Offshore flexible line installation and removal

A method of installing a flexible line between an offshore platform and a subsea structure and a method of removing a flexible line deployed between an offshore platform and a subsea structure. The method of installing includes at least partially deploying the line in the water using an installation vessel located outside of an exclusion zone in the vicinity of the platform, including lowering a topside end of the line either to the seabed or to an intermediate location between the seabed and the platform. The method further includes pulling the topside end to a position within the exclusion zone and winching the topside end up to the platform, and coupling the topside end to the platform. The method of removing includes decoupling a topside end of the riser from the platform, lowering the topside end from the platform and pulling the topside end to a position outside of an exclusion zone in the vicinity of the platform, and at least partially removing the line from the water using an installation vessel located outside of the exclusion zone, including raising the topside end of the line onto the installation vessel either from the seabed or from an intermediate location between the seabed and the platform.

TECHNICAL FIELD

The present invention relates to the installation and removal of flexible lines between an offshore platform or vessel and a subsea structure. The invention is applicable, for example, to the installation of flexible risers, power cables, umbilicals and the like.

BACKGROUND

In the context of offshore hydrocarbon production, a riser is a tubular conduit that extends from a subsea structure to a production platform, for the purpose of conveying produced hydrocarbons from the well to the platform. The subsea structure may be a “christmas tree”, riser base, Blowout Preventer (BOP), or some other structure. Flexible risers are particularly important for deep water wells.

Risers are configured in order to allow for movements of the platform relative to the subsea structure in both a horizontal and a vertical direction, and for the effects of ocean currents on the riser itself. Configurations include a “free hanging” configuration (where the riser extends substantially vertically down to the seabed and then horizontally along the seabed to the subsea structure, a reverse pliant wave configuration, a pliant wave configuration, a lazy S configuration, and other suitable configurations.

For riser removal and installation operations on platforms where the risers are exposed/hanging on the platform side, the normal methodology involves the use of an installation vessel (IV) in addition to the platform itself. The IV is equipped with industry-standard laying spread (normally Vertical Laying System—VLS) and reels/carousels/baskets for storage of risers. The IV can install the riser from both directions, i.e. either the subsea or the topside end of the riser may be installed first (subject for example to already in-place riser configuration, auxiliary equipment size, packing constraints etc.).

FIGS. 1 to 6illustrate schematically various stages in the riser installation process in the case of floating production storage and offloading (FPSO), where the riser is being installed to achieve a reversed pliant wave configuration. Very generally, the process involves the following steps:(1) An end of a pull-in wire101, suspended from the platform (attached to the seabed by tethers110), is transferred from the platform to the installation vessel (FIG. 1).FIG. 1also shows an existing riser111.(2) The transferred end of the pull-in wire101is attached to the bottom of the riser102such that the pull-in wire101is now suspended between the platform and the riser102. This bottom end of the riser102will subsequently be attached to the platform and so is referred to hereinafter as the “topside” end of the riser102.(3) The riser102is fed into the water from the installation vessel (FIG. 2). During this stage, the installation vessel steps away from the platform.(4) At this stage, the riser102has adopted a U-shaped configuration in the water (FIG. 3). At some intermediate point in this procedure, buoyancy modules103are installed around the riser, over a certain length (FIG. 4). A pull-down clump weight (PDCW) may be required to pull-down the buoyant section in this phase.(5) When the riser102has reached a certain depth into the water, the topside end is pulled up to the platform using the pull-in wire101. The topside end of the riser is then installed to the appropriate connection on the platform (“hang-off”).(6) The installation vessel continues to lower the riser102and, as a result of the buoyant section of the riser, the curvature inverts (FIG. 4).(7) The installation vessel then continues to lower the riser102into the water, stepping further away from the platform as it does so (FIG. 5).(8) The installation vessel installs anchor clamp(s), subsea bend restrictors, subsea tie-in head etc., onto the riser102and lowers the subsea end of the riser102into the water, suspended by a winch cable. The riser102is normally required to be pulled-down, by means of the PDCW or pull-down rope/wire.(9) When the riser102has been lowered to a position over the (previously installed) seabed anchor(s), an intermediate point on the riser102is connected to the anchor(s) via tether(s), typically using a remotely operated vehicle (ROV).(10) The installation vessel steps further away from the platform, laying the riser102along the seabed until, finally, the subsea end of the riser is located in close proximity to the subsea structure to which it is to be connected (FIG. 6, which shows only seabed layout of the new riser). The winch cable is then disconnected.(11) The subsea end of the riser may be connected to the subsea structure using an appropriate tie-in tool and/or divers and/or an ROV.

WO2011/099869 describes a method of installing a riser, similar to the method described above.

When implementing riser installation and removal using these known procedures, it is necessary for the installation vessel to move close in to the platform (typically 20-40 m away) during the transfer of the topside end of the riser to the platform (FIG. 1), in order to maintain the integrity of the guide tube loads, maintain the bending radius of the riser, and prevent any contact between the riser and the port in the installation vessel through which it is lowered. The fact that the installation vessel must move so close to the platform is a significant disadvantage due to the risk of collision, especially with the riser platform. Indeed, many operators define an exclusion zone in the area adjacent to the riser platform, e.g. typically extending up 250 m away from the riser platform. If vessels do need to enter the exclusion zone, production and other operations must be ceased. Any cessation of production represents a significant expense.

It is noted that procedures similar to those used to install risers may be used to install other types of flexible lines, such as power cables and umbilicals. As such, the same problem is encountered, i.e. the need for an installation vessel to enter the exclusion zone and consequential shut down of operations.

SUMMARY

It is an object of the present invention to overcome or at least mitigate the known riser installation procedures described above. In particular, it is an object of the invention to allow the installation of a riser without requiring a vessel to enter an exclusion zone around a platform.

According to a first aspect of the present invention there is provided a method of installing a flexible line between an offshore platform and a subsea structure. The method comprising at least partially deploying the line in the water using an installation vessel located outside of an exclusion zone in the vicinity of the platform, including lowering a topside end of the line either to the seabed or to an intermediate location between the seabed and the platform. The method further comprises pulling the topside end to a position within the exclusion zone and winching the topside end up to the platform, and coupling the topside end to the platform.

According to an embodiment, said step of pulling the topside end to a position within the exclusion zone may comprise attaching a winch cable to the topside end and winching the topside end to said position within the exclusion zone. The winch cable may be pulled by a subsea winch, a winch located on an assisting vessel, or a winch located on the platform. After pulling the topside end to a position within the exclusion zone, releasing the winch cable from the topside end, attaching a pull-in cable depending from the platform to the topside end either before or after releasing the winch cable from the topside end, and using the pull-in cable to winch the topside end up to the platform.

Where the topside end is lowered to the seabed, the method may further comprise attaching a protective structure and or buoyancy aid to the topside end to prevent damage while the topside end is pulled over the seabed.

The step of pulling the topside end to a position within the exclusion zone may comprise using one or more assisting vessels to pull the topside end into the exclusion zone. The method may further comprise suspending a cable between two assisting vessels located outside the exclusion zone, manoeuvring the assisting vessels to a location between the platform and the installation vessel, attaching the topside end of the line to the cable, and further manoeuvring the support vessels to pull the topside end into the exclusion zone whilst maintaining the assisting vessels outside of the exclusion zone. After pulling the topside end to a position within the exclusion zone, the method may comprise releasing the suspended cable from the topside end, attaching a pull-in cable depending from the platform to the topside end either before or after releasing the suspended cable, and using the pull-in cable to winch the topside end up to the platform. The steps of releasing and attaching may be carried out using an ROV.

Following winching of the topside end up to the platform, and either before or after coupling the topside end to the platform, the method may comprise completing deployment of the line if required and coupling a subsea end of the line to the subsea structure.

According to a second aspect of the present invention there is provided a method of removing a flexible line deployed between an offshore platform and a subsea structure. The method comprising decoupling a topside end of the riser from the platform, and lowering the topside end from the platform and pulling the topside end to a position outside of an exclusion zone in the vicinity of the platform. The method further comprises at least partially removing the line from the water using an installation vessel located outside of the exclusion zone, including raising the topside end of the line onto the installation vessel either from the seabed or from an intermediate location between the seabed and the platform.

The step of pulling the topside end to a position outside of the exclusion zone may comprise using the installation vessel to pull the topside end to said position outside of the exclusion zone. Where the topside end is raised from the seabed, the method may further comprise attaching a protective structure and or buoyancy aid to the topside end to prevent damage while the topside end is pulled over the seabed.

The said step of pulling the topside end to a position outside of the exclusion zone may comprise using one or more assisting vessels to pull the topside end out of the exclusion zone. The method may comprise suspending a cable between two assisting vessels located outside the exclusion zone, attaching the topside end of the line to the cable, and manoeuvring the support vessels to pull the topside end out of the exclusion zone whilst maintaining the assisting vessels outside of the exclusion zone.

The step of at least partially removing the line from the water may comprise attaching the topside end of the riser to a buoyancy module in order to cause the topside end to be located at said intermediate position within the water.

DETAILED DESCRIPTION

In the case of deep water hydrocarbon wells, floating platforms tethered to the seabed are most commonly used as a means of receiving hydrocarbons from the well, storing it if necessary, and delivering it to oil and/or gas tankers. The platforms may be of various types including, for example, semi-submersible platforms and floating production storage and offloading (FPSO) vessels. As has been described above, flexible risers are installed between the floating platform and a subsea structure, such as a christmas tree, using an installation vessel. The riser is generally pre-constructed and mounted on a reel on the installation vessel. The riser is then lowered into the sea from the installation vessel. According to the approach presented here, it is not necessary for the installation vessel to enter the exclusion zone surrounding the platform. As such, safety can be greatly enhanced and any ongoing production need not be interrupted.

FIG. 7illustrates an installation vessel1located outside of an exclusion zone adjacent to a platform2(the platform is not shown in the figure but is located to the right of the installation vessel out of view). As shown inFIG. 7, a flexible riser3has been partially deployed outside of the exclusion zone according a procedure that involves firstly lowering the topside end4of the riser to the seabed. The subsea end5of the riser is, at this stage, suspended by a crane wire6that depends from a crane mounted on the installation vessel1(or, possibly, the subsea end5is still located on the IV). The riser has a reverse pliant wave configuration, although this is merely exemplary.

FIG. 8illustrates a subsequent stage of the riser installation procedure. This involves the use of a winch (or crane)7mounted on an assisting vessel8. The assisting vessel is stationed on the opposite side of the platform2from the installation vessel1. As the exclusion zone extends only on that side of the platform where the installation vessel is located, the assisting vessel8can move close in to the platform if required. A winch cable9is suspended from the winch7and extends around an anchoring device10secured to the seabed. The anchoring device may be, for example, a clump weight or a suction anchor. In another implementation, rather than extending up to a support vessel, the winch cable9may extend up to the platform2itself, with the platform performing the winching operations. In an alternative implementation, the winch may comprise a powered winch fixed to the seabed, e.g. at the location of the anchoring device10shown inFIG. 8.

Prior to the configuration illustrated inFIG. 8, an end of the winch cable9has been pulled by a remotely operated vehicle (ROV, not shown) to a location adjacent to the topside end4of the riser. The ROV attaches the end of the winch cable9to the topside end.FIG. 9illustrates the platform2and its environs in plan view, illustrating in particular the installation vessel1, the riser3, and the winch cable9, as well as the exclusion zone11. The figure illustrates the process as the winch7has begun to pull the topside end4of the riser3through the exclusion zone11and towards a location on the seabed adjacent to the platform2. A protection and/or sliding structure may be installed to the topside end of the riser to reduce the seabed friction and protect the riser as it is pulled towards the platform. Buoyancy modules can also be attached to the riser to reduce the required pull force and to prevent the riser from becoming buried in the seabed. Once the topside end4is in the appropriate position close in to the platform, an ROV disconnects it from the winch cable9, and connects the topside end to a pull in wire13suspended from the platform (seeFIG. 10). Thereafter, the topside end of the riser is pulled up to the platform. The topside end of the riser is then connected to the platform as required. This configuration is illustrated inFIG. 11. [Note: In a modification to this procedure, the pull in wire13may be used to pull the riser topside end4towards the platform, avoiding the need for an additional winch. The anchoring device should be “module-based” as ROVs will need to lift/fly the modules into position inside the exclusion zone (at a location on the seabed underneath the platform wire exit location or guide tube on the platform), and build up the anchoring device until the required weight of the assembly is achieved.]

At this stage, the installation vessel installs anchor clamp(s), subsea bend restrictors, subsea tie-in head etc, onto the riser3and lowers the subsea end5of the riser into the water, suspended by the crane wire6. The riser is normally required to be pulled-down, e.g. by means of a pull-down wire18(or pull-down clump weight, PDCW). When the riser has been lowered to a position over the (previously installed) seabed anchor(s), an intermediate point on the riser is connected to the anchor(s) via tether(s), typically using a remotely operated vehicle (ROV). The next stage in the procedure is for the installation vessel1to step away from the platform, using the crane wire6to further lower the subsea end5of the riser3from the vessel, laying the riser horizontally along the seabed as it does so. The subsea end finally arrives at the subsea structure12, and can be connected to it, e.g. using a tie-in tool (or merely laid down on the seabed for connection to the subsea structure12at a later stage). The connected riser is illustrated inFIG. 12, which also shows the buoyancy modules14installed earlier in the process.

Whilst the riser installation procedure has been described above with a certain sequence or ordered steps, it will be appreciated that the order in which these steps are carried out may be varied. For example, it is possible to carry out the procedure by first lowering the subsea end5of the riser into the water first.

It will be appreciated that a procedure for removing a riser will follow the reverse procedure to that described with reference toFIGS. 7 to 12, i.e. commencing with disconnection of the subsea end of the riser from the subsea structure and raising of that end to the installation vessel, an intermediate step of pulling/stepping the topside end of the riser out of the exclusion zone towards the installation vessel, and a final step of raising the remainder of the riser up to the installation vessel is performed when it is located outside the exclusion zone. It is noted that the installation vessel itself may be used to pull the topside end of the riser out of the exclusion zone, avoiding the need for a winch (for the stage). Of course, a winch may be used (as perFIG. 8) to act as holdback to the pulling operation, affording further control.

The approach presented here provides a reversible method for the installation and removal of a flexible element comprising a riser or other flexible line (such as a power line or umbilical) or tube) between an offshore platform and a subsea structure. The approach avoids the need for the installation vessel to enter a defined exclusion zone in the vicinity of the platform during the installation or removal process and therefore may avoid the need to shut down platform operations, such as hydrocarbon production.

FIG. 13illustrates an alternative riser installation procedure which again relies on pulling/transferring a topside end4of the riser3from a location outside of the exclusion zone to a location adjacent to the platform2. This procedure requires the use of two off anchor handling vessels15a,15blight construction vessels (LCVs), platform supply vessels (PSVs) or similar, in addition to the riser installation vessel1. These two additional vessels are referred to here as assisting vessels (AVs). The riser3is installed outside of the exclusion zone as per the procedure described with reference toFIGS. 7 to 12. At some point, the topside end4of the riser is coupled to a towing cable16suspended between the AVs.

The AVs are located outside of and on each side of the exclusion zone, and move towards the platform2in synchrony, pulling the riser3into the exclusion zone as they move. The IV1follows the move whilst controlling the subsea end of the riser3. At a certain distance from the platform, a pull-in wire (PIW) from the platform (not shown) is connected to the topside end4of the riser and the riser is pulled into the platform hang-off area17. The IV then completes the riser installation (connecting up tether(s), land subsea end on seabed etc.) as per the above description.

FIG. 14illustrates a modification to the procedures described above with reference toFIGS. 7 to 13where the platform2is not shown but would be located to the right of the assisting vessel19as shown on the page. The modified procedure involves supporting the topside end4of the riser3by means of a surface or mid water buoy/floating device20. The floating device20is held in the correct position (e.g. using the AV(s) or the described winch) and pulled towards the platform2with either one or more assisting vessels or using a winch mounted on the platform or installation vessel (e.g. by means of a winch wire routed through a sheave on the floating device).

According to a further alternative riser installation procedure, the subsea end of the riser is lowered into the water first, and is landed on the seabed. The installation vessel then continues to install the riser (outside of the exclusion zone). The topside end is then transferred to the assisting vessel(s), buoy etc., when made available by the installation vessel. The installation vessel can, if required, re-connect to the subsea end. The remaining activities of the operation are as described above. The buoy/floating device can be designed such that the riser depth/configuration in the water column can be adjusted (sheave with wire/rope back to installation vessel, winch on buoy/floating device, flooding/air filling of compartments etc.). Additionally, the riser configuration during installation can be adjusted with temporary uplift equipment or weights if required.

FIG. 15is a flow diagram illustrating at a high level a method of installing a flexible line between an offshore platform and a subsea structure. The method comprises, at step S1, at least partially deploying the line in the water using an installation vessel located outside of an exclusion zone in the vicinity of the platform, including lowering a topside end of the line either to the seabed or to an intermediate location between the seabed and the platform. At step S2the topside end is pulled to a position within the exclusion zone and the topside end winched up to the platform. At step53the topside end is coupled to the platform.

FIG. 16is a flow diagram illustrating at a high level a method of removing a flexible line deployed between an offshore platform and a subsea structure. The method comprises, at step Sa1, decoupling a topside end of the riser from the platform and, at step5a2, lowering the topside end from the platform and pulling the topside end to a position outside of an exclusion10zone in the vicinity of the platform. At step5a3, the line is at least partially removed from the water using an installation vessel located outside of the exclusion zone, including raising the topside end of the line onto the installation vessel either from the seabed or from an intermediate location between the seabed and the platform.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.