Patent Description:
Subsea flowlines convey hydrocarbon fluids on their journey from a subsea well to a surface facility that receives the fluids for onward transport, such as a platform or a floating production, storage and offloading (FPSO) vessel. Other fluids such as water or chemicals may be conveyed in parallel pipes in the opposite direction for injection into the well or for supplying subsea processing systems.

It is not practical for a flowline to extend continuously as a unitary conduit along its entire length from a subsea well to a surface facility. In practice, therefore, a flowline typically comprises multiple sections that are connected in series and in fluid communication with each other. For example, a subsea flowline typically comprises a main pipeline that extends across the seabed from a subsea wellhead or manifold, potentially for tens of kilometres, a riser extending from the seabed toward the surface, piping on subsea structures such as manifolds or tees, and tie-in connections known as spools or jumpers to connect those sections.

The shape, structure and composition of each section of a flowline is chosen to suit its specific function. For example, spools and jumpers have to effect the necessary connections while complying with positional changes that arise from installation tolerances and from relative movement of subsea equipment post-installation. Such positional changes may, for example, result from settlement of heavy structures into low-strength seabed soils and from thermal expansion and contraction of lengthy flowline sections, which can result in 'walking' across the seabed over time. Also, a moored surface installation will move within a footprint area under the influence of currents, weather and surface dynamics.

Thus, the overall distance between the wellhead and the surface facility, and consequently also between at least some sections of the flowline, is not strictly defined at installation and will vary over time. Spools and jumpers are designed to accommodate these changes by being pliant in various planes.

Before considering the prior art in more detail, it is helpful to understand what the terms 'rigid pipe' and 'flexible pipe' mean to those skilled in the art. Conventional rigid pipes used in the subsea oil and gas industry are specified in the American Petroleum Institute (API) Specification <NUM> and Recommended Practice <NUM>. A rigid pipe usually consists of, or comprises, at least one pipe of solid steel or steel alloy. However, additional elements can be added, such as an internal liner layer or an outer coating layer. Such additional elements can comprise polymer, metal or composite materials. Rigid pipe joints are terminated by a bevel, a thread or a flange, and are assembled end-to-end by welding, screwing or bolting them together to form a pipe string or pipeline.

The allowable in-service deflection of rigid steel pipe is determined by the elastic limit of steel, which is around <NUM>% bending strain. It follows that the minimum bending radius or MBR of rigid pipe used in the subsea oil and gas industry is typically around <NUM> to <NUM> metres depending upon the cross-sectional dimensions of the pipe. Exceeding this limit caused plastic deformation of the steel, although some plastic deformation can be recovered by a subsequent straightening process.

Whilst they have enough flexibility to bend elastically along a substantial length, rigid pipes do not fall within the definition of flexible pipes as understood in the art. Flexible pipes used in the subsea oil and gas industry are specified in API Specification 17J and Recommended Practice 17B. The pipe body is composed of a composite structure of layered materials, in which each layer has its own function. In particular, bonded flexible pipes comprise bonded-together layers of steel, fabric and elastomer and are manufactured in short lengths in the order of tens of metres. Typically, polymer tubes and wraps ensure fluid-tightness and thermal insulation, whereas steel layers or elements provide mechanical strength. Conversely, unbonded flexible pipes can be manufactured in lengths of hundreds of metres but are very expensive and have limited strength.

The composite structure of a flexible pipe allows a large bending deflection without a similarly large increase in bending stresses. The bending limit of the structure is determined by the elastic limit of its outermost plastics layer, typically the outer sheath, which limit is typically <NUM>% to <NUM>% bending strain. Consequently, the MBR of flexible pipe used in the subsea oil and gas industry is typically between <NUM> and <NUM> metres. Exceeding that limit causes irreversible damage to the structure.

In recent years, the subsea oil and gas industry has begun to adopt composite pipes of polymer composite materials as an equivalent to, or replacement for, conventional rigid pipes of steel. Composite pipes have a tubular load-bearing structure that is principally of composite materials. This is to be distinguished from pipes having a composite structure, such as various layered configurations that may be used in both rigid and flexible pipes. Typically, the composite material used in a composite pipe comprises a polymer resin matrix reinforced by fibres such as glass fibres or carbon fibres. The polymer matrix may be of thermoplastic or thermoset materials. The former results in what is known in the art as thermoplastic composite pipe or, more simply, as thermo-composite pipe (TCP). TCP is classed as a bonded composite pipe.

The composite tube of a TCP has a solid, monolithic structure comprising a polymer liner, a polymer composite matrix and an optional outer coating that may also be of polymer. The polymer of the liner, the matrix and/or the coating may, for example, be polypropylene. The matrix is a true composite reinforced with fully-embedded reinforcing fibres.

Like-for-like, composite pipes are generally more pliant than conventional rigid pipes of steel and can be bent elastically to a smaller MBR. However, like conventional rigid pipes, composite pipes have a characteristic of rigidity that is largely absent from flexible pipes, namely a capability to deflect elastically along their length and a tendency to straighten under elastic recovery.

Most commonly, the lengthy main pipeline section of a flowline comprises a rigid pipe made primarily of steel. Steel achieves a good balance between structural strength and cost, and is less expensive than an equivalent length of TCP. However, steel has to be protected from corrosion. Consequently, where there is a risk of internal corrosion from sour fluids, steel pipelines are provided with corrosion-resistant liners which may, for example, be made of a polymer material. Polymer lining technology is offered by a sister company of the Applicant under the registered trade mark 'Swagelining'. This can be more cost-effective than lining rigid pipelines with corrosion-resistant alloy (CRA) liners, which have been used traditionally as an internal corrosion barrier.

Spools and jumpers can have various shapes, structures and compositions. For example, <CIT> discloses spools and jumpers made from rigid pipe. Whilst rigid, the pipe comprises bends that can accommodate slight angular deviations. However, spools and jumpers of rigid pipe must be designed with specific shapes and dimensions. Their final shape and dimensions can only be determined by a metrology process after the subsea structures to be connected by them have been installed on the seabed. These steps of measurement, design and manufacturing take valuable time and may require additional vessel campaigns, and so cost a lot of money.

In the context of corrosion-resistant flowlines, another drawback of rigid spools and jumpers is that it is difficult to manufacture lined bends, especially polymer-lined bends. So, even where the main pipeline is polymer-lined, rigid spools or jumpers at either end of the pipeline are typically still fabricated using CRA liners, which is expensive, or are made of solid CRA, which is even more expensive.

Metrology can be avoided by using a material for spools and jumpers that is substantially less stiff or more pliant than rigid pipe. This allows a spool or jumper to be bent underwater to an extent sufficient to cope with considerable positional variation between the subsea structures that are to be connected. For example, flexible pipe may be used as disclosed in <CIT> or pliant TCP may be used as disclosed in <CIT>. Alternatively, an articulated rigid pipe is disclosed in <CIT>.

Whatever their shape, structure or composition, at least one additional operation is required to complete the installation of a spool or jumper. This increases cost and business risk due to the high operational cost of an installation vessel, coupled with tie-up of valuable capital assets and reliance on the availability of a suitable weather window.

In principle, the main pipeline itself can be bent or deflected to enable direct tie-in to another subsea structure without an intermediate spool or jumper. This is the conventional way to connect an unbonded flexible pipeline, which is highly pliant and so can accommodate the necessary deflections without damage. However, unbonded flexible pipe is prohibitively expensive to manufacture in a length that would be useful for a full pipeline, and it cannot withstand significant compression or axial torsion without damage. Similarly, making an entire pipeline of TCP would also be very expensive.

Direct tie-in is proposed for a rigid pipeline in <CIT>. which is the simplest and least expensive tie-in solution in principle. However, in practice, direct tie-in of a rigid pipeline risks failure arising from irrecoverable plastic deformation of the pipeline and requires additional measures to manage thermal expansion and contraction.

<CIT> proposes terminating a rigid subsea pipeline with additional sections of flexible pipe. The sections of flexible pipe are pliant enough to be connected to a surface facility as a riser or to a static structure on the seabed as a jumper.

In <CIT>, the full length of the rigid pipeline is firstly laid on the seabed by a conventional technique such as S-lay, J-lay or reel-lay. Then, in a subsequent operation, an end of the pipeline is located and lifted to an installation vessel at the surface. There, the flexible pipe is coupled to the rigid pipeline, end-to-end, and the rigid pipeline is lowered back to the seabed suspended from an abandonment and recovery (A&R) wire of the installation vessel. Simultaneously, the appropriate length of flexible pipe is launched progressively from the vessel. The upper, free end of the flexible pipe may then be handed over for connection to a surface facility as a riser or may instead be abandoned to the seabed for subsea connection as appropriate.

The procedure described in <CIT> is useful for installing a section of flexible pipe that is long enough to be used as a riser. However, the procedure is inappropriately complex for installing a jumper pipe, which could be as short as <NUM> or less. Also, the procedure may be impractical in deep water, where a substantial weight load of the rigid pipeline would have to be suspended from the A&R wire via a short length of the less robust flexible pipe.

In more remote prior art, <CIT> proposes a method for laying a pipeline having a rigid portion and a flexible portion, to address a particular situation in which vessels capable of laying rigid pipeline are prohibited from entering certain areas, for example areas around drilling platforms.

In summary, the prior art has failed to provide an effective low-cost and simple to install solution to the challenge of maintaining internal corrosion protection substantially continuously along a multi-section flowline.

Against this background, the invention resides in a method of installing a subsea tie-in conduit. The method comprises: unspooling or manufacturing a rigid pipeline aboard an installation vessel; launching the pipeline progressively from the vessel into water; coupling a distal end of the tie-in conduit to a proximal end of the pipeline aboard the vessel, the tie-in conduit being pliant relative to the pipeline; and launching the tie-in conduit into the water coupled to the pipeline, while supporting a suspended weight load of the pipeline along a load path that substantially bypasses the tie-in conduit.

The tie-in conduit is launched fully into the water before landing the proximal end of the pipeline on the seabed. Thus, the tie-in conduit may have a length that is less than the depth of the water into which the tie-in conduit is launched.

The load path is suitably defined by an A&R wire that extends from the vessel and is coupled to the proximal end of the pipeline. For example, the A&R wire may conveniently be connected to a coupling at the proximal end of the pipeline, that coupling effecting fluid communication between the pipeline and the tie-in conduit. Substantially all of the suspended weight load of the pipeline may be borne by the A&R wire.

A proximal end of the tie-in conduit may be suspended from the A&R wire or from an auxiliary wire that extends from the vessel.

Advantageously, the pipeline and the tie-in conduit may be allowed to pivot together relative to the A&R wire about a substantially horizontal axis.

After being landed on the seabed, the tie-in conduit may be deflected relative to the pipeline. The deflected tie-in conduit may be connected to a subsea structure for fluid communication with the pipeline.

The tie-in conduit may be stored aboard the vessel, in a straight or curved configuration, before being coupled to the rigid pipeline.

The tie-in conduit preferably comprises a composite pipe of polymer composite material but may comprise a flexible pipe. The invention has particular benefit where the rigid pipeline needs to be lined with a corrosion-resistant liner.

The inventive concept extends to subsea flowlines. One such flowline of the invention comprises: a rigid pipeline lined with a corrosion-resistant liner; a corrosion-resistant tie-in conduit that is more pliant than the pipeline; and a coupling that joins the pipeline and the tie-in conduit on a common longitudinal axis. The coupling includes a pivoting anchorage for an A&R wire, the anchorage comprising a rigid yoke bridle that straddles the tie-in conduit and that is arranged to allow the pipeline and the tie-in conduit to pivot together relative to an anchored A&R wire.

The yoke bridle may, for example, comprise pivotable legs joined by a transverse bridge that has an anchor formation for connection to the A&R wire.

The invention allows continuity of corrosion-resistant material in the bore of a flowline, with lined rigid pipe in series with composite pipe or unbonded flexible pipe for the same corrosion-proof function. TCP is preferred. Whilst stiffer than flexible pipe, TCP is significantly more pliant than steel rigid pipe with a similar bore diameter.

In summary, before final laydown of a rigid pipeline installation, a more pliant spool or jumper of TCP or unbonded flexible pipe is connected to the second end of the rigid pipeline. A yoked bridle is used to attach an A&R wire to allow deployment without the spool or jumper taking the full installation load or being subjected to significant bending moments as a result of the interface between the rigid and pliant pipes.

The second end of the pliant spool or jumper may conveniently be deployed using a second connection to the A&R wire, allowing the spool or jumper to hang under the A&R wire but not to be subjected to the abandonment loading of the A&R wire.

This invention allows the single deployment of a rigid pipeline and pliant tie-in conduit from an installation vessel to the seabed. The installation vessel can then disconnect the rigid pipeline and the tie-in conduit from the A&R wire and proceed to tie-in the jumper to the relevant subsea architecture.

TCP offers a corrosion resistant pipeline solution and the benefits of flexibility or pliancy, hence eliminating the need for metrology. However, as TCP is more expensive than polymer-lined rigid pipe, the invention employs a cost-effective hybrid of both. The invention also proposes a more cost-effective installation of this hybrid system, allowing a single-vessel, single-trip solution.

Embodiments of the invention provide a corrosion-resistant flowline, comprising: a main section of corrosion-resistant pipeline, for example polymer-lined pipeline; and at least one thermoplastic composite spool for fluidly linking the polymer-lined pipe to a connection point.

The polymer-lined pipeline may comprise at least a steel pipeline and an inner polymer liner. An end of the polymer liner is suitably sealed to ensure internal continuity with the thermoplastic pipe.

The inventive concept may also be embodied in a subsea installation comprising one or more flowlines of the invention.

Embodiments of the invention also implement a method to perform direct tie-in of a corrosion-resistant flowline to a subsea connection point. The method comprises the following steps: installing the corrosion-resistant flowline from its first end; before abandonment of the second end close to the subsea connection point, connecting a pliant spool to the second end; abandoning the second end on its target by holding it back by a tensioned abandonment cable; simultaneously lowering the free end of the spool; and after landing the second end, deforming the spool to connect it to the subsea connection point. The spool may be much shorter and lighter than the main flowline section, for example less than <NUM> long.

The installation method may involve any pipelaying method, for example reel lay. Connections of the pliant spool may, for example, be effected by flanged connectors.

The connection between the lined pipe and the spool may comprise a yoke for connecting the tensioned abandonment line.

The free end of the spool may be lowered by a wire connected to the abandonment cable, for example via a shackle arrangement, a tri-plate or an equivalent rigging arrangement. Alternatively the free end of the spool may be lowered by a wire connected to a winch of the installation vessel.

Thus, the invention provides a method of installing a subsea tie-in conduit. The method comprises unspooling or manufacturing a steel rigid lined pipeline aboard an installation vessel and launching the pipeline progressively from the vessel into water. A distal end of the tie-in conduit is coupled to a proximal end of the pipeline above the surface and is then launched into the water coupled to the pipeline. The suspended weight load of the pipeline is supported by an A&R wire connected to the proximal end of the pipeline, defining a load path that bypasses the tie-in conduit. Elegantly, a proximal end of the tie-in conduit may be suspended from that wire.

The tie-in conduit is of composite or flexible pipe, hence being pliant relative to the lined rigid pipeline and maintaining internal corrosion resistance. After landing on the seabed, the tie-in conduit may be deflected relative to the pipeline for connection to a subsea connection point.

Referring firstly to <FIG> of the drawings, a pipelay vessel <NUM> is exemplified here as being configured for reel-lay operations. The invention could, however, be applied to pipelay vessels <NUM> that are configured differently, most notably for S-lay or J-lay operations.

As is conventional, the vessel <NUM> comprises a main reel <NUM> that stores a length of rigid pipeline <NUM>. The pipeline <NUM> is primarily of steel and is lined internally for corrosion resistance, for example with a polymer liner.

The main reel <NUM> turns to advance the pipeline <NUM> over a chute or guide wheel <NUM> at the top of an inclined laying ramp <NUM>. The pipeline <NUM> is launched from the bottom of the ramp <NUM> to extend beneath the surface <NUM>. The pipeline <NUM> is launched on a launch axis whose inclination corresponds to the inclination of the ramp <NUM>.

The inclination of the ramp <NUM> may be adjusted to suit different operational circumstances. For example, the ramp <NUM> may be pivoted to a shallower incline for transit and for loading, or to a steeper incline for pipelaying in deeper water.

As is also conventional, the ramp <NUM> supports a straightener system <NUM>, a tensioner system <NUM> and a hang-off or hold-back system <NUM>. The straightener system <NUM> reverses plastic deformation of the pipeline <NUM> advanced from the main reel <NUM>. The tensioner system <NUM> supports the suspended weight of the pipeline <NUM> as the pipeline <NUM> is being launched. The hold-back system <NUM> clamps or otherwise engages the pipeline <NUM> to allow connection of other equipment or conduits in fluid communication with the pipeline <NUM>.

The vessel <NUM> shown in <FIG> is also equipped with an auxiliary reel <NUM>. In this example, the auxiliary reel <NUM> is used to store a discrete tie-in conduit <NUM> as represented schematically by a dotted line in <FIG>. The tie-in conduit <NUM> is of a determinate length that is substantially shorter than the pipeline <NUM> stored on the main reel <NUM>. For example, the typical length of a discrete tie-in conduit <NUM> is in the range <NUM> to <NUM>, generally less than <NUM>. The tie-in conduit <NUM> is also substantially more pliant than the pipeline <NUM>, as will be explained.

<FIG> also show that the vessel <NUM> is equipped with an A&R system comprising an A&R winch <NUM>, from which an A&R wire <NUM> extends over a sheave <NUM> at the top of the ramp <NUM>. The A&R system is capable of supporting the suspended weight of the pipeline <NUM> in the water column when the trailing or proximal end of the pipeline <NUM> has passed through the tensioner system <NUM> and has been released from the hold-back system <NUM>.

<FIG> shows the pipeline <NUM> being unspooled from the main reel <NUM> over the guide wheel <NUM> at the top of the ramp <NUM>. The pipeline <NUM> is then straightened by the straightener system <NUM> and launched into the sea with its suspended weight supported by the tensioner system <NUM>. The hold-back system <NUM> is therefore disengaged to allow the pipeline <NUM> to pass through.

In <FIG>, most of the pipeline <NUM> has been launched and the hold-back system <NUM> is now engaged to hold the proximal end of the pipeline <NUM>. The tensioner system <NUM> is now disengaged to allow the pliant tie-in conduit <NUM> to be coupled to the proximal end of the pipeline <NUM>. For this purpose, a coupling <NUM> is shown schematically between the tie-in conduit <NUM> and the pipeline <NUM>. As will be explained with reference to <FIG>, the coupling <NUM> suitably comprises an end fitting of the tie-in conduit <NUM>.

The tie-in conduit <NUM> will serve as a spool or jumper to effect subsea connection between the pipeline <NUM>, after installation on the seabed, and another subsea structure to which the pipeline <NUM> is to be coupled for fluid communication.

In this example, the tie-in conduit <NUM> is a discrete length of TCP. This combines with the corrosion-resistant liner of the pipeline <NUM> to maintain internal corrosion resistance substantially continuously along the combined length of the pipeline <NUM> and the tie-in conduit <NUM>.

The tie-in conduit <NUM> of TCP is pliant enough to enable easy tie-in to other subsea structures without requiring significant deflection of, and hence risking failure of, the stiffer rigid pipeline <NUM>. Yet, the cost of the combination of the pipeline <NUM> and the much shorter tie-in conduit <NUM> is not much greater than the cost of an equivalent length of conventional lined rigid pipeline <NUM>.

<FIG> shows the tie-in conduit <NUM> being advanced from the auxiliary reel <NUM> on which it is stored. It would also be possible for the tie-in conduit <NUM> to be stored elsewhere on the vessel <NUM>, for example in a carousel or, if space permits, lying on a deck of the vessel <NUM>. In principle, the tie-in conduit <NUM> could be spooled onto the main reel <NUM> in addition to the pipeline <NUM>, for example beside the pipeline <NUM>. The tie-in conduit <NUM> could even be stored off the vessel <NUM>, for example towed or suspended behind, beside or under the vessel <NUM> or carried by a separate vessel <NUM>. In any event, the tie-in conduit <NUM> may be stored in straight or curved configurations, the latter being more compact.

By virtue of the invention, the combination of the pipeline <NUM> and the tie-in conduit <NUM> is also quick and simple for one vessel <NUM> to install in a single pipelaying operation. In this respect, <FIG> shows the full length of the pipeline <NUM> now launched beneath the surface <NUM>. The suspended weight of the pipeline <NUM> is borne by the A&R wire <NUM>, which is connected to the coupling <NUM> at the proximal end of the pipeline <NUM> but could instead be connected directly to the pipeline <NUM> close to the proximal end.

The coupling <NUM> connects a leading or distal end of the tie-in conduit <NUM> to the proximal end of the pipeline <NUM>. Conversely, an end fitting or connector hub <NUM> at the trailing or proximal end of the tie-in conduit <NUM> is connected to the A&R wire <NUM> at a location spaced proximally from the proximal end of the pipeline <NUM>. Consequently, the tie-in conduit <NUM> hangs as a catenary between its distal and proximal ends, carrying no tensile loads other than those arising from its self-weight. Instead, the load path between the pipeline <NUM> and the vessel <NUM> extends substantially exclusively along the parallel A&R wire <NUM> that bypasses the tie-in conduit <NUM>.

Turning next to <FIG>, these drawings show details of the coupling <NUM> between the pipeline <NUM> and the tie-in conduit <NUM>. Coupling is effected between flanges in this example, specifically a flanged end <NUM> of the pipeline <NUM> and a correspondingly-flanged end fitting <NUM> of the tie-in conduit <NUM>. The end fitting <NUM> may, for example, be a forging of steel containing a corrosion-resistant liner.

The end fitting <NUM> supports a yoke <NUM> that transfers the suspended weight load of the pipeline <NUM> to the A&R wire <NUM>. The yoke <NUM> also allows the pipeline <NUM> and the tie-in conduit <NUM> to pivot relative to the A&R wire <NUM> during the installation process as shown in <FIG>. This protects the pipeline <NUM> and the tie-in conduit <NUM> from excessive bending stresses while diverting the main load path away from the tie-in conduit <NUM>.

The end fitting <NUM> has a tubular body <NUM> with diametrically-opposed spigots aligned on a transverse pivot axis <NUM>. In this example, the pivot axis <NUM> intersects the central longitudinal axis <NUM> that extends through the end fitting <NUM> from the pipeline <NUM> to the tie-in conduit <NUM>.

A U-shaped yoke bridle <NUM> straddles the body <NUM> of the end fitting <NUM>. The yoke bridle <NUM> comprises a transverse bridge <NUM> that extends across the body <NUM> and parallel legs <NUM> joined by the bridge <NUM>. The spigots on the pivot axis <NUM> are received within mutually-aligned through-holes near the free ends of the legs <NUM>. This engagement defines a pivotable attachment between the yoke bridle <NUM> and the end fitting <NUM>.

The bridge <NUM> that extends between the legs <NUM> of the yoke bridle <NUM> supports an anchor formation <NUM> for removably attaching the A&R wire <NUM> as shown.

The arched shape of the yoke bridle <NUM> provides clearance for pivotal movement of the coupling <NUM> and of the attached pipeline <NUM> and tie-in conduit <NUM>. In particular, the arched shape avoids clashing of the yoke bridle <NUM> with the outer surface of the tie-in conduit <NUM>.

<FIG> shows the tie-in conduit <NUM> extending between the coupling <NUM> of <FIG> at its distal end and the flanged end fitting or connector hub <NUM> at its proximal end. The tie-in conduit <NUM> hangs beneath the A&R wire <NUM> in substantially the same vertical plane as the A&R wire <NUM>.

The distal end portion of the tie-in conduit <NUM> adopts the same shallower inclination relative to the A&R wire <NUM> as the proximal end portion of the pipeline <NUM>, which relative inclination is permitted by the pivotable yoke bridle <NUM> of the coupling <NUM>. Conversely, the proximal end portion of the tie-in conduit <NUM> is inclined more vertically, at a steeper inclination than the A&R wire <NUM>, hence converging upwardly toward the A&R wire <NUM> on an axis that, if projected, intersects the A&R wire <NUM>. Indeed, the proximal end portion of the tie-in conduit <NUM> may be oriented vertically or near-vertically as shown here.

In this example, the end fitting or connector hub <NUM> is connected to the A&R wire <NUM> proximally with respect to the pipeline <NUM> via a tri-plate connector <NUM> incorporated into the A&R wire <NUM>. Similar rigging arrangements are possible here instead, for example involving shackle connections.

<FIG> shows the pipeline <NUM> in the process of being laid down, curving distally from the coupling <NUM> to a touch-down point on the seabed <NUM>. More of the pipeline <NUM> is laid down as the vessel <NUM> moves from left to right as illustrated and pays out the A&R wire <NUM>. Eventually, the proximal end of the pipeline <NUM> is landed on the seabed <NUM>, immediately followed by the tie-in conduit <NUM>.

The pipeline <NUM> and the tie-in conduit <NUM> are landed on the seabed <NUM> on a heading that points generally toward a subsea structure <NUM> such as a manifold, to which the pipeline <NUM> is to be connected via the tie-in conduit <NUM>. The structure <NUM> also has a flanged fitting or connector hub <NUM> that complements the flanged end fitting or connector hub <NUM> at the proximal end of the tie-in conduit <NUM>.

Thus, the proximal end of the pipeline <NUM> is landed in the vicinity of the structure <NUM>, close enough for the tie-in conduit <NUM> to bridge the distance between the pipeline <NUM> and the structure <NUM>, while accommodating tolerances such as some misalignment between them. In this respect, <FIG> show the scene from above. It will be apparent that the pipeline <NUM> is not fully aligned with the flanged fitting or connector hub <NUM> of the structure <NUM>.

Initially, as shown in <FIG>, the tie-in conduit <NUM> is deflected to be laid on the seabed <NUM> on a path that curves away from the pipeline <NUM>. The A&R wire <NUM> is disconnected from the yoke bridle <NUM> of the coupling <NUM> and from the tie-in conduit <NUM>, and then is recovered to the vessel <NUM>.

Next, as shown in <FIG>, the tie-in conduit <NUM> is connected to the structure <NUM>, conveniently by the same vessel <NUM> during the same trip, using a crane or winch of the vessel <NUM> and supported by an ROV or divers. Specifically, the flanged end fitting or connector hub <NUM> at the proximal end of the tie-in conduit <NUM> is lifted into engagement with the complementary flanged fitting or connector hub <NUM> of the structure <NUM>. The tie-in conduit <NUM> bends elastically, or at least without localised plastic deformation, to accommodate the misalignment between the pipeline <NUM> and the structure <NUM>. The curvature of the tie-in conduit <NUM> also accommodates tolerances in the distance between the proximal end of the pipeline <NUM> and the structure <NUM>.

Of course, it would instead be possible for the structure <NUM> to be installed on the seabed <NUM> after the pipeline <NUM> and the tie-in conduit <NUM> have been installed, and therefore for connection with the structure <NUM> to be made only then.

A step may alternatively consist in disconnecting only the yoke bridle <NUM> of the A&R wire <NUM>, while the A&R wire <NUM> remains connected to the flanged end fitting or connector hub <NUM> of the tie-in conduit <NUM>. The A&R wire <NUM> may thereby be used for guiding the flanged end fitting or connector hub <NUM> into position for coupling with the flanged fitting or connector hub <NUM> of the structure <NUM>.

Many other variations are possible within the inventive concept. For example, <FIG> shows a variant of the invention in which the proximal end of the tie-in conduit <NUM> is supported by an auxiliary winch wire <NUM> extending from the vessel <NUM>, rather than being hung from the A&R wire <NUM> as in the preceding embodiment. The auxiliary winch wire <NUM> could extend from the A&R winch <NUM> or from another winch of the vessel <NUM>.

Finally, <FIG> of the drawings show an alternative to the coupling shown in the preceding embodiment. The coupling <NUM> shown here comprises a main body made up from a tubular pup piece <NUM>, a first forging <NUM>, another short length of pipe <NUM> and a second forging <NUM>, all of which are welded together. Alternatively, the forgings <NUM>, <NUM> and the pipe <NUM> between them could be replaced by a unitary forging, welded to the pup piece <NUM>. The pup piece <NUM> is generally made from the same pipe material as the rigid pipeline <NUM>.

The first forging <NUM> includes an annular lip <NUM>, which serves to support a catenary of the rigid pipeline <NUM> in conjunction with the hold-back system <NUM> on the vessel <NUM>. The second forging <NUM> includes an end flange <NUM>, which is suitable for connecting to a similarly-flanged end fitting of the tie-in conduit <NUM>.

A collar <NUM> comprises two halves connected together by bolts <NUM>. The collar <NUM> surrounds the pipe <NUM> and carries shackles <NUM> for the attachment of lifting rigging. The collar <NUM> is axially constrained by the annular lip <NUM> on the first forging <NUM> and by an annular lip <NUM> on the second forging <NUM> to transmit the load applied to the shackles <NUM> to the coupling <NUM> and the rigid pipeline <NUM>.

Lifting rigging connected to the shackles <NUM> is connected to an A&R wire like that shown in the preceding embodiments. Pivoting of the shackles <NUM> relative to the collar <NUM> allows the coupling <NUM> to pivot relative to the A&R wire in the manner of the yoke bridle <NUM> of the preceding embodiments.

The main body of the coupling <NUM>, comprising the pup piece <NUM>, the first forging <NUM>, the pipe <NUM>, the second forging <NUM> and the shackles <NUM>, is pre-assembled. The coupling <NUM> is then connected to the proximal end of the rigid pipeline <NUM> by welding. This connection is made on the vessel <NUM> after the rigid pipeline <NUM> has mostly been laid on the seabed <NUM>, before laying down the final portion of the rigid pipeline <NUM> on the seabed <NUM>, using an A&R winch connected to the shackles <NUM>.

Claim 1:
A method of installing a subsea tie-in conduit (<NUM>), the method comprising:
unspooling or manufacturing a rigid pipeline (<NUM>) aboard an installation vessel (<NUM>);
launching the pipeline (<NUM>) progressively from the vessel (<NUM>) into water; and
coupling a distal end of the tie-in conduit (<NUM>) to a proximal end of the pipeline (<NUM>) aboard the vessel (<NUM>), the tie-in conduit (<NUM>) being pliant relative to the pipeline (<NUM>);
the method being characterised by:
before landing the proximal end of the pipeline (<NUM>) on the seabed (<NUM>), launching the tie-in conduit (<NUM>) fully into the water coupled to the pipeline (<NUM>), while supporting a suspended weight load of the pipeline (<NUM>) along a load path that substantially bypasses the tie-in conduit (<NUM>).