Patent Description:
When installing, maintaining or decommissioning subsea oil and gas facilities, it is necessary to lower loads from the surface to a subsea location, usually on the seabed, and conversely to raise loads from a subsea location to the surface. Commonly, subsea operations involve laying pipelines and placing ancillary equipment such as wellheads or blowout preventers (BOPs) on or near the seabed. In this respect, pipelaying vessels generally comprise a working deck, winches, a service crane and a hold-back system such as tensioners or clamps for suspending a catenary of pipeline in water.

As winch systems do not conveniently allow loads to be lifted through the air from the deck of a vessel into the sea, some use of a crane remains necessary. However, small service cranes may not be capable of lifting bigger and heavier subsea structures such as wellheads or BOPs. Lowering such heavy loads in water may require the use of a separate crane vessel, which increases cost greatly and relies upon the availability of a suitable weather window.

As pipelaying vessels are available on field for pipeline installation, various proposals have been made to use the equipment that is already on board such vessels when lifting subsea loads. These proposals typically involve using pipelay equipment or a small crane to start lifting the load from the deck into the water and then handing over the load to a winch when the load is underwater. In general, a winch is more compact and can have higher capacity than a crane, and can be equipped with longer wires that can go deeper into the water. Several combinations of lifting equipment and pipelay equipment are known in the art but they often require complex operations to transfer the load underwater.

Before considering the prior art, 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 conventional rigid pipe usually consists of, or comprises, at least one pipe of solid steel or steel alloy. 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.

In recent years, the subsea oil and gas industry has begun to adopt rigid 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.

Whilst they have enough flexibility to bend elastically along a substantial length, rigid pipes and rigid composite 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.

Like-for-like, composite pipes are generally more pliant than conventional rigid pipes of steel and can be bent elastically to a smaller minimum bend radius. However, conventional rigid pipes and equivalent composite pipes share 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. Thus, for the purposes of this specification, a rigid composite pipe or other rigid elongate member of composite material may be regarded as equivalent to a conventional rigid pipe or other rigid elongate member of steel, and both may be described as rigid or at least as non-flexible.

<CIT> teaches using a composite pipe as a substitute for the wire of a winch, with the tensioners of a pipelaying vessel being used to provide extra hold-back force. Whilst it is not a flexible pipe as that term is understood in the art, the composite pipe of <CIT> is spooled on a reel carried by the vessel and is substantially pliant and non-rigid. The composite pipe is directed though a pipelay tower of the pipelaying vessel and the load is suspended from the pipe. Major drawbacks of this solution include that it requires the use of a reel and that it requires a great length of very expensive composite pipe.

Similarly, <CIT> discloses a method for lowering a wellhead from a drilling vessel by using a flexible pipe as a wire. Flexible pipe is also expensive and is mechanically weak, being prone to fatigue and damage. Additionally, a drilling vessel is expensive and its derrick is a huge support structure.

<CIT> also discloses the use of a flexible pipe to lower a load, in this case in abandonment and recovery (A&R) operations performed on a pipeline. The flexible pipe is supplemented by a parallel lifting wire, such that the flexible pipe and the lifting wire share the substantial load of the pipeline. The load of the pipeline may be handed over solely to the lifting wire at an intermediate depth where the load of the pipeline suspended above the seabed has reduced sufficiently. Similarly, in <CIT>, a transfer and connection device is used to connect a winch and a crane to perform a dual-lift operation or to hand over a load between the crane and the winch.

Pliant, flexible load-bearing links - such as a wire, a flexible pipe as disclosed in <CIT> and <CIT> or a highly-pliant composite pipe as disclosed in <CIT> - can allow the load to clash with the hull of the vessel when the load is being lowered through a moonpool or over the side of the vessel. This is a particular risk as the load traverses the splash zone that typically extends up to <NUM> to <NUM> beneath the water surface, where water dynamics and wave motion can cause the load to swing dangerously.

Another problem is that the above prior art installation methods may not be straightforward to reverse for recovering a load from a subsea location back onboard a vessel. In this respect, it will be apparent that the load must also traverse the splash zone on its upward journey before being lifted from the sea. Also, as pliant load-bearing links such as wires, flexible pipes or highly pliant composite pipes will tend to deflect under the influence of sea dynamics, it may be challenging to connect a load to them underwater.

<CIT> discloses the use of J-lay equipment to assist in abandonment and recovery of pipelines. An abandonment string attached to an end of a pipeline is assembled from multiple tendon elements, which are added successively to the top of the abandonment string in a J-lay tower as the end of the pipeline is lowered in the sea. When the end of the pipeline reaches a handover depth, the load of the pipeline is transferred from the abandonment string to a winch wire, which completes the abandonment operation. The abandonment string is then disassembled as it is raised back through the J-lay tower.

The solution described in <CIT> is advantageous for A&R operations performed on pipelines but is complex and inappropriate for lowering discrete, smaller loads such as wellheads or BOPs. It also requires the presence of a vessel that is configured for J-lay operations.

<CIT> describes a system for deploying a subsea well intervention system and a spoolable compliant guide (SCG) in a single stage from a surface vessel to a subsea location via a moon pool.

<CIT> teaches a method of lowering a subsea component to a subsea location without the requirement of a moon pool. The subsea component is lowered by a deployment frame that is cantilevered over the edge of a vessel.

<CIT> describes a method of abandoning a pipeline that is laid through a J-lay technique using a tensioning arrangement on a vessel. For abandonment, an intermediate string is welded to the upper end of the pipeline, and an abandonment string is coupled to the intermediate string via a pivoting joint. The intermediate and abandonment strings are lowered into the water using the tensioning arrangement. A steel rope connected to a winch on the vessel is attached to the upper end of the abandonment string, and the pipeline, intermediate string and abandonment string are lowered together on the steel rope to the seabed by unwinding the steel rope from the winch.

Against this background, the invention provides a method of lowering a load from a pipelay vessel floating on the surface of a body of water, according to claim <NUM>. The method comprises:.

The weight of the load is transferred from the hold-back system to the wire when the load reaches, or is at, a predetermined depth underwater, it being understood that this depth is typically less than <NUM>. In this respect, the splash zone typically extends up to about <NUM> deep; as long as the load is within the splash zone, the rod is the primary lifting interface.

Conveniently, the hold-back system may be used to hold the rod on the upright launch axis before coupling the rod to the load. In that case, the hold-back system may be operated to advance the rod toward the load for coupling the lower end of the rod to the load.

The load may be moved transversely relative to the launch axis before coupling the lower end of the rod to the load. For example, the load may be supported on a skid or on a moonpool hatch that is movable relative to a deck of the vessel.

The wire is preferably connected to the load before the hold-back system is operated to advance the rod.

The weight of the load may be transferred from the wire to a secondary wire when the load is underwater.

The load may be detached from the rod underwater while the rod is held by the hold-back system. In that case, the hold-back system may then be operated to retract the detached rod upwardly relative to the pipelay tower.

The wire is preferably tensioned while the rod is held by the hold-back system. The tensioned wire may extend substantially parallel to the launch axis or substantially along the launch axis.

The method of the invention is apt to be performed using a flex-lay vessel. The launch axis may extend through a moonpool of the vessel, or over the side of the vessel.

In a preliminary operation, the rod may be moved from a stowed position to a ready position extending through the hold-back system along the upright launch axis. The rod may, for example, be tilted into an upright orientation by lifting an end of the rod from a position lying horizontally on a working deck of the vessel. Alternatively, the rod may be lowered into the water before connecting the wire to an end of the rod underwater and winding in the wire to lift the rod into the ready position.

The inventive concept defined by claim <NUM> extends to a corresponding method of recovering a load from a subsea location to a pipelay vessel on the surface of the sea, according to claim <NUM>.

The recovery method comprises: winding in a wire suspended from the vessel to lift a load attached directly to the wire toward the vessel; underwater, transferring at least part of the weight of the load from the wire to a hold-back system on a pipelay tower of the vessel by mechanically coupling the load to a lower end of a rigid rod that is engaged with the hold-back system; and operating the hold-back system to retract the rod upwardly relative to the pipelay tower, lifting the load above the surface, while at least part of the weight of the load is suspended from the hold-back system via the rod. These recovery operations may follow the step of attaching the wire to the load at the subsea location.

The recovery method may be preceded by transferring the weight of the load to the wire, underwater, from a secondary wire used to raise the load from the subsea location.

The recovery method may further comprise: detaching the wire from the rod or the load aboard the vessel; detaching the load from the rod aboard the vessel; and/or removing the rod from the pipelay tower and stowing the rod.

The hold-back system is preferably a tensioner system that is operated by turning endless tracks that engage the rod, or may be or comprise a friction clamp that is able to travel along the pipelay tower and engages the rod by friction. The wire suitably extends through the hold-back system.

Also described is a device for hoisting a load underwater from a pipelay vessel. The device comprises a substantially rigid rod, preferably no greater than <NUM> in length, that has, at an end, a mechanical connector arranged to connect directly to the load. The mechanical connector is preferably arranged to connect rigidly to the load, thus preventing the load pivoting relative to the rod about an axis that is transverse to the length of the rod. The rod may further comprise a wire connection at an end opposed to the mechanical connector, to connect to at least one lifting wire.

The rod preferably has a substantially constant diameter extending along substantially its entire length, and more preferably is in a single continuous piece. The rod suitably has a surface state, texture or shape that is arranged to allow, or to facilitate, frictional gripping by a pipe hold-back apparatus of the pipelay vessel.

Exemplary embodiments of the invention implement a method for lifting and lowering a heavy load from a pipelay vessel into water. Preferably the vessel is a flexible pipelay or 'flex-lay' vessel. Flex-lay vessels are capable of installing flexible flowlines and risers, umbilicals and power cables. They are characterised by a substantially vertical ramp or tower that is equipped with one or more tensioners and an alignment chute or wheel on top. Some flex-lay systems can be adapted to perform J-lay or reel-lay operations.

Described are methods that comprise the following steps: preparing a rigid rod such as a pipe string; inserting the pipe string into the firing line or onto the launch axis of a pipelay tower; gripping the pipe string by a pipe-gripping means of the pipelay tower, such as a track tensioner and/or friction clamps; mechanically coupling a lower end of the pipe string to the load; starting to lower the load into the water by using the pipe-gripping means; handing over the load to a winch; and continuing to lower the load using a wire of the winch.

The load may be lowered through a moonpool of the vessel or may be lowered over a side of the vessel.

The winch wire may be connected to an upper end of the pipe string, which may be shorter than the pipelay tower. The pipe string may be uncoupled from the load and recovered after the load is installed at a destination, or may be uncoupled from the load and recovered after handing over the load to the winch. The winch wire could be connected to the load instead of to the top of the pipe string.

The winch wire may be connected to the load or to the pipe string before lowering begins, or may be connected to the load or to the pipe string when the upper end of the pipe string reaches the pipe-gripping means.

The winch or a sheave that supports the winch wire can be positioned at any suitable location on the vessel, including on a structure that can be displaced above a moonpool once the load has passed through. There can also be one or more subsequent handovers to other winches or to a crane or to a heave-compensated hoisting means for performing final landing of the load at a subsea location such as the seabed.

The load may be preinstalled on a movable platform such as a hatch of a moonpool that slides below the load for opening. The load may instead be mounted on a skid that is moved over the moonpool or otherwise relative to a deck of the vessel for connection to the pipe string.

More generally, the use of a structure installation tool that comprises a mechanical connector and an elongate structure such as a pipe is described. The tool is moved above a workstation by a crane lift or by deck handling, which need not involve lifting the tool overboard. The structure to be installed is moved to a working table of the workstation, for example on a skid or a pallet. The tool is then connected and locked to the structure, for example to a hub or socket on the structure.

Next, after taking the weight load of the structure and the tool with a tensioner system, a moonpool hatch or door or other structure supporting the load is opened or moved aside, or a supporting skid or pallet is pulled aside from below the load. The pipe is then advanced by turning the tracks of the tensioner system to lower the structure into the sea, for example through the moonpool. The structure is deployed through the splash zone and the load of the tool and the structure is transferred to an A&R winch, whereupon the load is lowered further on an A&R wire. Optionally the load is then transferred to a crane of the vessel. The structure is installed on reaching a subsea target location, preferably with the assistance of active heave compensation applied by a winch or by a crane as appropriate.

In summary, the invention provides a combination of innovative tooling and methodology for installing subsea structures such as wellheads, BOPs, Christmas trees and other temporary or permanent equipment. The invention employs a rigid pipe section of steel or rigid composite material, joined to a subsea connector by a flanged or welded connection. The pipe section can be supported and advanced along the launch axis by the tensioner system of a pipelaying support vessel (PLSV) and may extend through a moonpool of such a vessel.

An advantage of the invention is to enable the use of flex-lay vessels for lowering packages of, say, 50Te to 100Te weight that are too large and heavy for their cranes to handle effectively, but too small and light to justify using an expensive heavy-lift vessel or a drilling rig.

By using a tensioner system to support a rigid pipe/connector system, the invention enables safe and controlled deployment and recovery of a subsea structure through the splash zone. The invention also enables deployment and recovery of the structure through a sheltered moonpool. These factors allow subsea operations to progress with fewer weather restrictions.

As the tensioner system provides fine control over the vertical alignment of the subsea structure, the invention enables safe stack-up mounting of subsea equipment onboard the vessel for installation together as a unit.

The invention also allows easy access underneath a subsea structure or equipment before installation. This facilitates pre-deployment servicing, testing and checking of systems, such as those in instrumented wellhead equipment.

The invention provides a cost-effective solution that requires neither complex subsea equipment support vessel (SESV) equipment nor long drill pipe and rig vessels. The solution of the invention is safer and simpler than relying solely upon a crane supported by subsea ROV intervention. In comparison to the use of a crane for overboarding a load, the invention allows better control of the operation, improves the deck layout and enables easier and safer recovery in case of contingency.

<FIG> shows part of a flex-lay vessel <NUM> floating on the surface <NUM> of the sea. The vessel <NUM> is equipped with a vertical lay system (VLS) that comprises an upright ramp or pipelay tower <NUM> upstanding above a working deck <NUM> of the hull <NUM>. The pipelay tower <NUM> can typically be tilted several degrees from the vertical. The hull <NUM> also supports a main crane <NUM>, which is typically equipped with a heave compensation system.

The pipelay tower <NUM> is surmounted by a sheave <NUM> that guides and supports a winch wire <NUM>. The pipelay tower <NUM> also supports a tensioner system <NUM> that comprises upper and lower tensioners numbered as 26U and <NUM> respectively. The tensioners 26U, <NUM> are retractable but are shown here in a deployed position, aligned on a vertical launch axis. The tensioners 26U, <NUM> suitably comprise multiple endless tracks.

The launch axis extends through a moonpool <NUM> that extends vertically through the hull <NUM> of the vessel <NUM> from a top opening at the level of the working deck <NUM> to a bottom opening disposed underwater. A hatch or other equipment support can be deployed over, and retracted from, the top opening of the moonpool <NUM>.

An elongate, rigid and substantially straight column or rod <NUM> is shown in <FIG> extending along the launch axis and engaged with the tensioner system <NUM>. The winch wire <NUM> hanging from the sheave <NUM> is removably attached to the upper end of the rod <NUM> and is kept under tension.

An item of subsea equipment <NUM> is removably attached to the lower end of the rod <NUM> and so is suspended from the tensioner system <NUM> via the rod <NUM>. The equipment <NUM> that is suspended via the rod <NUM> is exemplified here by a blowout preventer (BOP). However, the invention may be used with other discrete items of subsea equipment or subsea structures, such as wellheads, Christmas trees or subsea production systems.

Preferably, the attachment between the rod <NUM> and the equipment <NUM> is a rigid attachment so that the equipment <NUM> cannot pivot or swing relative to the rod <NUM> about a horizontal axis. In the example shown in <FIG>, the rod <NUM> and the equipment <NUM> have complementary engagement formations that comprise an upwardly-facing socket <NUM> on top of the equipment <NUM> and a complementary downwardly-facing protrusion or spigot <NUM> at the lower end of the rod <NUM>. A downwardly-tapering guide funnel <NUM> suitably surrounds the socket <NUM> to guide the spigot <NUM> into the socket <NUM>.

When the spigot <NUM> is received in the socket <NUM>, a locking mechanism on the rod <NUM> and/or the equipment <NUM> is operated to lock the spigot <NUM> in the socket <NUM>. For example, pawls <NUM> on the spigot may move radially to engage with latch formations in the socket <NUM>. The pawls <NUM> may be operated at the surface aboard the vessel <NUM> or underwater, either to attach the equipment <NUM> to the rod <NUM> or to detach the equipment <NUM> from the rod <NUM> as appropriate. When operated underwater, the pawls <NUM> may be operated remotely or by intervention from a diver or an ROV.

The upper end of the rod <NUM> shown in <FIG> comprises a padeye coupling <NUM> for removably attaching the winch wire <NUM> that hangs from the sheave <NUM>.

The rod <NUM> shown in <FIG> is suitably fabricated from a series or string of tubular rigid pipe joints of steel joined end-to-end, for example by welding. Pipe joints used in the subsea oil and gas industry have a standard length of, nominally, <NUM>. Thus, the rod <NUM> may, for example, comprise a double joint that is <NUM> long, a triple joint that is <NUM> long or a quad joint that is <NUM> long. In any event, the rod <NUM> is preferably shorter than the pipelay tower <NUM>.

Moving on now to <FIG>, this sequence of drawings firstly shows the equipment <NUM> being attached to the rod <NUM> and then the equipment <NUM> and the rod <NUM> being lowered together through the moonpool <NUM> and beneath the surface <NUM> of the sea.

In <FIG>, the rod <NUM> is shown engaged with both tensioners 26U, <NUM> of the deployed tensioner system <NUM> and extends along the launch axis substantially parallel to the pipelay tower <NUM>. The tensioner system <NUM> restrains, drives and hence controls axial movement and positioning of the rod <NUM>. At this stage, the tensioner system <NUM> holds the bottom of the rod <NUM> clear of the working deck <NUM> by a vertical distance that is sufficient to accommodate the height of the equipment <NUM>.

Initially, the equipment <NUM> is shown supported on the working deck <NUM> to one side of the rod <NUM> and hence offset laterally with respect to the launch axis. Specifically, the equipment <NUM> is supported on a structure that is movable horizontally relative to the working deck <NUM>, such as on a skid or on a hatch that is deployable over the top opening of the moonpool <NUM>.

<FIG> shows the equipment <NUM> now moved horizontally relative to the working deck <NUM> to be brought into alignment with the launch axis, directly beneath the rod <NUM>. The rod <NUM> is then lowered slightly to bring the spigot <NUM> at the bottom of the rod into engagement with the socket <NUM> on top of the equipment <NUM>. The pawls <NUM> shown in <FIG> are then actuated to lock the spigot <NUM> in the socket <NUM> and hence to connect the equipment <NUM> rigidly to the rod <NUM>.

When the equipment <NUM> has been connected to the rod <NUM> in this way, the tensioner system <NUM> lifts the assembly of the rod <NUM> and the equipment <NUM> slightly away from the working deck <NUM> to allow a skid or hatch that previously supported the equipment <NUM> to be retracted away from over the moonpool <NUM>.

The rod <NUM> and the equipment <NUM> are now ready to be lowered into the moonpool <NUM> as shown in <FIG> and also in <FIG>. Here, the rod <NUM> has disengaged from the upper tensioner 26U and remains engaged only with the lower tensioner <NUM>. As a result, the rod <NUM> extends beneath the surface <NUM> within the moonpool <NUM> to the extent that the equipment <NUM> is suspended beneath the hull <NUM> of the vessel <NUM> and so is fully submerged.

The stiffness of the rod <NUM> and its non-pivoting engagement with the tensioner system <NUM> controls the position of the equipment <NUM> effectively during the launch operation. In particular, the resistance of the rod <NUM> to bending along its length keeps the equipment <NUM> substantially on the desired launch axis and clear from the surrounding walls of the moonpool <NUM>. Thus, there is no risk of clashing with the hull <NUM> of the vessel <NUM> as the equipment <NUM> transits the splash zone beneath the surface <NUM>. Also, the resistance of the rod <NUM> to compression and extension along its length resists the tendency of wave action to bounce the equipment <NUM> up and down during its transit through the splash zone.

<FIG> shows the assembly of the rod <NUM> and the equipment <NUM> now lowered fully beneath the surface <NUM>. The rod <NUM> has disengaged fully from the tensioner system <NUM> and so is suspended solely by the winch wire <NUM> that hangs from the sheave <NUM>. Whilst the rod <NUM> no longer provides the lateral location for the equipment <NUM> that is evident in <FIG>, it will be apparent that the equipment <NUM> is now fully beneath the splash zone and so is no longer susceptible to disturbance from wave action.

In <FIG>, the winch wire <NUM> has not yet been attached to the top of the rod <NUM> via the padeye coupling <NUM>. However, it would be possible to suspend the rod <NUM> from the winch wire <NUM> instead of from the tensioner system <NUM> when attaching the equipment to the rod <NUM> as shown in <FIG>. Thus, the tensioner system <NUM> need not be engaged with the rod <NUM> until after the equipment <NUM> has been attached to the rod <NUM> and the resulting assembly is ready to be launched into the sea as shown in <FIG>.

In principle, the winch wire <NUM> could be attached to the top of the rod <NUM> at any stage before the rod <NUM> passes through the tensioner system <NUM> and disengages from the lower tensioner <NUM>. In practice, however, the winch wire <NUM> should preferably be attached to the top of the rod <NUM> as a failsafe measure before the support of a skid or hatch over the moonpool <NUM> is removed from the equipment <NUM>.

In shallow water, the winch wire <NUM> could continue to be paid out to lower the assembly of the rod <NUM> and the equipment <NUM> all the way to the seabed. There, the equipment <NUM> can be landed at a target location, such as on top of a subsea wellhead. Then, the rod <NUM> can be detached from the equipment <NUM>, for example by retracting the pawls <NUM> shown in <FIG>, to deposit the equipment <NUM> before the rod <NUM> is winched back to the pipelay tower <NUM> through the moonpool <NUM> of the vessel <NUM>. The rod <NUM> can then be used again to launch or to retrieve other equipment or can be removed from the pipelay tower <NUM> and stowed on the working deck <NUM> of the vessel <NUM>.

Optionally, in deeper water, the load defined by the assembly of the rod <NUM> and the equipment <NUM> may be transferred underwater to the main crane <NUM> before that assembly is lowered further toward the seabed. One such transfer process is shown in <FIG> and will now be described.

<FIG> shows a lifting wire <NUM> that is suspended from the main crane <NUM>. A lower end of the lifting wire <NUM> is shown here being carried by an ROV <NUM> toward the winch wire <NUM> that suspends the rod <NUM> and the equipment <NUM> from the sheave <NUM> on the pipelay tower <NUM>.

<FIG> shows the lifting wire <NUM> now attached to the winch wire <NUM> by the ROV <NUM>. The lifting wire <NUM> is attached to the winch wire <NUM> at an intermediate coupling <NUM> above a short bottom portion 24B of the winch wire <NUM> that connects to the top of the rod <NUM>.

<FIG> shows a major portion of the winch wire <NUM> above the intermediate coupling <NUM> now detached from the lifting wire <NUM> by the ROV <NUM> and being raised back to the vessel <NUM>. The rod <NUM> remains attached to the lifting wire <NUM> via the bottom portion 24B of the winch wire <NUM>. The load of the rod <NUM> and the equipment <NUM> has now been transferred to the main crane <NUM>, which pays out the lifting wire <NUM> to lower that assembly toward the seabed as shown in <FIG>.

Again, once the equipment <NUM> has been landed at a target location on the seabed, the rod <NUM> is detached from the equipment <NUM> and lifted back to the vessel <NUM>. The rod <NUM> can be transferred to the winch wire <NUM> underwater to be lifted back into the pipelay tower <NUM> for re-use; alternatively, the main crane <NUM> can stow the rod <NUM> on the working deck <NUM>.

Recovery of equipment <NUM> from a subsea location can be achieved by lowering a rod <NUM> to the subsea location and then coupling the rod <NUM> to the equipment <NUM> at that location before recovering the rod <NUM> and the equipment <NUM> together to the vessel <NUM>.

In this respect, <FIG> show equipment <NUM> in the form of a BOP atop a subsea wellhead <NUM> on the seabed <NUM>. <FIG> shows the rod <NUM> suspended on a winch wire <NUM> and being lowered toward the equipment <NUM>. <FIG> shows the spigot <NUM> at the lower end of the rod <NUM> being engaged with the socket <NUM> on top of the equipment <NUM> with the assistance of an ROV <NUM>. Once the rod <NUM> has been locked to the equipment <NUM> in this way, the equipment <NUM> can be detached from the wellhead <NUM> and then pulled back up toward the surface <NUM> while being suspended from the rod <NUM> and the winch wire <NUM> as shown in <FIG>.

It will be apparent that when the top end of the rod <NUM> is pulled up through the moonpool <NUM> and engaged with the tensioner system <NUM>, the system returns to the state shown in <FIG> and in <FIG>. So, again, the stiffness of the rod <NUM> beneficially guides and stabilises the movement of the equipment <NUM> through the turbulent splash zone beneath the surface <NUM>, in this case in an upward direction, until the equipment <NUM> clears the surface <NUM> before reaching the level of the working deck <NUM>.

As in <FIG>, the rod <NUM> could be suspended from the lifting wire <NUM> of the main crane <NUM> instead of from the winch wire <NUM> when recovering the rod <NUM> and the equipment <NUM> to the vessel <NUM>. In that case, the load of the rod <NUM> and the equipment <NUM> could be transferred underwater from the lifting wire <NUM> to the winch wire <NUM> in a reversal of the process shown in <FIG>.

<FIG> show an embodiment of the invention, in which like numerals are used for like features. In this variant, the rod <NUM> is attached to the equipment <NUM> in the same way as described in the preceding paragraphs. For example, pawls <NUM> may be actuated to lock the spigot <NUM> of the rod <NUM> in the socket <NUM> of the equipment <NUM> as shown in <FIG>. However, the winch wire <NUM> is attached directly to the equipment <NUM> rather than to the rod <NUM>. Consequently, there is the option for the winch wire <NUM> to remain slack while the rod <NUM> that suspends the equipment <NUM> is clamped by and advanced through the tensioner system <NUM>, as shown in <FIG>.

Once the equipment <NUM> has safely transited the splash zone beneath the surface <NUM>, the winch wire <NUM> is made taut if necessary and then the equipment <NUM> is detached from the rod <NUM> as shown in <FIG>. For example, an ROV <NUM> may retract the pawls <NUM> that lock the spigot <NUM> into the socket <NUM> as shown in <FIG>. This transfers the load of the equipment <NUM> from the rod <NUM> to the winch wire <NUM>.

<FIG> shows that the equipment <NUM> may now be lowered away from the lower end of the rod <NUM>. As mentioned previously, the equipment <NUM> could be lowered to the seabed on the winch wire <NUM> or its load could be transferred underwater to the lifting wire <NUM> of the main crane <NUM> as described in <FIG>.

The rod <NUM> remains engaged with the tensioner system <NUM> and so can be lifted back up beside the pipelay tower <NUM> by reversing the drive direction of the tensioner system <NUM>. The rod <NUM> can then be used again to launch or to recover other equipment or can be removed from the pipelay tower <NUM> and laid onto the working deck <NUM> of the vessel <NUM> for storage.

It will be apparent that the principles of this embodiment may also be applied to recovery of equipment <NUM> from a subsea location, such as the subsea wellhead <NUM> shown in <FIG>. In this case, the rod <NUM> may be held by the tensioner system <NUM> with its lower end underwater, beneath the splash zone. The equipment <NUM> may then be raised on the winch wire <NUM> to be coupled to the lower end of the rod <NUM>, before the tensioner system <NUM> lifts the rod <NUM> and the equipment <NUM> together out of the water.

It would also be possible to raise the equipment <NUM> from a subsea location on the lifting wire <NUM> of the main crane <NUM> and then to transfer the equipment <NUM> underwater to the winch wire <NUM> before uniting the equipment <NUM> with the rod <NUM>.

Turning finally to <FIG> and <FIG>, these drawings show ways of moving the rod <NUM> from a stowed position lying on the working deck <NUM> of the vessel <NUM> into an upright orientation engaged with the tensioner system <NUM> of the pipelay tower <NUM>.

In <FIG>, the rod <NUM> is lifted from a horizontal position <NUM> through an intermediate position <NUM>, both shown in dashed lines, and into the vertical position shown in solid lines. Conveniently, the winch wire <NUM> hanging from the sheave <NUM> may be used to pull the rod <NUM> upright, optionally with assistance from the main crane <NUM> or another service crane of the vessel <NUM>.

The sequence of drawings in <FIG> shows another approach, which involves lowering the rod <NUM> into the sea and uniting the rod <NUM> with the winch wire <NUM> underwater. The winch wire <NUM> is then wound in to pull the rod <NUM> back above the surface <NUM> and into engagement with the tensioner system <NUM>.

In <FIG>, the rod <NUM> has been lifted from the working deck <NUM> and lowered over the side of the vessel <NUM> to a location under the surface <NUM> of the sea, at a depth beneath the influence of wave action. Initially, the rod <NUM> is supported by rigging <NUM> that is suspended from the lifting wire <NUM> of the main crane <NUM>. The rod <NUM> remains in a substantially horizontal orientation at this stage. The winch wire <NUM> extends through the moonpool <NUM> to hang underwater close to the rod <NUM>. A coupling <NUM> hangs at the bottom end of the winch wire <NUM>.

<FIG> shows an ROV <NUM> carrying a connecting line <NUM> from an end of the rod <NUM> to the coupling <NUM> at the nearby bottom end of the winch wire <NUM>. The connecting line <NUM> is shown attached to the coupling <NUM> in <FIG>. The winch wire <NUM> is then pulled upwardly as shown in <FIG> to transfer the load of the rod <NUM> from the lifting wire <NUM> to the winch wire <NUM> via the connecting line <NUM>.

<FIG> shows the rod <NUM> tilting toward a vertical orientation under the upward pull of the winch wire <NUM>. <FIG> also shows the lifting wire <NUM> and the rigging <NUM> going slack as the load of the rod <NUM> is transferred to the winch wire <NUM>.

When the rod <NUM> is substantially vertical, the ROV <NUM> disconnects the rigging <NUM> from the rod <NUM> as shown in <FIG>. The main crane <NUM> can then recover the rigging <NUM> to the vessel <NUM> by winding in the lifting wire <NUM>. The winch wire <NUM> is also wound in to pull the rod <NUM> up through the moonpool <NUM> and into alignment and engagement with the tensioner system <NUM> of the pipelay tower <NUM>.

Many other variations are possible within the inventive concept. For example, the use of a moonpool is optional; it would be possible instead for the launch axis to lie outboard of the vessel so that the equipment can be launched to either side of the vessel.

Various inter-engagement arrangements are possible instead of a spigot on the rod and a socket on the equipment to be supported by the rod. For example, the arrangement could be reversed so that a male formation on the equipment engages into a female formation on the rod. Similarly, pawls or other locking provisions that lock the equipment to the rod could be provided on the equipment rather than on the rod.

Whilst it is preferred for the rod to be tubular and it is convenient for the rod to be formed of a series of steel pipe joints welded or otherwise joined end-to-end, this is not essential. For example, the rod could be solid, could be made from a single piece of material, or could be made of a polymer composite or other material that is different to steel.

Claim 1:
A method of lowering a load (<NUM>) from a pipelay vessel on the surface (<NUM>) of a body of water, the method comprising:
holding a rigid rod (<NUM>) on an upright launch axis, extending through a hold-back system on a pipelay tower (<NUM>) of the vessel;
mechanically coupling a lower end of the rod (<NUM>) to the load (<NUM>);
above the surface (<NUM>), connecting a wire (<NUM>) to the load (<NUM>) directly;
operating the hold-back system to advance the rod (<NUM>) downwardly relative to the pipelay tower (<NUM>), submerging the load (<NUM>), while the weight of the load (<NUM>) is suspended from the hold-back system via the rod (<NUM>);
when the load (<NUM>) reaches a predetermined depth underwater, transferring the weight of the load (<NUM>) from the hold-back system to the wire (<NUM>) by detaching the load (<NUM>) from the rod (<NUM>) while the rod (<NUM>) is held by the hold-back system; and
continuing to lower the load (<NUM>) in the water, suspended from the wire (<NUM>).