figures 4 and 5 show the segments 12 of the spacer 10 seated against the inner pipe 22. It will be apparent that the ball bearing units 42 are equi-angularly spaced around the circumference of the spacer 10 and hence around the circumference of the inner pipe 22.

Figures 4 and 5 also show an outer pipe 56 of the PiP system, whose cross-section is concentric with the inner pipe 22. The spacer 10 is situated in an annulus 58 defined between the inner pipe 22 and the outer pipe 56. In typical offshore applications, both the inner pipe 22 and the outer pipe 56 will be of steel. However, other materials such as composites are within the inventive concept.

Figure 4 best shows how the collar 50 and the exposed portion of the ball 48 protrude radially outwardly from the outer face 16 of the segment 12, with respect to the axis of curvature 24. It will be apparent how the frusto-conical ramp surface 54 of the collar 50 provides a smooth transition between the outer face 16 of the segment 12 and the exposed portion of the ball 48. This reduces the risk of the spacer 10 jamming against an obstruction on the inner surface of the outer pipe 56.

The risk of jamming against an obstruction on the outer pipe 56 is reduced further by the large circumferential gaps between the balls 48 of the ball bearing units 42 and by the small radial clearances that are evident between the inner surface of the outer pipe 56 and the balls 48.

In use when assembling a PiP system, a series of spacers 10 are fitted at suitable intervals along the length of the inner pipe 22. The inner pipe 22 carrying the spacers is then pulled and/or pushed telescopically into the outer pipe 56. Upon insertion of each spacer 10, the balls 48 bear against the inner surface of the outer pipe 56. The balls 48 also separate the outer faces 16 of the segments 12 from the inner surface of the outer pipe 56.

It will be apparent that the weight of the inner pipe 22 is directed solely through the balls 48, which minimises friction by ensuring purely rolling contact with the inner surface of the outer pipe. This enables a very long inner pipe 22 to be inserted into a similarly long outer pipe 56 and so exploits the land space that is available at the largest fabrication sites. Also, the balls 48 are not constrained to turn only about one pivot axis: they can turn about any pivot axis, including pivot axes that are not orthogonal to the general longitudinal direction of insertion. This allows for relative twisting motion between the inner pipe 22 and the outer pipe 56 during insertion.

By facilitating twisting of the inner pipe 22 during longitudinal insertion movement, the ball bearing units 42 help the inner pipe 22 to bypass or overcome obstructions within the outer pipe 56. The ball bearing units 42 also resist jamming of the spacer 10 within the outer pipe 56.

Many variations are possible within the inventive concept. For example, balls of ball bearings or other contact elements could be biased radially outwardly by springs or by flexible mountings. In this way, the balls could be deflected radially inwardly against that bias to overcome an obstruction within the outer pipe. This may reduce or obviate a clearance between the balls and the inner surface of the outer pipe.

Ball bearings could be replaced by other rolling elements such as wheels or rollers that are supported for rotation about axes whose orientation can vary or float. For example, rolling elements could be mounted on axles or pivots, which axles or pivots are in turn pivotably mounted to the spacer. Such pivotable mountings would allow the rolling elements to tilt relative to the general longitudinal direction of insertion of an inner section into an outer pipe string, hence to allow relative twisting motion between the inner section and the outer pipe string during insertion.

Whilst rolling elements are preferred as bearings, non-rolling friction-reducing elements such as discrete PTFE pads are also possible for use as bearings to define contact points between the spacer and the outer pipe. Such elements may also be spaced around the circumference of the spacer and protrude radially outwardly from the spacer toward the outer pipe.

The skilled reader will appreciate that although described above in relation to a PiP system, the invention may also be used beneficially when inserting a bundle of pipes and/or other elongate elements into a surrounding carrier pipe.

Whilst the invention is particularly suitable for allowing freedom of relative movement between an inner section and an outer pipe string during insertion of the former into the latter upon assembly, the invention is also apt to allow such freedom when the resulting assembly is being transported and installed and is in service. This reduces stress and fatigue on the components of the assembly when in use.

Claims (18)

14 Claims 1. A pipeline assembly, comprising: an outer pipe string; an inner section arranged for telescopic insertion into the outer pipe string; and at least one spacer mounted on and surrounding the inner section to lie between the inner section and the outer pipe string when the inner section is inserted into the outer pipe string; wherein the spacer comprises contact points for contacting the outer pipe string, each contact point being defined by a respective friction-reducing element supported by the spacer and angularly spaced around the spacer, each friction-reducing element being adapted to minimise friction on movement relative to the outer pipe string both longitudinally, as during insertion of the inner section into the outer pipe string, and circumferentially, as during twisting of the inner section relative to the outer pipe string.

2. The assembly of Claim 1, wherein the inner section comprises at least one pipe string arranged to carry hot production fluid.

3. The assembly of Claim 1 or Claim 2, wherein the inner section is a bundle comprising a plurality of elongate elements. 25

4. The assembly of any preceding claim, wherein the friction-reducing elements are spaced around the circumference of the spacer and protrude radially outwardly from the spacer toward the outer pipe string, said protrusion being to an extent that maintains radial clearance between the outer pipe string and a circumferential portion of the spacer between the friction-reducing elements.

5. The assembly of Claim 4, wherein circumferential portions of the spacer between the protruding friction-reducing elements together extend around a majority of the circumference of the spacer.

6. The assembly of Claim 4 or Claim 5, wherein radial clearance between the outer pipe string and the circumferential portions of the spacer between the protruding friction-reducing elements is less than 5mm.

7. The assembly of any of Claims 4 to 6, wherein the spacer comprises opposed segments each supporting at least two friction-reducing elements.

8. The assembly of any preceding claim, wherein at least one friction-reducing element is a rolling element mounted for rotation with respect to the spacer.

9. The assembly of Claim 8, wherein the rolling element is supported for rotation about an axis whose orientation relative to the spacer can vary or float.

10. The assembly of Claim 8 or Claim 9, wherein the rolling element is a ball of a ball bearing, which ball protrudes radially outwardly from the spacer.

11. The assembly of Claim 10, wherein the ball is surrounded by a frusto-conical collar extending between the ball and the spacer.

12. The assembly of any preceding claim, wherein the spacer is integral with the inner section.

13. The assembly of any of Claims 1 to 11, wherein the spacer is frictionally engaged with the inner section and a coefficient of friction between the spacer and the inner section exceeds a coefficient of friction at the contact points between the spacer and the outer pipe string.

14. The assembly of any preceding claim, having a coefficient of friction below 0.1 between the friction-reducing element and the outer pipe string.

15. The assembly of any preceding claim, having a length of at least 500m. 30

16. A method of assembling a pipeline, comprising: telescopically inserting an inner section into an outer pipe string; supporting the inner section within the outer pipe string by at least one spacer that lies between the inner section and the outer pipe string and defines contact points between the spacer and the outer pipe string; and during insertion of the inner section into the outer pipe string, minimising frictional resistance at each contact point to movement of the inner section relative to the outer pipe string both longitudinally and circumferentially.

17. The method of Claim 16, comprising effecting rolling contact between the spacer and the outer pipe string at the contact points.

18. The assembly of Claim 17, comprising rolling a rolling element while allowing the orientation of an axis of rotation of that element to vary or to float relative to the spacer.