Technical Field

The present invention relates to an apparatus and method for the protection of underwater equipment, and in particular the protection of equipment in a subsea production system.

Background

Underwater equipment is at risk of damage from collisions with e.g. vessel anchors or trawling equipment, or movement of the seabed. Examples of such underwater equipment include seabed structures associated with oil and/or gas production systems, including wellheads, Christmas trees, templates, manifolds, spools and pipelines. A seabed structure may be protected using a heavy metal protection structure, typically in the form of a cage. Such protection structures are costly, and must be installed using a dedicated vessel. It is difficult to access a subsea structure for maintenance or repair, e.g. using either a diver or an ROV, if such a protection structure is in place. There is also a risk that trawling equipment may snag on the protection structure.

Service fluids and chemicals, e.g. mono ethylene glycol (MEG), are typically provided to a subsea structure in an oil and/or gas production system using an umbilical. An umbilical is a cable that extends from a host, e.g. a production platform, to a subsea structure, and typically includes multiple fluid, power and fiber lines. The installation and maintenance of such an umbilical can be costly, due to the structural and operational complexity of the umbilical.

Summary of Invention

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 an apparatus for protecting an underwater structure comprising a flexible, non-permeable first sheet configured to cover at least a portion of the underwater structure. The first sheet is further configured to be retained in position at a plurality of points around the underwater structure, and the first sheet is still further configured to contain a first buoyant fluid having a density lower than the surrounding water, such that the fluid exerts a buoyancy force on the first sheet.

The underwater structure may be a subsea structure that forms part of an oil and/or gas production system.

The apparatus may further comprise a flexible second sheet joined to the first sheet to form a closed first volume between the first sheet and the second sheet, and a second volume below the second sheet.

The apparatus may be configured to contain the first buoyant fluid between the first sheet and the second sheet. The second sheet may be non-permeable, and the apparatus may be configured to contain the first buoyant fluid in the second volume, and to contain a second fluid is the closed first volume. The second sheet may be non- permeable, and the apparatus may be configured to contain a second, different buoyant fluid in the second volume. The apparatus may comprise a third sheet joined to the first sheet and/or the second sheet, and the second volume may be a closed volume between the second sheet and the third sheet.

The apparatus may further comprise a port in the first sheet configured to provide access to the underwater structure for a diver or ROV.

The apparatus may further comprise one or more ports, each port configured to be coupled to a fluid line for adding and/or removing fluid.

The apparatus may be for storing fluid.

According to a second aspect of the present invention there is provided an underwater oil and/or gas production system, comprising: an underwater structure; the apparatus of the first aspect of the invention, retained in position at a plurality of points around the underwater structure; and a buoyant fluid having a density lower than the surrounding water, wherein the buoyant fluid is contained by the first sheet and exerts a buoyancy force on the sheet.

The buoyant fluid may comprise ethylene glycol.

The surrounding water may be salt water, and the apparatus according to the first aspect of the invention may further contain fresh water for preventing corrosion of the underwater structure.

The apparatus according to the first aspect of the invention may be held in position at at least one of the plurality of points using a releasable fixing device, and the apparatus may be configured to be released at the at least one point to provide access to the underwater structure.

The apparatus according to the first aspect of the invention may have a substantially round shape for deflecting trawling equipment.

According to a third aspect of the present invention there is provided a method of protecting an underwater structure, comprising: covering the underwater structure with a flexible, non-permeable first sheet; retaining the first sheet in position at a plurality of points around the underwater structure; and filling a volume under the first sheet with a buoyant fluid having a density lower than the surrounding water, such that the buoyant fluid is contained by the first sheet and exerts a buoyancy force on the first sheet.

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 Drawinqs

Figure 1 shows a subsea oil and/or gas production system including an apparatus in accordance with the invention.

Figure 2A shows an apparatus in accordance with the invention in situ, containing a first buoyant fluid. Figure 2B shows an apparatus in accordance with the invention in situ, containing a first buoyant fluid, and second and third fluids.

Figure 3 illustrates an apparatus according to the invention being partially released to provide access to an underwater structure.

Figure 4A shows fluid being added to an apparatus according to the invention using a vessel, and fluid being removed from the apparatus and being provided to a subsea oil and/or gas production system.

Figure 4B shows fluid being added to an apparatus according to the invention using an umbilical, and fluid being removed from the apparatus and being provided to a subsea oil and/or gas production system.

Figure 5 shows a high-level flow diagram describing a method in accordance with the invention.

Detailed Description

The apparatus of the invention comprises a flexible, non-permeable first sheet. The first sheet is e.g. a tarpaulin or other similar strong, damage-resistant, flexible, non-fluid- permeable material. The first sheet is configured to at least partially cover an underwater structure, which is e.g. a subsea structure such as a wellhead, Christmas tree (XMT) or template in a subsea oil and/or gas production system. The subsea structure may be any structure that requires protection in a subsea oil and/or gas production system. The first sheet is light, cheap, and simple to operate and maintain, relative to metal cage protection structures that are typically deployed to cover underwater structures. The first sheet is configured to be retained in position at a plurality of points around the underwater structure, and to contain a first buoyant fluid having a density lower than the surrounding water, such that the fluid exerts a buoyancy force on the first sheet. When the first sheet is in situ, therefore, the first buoyant fluid provides the shape and structure of the apparatus of the invention. The buoyancy force exerted on the first sheet by the first fluid is preferably large enough that the apparatus is able to deflect falling objects or trawling equipment being dragged by a surface vessel, to thereby protect the underwater structure from damage. The apparatus of the invention may also be used as a storage tank for service fluids and/or chemicals, or any other fluid. In this case it is not necessary for the apparatus to cover an underwater structure.

Figure 1 shows a subsea oil and/or gas production system 150 comprising a manifold 180, Christmas trees 190, a subsea structure 106, and an apparatus 100 in accordance with the invention. The apparatus of the invention 100, which is referred to here as a protection structure, is shown covering the subsea structure 106. The protection structure comprises a first sheet 102, which is made from a flexible and non- permeable material. The first sheet 102 may be e.g. a tarpaulin, or any other suitable material, e.g. plastic or polymer-coated fabric. The first sheet is retained in position at a plurality of points 1 12, referred to henceforth as attachment points 1 12, around the subsea structure 106. In particular, each corner of the first sheet is coupled to the seabed 108, e.g. via attachment to the seabed or to a fixed structure that is itself fixed to the seabed, at an attachment point 1 12. The fixed structure that is itself fixed to the seabed may be any structure of the subsea oil and/or gas production system, e.g. a Christmas tree, a manifold, or a wellhead, or a structure specifically configured to provide coupling of the first sheet to the seabed.

The first sheet 102 contains a first buoyant fluid 104 which has a density lower than the density of the surrounding water 1 10, which in an embodiment is seawater 1 10. The first fluid 104 cannot pass through the non-permeable first sheet 102. Due to its buoyant nature the first fluid 104 has a tendency to rise, and thereby exerts an upward buoyant force on the first sheet. The first sheet is pulled upwards and outwards by the first fluid and is retained in place via coupling to the seabed at the attachment points 1 12. The shape and structure of the apparatus is therefore provided by the buoyant fluid. In the embodiment of Figure 1 , the apparatus has a dome shape that may be advantageous for deflecting falling objects or trawling equipment. In particular, such a dome shape may cause a trawl being towed by a vessel to‘jump’ over a subsea structure when the trawl board and lump weight hit the sheet. The surface of the apparatus, i.e. the surface of the first sheet, is preferably smooth to prevent any snagging with incident objects.

The first sheet shown in Figure 1 has four corners. Other configurations are possible, and the first sheet may have any number of corners equal to or greater than three. In one embodiment each corner is permanently coupled at its respective attachment point 1 12. Alternatively, and preferably, one or more of the corners is releasably coupled to the seabed at its respective attachment point 1 12. In this way the protection structure can be moved to provide access to an ROV 120 or diver, or other device, for repair and/or maintenance of the subsea structure 106. Further details are set out in relation to Figure 3. In an embodiment there is a gap between at least one bottom edge of the first sheet and the seabed that provides enough space for an ROV to access the volume under the first sheet (and therefore access the subsea structure) without moving, de-coupling or detaching the first sheet. The size of the gap required depends on the size of the ROV used for maintenance and/or repair of the subsea structure.

Figure 2A shows a side elevation schematic view of a protection structure 210 in accordance with the invention. Reference numbers referring to features shown in Figure 1 are incremented by one hundred. In the embodiment shown in Figure 2A the protection structure 210 comprises a first sheet 202 with no additional sheet enclosing the volume beneath the first sheet, i.e. there is an open volume under the first sheet containing the first fluid 204. An access port 214 provides access to the internal volume of the protection structure, i.e. the volume beneath the first sheet, preferably at a position below the level of the first fluid 204. The access port may be used to provide access to, for example an ROV 220 or diver to perform repairs or maintenance work on the subsea structure 206. The ROV may also be used to add fluid 204, either by accessing the internal volume of the protection structure through access port 214 or by connecting to fluid port 216, which is for providing connection to a fluid line to add or remove fluid.

Figure 2B shows an alternative embodiment in which the protection structure includes a second sheet 203 joined to the first sheet 202 to form a closed first volume containing the first fluid 204. Figure 2B also shows a third sheet 205 joined to the first sheet 202 and/or second sheet 203 to form a closed second volume between the second and third sheets that contains a second fluid 207. A third fluid 209 is contained in an open third volume under the third sheet. This is an illustrative example, and the protection structure may include only a first sheet and a second sheet (and not a third sheet). Buoyant fluid may be contained in the first volume and/or the second volume. For example, a less buoyant fluid (which may in fact not be buoyant) could be contained in the first volume if the fluid in the second and/or third volume is sufficiently buoyant to maintain the shape and structure of the protection structure. The fluid is the third volume may be buoyant or non-buoyant. The protection structure optionally has more than three sheets, forming a multi-layer structure with multiple‘tanks’ containing fluid. Preferably, each closed volume in the protection structure has a corresponding fluid port in the first sheet to allow fluid to be added to or removed from the closed volume. In one embodiment, a liquid for preventing or mitigating corrosion of the subsea structure is stored in the lowermost, open volume (in Figure 2B this is the third volume), e.g. fresh water, optionally including anti-corrosion agent(s), or any other suitable buoyant (e.g. methanol) or non-buoyant fluid that will prevent or mitigate corrosion of the subsea structure. In the case that the liquid is fresh water, the subsea structure will be surrounded by fresh water instead of salt water, which may mitigate corrosion due to salt water. The fresh water, or any other non-buoyant fluid, may be negatively buoyant relative to the salt water, and may therefore leak out through any gaps between the protection structure and the seabed. To prevent this, in one embodiment the apparatus is installed flush with the seabed to minimise leak paths. Alternatively or additionally, the fresh water is replaced periodically, or is continuously generated using seawater outside the protection structure, or seawater that is drawn inside the structure as the fresh water exits the structure.

Figure 3 illustrates an embodiment in which the protection structure has been uncoupled at one or more of its corners to provide access to the subsea structure 306 for an ROV 320 or diver, or for heavier inspection, maintenance and repair (IMR) procedures. In particular, the first sheet 302 has been reversibly detached from one or more of the attachment points 312 while remaining attached at at least one attachment point, and the protection structure has moved upward under the influence of the buoyancy force provided by the first fluid 304. In Figure 3 the protection structure still partially covers the subsea structure 306. In an alternative embodiment the protection structure is configured to provide vertical intervention access to the subsea structure 306 when the protection structure is reversibly detached from one or more (but not all) of the attachment points 312. In one embodiment the protection structure is re-attached to the attachment points 312 using an ROV 320 to pull the first sheet 302 down into position. Alternatively, an ROV may be used to attach a pull-down cable deployed from a vessel to the first sheet, and the pull-down cable may be used (in combination with a winch, pulleys and any other necessary equipment) to pull the first sheet into place. Figures 4A and Figure 4B illustrate alternative ways of adding fluid to, and/or removing fluid from, the protection structure.

Figure 4A shows a surface vessel 460 adding a first fluid 404 to a first volume beneath the first sheet 402 of the apparatus, via a fluid line 462 that is attached to fluid port 416. Alternatively, as set out above in relation to Figure 2A, the fluid is added using an ROV. In Figure 4A a fluid line is attached to a second fluid port 417 and the first fluid 404 is being extracted from the apparatus and conveyed to the production system 150. The production system 150 optionally includes a pump for extracting the first fluid from the apparatus and feeding the first fluid into the wellstream, or into any other point necessary. It is of course not necessary to have separate fluid ports for adding and removing fluid. In an embodiment the same fluid port 416 is used for adding fluid and for extracting fluid. In this case adding fluid and extracting fluid are separate operations that cannot be performed simultaneously (in relation to one internal volume of the apparatus). This description of the procedures for the first internal volume of the apparatus applies equally to any further internal volumes of the apparatus, each of which preferably has its own respective fluid port.

Figure 4B illustrates a similar procedure of adding and/or removing fluid from the apparatus. Flowever, in this embodiment the fluid is added using a subsea fluid line or umbilical 470.

In addition to providing protection for a subsea structure located underneath the apparatus, the apparatus also provides a way to store fluids for use in a subsea production system (e.g. the production system 150 shown in Figure 1 ). In particular, the internal volume or volumes of the apparatus, which may be closed or open volumes, can be used as storage tanks for fluids. The fluids may be wellstream fluids, service fluids or chemicals, e.g. methanol or ethylene glycol (MEG). Preferably, a single type of fluid is stored in each tank, i.e. each separate internal volume, of the apparatus. Preferably, each of the fluids is buoyant under subsea conditions. Flowever, as long as the buoyancy force on the apparatus (due to the combination of the fluids stored in the apparatus) is sufficient to maintain the shape and structure of the apparatus for protecting the subsea structure, one or more of the fluids stored in the apparatus can have a neutral or negative buoyancy. Storing fluids in the apparatus in this way may eliminate the need for fluid lines in an umbilical servicing the production system. In particular, having in situ storage tanks for wellstream fluids and/or other fluids for use in the production system may mean that it is not necessary to transport fluids from a remote location to the production system. Removing fluid lines from such an umbilical will reduce the size of the umbilical, which will in turn significantly reduce the cost of providing fluids to the production system. When used as a protection structure, the invention also removes the need for heavy, expensive protection cages. The protection structure of the invention is cheaper, lighter and easier to install, and is also less likely to result in snagging of e.g. trawling equipment.

In an embodiment the apparatus is used only as a storage tank (or storage tanks) for fluids, and is not used as a protection structure. In this case, no subsea structure is located below the apparatus. When used only as a storage tank the shape and structure of the apparatus is less important, and any combination of fluids may be stored in the tanks. In particular, fluids that are denser than seawater may be stored in the tanks, and the overall buoyancy force on the apparatus (due to the fluid(s) stored in the tank(s)) may be zero, or may be negative. The description set out above in relation to the apparatus applies equally in the embodiment in which the apparatus is used solely as a storage tank, with the exception that it is not strictly necessary for any of the fluids contained in the apparatus to be buoyant, i.e. it is optional for any one of the fluids to be buoyant. Further, in one embodiment there is no requirement for an access port, since the apparatus does not cover any subsea structure requiring maintenance or repair.

Figure 5 shows a high-level flow diagram describing a method in accordance with the invention. In step S502 an underwater structure is covered with a flexible, non- permeable first sheet. In step S504, the first sheet is retained in position at a plurality of points around the underwater structure. In step S506, a volume under the first sheet is filled with a buoyant fluid having a density lower than the surrounding water, such that the buoyant fluid is contained by the first sheet and exerts and buoyancy force on the first sheet. Whilst the description above relates to an oil and/or gas production system, the apparatus of the invention could of course be applied to protect any suitable underwater structure. 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.