Incorporating structures into reeled pipelines

Abstract

An installation vessel for laying a subsea pipeline comprises a tensioner 30 that is operable to maintain top tension while controlling lay motion of the pipeline. A hang‑off module 34 beneath the tensioner 30 is operable at that position to engage the pipeline when the lay motion is paused. The hang‑off module 34 is movable downwardly and upwardly, and optionally also horizontally, while holding a catenary portion of the pipeline suspended beneath. After engaging the pipeline with the hang‑off module 34, the pipeline is cut to define an upper pipeline portion 14U and a lower pipeline portion 14L. The hang‑off module 34 is lowered with the lower pipeline portion 14L to create a gap 40 between the upper and lower pipeline portions. After inserting an accessory 42 into the gap 40 and connecting the accessory 42 to the upper and lower pipeline portions, the hang‑off module 34 is disengaged from the lower pipeline portion 14L and lay motion of the pipeline is resumed.

Description

[0001] Incorporating structures into reeled pipelines

[0002] This invention addresses the challenges of incorporating structures such as pipeline accessories into pipelines being installed offshore. The main application of the invention is the installation of subsea pipelines by the reel-lay method, in which a pipeline is spooled onto a reel for transport and unspooled from the reel for installation. However, the invention could also be used in S-lay or J-lay operations. Aspects of the invention are particularly advantageous when used with mechanically-lined pipelines, especially in a reel-lay context.

[0003] Reel-lay operations involve winding or spooling a continuous pipe formed of welded elements onto a reel of a pipelaying vessel, to be unwound or unspooled subsequently during pipelaying at sea. The vessel comprises at least one transport reel and a lay ramp or lay tower that supports a straightener to reverse plastic deformation experienced by the pipeline upon spooling.

[0004] A radius controller or aligner at the top of the lay tower guides the unspooled pipeline from the reel onto a lay axis that extends along the lay tower and into the sea. The aligner may be a wheel or a chute that is convex-curved along its length, around which the pipeline is bent toward the lay axis to enter the straightener.

[0005] Beneath the straightener, the lay tower supports a tensioner that controls the speed of pipelaying and bears the weight load of the catenary of pipeline suspended in the water column between the vessel and the seabed. Some of the weight load can also be borne by back-tension applied to the pipeline by the reel and by frictional engagement with various guides acting on the pipeline between the reel and the tensioner. In some cases, a travelling clamp can be used instead of, or in addition to, a tensioner.

[0006] A hang-off module beneath the tensioner bears the weight load of the catenary temporarily if the tensioner is required to release the pipeline or if the pipeline is to be cut beneath the tensioner, for example to incorporate an accessory into the pipeline as will be explained. To maintain the necessary top tension, the hang-off module can clamp around the pipeline and / or can engage a flange or other engagement formation provided on or welded to the pipeline.

[0007] It is desirable for stability that the centre of gravity of a vessel is kept as low as possible.

[0008] However, the size, weight and height of the lay tower, and of heavy structures such as tensioners on the lay tower, makes it challenging to ensure stability without enlarging the hull of the vessel and hence correspondingly increasing the cost of the vessel.

[0009] The reel-lay technique depends upon the ability of nominally rigid pipes to be bent along their length and subsequently straightened, provided that a minimum bend radius is observed throughout. When spooling, bending extends beyond elastic limits into plastic deformation of the pipe that must be recovered by straightening processes during laying. Bending deformation of a pipe upon spooling and unspooling develops considerable stresses and strains in the pipe wall, including ovalisation in transverse cross-section. Particular problems arise when bending a lined pipe, which may be required for handling well fluids containing corrosive agents such as hydrogen sulphide and chlorides.

[0010] A lined pipe typically comprises a load-bearing, thick-walled, high-strength outer pipe of low-alloy carbon steel, lined with a thin-walled liner sleeve of a corrosion-resistant alloy (CRA). Plastics liner sleeves are also known. The outer pipe resists buckling during spooling and unspooling and resists hydrostatic pressure when underwater. Conversely, the inner sleeve provides little mechanical strength, being just a few millimetres thick, but protects the outer pipe from corrosive constituents of fluids carried by the pipe in use.

[0011] The use of two different materials in this way recognises that a pipe made entirely from corrosion-resistant material would be prohibitively expensive and yet could lack the essential mechanical properties that are provided by the strong outer wall of a lined Pipe.

[0012] CRA-lined bimetallic pipes take two forms. The first is ‘clad’ pipe, in which an internal CRA liner sleeve is metallurgically bonded to the outer pipe. The second is ‘mechanically lined pipe’ or ‘MLP’, in which an interference fit between the liner sleeve and the outer pipe fixes the liner sleeve without metallurgical bonding. An example of MLP is supplied by H. Butting GmbH & Co. KG of Germany under the trade mark ‘BuBi’.

[0013] To produce a length of MLP, a tubular liner sleeve is inserted telescopically into an outer pipe as a sliding fit and both are expanded radially by internal hydraulic pressure applied to the liner sleeve. The expanding liner sleeve undergoes radially-outward plastic deformation to apply radial expansion force to the outer pipe, which undergoes radially-outward plastic or elastic deformation as a result. Once the internal pressure is relaxed, radially-inward elastic shrinkage of the outer pipe onto the plastically-expanded liner sleeve effects a mechanical bond between the outer pipe and the liner sleeve.

[0014] MLP benefits from an economical production process that makes it much less expensive than clad pipe. This can save tens of millions of US dollars in a large subsea project, considering the many kilometres of lined pipe that may be required. However, MLP is susceptible to problems during spooling and unspooling that make its use challenging in reel-lay applications. Specifically, under bending deformation, the thickwalled outer pipe may be structurally stable while the thin-walled inner liner sleeve of lower yield strength suffers significant deformation under the combined action of bending and external pressure from the outer pipe. This deformation manifests itself as buckling or wrinkling of the liner sleeve, especially around the intrados or inner curve of the pipe bend. A wrinkled liner sleeve could hinder the smooth flow of well fluids, decrease fatigue life due to stress concentration, and preclude effective pigging of the pipeline.

[0015] In view of the wrinkling problem, the present invention is mainly concerned with installation of MLP as opposed to metallurgically-bonded clad pipe for which wrinkling of the liner is not such a challenge. The invention is also concerned with plastics-lined pipes. Both MLP and plastics-lined pipes are characterised by the possibility of localised longitudinal slippage of the liner sleeve relative to the outer pipe upon bending deformation of the lined pipe, with consequent undesirable deformation of the liner sleeve manifested as wrinkling. However, in a broad sense, the invention could also have benefit when bending clad pipe and indeed when bending unlined pipe, if bending is such as to give rise to unwanted inward deformation of the pipe wall.

[0016] Wrinkling of the intrados of a pipe bend during bending is a well-known phenomenon. Standards such asASME B31.3, API 5L and DNV-OS-F-101 recommend that wrinkling should be mitigated by choosing a pipe wall of suitable thickness. However, this approach cannot be applied to double-walled lined pipe in which the outer wall is designed primarily for mechanical strength but the inner wall is designed primarily for corrosion resistance and is made of a material with lower yield strength.

[0017] Pipeline accessories must be incorporated into and installed with a pipeline from time to time. A pipeline accessory may be an end accessory such as a pipeline end termination (PLET) or a pig launcher / receiver, or an in-line accessory such as an in-line tee (I LT). An end accessory is welded to an end of a pipeline whereas an in-line accessory is welded to opposed upper and lower portions of the pipeline at an intermediate location between the ends of the pipeline. In that case, the pipeline is cut at a workstation in the tower, downstream of or below the straightener at a level between the tensioner and the hang-off module, and the accessory structure is welded between the opposed cut ends of the portions of the pipeline.

[0018] To reduce the number of trips that a reel-lay vessel must make between a coastal loading quay or spoolbase and an offshore installation site, it can be convenient for the vessel to carry first and second pipelines spooled in series on the same reel if the capacity of the reel is sufficient. A transition joint effects the transition between successively-reeled pipelines that may have different characteristics, such as different diameters. The transition joint is cut out before abandoning the upper end of the first pipeline to the seabed and then initiating installation of the second pipeline. Pipeline end accessories may be added to the cut ends of either or both of the pipelines.

[0019] When adding an in-line accessory, a length of the pipeline between the tensioner and the hang-off module is removed to create a gap that is slighter longer than the length of the accessory. For this purpose, the accessory typically includes short stub pipes that align coaxially with, and are welded to, respective portions of the pipeline. The accessory is first welded to one of the cut ends, most typically to the cut end of the lower portion of the pipeline that hangs as a catenary from the hang-off module. As the gap is longer than the accessory, some of the length of the gap remains between the top of the accessory and the cut end of the upper portion of the pipeline that extends down the lay tower from the aligner and through the straightener. The residual gap is closed by advancing the pipeline around the aligner to lower the cut end of the upper portion to the level of the top of the accessory, before that cut end is welded to the accessory. Typically, the pipeline must be advanced over a longitudinal distance of one to three metres.

[0020] WO 2008 / 072970, WO 2010 / 010390 and WO 2011 / 051218 address the problem of liner wrinkling by filling at least a substantial part of a lined pipeline with a pressurised liquid such as water during spooling and unspooling. However, as explained in WO 2020 / 118404, the presence of a pressurising liquid complicates the addition of an accessory to a lined pipeline, noting that the internal fluid pressure must be reduced when cutting the pipeline. Also, the liquid must be drained at least from the upper portion defining the free end of the pipeline that extends down the lay tower from the aligner to the cut end, noting that the presence of water in the free end of the pipeline would disrupt welding operations. This presents a problem because the pipeline will undergo plastic deformation as it travels around the aligner and through the straightener but, in this instance, the pipeline must experience that deformation without the protection of internal fluid pressure. The absence of internal fluid pressure makes the liner vulnerable to delamination or wrinkling during the deformation of the pipeline.

[0021] WO 2009 / 147168 is an example of prior art that teaches cutting out a length of the pipeline to accommodate an accessory, as described above, hence making two cuts through the pipeline with longitudinal spacing between them to define the necessary gap. WO 2009 / 147168 also teaches the alternative of making a single cut through the pipeline and then reversing the reel to lift the upper portion of the pipeline away from the lower portion to define the necessary gap. In that case, the upper portion of the pipeline therefore passes back through the straightener and around the aligner. If performed on a lined pipeline, this would subject the pipeline to yet more deformation and so would expose the liner to an even greater risk of delamination or wrinkling.

[0022] WO 2009 / 147168 discloses pipe-tensioning cylinders that could facilitate limited axial alignment between mutually opposed pipe ends in a fit-up operation before welding. However, this is a secondary purpose of the cylinders; their stroke would be far too short to close a longitudinal gap of any significance for accommodating the overall length of an accessory.

[0023] US 7955028 and US 8807872 disclose reel-lay vessels whose lay towers are pivotable about a horizontal axis so that their angle of inclination can be varied to adjust the angle at which the pipeline enters the water. A hang-off module, correspondingly pivotable, is movable between an operative position on the launch axis and a retracted position away from the launch axis to allow for the passage of accessories along the pipeline. When moved away from the operative position, the hang-off module does not engage the pipeline. Again, there is no provision for closing any residual gap between an accessory and an upper portion of pipeline held on the lay tower, save for longitudinal movement of the pipeline with the consequent risk of delamination or wrinkling of any liner.

[0024] In US 7806628, a hang-off module is mounted on a trolley that is movable along a transverse horizontal beam. The beam is mounted, in turn, on horizontal rails extending along parallel outriggers that are cantilevered aft from the stern of a reel-lay vessel, above the waterline. The beam can move fore and aft along the rails and the trolley can move laterally along the beam, hence providing freedom for the hang-off module to be moved in a horizontal plane throughout the working area that is delimited by the outriggers.

[0025] Consequently, the hang-off module of US 7806628 can be moved out of alignment with the lay axis to receive an accessory and then can be moved back onto the lay axis to align the accessory with an end of a pipeline held in a lay tower or tower, enabling the accessory to be welded to the pipeline. While out of alignment with the lay axis, the hang-off module can also be used to support a catenary, being a lower portion of a pipeline. This makes it convenient for an accessory to be lifted onto and welded to the top of the catenary. The hang-off module can then be moved back onto the lay axis to align the accessory with an end of an upper portion of the pipeline held in a lay tower or tower. This enables the accessory to be welded in line with the pipeline, disposed between the upper and lower portions. However, again, closing any residual gap between the upper portion of the pipeline and the top of the accessory would still require longitudinal movement of the upper portion and hence a risk of delamination or wrinkling of any liner.

[0026] In a similar approach to US 7806628, WO 03 / 067019 discloses a hang-off module that can translate horizontally relative to a hull of a reel-lay vessel while supporting a catenary suspended beneath. In this case, however, the hang-off module can move only laterally between two positions, namely: an accessory connection position spaced to one side of the lay axis, where an accessory can be lifted onto and welded to the top of the catenary; and a pipeline feeding position beneath a lay tower, aligned with the lay axis, where the accessory can be welded to the end of an upper portion of the pipeline held in the lay tower.

[0027] After welding the accessory to the top of the catenary in WO 03 / 067019, the accessory and the catenary are released from the hang-off clamp, lowered by abandonment and recovery (A&R) lines and re-engaged with the hang-off clamp when the accessory is largely beneath the hang-off clamp. The hang-off clamp is then moved onto the lay axis for the accessory to be welded to the upper portion of the pipeline held in the lay tower. Even this complex operation involving vertical movement of the accessory cannot provide the accuracy of vertical alignment that is necessary for the accessory to be welded to the upper portion of the pipeline. US 2011 / 0123272 discloses a reel-lay vessel in which a tensioner on a lay tower or tower works in conjunction with a travelling clamp, additional to a fixed hang-off module. The travelling clamp is mounted on a hoist, comprising a winch and a cable, to move reciprocably along the lay axis. The arrangement allows for semi-continuous deployment of the pipeline by alternating between the clamps while potentially allowing the use of a smaller tensioner of lesser capacity, to the benefit of stability but at a cost of greater complexity. However, if an in-line accessory is to be deployed, the fixed hang- off module must be disposed above the tensioner to allow the stroke length of the travelling clamp to be sufficient for the hang-off module to close around the pipeline above the accessory. In that case, the elevated position of the hang-off module could be detrimental to stability.

[0028] Against this background, the invention resides in a method of incorporating an in-line accessory into a subsea pipeline being laid from a lay tower of an installation vessel. The method comprises: pausing lay motion of the pipeline; engaging the pipeline with a hang-off module; cutting through the pipeline above the hang-off module to define an upper pipeline portion held in the lay tower and a lower pipeline portion engaged by and suspended from the hang-off module; lowering the hang-off module with the lower pipeline portion to create or to enlarge a gap between mutually-opposed cut ends of the upper and lower pipeline portions; inserting the accessory into the gap; connecting the accessory to the upper and lower pipeline portions; disengaging the hang-off module from the lower pipeline portion; and resuming the lay motion of the pipeline.

[0029] The method may then further comprise raising the hang-off module to close the gap for connection of the accessory to the upper and lower pipeline portions. For example, the accessory can be connected to the lower pipeline portion before raising the hang-off module to raise the accessory with the lower pipeline portion, and then connecting the accessory to the upper pipeline portion. Alternatively, the accessory can be connected to the upper pipeline portion before raising the hang-off module to raise the lower pipeline portion, and then connecting the accessory to the lower pipeline portion.

[0030] The hang-off module can also be moved horizontally relative to the lay tower to bring the lower pipeline portion out of coaxial alignment with the upper pipeline portion. In one such arrangement, the hang-off module can be moved in an aft direction with respect to the vessel, for example by extending a cantilever mount disposed between the hang-off module and a hull of the vessel. It is also possible to move the hang-off module in a transverse direction with respect to the vessel or to rotate the hang-off module about an upright axis.

[0031] A corresponding method involving a jig comprises: pausing lay motion of the pipeline; engaging the pipeline with a jig supported by a hang-off module; cutting through the pipeline above the jig to define an upper pipeline portion held in the lay tower and a lower pipeline portion engaged by and suspended from the jig; lowering the jig with the lower pipeline portion to create or to enlarge a gap between mutually-opposed cut ends of the upper and lower pipeline portions; inserting the accessory into the gap; connecting the accessory to the upper and lower pipeline portions; disengaging the jig from the lower pipeline portion; and resuming the lay motion of the pipeline.

[0032] The jig can be lowered relative to the hang-off module, which can remain fixed relative to a hull of the vessel. The jig can then be raised to close the gap for connection of the accessory to the upper and lower pipeline portions. The jig can be moved horizontally relative to the lay tower to bring the lower pipeline portion out of coaxial alignment with the upper pipeline portion, for example by extending a cantilever mount disposed between the jig and the hang-off module. The jig and / or the hang-off module can be moved in a transverse direction with respect to the vessel or can be rotated about an upright axis.

[0033] The upper pipeline portion can be held substantially stationary relative to the lay tower between engaging and disengaging the hang-off module. The pipeline can be depressurised internally before cutting through the pipeline and can be repressurised internally before resuming the lay motion.

[0034] A corresponding method of incorporating an end accessory into a subsea pipeline comprises: engaging the pipeline with a hang-off module; cutting through the pipeline above the hang-off module to leave a catenary portion of the pipeline engaged by and suspended from the hang-off module; lowering the hang-off module with the catenary portion; connecting the accessory to the catenary portion; disengaging the hang-off module from the catenary portion; and lowering the accessory and the catenary portion to abandon the pipeline.

[0035] A corresponding method of incorporating an end accessory into a subsea pipeline employing a jig comprises: engaging the pipeline with a jig supported by a hang-off module; cutting through the pipeline above the jig to leave a catenary portion of the pipeline engaged by and suspended from the jig; lowering the jig with the catenary portion; connecting the accessory to the catenary portion; disengaging the jig from the catenary portion; and lowering the accessory and the catenary portion to abandon the pipeline.

[0036] The inventive concept also embraces an installation vessel for laying a subsea pipeline, the vessel comprising: a lay tower supporting a tensioner or travelling clamp, the tensioner or travelling clamp being operable to maintain top tension in the pipeline while controlling lay motion of the pipeline; and a hang-off module positioned beneath the tensioner or travelling clamp and being operable at that position to engage the pipeline when the lay motion is paused; wherein the hang-off module is movable downwardly and upwardly relative to the lay tower while holding a catenary portion of the pipeline suspended beneath the hang-off module.

[0037] The installation vessel can employ a jig, in that case comprising: a lay tower supporting a tensioner or travelling clamp, the tensioner or travelling clamp being operable to maintain top tension in the pipeline while controlling lay motion of the pipeline; a hang- off module positioned beneath the tensioner or travelling clamp; and a jig movable relative to the hang-off module and being operable to engage the pipeline when the lay motion is paused; wherein the jig is movable downwardly and upwardly relative to the hang-off module while holding a catenary portion of the pipeline suspended beneath the jig-

[0038] In summary, the invention relates to a hang-off module, or to an insert or other attachment for a hang-off module, that can move up and down or axially, parallel to the lay tower and the lay axis, to open and then close a gap for insertion of an accessory or other in-line structure. Rather than simply opening the gap by axial retraction of a lower pipe portion, it is also or alternatively possible for the hang-off module to move laterally by misaligning the lower pipe portion relative to an upper pipe portion 14L. A combination of both axial and lateral movements is possible.

[0039] Thus, the invention involves a movable hang-off module, or a movable insert for a preexisting hang-off module, to provide movement of a clamped pipeline catenary in alignment with the lay tower angle while maintaining a top tension of, for example, up to 600 tonnes. An existing hang-off system is modified or adapted to lower the hung-off catenary portion of the pipeline, thereby forming a gap between the hung-off portion and the upper portion of the pipeline in the firing line or lay axis of the lay tower, while maintaining pre-existing system capacity.

[0040] The invention contemplates pipe handling modifications to existing hang-off systems, conferring an independent ability for the system to: move up and down, for example parallel to the firing line or lay axis, by moving the hang-off module or by moving a lifting jig or insert supported by the hang-off module; rotate or translate away from the firing line or lay axis, and optionally also to move down; and / or step away from the firing line or lay axis, for example by moving aft and optionally also by moving down.

[0041] All of these movements are possible while the hang-off system supports the weight of a catenary.

[0042] The invention contemplates precise movement of the pipeline through a longitudinal stroke of three metres or more, while also achieving millimetric accuracy to facilitate welding fit-up. By way of example, a hang-off module or insert of the invention could handle pipe with a nominal outer diameter of from six inches to eighteen inches (approximately 15cm to 45cm), which covers most wet-coated CRA-lined pipelines apt to be laid by reel-lay.

[0043] In this way, the invention solves the problem of longitudinal movement of a pipeline made of MLP when the pipeline must be unpressurised during a reel-lay operation, for example when handling and inserting structures or accessories, or when opening a pipe-in-pipe annulus to insert pipe end plugs, or when lining up abutting lengths of pipe for welding.

[0044] In embodiments of the invention to be described, an installation vessel for laying a subsea pipeline comprises a tensioner that is operable to maintain top tension while controlling lay motion of the pipeline. A hang-off module beneath the tensioner is operable at that position to engage the pipeline when the lay motion is paused. The hang-off module is movable downwardly and upwardly, and optionally also horizontally, while holding a catenary portion of the pipeline suspended beneath.

[0045] After engaging the pipeline with the hang-off module, the pipeline is cut to define an upper pipeline portion and a lower pipeline portion. The hang-off module is lowered with the lower pipeline portion to create a gap between the upper and lower pipeline portions. After inserting an accessory into the gap and connecting the accessory to the upper and lower pipeline portions, the hang-off module is disengaged from the lower pipeline portion and lay motion of the pipeline is resumed.

[0046] 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:

[0047] Figure 1 is a schematic side view of a pipelay support vessel of the invention while laying a subsea pipeline by the reel-lay method;

[0048] Figures 2a and 2b are schematic detail side views of a stern portion of the vessel of Figure 1 , showing the operation of a hang-off module modified in accordance with the invention;

[0049] Figures 3a and 3b correspond to Figures 2a and 2b but instead show a lifting jig of the invention added to a conventional hang-off module;

[0050] Figures 4a and 4b correspond to Figures 2a and 2b but show another hang-off module modified in accordance with the invention; and

[0051] Figures 5a and 5b are schematic detail rear views of the vessel of Figure 1 , showing a further hang-off module modified in accordance with the invention.

[0052] Referring firstly to Figure 1 , this drawing shows a pipelay support vessel 10 travelling across the surface 12 of a sea while reel-laying a pipeline 14 onto the seabed 16. The pipeline 14 is unspooled from a reel 18 and then bends over a spool aligner 20 atop a lay tower 22 upstanding from the hull 24 of the vessel 10. The pipeline 14 follows a lay axis 26 corresponding to the orientation of the lay tower 22 before entering the sea on the lay axis 26 and then extends from the surface 12 to the seabed 16 as a catenary suspended from the vessel 10. The lay tower 22, and therefore the lay axis 26, are shown in the drawings in an upright, substantially vertical orientation but the lay tower 22 could instead be pivoted relative to the hull 24 about a transverse horizontal axis, hence away from the vertical. Tilting the lay tower 22 in this way may, for example, be appropriate when laying the pipeline 14 in shallower water or to facilitate spooling operations while the vessel 10 is being loaded with pipe stalks at a coastal spoolbase.

[0053] In downward succession beneath the aligner 20, the lay tower 22 supports a straightener 28, a tensioner 30 and a workstation 32. There could be more than one tensioner 30 in series; similarly, there could be more than one workstation 32 to perform operations at respective different longitudinal positions along the pipeline 14.

[0054] A hang-off module 34 adapted in accordance with the invention is shown here mounted on the hull 24 beneath the lay tower 22, although the hang-off module 34 could instead be mounted on the lay tower 22 at a level beneath the workstation 32. In either case, the hang-off module 34 can be pivotable relative to the hull 24 to match any adjustment of the inclination of the lay axis 26 relative to the hull 24.

[0055] The tensioner 30, workstation 32 and hang-off module 34 are shown in more detail in the remaining drawings, which show various modifications of, or additions to, the hang- off module 34 in accordance with the invention. Those drawings correspond to Detail A of Figure 1. In each case, the tensioner 30, or at least a lower tensioner 30 if more than one, is shown engaged with the pipeline 14 extending along the lay axis 26 of the lay tower 22. The workstation 32 is shown located beneath the tensioner 30, and may conveniently be positioned at the base of the lay tower 22, level with or above a working deck 36 of the vessel 10 as shown. The hang-off module 34 is shown mounted to and projecting aft from the hull 24 of the vessel 10 in Figures 2a to 4b but is instead shown mounted to a lower end of the lay tower 22 in Figures 5a and 5b, outboard and aft of the hull 24.

[0056] In Figures 2a and 2b, the hang-off module 34 is movable vertically relative to the hull 24 and the workstation 32 between an upper position shown in Figure 2a and a lower position shown in Figure 2b. Optionally, as in this example, the hang-off module 34 lies beneath the surface 12 when in the lower position and so is designed to be fully submersible. When the hang-off module 34 is in the upper position and engaged with the pipeline 14 as shown in Figure 2a, a cut 38 is made through the pipeline 14 at the workstation 32, hence dividing the pipeline 14 into upper and lower pipeline portions 14U, 14L, the latter including the catenary that hangs between the surface 12 and the seabed 16.

[0057] The hang-off module 34 is then lowered to the lower position, separating the lower pipeline portion 14L from the upper pipeline portion 14U to create a longitudinal gap 40 between the facing ends of the pipeline portions 14U, 14L as shown in Figure 2b. The gap 40 is long enough to accommodate insertion of an in-line accessory 42, which can be lifted into place using a crane or an A&R winch of the vessel 10. Such lifting provisions are conventional and so are not shown here.

[0058] As the hang-off module 34 moves downwardly to create the necessary gap 40 by lowering the lower pipeline portion, there is no need to raise the upper pipeline portion 14U or, therefore, to risk wrinkling of a liner as the upper pipeline portion 14U bends back through the straightener 28 and over the aligner 20. However, once the accessory 42 has been inserted, pressurising liquid can be reintroduced in the pipeline 14 and repressurised to protect the liner against wrinkling as the pipeline 14 moves longitudinally in either direction.

[0059] The bottom of the accessory 42 can be welded to the lower pipeline portion 14L before the hang-off module 34 is raised to an intermediate position at which the top of the accessory 42 is brought into abutment with and welded to the upper pipeline portion 14U. However, the top of the accessory 42 could instead be welded to the upper pipeline portion 14U first before raising the hang-off module 34 to bring the lower pipeline portion 14L into abutment with the bottom of the accessory 42, ready for welding. In principle, it would also be possible to weld the accessory 42 to the upper and lower pipeline portions 14U, 14L simultaneously.

[0060] An additional workstation may be provided for welding and testing the joint between the accessory 42 and the lower pipeline portion 14L. Such a workstation could be provided on top of the hang-off module 34 or on the hull 24 of the vessel 10. Provision may also be made for local movement of the hang-off module 34 during pipe alignment, for example by use of a walk-around box.

[0061] Figures 3a and 3b show how a pre-existing conventional hang-off module 34 can be adapted by the addition of an attachment or insert, namely a jig 44 that can support the lower pipeline portion 14L. As, conventionally, the hang-off module 34 is at a fixed height relative to the lay tower 22 or the hull 24 of the vessel 10, the jig 44 can be driven up or down relative to the hang-off module 34 to effect movement of the lower pipeline portion 14L between the upper and lower positions. The jig 44 thereby accommodates the length of an accessory 42 in the resulting gap 40 between the upper and lower pipeline portions 14U, 14L and then effects the further, smaller longitudinal movements required to position the accessory 42 for welding to the pipeline portions. Advantageously, the jig 44 can be used interchangeably on two or more pipelay support vessels.

[0062] Figures 4a and 4b show a variant of the embodiment shown in Figures 2a and 2b in which the hang-off module 34 can not only move the lower pipeline portion 14L vertically but also horizontally. In this example, a movement mechanism 46 acts on the hang-off module 34 to move the hang-off module 34 aft relative to the hull 24, before or after lowering, by extending a cantilever mounting disposed between the hang-off mechanism 46 and the hull 24. The hang-off module 34 thereby translates in the aft direction. In other examples, a movement mechanism 46 could move the hang-off module 34 laterally relative to the hull 24 by translation or otherwise.

[0063] Finally, Figures 5a and 5b show a further variant of the embodiment shown in Figures 2a and 2b in which the hang-off module 34 is moved both aft and laterally. In this example, the aft and lateral movements are effected by a movement mechanism 48 that rotates or pivots the hang-off module 34 about an upright pivot axis 50 offset from the axis of the lower pipeline portion 14L.

[0064] The arrangements illustrated in Figures 4a to 5b envisage achieving a minimum horizontal separation of, for example, 610mm between the upper and lower pipeline portions 14U, 14L. Horizontal separation could be sufficient in itself to accommodate an accessory 42. However, when used in conjunction with vertical separation, horizontal separation can allow the vertical separation between the upper and lower pipeline portions 14U, 14L to be reduced to, for example, 500mm while maintaining a wide enough gap 40 between the pipe ends to accommodate the accessory 42.

[0065] Many other variations are possible within the inventive concept. For example, the lay tower is exemplified in the drawings as overhanging the stern of the vessel but could instead be mounted on a side of the vessel or in alignment with a moonpool extending through the hull of the vessel. Also, the tensioner could be replaced or supplemented by a travelling clamp.

[0066] A jig could adapt a conventional hang-off module not only to move the lower pipeline portion up and down along the lay axis but also to move the lower pipeline portion horizontally relative to the lay axis on which the upper pipeline portion lies.

[0067] A hang-off module or jig of the invention could also be used to facilitate adding an end accessory to an end of a subsea pipeline, if it could be helpful to lower a catenary portion of the pipeline suspended from the hang-off module or jig before the accessory is connected to the catenary portion.

Claims

Claims1. A method of incorporating an in-line accessory into a subsea pipeline being laid from a lay tower of an installation vessel, the method comprising: pausing lay motion of the pipeline; engaging the pipeline with a hang-off module; cutting through the pipeline above the hang-off module to define an upper pipeline portion held in the lay tower and a lower pipeline portion engaged by and suspended from the hang-off module; lowering the hang-off module with the lower pipeline portion to create or to enlarge a gap between mutually-opposed cut ends of the upper and lower pipeline portions; inserting the accessory into the gap; connecting the accessory to the upper and lower pipeline portions; disengaging the hang-off module from the lower pipeline portion; and resuming the lay motion of the pipeline.

2. The method of Claim 1 , further comprising raising the hang-off module to close the gap for connection of the accessory to the upper and lower pipeline portions.

3. The method of Claim 2, comprising connecting the accessory to the lower pipeline portion, raising the hang-off module to raise the accessory with the lower pipeline portion, and connecting the accessory to the upper pipeline portion.

4. The method of Claim 2, comprising connecting the accessory to the upper pipeline portion, raising the hang-off module to raise the lower pipeline portion, and connecting the accessory to the lower pipeline portion.

5. The method of any preceding claim, further comprising moving the hang-off module horizontally relative to the lay tower to bring the lower pipeline portion out of coaxial alignment with the upper pipeline portion.

6. The method of Claim 5, comprising moving the hang-off module in an aft direction with respect to the vessel.

7. The method of Claim 6, comprising extending a cantilever mount disposed between the hang-off module and a hull of the vessel.

8. The method of any of Claims 5 to 7, comprising moving the hang-off module in a transverse direction with respect to the vessel.

9. The method of any of Claims 5 to 8, comprising rotating the hang-off module about an upright axis.

10. A method of incorporating an in-line accessory into a subsea pipeline being laid from a lay tower of an installation vessel, the method comprising: pausing lay motion of the pipeline; engaging the pipeline with a jig supported by a hang-off module; cutting through the pipeline above the jig to define an upper pipeline portion held in the lay tower and a lower pipeline portion engaged by and suspended from the jig; lowering the jig with the lower pipeline portion to create or to enlarge a gap between mutually-opposed cut ends of the upper and lower pipeline portions; inserting the accessory into the gap; connecting the accessory to the upper and lower pipeline portions; disengaging the jig from the lower pipeline portion; and resuming the lay motion of the pipeline.

11. The method of Claim 10, comprising lowering the jig relative to the hang-off module.

12. The method of Claim 11 , wherein the hang-off module remains fixed relative to a hull of the vessel.

13. The method of any of Claims 10 to 12, further comprising raising the jig to close the gap for connection of the accessory to the upper and lower pipeline portions.

14. The method of Claim 13, comprising connecting the accessory to the lower pipeline portion, raising the hang-off module to raise the accessory with the lower pipeline portion, and connecting the accessory to the upper pipeline portion.

15. The method of Claim 13, comprising connecting the accessory to the upper pipeline portion, raising the hang-off module to raise the lower pipeline portion, and connecting the accessory to the lower pipeline portion.

16. The method of any preceding claim, further comprising moving the jig horizontally relative to the lay tower to bring the lower pipeline portion out of coaxial alignment with the upper pipeline portion.

17. The method of Claim 16, comprising moving the jig in an aft direction with respect to the vessel.

18. The method of Claim 17, comprising extending a cantilever mount disposed between the jig and the hang-off module.

19. The method of any of Claims 16 to 18, comprising moving the hang-off module in a transverse direction with respect to the vessel.

20. The method of any of Claims 16 to 19, comprising rotating the hang-off module about an upright axis.

21. The method of any preceding claim, comprising holding the upper pipeline portion substantially stationary relative to the lay tower between engaging and disengaging the hang-off module.

22. The method of any preceding claim, comprising depressurising the pipeline internally before cutting through the pipeline and repressurising the pipeline internally before resuming the lay motion.

23. A method of incorporating an end accessory into a subsea pipeline being laid from an installation vessel, the method comprising: engaging the pipeline with a hang-off module; cutting through the pipeline above the hang-off module to leave a catenary portion of the pipeline engaged by and suspended from the hang-off module; lowering the hang-off module with the catenary portion; connecting the accessory to the catenary portion; disengaging the hang-off module from the catenary portion; and lowering the accessory and the catenary portion to abandon the pipeline.

24. A method of incorporating an end accessory into a subsea pipeline being laid from an installation vessel, the method comprising: engaging the pipeline with a jig supported by a hang-off module; cutting through the pipeline above the jig to leave a catenary portion of the pipeline engaged by and suspended from the jig; lowering the jig with the catenary portion; connecting the accessory to the catenary portion; disengaging the jig from the catenary portion; and lowering the accessory and the catenary portion to abandon the pipeline.

25. An installation vessel for laying a subsea pipeline, the vessel comprising:a lay tower supporting a tensioner or travelling clamp, the tensioner or travelling clamp being operable to maintain top tension in the pipeline while controlling lay motion of the pipeline; and a hang-off module positioned beneath the tensioner or travelling clamp and being operable at that position to engage the pipeline when the lay motion is paused; wherein the hang-off module is movable downwardly and upwardly relative to the lay tower while holding a catenary portion of the pipeline suspended beneath the hang-off module.

26. The vessel of Claim 25, wherein the hang-off module is also movable horizontally relative to the lay tower while holding a catenary portion of the pipeline suspended beneath the hang-off module.

27. The vessel of Claim 26, wherein the hang-off module is movable in an aft direction relative to the vessel.

28. The vessel of Claim 27, comprising an extensible cantilever mount disposed between the hang-off module and a hull of the vessel.

29. The vessel of any of Claims 26 to 28, wherein the hang-off module is movable in a transverse direction relative to the vessel.

30. The vessel of any of Claims 26 to 29, wherein the hang-off module is movable by rotation about an upright axis.

31. An installation vessel for laying a subsea pipeline, the vessel comprising: a lay tower supporting a tensioner or travelling clamp, the tensioner or travelling clamp being operable to maintain top tension in the pipeline while controlling lay motion of the pipeline; a hang-off module positioned beneath the tensioner or travelling clamp; anda jig movable relative to the hang-off module and being operable to engage the pipeline when the lay motion is paused; wherein the jig is movable downwardly and upwardly relative to the hang-off module while holding a catenary portion of the pipeline suspended beneath the jig-32. The vessel of Claim 31, wherein the jig is also movable horizontally relative to the hang-off module while holding a catenary portion of the pipeline suspended beneath the jig-33. The vessel of Claim 32, wherein the jig is movable in an aft direction relative to the hang-off module.

34. The vessel of Claim 33, comprising an extensible cantilever mount disposed between the jig and the hang-off module.

35. The vessel of any of Claims 31 to 34, wherein the jig is movable in a transverse direction relative to the hang-off module.

36. The vessel of any of Claims 31 to 35, wherein the jig is movable by rotation about an upright axis.

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