GB2591473A

GB2591473A - Attaching hold-back fixings to subsea pipelines

Info

Publication number: GB2591473A
Application number: GB2001191.2A
Authority: GB
Inventors: Rajaratnam Sabesan; Gunawan Suwarno; Kumar Deepesh
Original assignee: Subsea 7 Ltd
Current assignee: Subsea 7 Ltd
Priority date: 2020-01-28
Filing date: 2020-01-28
Publication date: 2021-08-04
Anticipated expiration: 2040-01-28
Also published as: GB202001191D0; GB2591473B

Abstract

A hold-back fixing for a subsea pipeline, the fixing comprising a pipeline section (such as a tubular forged insert 10) shaped with at least one outer radially-protruding stopper formation 60, 70, the or each stopper formation having at least one ramp face (68) that is inclined relative to a central longitudinal axis of the pipeline section; and a fixing interface (clamp sleeve 12) that is arranged to be mounted to the pipeline section and that is shaped to bear against the or each stopper formation. Embodiments include a method of installing a subsea pipeline fitted with a hold-back fixing, the method comprising: unspooling a portion of a pipeline from a reel aboard an installation vessel, the unspooled pipeline portion comprising a pipeline section that has at least one external stopper formation; mounting a hold-back fixing interface to the pipeline section aboard the vessel to be engaged by the or each stopper formation; and lowering the pipeline section and the hold-back fixing interface from the vessel to a seabed location.

Description

Attaching hold-back fixings to subsea pipelines This invention relates to the challenges of attaching fixings such as hold-back clamps to subsea pipelines, especially to pipelines that are to be installed by a reel-lay method.

Subsea pipelines laid on the seabed tend to move across the seabed over time due to sea dynamics, such as tidal currents or the effects of storms, and cycles of thermal elongation and contraction caused by temperature fluctuations between operation and shut-down. Eventually, this can lead to an effect known as 'walking' of the pipeline laterally across the seabed away from its original position.

A subsea pipeline can also be deflected by other external influences, such as by motion of a riser connecting the pipeline to the surface, or by impact due to over-trawling. Whatever its cause, uncontrolled displacement of a subsea pipeline can cause the pipeline to buckle and makes connections to other subsea infrastructure problematic.

There are various approaches to mitigate buckling of a subsea pipeline, both preventive and corrective. For example, pipeline motion may be restrained by anchoring the pipeline as exemplified in US 8882392, in which anchor chains connect one or more hold-back clamps, attached to the exterior of the pipeline, to anchors embedded in the seabed. Typically the pipeline is anchored at intervals along its length via a longitudinal series of hold-back clamps. Each hold-back clamp provides a padeye or other attachment point for attachment of a respective anchor chain.

A hold-back clamp has to be fixed immovably to a subsea pipeline in order to anchor the pipeline securely. In this respect, two types of hold-back clamps are in common use, namely friction clamps and slip-ring clamps. Each of these types of hold-back clamp comprises two half-cylinder portions that are bolted together in mutual opposition around the pipeline to define a bore that receives the pipeline. The half-cylinder portions clamp together to engage an outer coating of the pipeline so as to prevent movement of the hold-back clamp along the pipeline.

In friction clamps, the half-cylinder portions each have a substantially smooth inner surface. Resistance to movement is proportional to the area of contact between the inner surfaces of the half-cylinder portions and the outer surface of the pipeline. Thus, to ensure a sufficient grip on the pipe, friction clamps have to be relatively long so as to maximise the area of contact. This increases their bulk and their cost.

In slip-ring clamps, the half-cylinder portions are provided with slip rings having teeth that project radially inwardly toward the pipeline. When the half-cylinder portions are bolted together around the pipeline, the teeth on the slip rings bite into the coating of the pipeline. Whilst this additional mechanical engagement allows slip-ring clamps to be more compact than equivalent friction clamps that rely entirely upon friction, the teeth can impart high local stresses in the pipeline and its coating.

Another conventional fastening method for a hold-back clamp employs a stopper or collar plate as described in EP 2250415. Alternatively, EP 2510270 teaches grouting a hold-back clamp onto a pipeline, typically by injecting epoxy grout.

The grouting technique taught in EP 2510270 is useful for retrofitting hold-back clamps onto an existing pipeline, but it is not suitable for initial pipelay because it is performed underwater only after installation of the pipeline. If such a hold-back clamp were instead fitted to a pipeline aboard an installation vessel, the pipeline fitted with the clamp could damage the guide rollers and other pipelaying equipment of the vessel.

Similarly, a temporary layer of grout without the clamp would also be damaged.

The present invention is particularly concerned with hold-back clamps for restraining pipelines that are laid using a reel-lay technique. Reel-lay operations involve winding or spooling a continuous rigid pipe formed of welded elements onto a reel of a pipelaying vessel in successively layered coils, to be unwound or unspooled subsequently during laying at sea. This exploits the fact that nominally rigid pipes have enough flexibility to be bent reversibly along their length provided that a minimum bend radius is observed. When spooling or reeling, bending extends beyond elastic limits into plastic deformation of the pipe that must be recovered by a subsequent straightening process during laying.

Fabrication and spooling of the pipe typically takes place at a spoolbase that the pipelaying vessel visits when necessary for loading. It is also possible for a pipe to be wound onto an intermediate storage reel after fabrication at a spoolbase, to be unwound subsequently from the storage reel and wound simultaneously onto a reel of a pipelaying vessel A key benefit of the reel-lay technique is that, in principle, offshore installation of the prefabricated pipeline can take place continuously. This saves valuable vessel time compared to S-lay and J-lay techniques that instead involve stepwise fabrication of pipelines aboard a pipelaying vessel. Reducing vessel time reduces huge operational costs and tie-up of capital, and enables pipelaying operations to be completed safely within a short weather window.

In reality, however, reel-lay operations must be interrupted whenever it is necessary to incorporate a structure into a pipeline, or to attach a structure to a pipeline, that cannot be spooled with the pipeline onto the reel. In this respect, the outer diameter of a reelable pipe product is ideally constant; but if it is not constant, the outer diameter must change only with gentle and smooth longitudinal transitions.

A discrete structure that is substantially wider than the diameter of the pipe, such as a hold-back clamp or other fixing, is too bulky and has the wrong shape for spooling. For example, a friction clamp that is long enough and rigid enough to grip the pipeline would prevent the pipeline from following the bending radius of a typical reel. Thus, attempting to spool such a structure attached to a pipeline would disrupt coiling of the pipeline onto the reel and would over-stress the adjoining lengths of pipe.

Consequently, known hold-back clamps can only be attached to the pipeline after unspooling and straightening an appropriate portion of the pipeline.

Attaching a hold-back clamp to a pipeline lies on the critical path of the reel-lay process. In particular, pipelaying has to be interrupted while a hold-back clamp is assembled and tightened around the unspooled and straightened portion of the pipeline. Whilst some resulting delay is acceptable, a balance has to be struck between saving time and sacrificing security of fixing.

Specifically, a hold-back clamp that can be attached quickly to a pipeline aboard an installation vessel, such as a friction clamp, also risks slippage along the pipeline in service underwater. In this respect, disadvantages of a friction clamp include its uncertain friction value, compounded by creep losses in its structure and fastenings and in the pipeline and its coating over the lengthy design life of the pipeline system. Additionally, friction clamps may require onshore testing.

Another approach that avoids the risks of clamping is to incorporate a hold-back structure into the unspooled and straightened portion of the pipeline, which involves cutting through the pipe and then welding the structure between the cut ends of the pipe. Producing, testing and coating those critical welds takes a long time and so undermines the potential time savings allowed by reel-lay. Indeed, typically, twenty to thirty hours of vessel time would be required to incorporate each hold-back structure into a pipeline.

A typical subsea pipeline requires a minimum of two hold-back structures, one at each end, and potentially also one or more intermediate hold-back structures along its length. On aggregate, incorporating such structures consumes vessel time that can cost hundreds of thousands of US dollars.

EP 2035734 provides an interface for mounting buoyancy modules on a pipeline being installed by the reel-lay method. Coils of resilient material are wound around the pipeline, which provides both a mechanical interface and a diameter variation suitable for reel-lay. However, such an arrangement may not be able to withstand loads generated by pipeline motions, which may be significantly greater than loads exerted by buoyancy modules. Also, the teaching of EP 2035734 is a solution for flexible pipelines that comprise a flexible outer layer and is not applicable to rigid pipelines with which the present invention is primarily concerned.

Against this background, the invention may be expressed as a hold-back fixing for a subsea pipeline, the fixing comprising: a pipeline section shaped with at least one outer radially-protruding stopper formation, the or each stopper formation having at least one ramp face that is inclined relative to a central longitudinal axis of the pipeline section; and a fixing interface that is arranged to be mounted to the pipeline section and that is shaped to bear against the or each stopper formation.

The or each stopper formation may protrude radially beyond an outer diameter of end portions of the pipeline section. For example, the or each stopper formation may extend radially beyond the outer diameter of the end portions by less than 10% of the overall diameter of the pipeline section. Similarly, the or each ramp face is preferably at a shallow inclination of less than 20° relative to the central longitudinal axis.

The or each stopper formation is suitably rotationally symmetrical about the central longitudinal axis. For example, the or each ramp face may be frusto-conical.

The fixing interface suitably comprises a tubular sleeve having sleeve parts that are arranged for assembly together around the pipeline section. The sleeve may comprise inner engagement formations that are shaped to complement and engage with the or each stopper formation of the pipeline section, when so assembled. For example, a radially inner side of the sleeve may comprise one or more sloped faces whose inclination relative to the central longitudinal axis matches that of the or each ramp face of the stopper formation.

The or each stopper formation may comprise longitudinally-opposed ramp faces. In that case, the or each stopper formation may further comprise a central face between the ramp faces that extends substantially parallel to the central longitudinal axis.

Advantageously, the pipeline section may comprise longitudinally-spaced stopper formations that define a depression between them. For example, the depression may be a circumferential groove that encircles the pipeline section. The fixing interface may comprise a protrusion that is shaped to fit into the depression and to engage between the stopper formations.

Conveniently, the fixing interface may also be arranged to apply radially-inward clamping pressure to the pipeline section.

In an embodiment to be described, the pipeline section comprises a tubular insert for incorporation between opposed pipe lengths of a pipeline. That insert is a monolithic forged or machined piece that is welded to the opposed pipe lengths.

The fixing interface may further comprise a layer of thermal insulation that may be sandwiched between inner and outer shell walls of the fixing interface. Preferably the thermal insulation is sealed in a watertight chamber between the inner and outer shell walls.

The fixing interface suitably further comprises an anchoring formation that is arranged for attachment of an anchor line.

The inventive concept also embraces a subsea pipeline that comprises at least one hold-back fixing of the invention, as well as a method of installing such a pipeline. That method comprises: unspooling the pipeline section from a reel aboard an installation vessel; mounting the fixing interface to the pipeline section aboard the vessel; and lowering the pipeline section and the fixing interface from the vessel to a seabed location.

The method of the invention may be expressed more generally as a method of installing a subsea pipeline fitted with a hold-back fixing, the method comprising: unspooling a portion of a pipeline from a reel aboard an installation vessel, the unspooled pipeline portion comprising a pipeline section that has at least one external stopper formation; mounting a hold-back fixing interface to the pipeline section aboard the vessel to be engaged by the or each stopper formation; and lowering the pipeline section and the hold-back fixing interface from the vessel to a seabed location. The pipeline may then be anchored via the hold-back fixing interface at the seabed location.

It may be possible to mount the hold-back fixing interface to the pipeline section while continuing to unspool the pipeline portion, or while unspooling is briefly paused.

Thus, the invention solves the problems of the prior art to mitigate pipeline walking with a reelable anchor design that can be assembled quickly onboard a vessel to avoid the delay of offshore welding and yet is substantially unaffected by loss of clamping force over time due to bolt creep. As the design relies upon shear/bearing resistance rather than frictional resistance, there is no risk of slippage of the clamp along the pipeline.

This provides a robust solution even for very high pipeline walking loads.

The anchor design of the invention also provides for better thermal insulation than in the prior art to mitigate a 'cold spot' at which heat would transfer preferentially from hot production fluid within the pipeline into the cold surrounding sea.

Embodiments of the invention provide an interface for securing a hold-back clamp to a pipeline, the interface comprising: a pipeline section integral to the pipeline comprising at least one outer stopper bead, wherein the bead has a shape and slope that are suitable for reeling the pipeline; and a clamp interface comprising at least one interface bearing surface to be mounted in contact with the bead of the pipeline section.

The pipeline section may, for example, be a forged or machined piece that may comprise a wall and an inner bore, with opposed ends of the wall welded to respective pipes of the pipeline.

The pipeline section suitably comprises two longitudinally-spaced beads. The or each bead may be a ring around the pipeline having a trapezium shape in longitudinal section. The or each bead is suitably integral with the wall of the pipeline section.

The clamp interface may be a part of a clamp, such as an inner shoe of the clamp. The clamp interface may advantageously comprise a thermally insulating material.

Embodiments of the invention also implement a method for assembling a hold-back clamp with a pipeline, the method comprising: providing a pipeline comprising at least one interface pipeline section that has at least one stopper bead; spooling the pipeline on a reel for transportation; unspooling and straightening the pipeline for installation by the reel lay method; providing a hold-back clamp comprising at least an inner interface with a sloped abutting surface; and, during or after unspooling, clamping the hold-back clamp around the stopper bead of the pipeline so that the sloped abutting surface abuts the stopper bead.

In summary, the invention provides a hold-back fixing for a subsea pipeline. The hold-back fixing comprises a pipeline section, such as a tubular forged insert, that has external radially-protruding stopper formations. Each stopper formation has ramp faces that are oppositely inclined relative to a central longitudinal axis to facilitate spooling of the pipeline section with the remainder of the pipeline. After or during unspooling a portion of the pipeline comprising the pipeline section, a fixing interface is mounted to the pipeline section. The fixing interface has an internal profile that is shaped to engage with the stopper formations of the pipeline section.

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 cut-away perspective view of a hold-back clamp assembly of the invention; Figure 2 shows the assembly of Figure 1 in longitudinal section; Figure 3 is a side view of an insert of the assembly of Figures 1 and 2, welded between lengths of pipe as part of a pipeline; Figure 4 corresponds to Figure 3 but shows the parts in longitudinal section; Figures 5a and 5b are sectional side views that show a clamp sleeve being assembled around the insert as shown in Figure 4 to complete a hold-back clamp assembly of the invention, fixed to a pipeline that incorporates the insert; Figure 6 is a schematic side view of a reel-lay vessel laying a pipeline of the invention onto the seabed; and Figure 7 is a schematic side view of a pipeline of the invention being spooled onto or unspooled from a reel of the vessel.

Referring initially to Figures 1 and 2 of the drawings, a hold-back clamp assembly of the invention comprises a radially-inner tubular insert 10 that is surrounded by a radially-outer tubular clamp sleeve 12.

The insert 10 and the clamp sleeve 12 are both apt to be made of steel. The insert 10 is suitably forged or machined and may have a corrosion-resistant liner and coating. Conversely, the clamp sleeve 12 is suitably fabricated from components that are welded together.

With further reference to Figures 3 and 4, the insert 10 is incorporated between the opposed ends of two lengths of pipe 14 when fabricating a pipeline 16 at a spoolbase onshore, before the pipeline 16 including the insert 10 is spooled onto a reel. In Figures 3 and 4, the insert 10 is shown attached to the lengths of pipe 14 by respective circumferential butt welds 18.

Conversely, the clamp sleeve 12 is placed around the insert 10 after a portion of the pipeline 16 including the insert 10 has been unspooled from the reel aboard a pipelay vessel offshore. In this respect, Figures 1 and 2 show that the clamp sleeve 12 is divided longitudinally into two half-tubular shells 20 that are assembled together in mutual opposition around the insert 10, as shown in Figures 5a and 5b. The insert 10 and the clamp sleeve 12 are curved about a central longitudinal axis 22 that is common to them both.

Figure 6 shows a pipelay vessel 24 configured for reel-lay operations, carrying a pipeline 16 spooled onto a reel 26 of the vessel 24. The vessel 24 is shown at the surface 28 in the process of unspooling the pipeline 16 from the reel 26 and laying the pipeline 16 on the seabed 30.

The vessel 24 comprises an inclined laying ramp 32 that supports a guide system 34 for guiding the unspooled pipeline 16 onto a correspondingly-inclined launch axis, a straightening system 36 and a hang-off system 38 for supporting the weight of the pipeline 16 extending between the surface 28 and the seabed 30. As is conventional, the hang-off system 38 may, for example, comprise tensioners and/or a hang-off clamp.

An insert 10 is shown here incorporated into an unspooled portion of the pipeline 16 between the reel 26 and the guide system 34. Another insert 10 is shown incorporated into a portion of the pipeline 16 between the surface 28 and the seabed 30. That latter insert 10 is surrounded by a clamp sleeve 12 that was attached to the pipeline 16 at a work station 40 on the laying ramp 32 downstream of the hang-off system 38.

After the pipeline 16 carrying the clamp sleeve 12 is landed on the seabed 30, the clamp sleeve 12 is attached to a subsea anchor, for example via an anchor chain, to restrain unwanted walking movement of the pipeline 16 relative to the seabed 30.

Various subsea anchoring arrangements are well known in the art and so have been omitted from the drawings.

In this example, each shell 20 comprises a layer of thermal insulation 42 that helps to retain heat in hydrocarbon fluid contained within the pipeline 16 in use. This mitigates the creation of a cold spot that could otherwise promote the formation or deposition of plugging solids within the pipeline 16, especially during a shut-down interval.

Conveniently, as shown, the layer of thermal insulation 42 may be sandwiched between inner and outer walls 44, 46 of the shells 20, within a part-annular chamber that is closed by end rings 48 disposed between the walls 44, 46. By virtue of the end rings 48, the chamber containing the layer of thermal insulation 42 is suitably sealed against ingress of water in use. This allows more efficient 'dry' insulation material to be used, such as an aerogel. If the chamber is not sealed, a 'wet' insulation material such as polypropylene could be used instead. Unlike dry insulation, wet insulation retains its thermal insulation properties when in contact with water.

The shells 20 comprise respective pairs of longitudinal flanges 50 that each project radially outwardly from a respective longitudinal edge of each outer wall 46. The flanges 50 of each shell 20 lie in a common plane that is parallel to the central longitudinal axis 22. When the shells 20 are assembled together to bring their flanges 50 into mutual opposition, the opposed parallel flanges 50 are held together by a longitudinal series of bolts 52 as shown in Figure 1.

Each shell 20 further comprises an anchoring structure that provides for anchor chains to be attached to the clamp sleeve 12 on the seabed 30. In this example, each anchoring structure comprises a longitudinally-extending rib 54 that projects radially outwardly from the respective outer wall 46. The rib 54 lies in a plane that is substantially orthogonal to the plane of the flanges 50. The rib 54 comprises an anchoring formation such as a hole 56, as shown, for receiving a padeye that will secure an anchor chain in use.

As best appreciated in Figures 2 and 4, the radially inner surface of the insert 10 is suitably of smooth, constant cross-section so as to promote the flow of hydrocarbon fluid through a pipeline 16 incorporating the insert 10. Conversely, the radially outer surface of the insert 10 and the radially inner surface of the clamp sleeve 12 are shaped with complementary interengaging formations. Those interengaging formations effect mechanical engagement between the insert 10 and the assembled clamp sleeve 12 to prevent axial movement of the clamp sleeve 12 relative to the insert 10, and hence relative to a pipeline 16 that incorporates the insert 10.

In the example shown, the interengaging formations of the insert 10 and the clamp sleeve 12 are, substantially, rotationally symmetrical about the central longitudinal axis 22. This allows the clamp sleeve 12 to adopt any angular position about the insert 10, hence easing assembly of the clamp sleeve 12 around the insert 10 offshore and, if desired, allowing the pipeline 16 comprising the insert 10 to twist relative to the clamp sleeve 12 when installed on the seabed 30. Also, in this example, the interengaging formations of the insert 10 and the clamp sleeve 12 are symmetrical about a central plane that is orthogonal to the central longitudinal axis 22. The insert 10 therefore has bidirectional properties and hence the same effect on the pipeline 16 during spooling and unspooling The interengaging formations of the insert 10 are disposed longitudinally between tubular end portions 58 at opposed ends of the insert 10. Those end portions 58 are of uniform diameter and thickness and extend parallel to the central longitudinal axis 22. They suitably match the diameter and wall thickness of the pipes 14 to which the insert is welded on its incorporation into a pipeline 16, as shown in Figures 3 and 4.

The interengaging formations of the insert 10 comprise at least one circumferential male ring or band 60 that encircles the insert 10, bulging radially outwardly from the underlying outer contour of the insert 10 corresponding to the outer diameter of the end portions 58. Conveniently, the male band 60 is defined by a locally-thickened portion of the tubular wall of the insert 10 as shown, hence being integral with that wall. In this example, a pair of longitudinally-spaced, circumferentially-continuous male bands 60 define between them a circumferentially-continuous groove 62. Thus, in longitudinal section, the male bands 60 are convexities or protrusions that define between them a concavity or depression in the form of the groove 62.

Correspondingly, the complementary interengaging formations of the clamp sleeve 12 comprise a circumferential female ring or band 64 that encircles the insert 10. The female band 64 bulges radially inwardly from, and is disposed longitudinally between, tubular end portions 66 at opposed ends of the clamp sleeve 12 that are of uniform diameter and thickness and extend parallel to the central longitudinal axis 22. Again, conveniently, the female band 64 is defined by a locally-thickened portion of the inner wall 44 of the shell 20 as shown, hence being integral with the inner wall 44.

The female band 64 of the clamp sleeve 12 is shaped to fit closely within the complementary groove 62 that is defined between the male bands 60 of the insert 10.

As best appreciated in Figure 5a, the tubular end portions 66 of the clamp sleeve 12 define circumferential recesses, one on each longitudinal side of the female band 64, that are shaped to fit closely against the complementary male bands 60 of the insert 10. Thus, the male bands 60 of the insert 10 serve as stopper beads for the female band 64 of the clamp sleeve 12 that is received in the recess 62 between them.

In the example shown, the male bands 60 and the groove 62 of the insert 10 and the female band 64 of the clamp sleeve 12 each have a faceted shape. Specifically, the bands 60, 64 and the groove 62 each comprise opposed frusto-conical ramp faces 68, oppositely inclined relative to the central longitudinal axis 22, that meet at a mutual apex. In this example, the apex is defined by a central face 70 of uniform diameter extending parallel to the central longitudinal axis 22.

Thus, in longitudinal section, each band 60, 64 and groove 62 follows the shape of a quadrilateral with radially-inner and radially-outer sides that are parallel, that shape being described as a trapezoid in North American English or as a trapezium in English as used elsewhere. As the opposed ramp faces 68 have equal inclination in this example, the shape may be described more specifically as a regular or isosceles trapezium or trapezoid.

The bands 60, 64 and the groove 62 are each symmetrical about a respective central plane that is orthogonal to the central longitudinal axis 22. The frusto-conical ramp faces 68 exemplify how, in opposed longitudinal directions from their central planes, each male band 60 tapers toward the central longitudinal axis 22 whereas the groove 62 and the female band 64 splay away from the central longitudinal axis 22.

The inclination of the ramp faces 68 relative to the central longitudinal axis 22 is shallow, typically less than 200, or 15°, or 10°. Also, the male bands 60 of the insert 10 project radially outwardly from the outer diameter of the adjoining pipes 14 to only a minor extent, for example less than 10% or 5% of the overall diameter of the insert 10. These features minimise outward projections and define smooth longitudinal transitions to facilitate spooling of a pipeline 16 that comprises the insert 10.

The importance of these minimal outward projections and smooth longitudinal transitions of the insert 10 can be appreciated in Figure 7 of the drawings. This shows the insert 10 about to be spooled with the pipeline 16 onto the reel 26 or, conversely, immediately after being unspooled with the pipeline 16 from the reel 26. It will be apparent that the pipeline 16 is coiled around the reel 26 in a radial succession of layers 72 and that, when on the reel 26, the insert 10 may lie beneath one or more radially-outer layers 72 of coils. If the insert 10 was too wide and the longitudinal transitions were too abrupt, coiling could be disrupted and the lengths of pipe 14 adjoining or contacting the insert 10 could be overstressed.

On assembly of the clamp sleeve 12, tension in the bolts 52 can deform the shells 20 elastically to apply radially-inward clamping force to the insert 10. However, unlike the friction clamps of the prior art, clamping force is not the main fixing mechanism that locks the clamp sleeve 12 against longitudinal movement. Thus, the clamp assembly of the invention is invulnerable to a reduction in clamping force as a consequence of creep over time. Also, the clamp sleeve 12 can be made more compact than in a friction clamp. This allows the insert 10 to be correspondingly compact and, in particular, to be kept as short as possible in the longitudinal direction.

Shortening the insert 10 minimises its local stiffening effect on the pipeline 16, which could otherwise resist bending of the pipeline 16 along its length. The compactness of the arrangement is therefore a further factor that facilitates spooling of a pipeline 16 comprising the insert 10.

Many variations are possible within the inventive concept. For example, the layer of thermal insulation could be omitted or could be applied to an outer surface of the clamp sleeve.

The interengaging formations are not necessarily integral with the insert and the clamp sleeve. They could instead be defined by separate components that are attached, respectively, to the outer surface of the insert and/or to the inner surface of the clamp sleeve.

The insert could be formed integrally with the pipeline as a pipeline section that is provided with an outer shape defining the interengaging formations of the insert. For example, a sleeve or coating that extends around and along the pipeline section may be applied or shaped to define an external profile that complements the internal profile of the clamp sleeve. Such a profile may, for example, comprise longitudinally-spaced circumferential bands with a circumferential groove between them, which may be disposed or shaped like those described above.

Claims

Claims 1. A hold-back fixing for a subsea pipeline, the fixing comprising: a pipeline section shaped with at least one outer radially-protruding stopper formation, the or each stopper formation having at least one ramp face that is inclined relative to a central longitudinal axis of the pipeline section; and a fixing interface that is arranged to be mounted to the pipeline section and that is shaped to bear against the or each stopper formation.
2. The fixing of Claim 1, wherein the or each stopper formation protrudes radially beyond an outer diameter of end portions of the pipeline section.
3. The fixing of Claim 2, wherein the or each stopper formation extends radially beyond the outer diameter of the end portions by less than 10% of an overall diameter of the pipeline section.
4. The fixing of any preceding claim, wherein the or each ramp face is at an inclination of less than 200 relative to the central longitudinal axis.
5. The fixing of any preceding claim, wherein the fixing interface comprises a tubular sleeve having sleeve parts that are arranged for assembly together around the pipeline section.
6. The fixing of Claim 5, wherein the sleeve comprises inner engagement formations that are shaped to complement and engage with the or each stopper formation of the pipeline section, when so assembled.
7. The fixing of Claim 6, wherein a radially inner side of the sleeve comprises one or more sloped faces whose inclination relative to the central longitudinal axis matches that of the or each ramp face of the stopper formation.
8. The fixing of any preceding claim, wherein the or each stopper formation comprises longitudinally-opposed ramp faces.
9. The fixing of Claim 8, wherein the or each stopper formation further comprises a central face between the ramp faces that extends substantially parallel to the central longitudinal axis.
10. The fixing of any preceding claim, wherein the pipeline section comprises longitudinally-spaced stopper formations defining a depression between them and the fixing interface comprises a protrusion that is shaped to fit into the depression and to engage between the stopper formations.
11. The fixing of Claim 10, wherein the depression is a circumferential groove that encircles the pipeline section.
12. The fixing of any preceding claim, wherein the fixing interface is also arranged to apply radially-inward clamping pressure to the pipeline section.
13. The fixing of any preceding claim, wherein the pipeline section comprises a tubular insert for incorporation between opposed pipe lengths of a pipeline.
14. The fixing of Claim 13, wherein the insert is a monolithic forged or machined piece that is welded to the opposed pipe lengths.
15. The fixing of any preceding claim, wherein the fixing interface further comprises a layer of thermal insulation.
16. The fixing of Claim 15, wherein the thermal insulation is sandwiched between inner and outer shell walls of the fixing interface.
17. The fixing of Claim 16, wherein the thermal insulation is sealed in a watertight chamber between the inner and outer shell walls.
18. The fixing of any preceding claim, wherein the fixing interface further comprises an anchoring formation arranged for attachment of an anchor line.
19. The fixing of any preceding claim, wherein the or each stopper formation is rotationally symmetrical about the central longitudinal axis.
20. The fixing of any preceding claim, wherein the or each ramp face is frusto-conical.
21. A subsea pipeline comprising at least one hold-back fixing of any preceding claim.
22. A method of installing the pipeline of Claim 21, the method comprising: 5 unspooling the pipeline section from a reel aboard an installation vessel; mounting the fixing interface to the pipeline section aboard the vessel; and lowering the pipeline section and the fixing interface from the vessel to a seabed location.
23. The method of Claim 22, comprising mounting the fixing interface to the pipeline section while continuing to unspool a length of pipeline that is attached to or integral with the pipeline section.
24. The method of Claim 22 or Claim 23, further comprising anchoring the pipeline via the fixing interface at the seabed location.
25. A method of installing a subsea pipeline fitted with a hold-back fixing, the method comprising: unspooling a portion of a pipeline from a reel aboard an installation vessel, the unspooled pipeline portion comprising a pipeline section that has at least one external stopper formation; mounting a hold-back fixing interface to the pipeline section aboard the vessel to be engaged by the or each stopper formation; and lowering the pipeline section and the hold-back fixing interface from the vessel to a seabed location.
26. The method of Claim 25, comprising mounting the hold-back fixing interface to the pipeline section while continuing to unspool the pipeline portion.
27. The method of Claim 25 or Claim 26, further comprising anchoring the pipeline via the hold-back fixing interface at the seabed location.