GUIDING FLEXIBLE PIPELINES ABOARD INSTALLATION VESSELS

An assembly of a rigid accessory such as an end fitting and a flexible elongate element such as a pipeline is laid from an installation vessel into water. The accessory and the element are in longitudinal series. An elongate longitudinally-curved cradle is attached to the assembly, the cradle overlapping longitudinally with both the element and the accessory. The assembly and the cradle are advanced together over a longitudinally-convex guide mounted on a lay tower of the vessel, with the cradle interposed between the assembly on an upper side of the cradle and the guide on a lower side of the cradle. The cradle supports the flexible element to control its curvature and stress.

This invention relates to the installation of elongate flexible elements such as flexible subsea pipelines, in particular to how such elements are guided aboard installation vessels as they are advanced onto a launch axis during installation. The invention particularly addresses the challenges of passing a rigid accessory of a flexible pipeline, such as a terminal head or end fitting at an end of the pipeline, over a guide of a pipelay tower while controlling local curvature and stress of the pipeline.

It is common for a flexible subsea pipeline to be laid on a nominally vertical launch axis from a pipelay tower of an installation vessel. Conventionally, this involves bending the pipeline around a circular or part-circular guide mounted high on the tower during movement of the pipeline between a storage location and the nominally vertical lay direction.

In this respect,FIGS.1and2show conventional pipelay support vessels (PLSVs)10laying a flexible pipeline12beneath the surface14. Each PLSV10has a pipeline store16, that ofFIG.1being a reel turning on a horizontal axis and that ofFIG.2being a carousel in which the pipeline12is wound around a vertical axis. Each PLSV10also has an upright pipelay tower18over which the pipeline12extends upwardly from the store16before curving onto a downward path and then extending through a hold-back system, such as a tensioner20, to enter the sea on a launch axis corresponding to the inclination of the tower18.

The pipeline12is guided as it bends over the top of the tower18by a curved guide22that permits longitudinal movement but restrains lateral movement of the pipeline12. For this purpose, the guide22is upwardly convex in a longitudinal direction to define a curved transition path between upward and downward movement of the pipeline12. The radius of longitudinal curvature of the guide22is large enough to respect the minimum bend radius (MBR) of the pipeline12.

Conversely, the guide22is upwardly concave in a transverse direction as shown schematically in the cross-section ofFIG.4. Thus, the guide22defines an upwardly-facing, longitudinally- or circumferentially-extending groove or channel24that receives and restrains lateral movement of the pipeline12between upwardly-extending side walls26of the channel24.

InFIG.1, the guide22is a guide wheel that turns about a horizontal axis as the pipeline12advances. Conversely, inFIG.2, the guide22is a static guide chute through which the advancing pipeline12slides. The guide chute embodying the guide22is shown here as having constant part-circular curvature along its length but it could instead have variable curvature along its length.

PLSVs10have various configurations. For example, the launch axis defined by the pipelay tower18inFIG.1is inclined slightly away from the vertical whereas the launch axis defined by the pipelay tower18inFIG.2is substantially vertical. Also, the pipelay tower18inFIG.1is at the stern of the PLSV10so that the launch axis is aft of the PLSV10whereas the pipelay tower18inFIG.1is amidships so that the launch axis is over the side of the PLSV10or extends through a moonpool in the hull of the PLSV10.

The abovementioned features of the PLSVs10exemplified inFIGS.1and2can of course be combined in various ways. For example, a guide wheel serving as the guide22inFIG.1could surmount a vertical pipelay tower18like that ofFIG.2.

A pipeline12may have in-line or terminal accessories that create discontinuities due to their different diameter, stiffness and weight relative to the adjoining sections of the pipeline12. For example, a flexible pipeline12is characterised by a pliant composite layered structure of metallic wire armour and polymer layers and spacers terminating in rigid elongate flanged end fittings of steel that are typically of greater diameter than the adjoining composite structure. Other head structures, terminal modules or end fittings are possible.FIG.3shows such a head28in addition to an optional bend stiffener30tapering away from the head28around the adjoining part of the pipeline12. The head28, the bend stiffener30and the pipeline12are co-axial along a central longitudinal axis32. The head28has an anchor formation34at its distal end for the attachment of a line or link.

FIG.4shows the head28, bend stiffener30and pipeline12received in the upwardly-concave channel24of the guide22. The rigidity and length of a terminal head28of a flexible pipeline12means that the head28cannot follow the longitudinal curvature of the guide22. The head28could even be so wide that it cannot fit fully into the channel24of the guide22. Thus, handling such heads28presents a challenge when advancing a flexible pipeline12along a longitudinally-curved path as it risks over-stressing and over-bending the pipeline12, especially close to the interface between the pipeline12and the head28. In this respect, flexible pipe has a smaller MBR than nominally rigid pipe but that MBR cannot be exceeded without causing irreversible damage to the pipe, unlike a rigid pipe that can be straightened following plastic deformation.

It will be apparent fromFIG.4that the greater cross-sectional radius of the head28holds the bend stiffener30and the pipeline12spaced apart from the base of the channel24in the guide22. Thus, the bend stiffener30and the pipeline12are left suspended and hence unsupported by the guide22, especially in the vulnerable portion of the pipeline12close to the interface between the pipeline12and the head28.

The difficulty of handling the head28is compounded where successive pipelines12are linked via opposing heads28and stored end-to-end to be installed in the same installation campaign. In that case, a head28at the trailing end of the leading pipeline12is linked to a head28at the leading end of the trailing pipeline12. Consequently, the leading pipeline12pulls the trailing pipeline12out of the pipe store14such as a reel or carousel, and the linked heads28must then pass over the guide22simultaneously. The leading pipeline12is subsequently separated from the trailing pipeline12and lowered to the seabed in an abandonment operation before laying of the trailing pipeline12is initiated at another installation site.

One way to handle a head28at the leading end of a flexible pipeline12is to use a crane or winch to lift the head28over the guide22and then to lower the adjoining part of the pipeline12into the channel24of the guide22. This can be done using the main crane of the vessel or a dedicated hoist, like the crane or winches of WO 2016/133388 and WO 2014/148907, which can also be used to hold and lower the trailing end of the pipeline catenary during abandonment. However, it is an awkward operation to lift the head28and to insert the adjoining part of the pipeline12into the channel24while avoiding inadvertent over-stressing or over-bending of the pipeline12. If opposed heads28are linked where pipelines12are arranged end-to-end, the operation is even more challenging.

Another approach disclosed in WO 2016/053094 and WO 2015/075511 is to use a swinging arm to displace the head28of the pipeline12along the curvature of the guide22. However, this requires extensive and costly modification of the pipelay equipment.

In WO 2015/075555, a transport collar connects a point along the head28to a guide wheel. However, such an arrangement is unsuitable for use where the guide22is a static guide chute rather than a wheel.

GB 2520512 describes a transfer mechanism for transferring a fitting on a flexible tubular conduit about a guiding means of a pipelay tower.

In U.S. Pat. No. 5,947,641, a series of packing elements support a pipeline on either side of a localised zone of increased stiffness of the pipeline, so as to guard against buckling in these regions when the pipeline is bent around an arcuate path.

GB 2173760 describes a handling device that guides a rigid pipeline accessory about a guide wheel.

Against this background, the invention resides in a method of laying an assembly of a flexible elongate element and a rigid accessory into water from an installation vessel. The accessory may, for example, be a head at an end of the element. The method comprises: advancing the assembly with the accessory and the element in longitudinal series and with an elongate longitudinally-curved cradle attached to the assembly, the cradle overlapping longitudinally with both the element and the accessory; and advancing the assembly and the cradle together over a longitudinally-convex guide mounted on a lay tower of the installation vessel, with the cradle interposed between the assembly on an upper side of the cradle and the guide on a lower side of the cradle.

The lower side of the cradle may suitably have a radius of longitudinally concave curvature that substantially matches a radius of longitudinally convex curvature of the guide. The lower side of the cradle may also have transversely convex curvature. In that case, where the guide comprises a longitudinally-extending channel, the lower side of the cradle may have a radius of transversely convex curvature that substantially matches a radius of transversely concave curvature of the channel.

Conversely, the upper side of the cradle may have a radius of transversely concave curvature that substantially matches a radius of transversely convex curvature of the accessory. More generally, the upper side of the cradle may have a radius of transversely concave curvature that is centred on an axis parallel to, or coincident with, a central longitudinal axis of the element.

The element may be supported on one or more supports disposed on a proximal side of the accessory, the or each support extending radially between the upper side of the cradle and an outer surface of the element. The or each support may, for example, have an end face with transversely concave curvature. The or each support may extend radially to a height corresponding to a cross-sectional radius of the accessory minus a cross-sectional radius of the element.

The element may be allowed to move away from the upper side of the cradle as the assembly and the cradle are advanced together toward or away from the guide. For example, the element can diverge from the cradle in a proximal direction. This may be achieved by allowing the element to lift away from the or each support fixed to the cradle, or by allowing the or each support fixed to the element to lift away from the cradle, or by causing the or each support fixed to the element and to the cradle to extend in a radially outward direction. Conversely, movement of the element toward the upper side of the cradle may be restrained.

First and second assemblies may be advanced together over the guide, the assemblies being linked end-to-end and each having a respective cradle interposed between the assembly and the guide. The first and second assemblies may, for example, be linked via their respective accessories. A link between the assemblies may be held spaced radially from the guide.

The cradle can be removed from the assembly before advancing the assembly through a hold-back system of the installation vessel, and more generally can be removed from the assembly and attached to another assembly for re-use.

The inventive concept embraces an assembly for subsea installation comprising a flexible elongate element and a rigid accessory in longitudinal series, in combination with an elongate longitudinally-curved cradle attached to, and laterally offset from a longitudinal axis of, the assembly. The cradle extends between and overlaps longitudinally with the element and the accessory and presents a first, longitudinally-convex side to the assembly opposed to a second, longitudinally-concave side extending along a length of the cradle. Equipment may be included to effect a transition between the head and the element.

One or more supports may be disposed on a proximal side of the accessory and may extend between the first side of the cradle and an outer surface of the element. The or each support may have an end face with transversely concave curvature, or may extend to a height corresponding to a cross-sectional radius of the accessory minus a cross-sectional radius of the element.

The or each support may be fixed to the element and separable from the cradle, or may be fixed to the cradle and separable from the element, or may be fixed to the cradle and to the element and of variable height.

The second side of the cradle suitably has transversely convex curvature. Conversely, the first side of the cradle may have a radius of transversely concave curvature that substantially matches a radius of transversely convex curvature of the accessory or that is centred on an axis parallel to, or coincident with, a central longitudinal axis of the element.

The inventive concept also embraces a combination of assemblies of the invention, linked end-to-end and each fitted with one of the cradles. The assemblies may, for example, be linked via their respective accessories.

In summary, the invention facilitates safe and speedy transfer of flexible pipeline sections over a guide such as a guide wheel of a PLSV pipelay tower, especially when performing twin transfer of adjoining, interconnected sections of flexible pipe. The solution of the invention involves a chute or cradle with the same radius of curvature as the guide, for example 5.6 m, to accommodate a head or end fitting at the end of a pipeline section. The cradle is interposed between the head and the guide. The cradle may, for example, be made of a composite material to be light and easy to be handle.

The flexible pipeline and optionally also the head may be accommodated on supports on the cradle that limit curvature of the pipeline product. The supports may be fabricated based on each project specification, taking account of the outer diameter of the head and the flexible pipeline and the MBR of the flexible pipeline. The supports may also be made of a composite material.

The invention enables a major reduction in operational time and hence operational cost and project duration. Assuming six hours for a single transfer operation of the prior art versus up to two hours for a twin transfer manoeuvre of the invention, the time reduction expected for each manoeuvre is close to 66%. Assuming twenty-five such operations per year per PLSV, an aggregate time reduction of one hundred to one hundred and twenty-five hours per year will result, per PLSV.

The invention also has other benefits, such as removal of a crane or winch from the firing line and reduction or elimination of rigging. In addition to improved performance during installation, the installed pipeline can achieve production sooner than if installed by prior art methods.

Embodiments of the invention implement a method to guide at least one head of a flexible pipeline on a circular path groove of a flexible lay tower guide or wheel. The method comprises: attaching supports to the flexible pipeline along a predetermined section close to the head; and attaching an arcuate section of gutter or cradle along the head, wherein the cradle has the same radius of curvature as the guide or wheel. The supports are within a space in the transverse direction between the flexible pipeline and the cradle when the head and the flexible pipeline are on the guide or wheel.

The transverse radius of the cradle may correspond to the maximum radius of the head. The cradle is preferably longer than the head. The height of a support may equate to the radius of the cradle minus the radius of the flexible pipeline.

The supports could be free to move radially inside the cradle. The supports may, for example, be strapped to the pipeline. The supports could also partially support the head and any equipment effecting a transition between the head and the pipeline, such as a bend restrictor, stiffener, limiter or taper.

Embodiments of the invention also implement a method to guide the transition between two flexible pipelines on a circular path groove of a flexible lay tower guide or wheel. The method comprises: connecting the trailing end of a first flexible pipeline to the leading end of a second flexible pipeline by a transition device, the transition device being a pliant link such as a short chain or wire; and attaching a section of gutter or cradle to each end and supports between the section of cradle and flexible pipeline close to the end, so that the height of the supports allows the pliant link to remain taut between the two ends without touching the guide or wheel.

Thus, in the invention, an assembly of a rigid accessory such as an end fitting and a flexible elongate element such as a pipeline is laid from an installation vessel into water. The accessory and the element are in longitudinal series. An elongate longitudinally-curved cradle is attached to the assembly, the cradle overlapping longitudinally with both the element and the accessory. The assembly and the cradle are advanced together over a longitudinally-convex guide mounted on a lay tower of the vessel, with the cradle interposed between the assembly on one side of the cradle and the guide on the opposite side of the cradle. The cradle supports the flexible element to control its curvature and stress.

Referring next, then, toFIG.5, this shows a flexible pipeline12that terminates in an enlarged rigid head28. In this example, a bend stiffener has been omitted for simplicity. In accordance with the invention, the head28and the pipeline12are both supported by an elongate curved rigid cradle36that underlies at least part of the head28and that extends proximally away from the head28to underlie the part of the pipeline12adjoining the head28.

The cradle36is shaped to complement the shape of a channel24in the curved guide22of a pipelay tower. Specifically, the cradle36is upwardly convex and downwardly concave in a longitudinal direction. The underside38of the cradle36has a radius of longitudinal curvature that substantially matches or complements the radius of longitudinal or circumferential curvature of the base of the channel24in the guide22.

The cradle36is also upwardly concave and downwardly convex in a transverse direction. The underside38of the cradle36has a radius of transverse curvature that substantially matches or complements the radius of transverse curvature of the base of the channel24in the guide22, as best appreciated inFIGS.11ato11cand13ato13c.

The portion of the pipeline12adjoining the head28is supported by a longitudinally-spaced series or array of supports40. The head28may be supported directly by the cradle36; alternatively, or additionally, one or more supports40could also be interposed between the cradle36and the head28.

The supports40bridge the radial gap, with respect to the longitudinal curvature of the cradle36, between the radially outer or upper side42of the cradle36and the outer diameter of the pipeline12. This ensures that the pipeline12is supported along its length up its interface with the head28.

Each support40has an upper end face44that is shaped to complement and engage with the external shape of the pipeline12and/or the head28as appropriate. Thus, as best appreciated inFIGS.11b,11cand13ato13c, the upper end face44of each support40is upwardly concave, with a part-cylindrical surface curved about a longitudinal axis. That axis is suitably coaxial with the central longitudinal axis32of the pipeline12and/or the head28as shown inFIGS.11ato11c.

The supports40may be separate from or integral with the cradle36but are preferably separate from the cradle36, removably attachable and/or interchangeable. In this way, the longitudinal position, shape and/or height of the supports40can be tailored to suit the radius of the pipeline12and/or the head28or the MBR of the pipeline12.

Whilst the cradle36of the invention can be used beneficially with a single pipeline12, the invention has special benefits when used in a twin transfer operation involving pipelines12that are interconnected end-to-end. Such an arrangement is shown inFIG.6, where the pipelines12and their associated cradles36are in mirrored relation about a central plane between them. An articulated pliant link46comprising shackles and a central ring joins the anchor formations34of the heads28and is held in tension.

FIG.7shows the interconnected pipelines12with their respective cradles36being pulled out of a carousel store16beneath a working deck of a PLSV10. The cradles36may be fitted to the pipelines12before coiling the pipelines12into storage or after uncoiling the pipelines12from storage.

It will be noted that as the curvature of the pipelines12changes or straightens on their path from storage, the pipelines12can bend or pivot away from the cradles36and hence from the supports40, which in this example are fixed to the cradles36and are not fixed to the pipelines12. Thus, by decoupling the cradles36from the pipelines12, the fixed curvature of the cradles36will not stress the pipelines12if the longitudinal curvature of the pipelines12tends to depart from that of the cradles36. Conversely, the pipelines12can bend or pivot away from the cradles36but not from the supports40, when they are not fixed to the pipelines12.

FIGS.8and9show the cradles36and the pipelines12lying on concentric part-circular curves that complement the circumferential curvature of the guide22of the PLSV10, exemplified here by a guide wheel.

As noted above, the supports are not fixed to the pipelines12in the example shown inFIGS.5to9. However,FIGS.10and11ato11cshow another possibility, in which each support40is fixed to the pipeline12, for example by a strap48that wraps over the pipeline12.

FIG.11ashows that the transversely-concave upper surface42of the cradle36receives and directly supports the head28. The upper surface42of the cradle36has a radius of curvature corresponding to that of the outer surface of the head28, centred on an axis of curvature that is coaxial with the central longitudinal axis32of the pipeline12and the head28.

In this example, a proximally-tapering bend stiffener30adjoins the head28and is restrained by a shallow support40, also shown inFIG.11b. The height or radial thickness of the support40corresponds to the difference in radius between the outside of the bend stiffener30and the upper surface42of the cradle36, with respect to the central longitudinal axis32of the pipeline12.

Further, relatively deep, supports40extend between the pipeline12and the upper surface42of the cradle36as also shown inFIG.11c. The height or radial thickness of those supports40corresponds to the greater difference in radius between the outside of the pipeline12and the upper surface42of the cradle36, with respect to the central longitudinal axis32of the pipeline12.

Decoupling the cradle36from a pipeline12is also shown inFIG.12, where the pipeline12straightens and lifts away from the supports40as also shown inFIG.13a. Conversely, inFIG.13b, the supports40are fixed to the pipeline12, for example by straps48that wrap over the pipeline12, but are free to lift away from the cradle36. InFIG.13c, the supports40can extend, for example telescopically, while remaining attached to both the pipeline12and the cradle36. The supports40could have springs that bias the pipeline12toward the cradle36and/or restraints that limit movement of the pipeline12away from the cradle36. In all of those cases, the cradle36is suitably fixed to the head28, for example by one or more straps48that wrap over the head28as shown inFIGS.10,11aand12.

Turning finally toFIGS.14aand14b, these graphs plot maximum curvature of the pipeline12against distance from the proximal end of a rigid end fitting of the pipeline12such as a head28, modelled in each case when passing over a curved guide22such as a guide wheel.FIG.14arepresents the second or trailing end of a first pipeline12andFIG.14brepresents the first or leading end of a second pipeline12joined end-to-end to the trailing end of the first pipeline12by a link46as shown inFIGS.6and7.

In each graph, the maximum allowable curvature of the pipeline12is represented by a horizontal dashed line50. The ‘equipment length’ of the cradle36is represented by a first pair of vertical dashed lines52, shown to the left. The ‘aligner contact’ length over which the pipeline12contacts the guide22is represented by a second pair of vertical dashed lines54, shown to the right. A short gap between those pairs of lines52,54corresponds to where the pipeline12hangs between the cradle36and the guide22.

It will be noted in both cases that when modelling the prior art in the absence of the cradle36of the invention, as shown by plot56, the curvature of the pipeline12exceeds the allowable maximum50when approaching the head28. In contrast, when the pipeline12and the head28are supported by the cradle36of the invention, as shown by plot58, the curvature of the pipeline12always remains within the allowable maximum50.

When a cradle36supporting a pipeline12has passed over a guide22, the cradle36is suitably removed from the pipeline12before that part of the pipeline12passes through the hold-back system20on the tower18. However, in principle, the cradle36could remain attached to the pipeline12as the pipeline12is launched into the water.

Many variations are possible within the inventive concept. For example, an upper or radially outer surface42of the cradle36could be shaped with one or more protrusions or recesses to complement and engage with the external shape of the head28.

There could be any suitable number of supports40between the cradle36and the pipeline12, from one support40upwards.

The cradle36preferably underlies all, or substantially all, of the length of the head28but could instead support only part or a major part of the length of the head28, hence terminating proximally relative to a distal end of the head28.

The pipeline12could converge with the cradle in a proximal direction to minimise stress on the pipeline12. For example, the supports40could reduce or step down in height progressively in a proximal direction away from the head28.

The cradle36and supports40can be made suitable for accessories other than the head28, such as bend stiffeners, buoys, clamps or ballast weights.

The inventive concept could be applied when laying flexible elongate subsea elements other than pipelines, such as cables or umbilicals.