Technical Field

The present invention relates to the subsea mounting of ancillary equipment on an elongate member, and, in particular, post-deployment subsea mounting of such ancillary equipment.

Background

The production or exploration of hydrocarbons usually involves the use of elongate members, including tubulars and/or flexible lines. Hydrocarbons are organic compounds consisting of hydrogen and carbon. Examples of hydrocarbons, which are used as an energy source, are methane, ethane, butane, pentane and hexane, but many other hydrocarbons are used as an energy source or for other industrial purposes. Crude oil includes hydrocarbons and crude oil is commonly produced from reservoirs in geological formations, typically below the seabed. Elongate members such as risers, unbonded flexible pipe, steel pipe, umbilicals and power cables are often used offshore to convey power and/or fluids, including hydrocarbons between a subsea structure, e.g. a wellhead or manifold, and a surface platform, e.g. a drilling platform or floating production storage and offloading (FPSO) vessel.

Existing techniques for the offshore installation of such an elongate member typically require the use of a complex and expensive installation vessel, where the operating costs of such a vessel may represent the majority of the total installation cost. The vessel time, and hence vessel operating cost, required to complete installation is a function of a variety of factors. These factors include line laying speed, and the time taken for additional activities including the onboard mounting of ancillary equipment on the elongate member before the elongate member is completely deployed into the sea and finally installed. Such ancillary equipment typically includes buoyancy modules, to maintain the elongate member in a configuration that minimises tension loads, vortex induced vibration (VIV) strakes and/or clamps to couple elongate members together or to couple equipment to the elongate member. Summary

It is an object of the present invention to overcome or at least mitigate the problems identified above.

In accordance with a first aspect of the present invention there is provided a method for the subsea mounting of ancillary equipment on an elongate member. The method comprises mounting ancillary equipment on an elongate member, after the elongate member has been deployed from a deployment vessel and before final installation of the elongate member.

After the elongate member has been deployed from the deployment vessel, the elongate member may not be coupled to the deployment vessel.

The method may further comprise, before mounting the ancillary equipment, deploying the elongate member from the deployment vessel. Deploying the elongate member from the deployment vessel may comprise feeding out the elongate member from a storage device on the deployment vessel, and deploying the elongate member into the water.

The step of mounting the ancillary equipment may be performed while the elongate member is located subsea in a temporary position in which at least a portion of the elongate member is located at or near the seabed and at least one end of the elongate member is not connected. The at least one end that is not connected may be tied down to the seabed, or to a structure at or near the seabed.

The elongate member may be deployed into the temporary position, and in the temporary position at least a portion of the elongate member may be located on a platform that supports the elongate member above the seabed, thereby providing access for mounting the ancillary equipment.

The method may further comprise, after mounting the ancillary equipment, performing final installation of the elongate member. Performing final installation of the elongate member may comprise: connecting the elongate member to a subsea structure and/or a surface platform, wherein, once so connected, the elongate member is configured to transport fluids and/or power between the subsea structure and the surface platform. The method may further comprise transporting hydrocarbon fluids through the line.

The deployment vessel may be a first vessel, and the step of mounting the ancillary equipment may be performed and/or managed using a second vessel.

The ancillary equipment may be mounted on the elongate member using an ROV or AUV.

The ancillary equipment may comprise at least one of a buoyancy module, a vortex induced vibration strake and a clamp.

The line may be one of: a riser, a pipeline, a tubular, a flexible steel pipe, an electrical cable or a hydraulic cable.

The step of mounting ancillary equipment may comprise mounting: at least 30, and preferably between 30 and 50; or at least 100, and preferably between 100 and 200, items of ancillary equipment.

The elongate member may be a tubular or a flexible line.

The elongate member may be a riser, an unbonded flexible pipe, a bonded flexible pipe, a steel pipe, an umbilical or a power cable.

Said ancillary equipment may comprise buoyancy modules, and the method may further comprise, after said mounting, raising the line from the seabed by increasing the buoyancy in the buoyancy modules.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Brief Description of Drawings

Figure 1 illustrates a method of mounting ancillary equipment to an elongate member in accordance with the prior art. Figure 2 illustrates an initial stage of the method in which an elongate member is deployed from a deployment vessel into a body of water.

Figure 3 shows the elongate member following deployment from the deployment vessel.

Figure 4 illustrates a method step in which ancillary equipment is mounted subsea on the elongate member.

Figure 5 shows the elongate member after a final installation process.

Figure 6 shows the elongate member following deployment from the deployment vessel, where at least a portion of the elongate member is supported by a platform.

Figure 7 shows a high-level flow diagram describing a method in accordance with the invention.

Detailed Description

Figure 1 illustrates an initial stage of a procedure of installing an elongate member in accordance with existing techniques. An installation vessel 910 carries a storage device 914, e.g. a reel or built-in carousel, on which an elongate member 912 is carried. The elongate member 912 is fed out from the storage device 914 and through installation spread 916, where the installation spread 916 supports the load imposed by the deployed elongate member, and may remove residual curvature from the elongate member if necessary, e.g. if the elongate member is a steel pipe. Before the elongate member is deployed into the water, ancillary equipment 920 is mounted onto the elongate member, e.g. on an installation platform 918. The ancillary equipment may include buoyancy modules, VIV strakes and/or clamps. In this way, the ancillary equipment is mounted on a portion of the elongate member before said portion enters the water, i.e. during deployment of the elongate member. In a typical installation procedure the same installation vessel will go on to perform a final installation of the elongate member, in which the elongate member is connected between e.g. a subsea structure, such as a wellhead or manifold, and a surface platform, such as a drilling platform or FPSO vessel. Alternatively, the elongate member may be deployed to a temporary position and left for future final installation.

The inventors have realised that installation methods such as illustrated in Fig. 1 have significant drawbacks. For example, during installation in deeper waters using a longer elongate member, or for risers requiring a significant number of buoyancy modules, the time required for the onboard mounting of ancillary equipment may increase the total installation or deployment time significantly, resulting in a total installation time on the order of days. For an installation or deployment procedure to start, the planned installation or deployment duration must be within a forecasted window of good weather and sufficiently low wave-height, within a predetermined safety factor. Longer total installation or deployment times decrease the likelihood of a suitable weather window, and if installation or deployment cannot start the installation vessel will remain on stand-by, incurring additional costs, until the forecast confirms a satisfactory window. This typically limits the installation of elongate members to the summer season, due to the increased likelihood of a suitable weather window.

Further, the ancillary equipment installed on an elongate member typically increases the load on an elongate member during deployment, reducing the allowable waveheight during an installation procedure.

The present invention relates to a method for the subsea mounting of ancillary equipment on an elongate member, in which the ancillary equipment is mounted on the elongate member after the elongate member has been deployed from a deployment vessel, i.e. once a deployment procedure is complete, and before final installation of the elongate member.

The method of the present invention therefore fully or partly decouples the procedure of mounting ancillary equipment on the elongate member from the procedure of deploying the elongate member. Removing the necessity to mount ancillary equipment during deployment of the elongate member reduces the duration of the deployment procedure and reduces the load on the elongate member during deployment. Such reductions will be significant for the deployment of longer elongate members in deeper waters, or for risers requiring a large number of buoyancy modules. The deployment procedure is critically dependent on weather conditions and wave height, and reducing the duration of the deployment procedure and the load on the elongate member during deployment means that a shorter forecasted window of weather is required and a larger wave height may be allowed for deployment to begin. This will increase the proportion of vessel time that can be used for deployment, thereby increasing the efficiency of the deployment procedure.

The season suitable for elongate member deployment may be extended, and vessel stand-by time and costs will be decreased. Further, a smaller, cheaper vessel may be used to mount the ancillary equipment on the elongate member, further reducing vessel costs and increasing efficiency of the installation of elongate members. The load design requirements may be less stringent for ancillary equipment used in the method, because the load imposed by the ancillary equipment on the elongate member will no longer critically affect the window for deployment, and hence the efficiency and cost of the overall installation. It is recognised that the lack of buoyancy modules during deployment may affect the top tension on the elongate member, and hence vessel installation spread capacity, for deployment in very deep waters. The post-deployment subsea mounting of ancillary equipment on an elongate member in accordance with the invention will be particularly useful in harsh environments, and for optimising planned, large-scale installation campaigns. In one embodiment, for example, at least 100 items of ancillary equipment are mounted on the elongate member.

Figure 2 illustrates an initial step of the method in which an elongate member 202 is deployed into the water from a deployment vessel 250. The elongate member is a tubular or a flexible line, e.g. a riser, an unbonded flexible pipe, a bonded flexible pipe, a steel pipe, an umbilical or a power cable. It is noted that in some cases the definition of the terms “tubular” and “flexible line” may overlap, and these terms are therefore not to be considered as mutually exclusive. For example, an unbonded flexible pipe is a tubular that is also a flexible line. In this embodiment the deployment is related to the offshore installation of the elongate member between a platform 240 at the sea surface 270 and a subsea structure 230 at or near the seabed 260. The platform is e.g. a drilling platform or an FPSO vessel. The subsea structure is e.g. a wellhead or a manifold. The invention is not limited to connection between such entities, and the elongate member may be intended for final installation between any sea-surface and/or subsea structures relating to the production and/or conveyance of hydrocarbons. The deployment vessel 250 carries a storage device 252, e.g. a reel or larger built-in carousel, on which the elongate member 202 is wound. The storage device 252 is referred to here as a reel. In an alternative embodiment, the deployment vessel 250 may carry separate elongate member sections, e.g. stacked on a deck of the deployment vessel, which are sequentially assembled on the vessel to form the elongate member as the elongate member is deployed. The elongate member is fed out from the reel (or assembled from separate elongate member sections) and through installation spread 254, which supports the load imposed by any portion of the elongate member that has been deployed into the water, and may remove any residual curvature from the elongate member produced by the configuration of the elongate member on the reel if necessary, e.g. if the elongate member is a steel pipe. The elongate member is then deployed into the water. In this embodiment the elongate member is deployed into a position at the seabed 260, and a first end of the elongate member 206 is at or near the seabed.

In the Figures subsequent to Figure 2, like features are denoted by reference numerals incremented by 100.

Figure 3 illustrates the elongate member in a temporary position at the seabed 360 after the deployment procedure has been completed and the deployment vessel has departed. In an alternative embodiment, the deployment vessel may remain in place to continue with subsequent mounting and/or installation operations. The temporary position is a position in which the elongate member is stable, after completion of the deployment procedure and before a final installation procedure. After completion of the deployment procedure the elongate member is entirely subsea, and is not coupled or attached to the deployment vessel in any way.

In this embodiment, the first end 306 of the elongate member is at or near the seabed. In an alternative embodiment, the first end may be more distant from the seabed. In an embodiment the first end is tied down to the seabed, or to a structure at or near the seabed. Tying down the elongate member in this way retains the elongate member in the temporary position while ancillary equipment is attached in a subsequent step (described below with reference to Figure 4). Where necessary, e.g. if the ancillary equipment to be mounted includes one or more buoyancy modules, one or more intermediate portions of the elongate member may also be tied down. The first end of the elongate member is not connected to any subsea structure or surface platform, and the elongate member is not ready to convey fluid or power. In this embodiment, a second end of the elongate member 308 is connected to the subsea structure 330. In an alternative embodiment, once the deployment procedure is complete, the second end is not connected to any subsea structure or surface platform, in which case the second end may be tied down to the seabed or to a structure at or near the seabed. In an intermediate step between Figure 2 and Figure 3, the second end 308 is lowered to the seabed using e.g. a winch located on the deployment vessel, although any suitable method may be used. In the temporary position shown in Figure 3, the elongate member is ready for the mounting of ancillary equipment. The elongate member can remain in the temporary position for as long as is necessary.

Figure 4 illustrates a step of the method in which ancillary equipment 404 is mounted on the elongate member 402. The ancillary equipment 404 includes at least one of a buoyancy module, a VIV strake and a clamp. Before the mounting procedure begins, the elongate member 406 is in the temporary position shown in Figure 3. In this embodiment, the mounting procedure is performed, controlled and/or managed using a second vessel 410 that is different from the deployment vessel 250. The second vessel shown in Figure 4 is smaller than the deployment vessel 250, and therefore may have lower operating costs. Of course, any suitable vessel can be used to perform, control and/or manage the mounting procedure, including a deployment vessel such as that shown in Figure 2.

In the embodiment shown in Figure 4, the ancillary equipment 404 is mounted on the elongate member 406 using an ROV 420 that is controlled from the second vessel 410. In alternative embodiments, different methods are used for mounting the ancillary equipment, depending on conditions. For example, an AUV may be used to mount the ancillary equipment, or it may be possible for a diver to attach the ancillary equipment in shallower waters.

In the schematic illustration shown in Figure 4, five items of ancillary equipment 404 have been mounted on the elongate member. In practice, of course, the number of items of ancillary equipment mounted on the elongate member may be larger. The number of items of ancillary equipment required is, in some cases, positively correlated with the length of the elongate member, which is itself dependent on the water depth. For example, more buoyancy modules are typically required for a longer elongate member, e.g. a riser, in deeper water.

In one embodiment, at least 100 items, and preferably between 100 and 200 items, of ancillary equipment are mounted on the elongate member. In this embodiment the water depth is e.g. 500 m to 3000 m deep, and the elongate member is a correspondingly longer elongate member. Where the ancillary equipment includes one or more buoyancy modules, each buoyancy module is e.g. 2 m in diameter and 2 m in length. In one embodiment, the at least 100, and preferably between 100 and 200, items of ancillary equipment are buoyancy modules, and the buoyancy modules provide a net buoyancy of between 100 and 300 Te (100000 to 300000 kg) on the elongate member. It is known for mass to be used as a unit of buoyancy, and the person skilled in the art will understand that the buoyancy force is simply the product of the mass and the acceleration due to gravity.

In an alternative embodiment, at least 30 items, and preferably between 30 and 50 items, of ancillary equipment are mounted on the elongate member. In this embodiment the water depth is e.g. equal to or less than 500 m, and the elongate member is a correspondingly shorter elongate member. For such a shorter elongate member the buoyancy required is less.

It is envisaged that mounting of such large numbers of items of ancillary equipment (e.g. between 30 and 50 items, or between 100 and 200 items) will take place as part of a planned, large-scale installation campaign. Such planned, large-scale installation campaigns are in contrast to repair, replacement and/or maintenance operations in which a smaller number of items of ancillary equipment may be mounted on an elongate member.

Once the ancillary equipment has been mounted on the elongate member, a final installation procedure is carried out to connect the elongate member. Figure 5 shows the elongate member 502 after the final installation procedure. After the final installation procedure is complete, the elongate member is ready to convey fluids and/or power. The fluids are e.g. one or more of liquid or gaseous hydrocarbons, well fluids, service fluids and chemicals. During the final installation procedure, the elongate member is untied (if the first end, second end and/or an intermediate portion of the elongate member was tied down), and the first end 506 of the elongate member is pulled up to the surface platform 540 and connected to the platform, using any suitable method known to the skilled person. In the embodiment shown in Figure 5, the ancillary equipment 504 includes buoyancy modules, and the buoyancy modules hold the elongate member in a lazy wave configuration. Of course, any suitable configuration of the elongate member after final installation is possible, as determined by the configuration of the entities between which the elongate member is connected and the ancillary equipment mounted on the elongate member. For example, the elongate member may be in a vertical, free-hanging catenary, or lazy-S configuration. The elongate member may be raised from the seabed by increasing the buoyancy in the buoyancy modules. In one embodiment this is achieved by releasing compressed gas into each buoyancy module from a compressed gas container in the buoyancy module. Of course, any other suitable method, e.g. use of a chemical reaction that releases gas, may be used to increase the buoyancy in each buoyancy module.

Figure 6 illustrates the elongate member 606 in the temporary position in an alternative embodiment, following mounting of the ancillary equipment 604. In the embodiment shown in Figure 6 the elongate member 606 is deployed such that, in the temporary position, at least a portion of the elongate member is supported above the seabed 660 by a platform 680, thereby providing improved access to the elongate member for mounting the ancillary equipment 604. The platform 680 may be in the form of a rack, to provide easier access to the underside of the elongate member.

Figure 7 shows a high-level flow diagram describing a method in accordance with the invention. In step S702, ancillary equipment is mounted on an elongate member, after the elongate member has been deployed from a deployment vessel and before final installation of the elongate member.

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.