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Title:
FACILITATING UNDERWATER OPTICAL WIRELESS COMMUNICATION
Document Type and Number:
WIPO Patent Application WO/2020/251364
Kind Code:
A1
Abstract:
An apparatus for facilitating optical wireless communication between underwater equipment. The apparatus comprises a flexible, non-permeable first sheet configured to cover at least a portion of the underwater equipment. The first sheet is further configured to be retained in position at a plurality of points around the underwater equipment, and the apparatus is 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 apparatus. An inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals.

Inventors:
SOLHEIMSNES PÅL ATLE (NO)
Application Number:
PCT/NO2020/050111
Publication Date:
December 17, 2020
Filing Date:
April 30, 2020
Export Citation:
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Assignee:
EQUINOR ENERGY AS (NO)
International Classes:
E21B41/00; E21B33/037; H04B10/80; H04B13/02
Domestic Patent References:
WO2019013628A22019-01-17
Foreign References:
US20050232638A12005-10-20
GB2489279A2012-09-26
Attorney, Agent or Firm:
ROBERTS, Gregory (GB)
Download PDF:
Claims:
Claims

1 . An apparatus for facilitating optical wireless communication between underwater equipment, comprising:

a flexible, non-permeable first sheet configured to cover at least a portion of the underwater equipment,

wherein the first sheet is further configured to be retained in position at a plurality of points around the underwater equipment,

and the apparatus is 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 apparatus,

and wherein an inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals.

2. An apparatus according to claim 1 , further comprising 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, wherein

optionally, the second sheet is non-permeable.

3. An apparatus according to claim 1 or 2, wherein the inner surface of the apparatus comprises an inner surface of the first sheet.

4. An apparatus according to claim 2 or 3, wherein the inner surface of the apparatus comprises an inner surface of the second sheet.

5. An apparatus according to any one of claims 2 to 4, wherein the apparatus is configured to contain the first buoyant fluid between the first sheet and the second sheet.

6. An apparatus according to any one of the preceding claims, wherein the reflective material comprises one or more of a metal foil, a light-coloured paint, a glossy paint, and a glossy and/or light-coloured polymer material.

35624574-1

7. An apparatus according to any one of claims 2 to 6, wherein the second sheet is non-permeable, and the apparatus is configured to contain the first buoyant fluid in the second volume, and to contain a second fluid in the closed first volume.

8. An apparatus according to any one of claims 2 to 6, wherein the second sheet is non-permeable, and the apparatus is configured to contain a second, different buoyant fluid in the second volume.

9. An apparatus according to any one of the preceding claims, further comprising a port in the first sheet configured to provide access to the underwater equipment for a diver or ROV.

10. An apparatus according to any one of the preceding claims, further comprising one or more ports, each port configured to be coupled to a fluid line for adding and/or removing fluid.

1 1. An apparatus according to any one of the preceding claims, wherein the underwater equipment is subsea equipment that forms part of an oil and/or gas production system.

12. An apparatus according to any one of the preceding claims, wherein the apparatus is configured to prevent or mitigate the passage of visible light through the apparatus.

13. An underwater oil and/or gas production system, comprising:

underwater equipment;

the apparatus of claim 1 , retained in position at a plurality of points around the underwater equipment; and

a buoyant fluid having a density lower than the surrounding water,

wherein the buoyant fluid is contained by the apparatus and exerts a buoyancy force on the apparatus.

14. A system according to claim 13, wherein the underwater equipment comprises a plurality of equipment units, each unit comprising a transceiver for transmitting and/or receiving optical wireless communication signals.

35624574-1

15. A system according to claim 14, wherein the plurality of equipment units includes one or more of a manifold, a subsea drone and a Christmas tree.

16. A system according to any one of claims 13 to 15, further comprising one or more reflectors for increasing optical wireless communication signal coverage.

17. A system according to any one of claims 13 to 16, wherein the buoyant fluid comprises ethylene glycol.

18. A system according to any one of claims 13 to 17, wherein the surrounding water is salt water, and the apparatus of claim 1 further contains fresh water for preventing corrosion of the underwater structure.

19. A system according to any one of claims 13 to 18, wherein the apparatus of claim 1 is held in position at at least one of the plurality of points using a releasable fixing device, and the apparatus is configured to be released at the at least one point to provide access to the underwater equipment.

20. A system according to any one of claims 13 to 19, wherein the apparatus of claim 1 has a substantially round shape for deflecting trawling equipment.

21. A method of facilitating optical wireless communication between underwater equipment, comprising:

covering the underwater equipment with an apparatus comprising a flexible, non-permeable first sheet, wherein an inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals;

retaining the first sheet in position at a plurality of points around the underwater equipment; 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 apparatus and exerts a buoyancy force on the apparatus.

35624574-1

Description:
FACILITATING UNDERWATER OPTICAL WIRELESS COMMUNICATION

Technical Field

The present invention relates to an apparatus and method for the facilitating optical wireless communication between underwater equipment, and in particular between equipment in a subsea hydrocarbon production system.

Background

Recently, optical wireless subsea communication has been developed for subsea applications. Such optical wireless subsea communication is typically achieved using LED light or lasers for data transfer. It is envisaged that the technology can be used, for example, in wireless ROV telemetry, local control of AUV systems etc. Examples of systems for such optical wireless subsea communication are Sonardyne BlueComm™ and Hydromea Luma™.

The transmission of wireless communication signals underwater can be challenging, due to attenuation, refraction, absorption and scattering produced by the transmission medium. In subsea hydrocarbon production systems, such issues are typically avoided by the use of electrical jumper cables for communication signal transmission between underwater equipment (where 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). However, such jumper cables are costly, and may have a limited lifetime due to seawater corrosion and/or salt deposition in jumper connectors.

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 facilitating optical wireless communication between underwater equipment, comprising: a flexible, non-permeable first sheet configured to cover at

35624574-1 least a portion of the underwater equipment, wherein the first sheet is further configured to be retained in position at a plurality of points around the underwater equipment, and the apparatus is 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 apparatus, and wherein an inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals.

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, wherein, optionally, the second sheet is non- permeable.

The inner surface of the apparatus may comprise an inner surface of the first sheet.

The inner surface of the apparatus may comprise an inner surface of the second sheet.

The apparatus may be configured to contain the first buoyant fluid between the first sheet and the second sheet.

The reflective material may comprise one or more of a metal foil, a light-coloured paint, a glossy paint, and a glossy and/or light-coloured polymer material.

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 in 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 further comprise a port in the first sheet configured to provide access to the underwater equipment 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.

35624574-1 The underwater equipment may be subsea equipment that forms part of an oil and/or gas production system.

The apparatus may be configured to prevent or mitigate the passage of visible light through the apparatus.

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

The underwater equipment may comprise a plurality of equipment units, each unit comprising a transceiver for transmitting and/or receiving optical wireless communication signals. The plurality of equipment units may include one or more of a manifold, a subsea drone and a Christmas tree.

The system may further comprise one or more reflectors for increasing optical wireless communication signal coverage.

The buoyant fluid may comprise ethylene glycol.

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

The apparatus of the first aspect 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 equipment.

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

In accordance with a third aspect of the present invention there is provided a method of facilitating optical wireless communication between underwater equipment, comprising:

35624574-1 covering the underwater equipment with an apparatus comprising a flexible, non- permeable first sheet, wherein an inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals; retaining the first sheet in position at a plurality of points around the underwater equipment; 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 apparatus and exerts a buoyancy force on the apparatus.

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 2 shows an apparatus in accordance with the invention in situ, containing a first buoyant fluid and with an inner surface of the first flexible sheet comprising a reflective material.

Figure 3 shows an apparatus in accordance with the invention in situ, containing a first buoyant fluid between two flexible sheets and with an inner surface of the second flexible sheet comprising a reflective material.

Figure 4 shows an apparatus in accordance with the invention in situ, containing a first buoyant fluid, and second and third fluids.

Figure 5 illustrates an apparatus according to the invention being partially released to provide access to an underwater structure.

Figure 6A 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.

35624574-1 Figure 6B 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 7 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 flexible, non-fluid-permeable, and preferably strong and damage-resistant material. The first sheet is configured to at least partially cover underwater equipment. The underwater equipment includes a plurality of underwater equipment units which include, for example, one or more of a wellhead, a Christmas tree (XMT), a manifold (e.g. a subsea production system [SPS] manifold), a template (e.g. an SPS template), and a drone, e.g. a subsea intervention drone, in a subsea oil and/or gas production system. The subsea equipment may be any equipment that uses optical wireless communication in a subsea oil and/or gas production system. The first sheet is light, cheap, and simple to operate and maintain. The first sheet is configured to be retained in position at a plurality of points around the underwater equipment, and the apparatus is 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 apparatus. In particular, the first sheet is configured to contain the first buoyant fluid, and in an embodiment the buoyant 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. An inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals. The apparatus is thereby configured to facilitate optical wireless communication between the underwater equipment, by providing a reflective dome-like structure covering the underwater equipment. In particular, the reflection of the optical wireless communication signals increases optical wireless communication signal coverage, and/or signal strength, in the area covered by the apparatus.

The apparatus further facilitates optical wireless communication because the first sheet is configured to provide a continuous physical barrier/cover over the underwater

35624574-1 equipment. This physical barrier prevents or mitigates the entry of debris and/or smaller particles (of biological or non-biological origin, which may descend through the water column under the influence of gravity) into the volume covered by the apparatus. This reduces the turbidity of the fluid in the volume covered by the apparatus, thereby reducing at least the absorption and scattering of optical wireless communication signals in the transmission medium, and providing a clearer medium for transmission of the signals.

The apparatus still further facilitates optical wireless communication between the underwater equipment because the apparatus is configured to eliminate or reduce the intensity of ambient light in the volume covered by the apparatus. In some situations, underwater equipment is located at shallower depths where ambient light levels are high, or at least non-zero, due to sunlight or moonlight from e.g. a full moon. The presence of ambient light may be detrimental for the underwater transmission of optical wireless communication signals, because the ambient light may result in increased noise levels, or interference, in the signals. The apparatus, and in particular the first sheet, provides a shade structure that covers the underwater equipment and reduces or prevents the transmission of ambient light through the apparatus and into the volume covered by the apparatus. This in turn reduces interference and/or noise in the optical wireless communication signals.

The buoyancy force exerted on the apparatus 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 equipment 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 an embodiment, the apparatus also provides benefits for the start-up of a hydrocarbon production system. In particular, in an embodiment where the volume under the apparatus is closed, the fluid in the internal volume will be contained by the apparatus and will not be subject to mixing with the surrounding seawater. The fluid in the internal volume will therefore be maintained at a higher temperature than the surrounding seawater, and this will reduce the need for chemicals, e.g. hydrate inhibitors, in a start-up process.

35624574-1 Figure 1 shows a subsea oil and/or gas production system 150 comprising a manifold 180, Christmas trees 190, subsea equipment 106, and an apparatus 100 in accordance with the invention. The apparatus of the invention 100 is shown covering the subsea equipment 106. The apparatus 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 configured to provide a shade over the underwater equipment. In an embodiment the apparatus, and in particular the first sheet, is opaque, or substantially opaque, to the transmission of ambient light. The first sheet also provides a physical barrier covering the underwater equipment, minimising the presence of debris in the volume covered by the apparatus. An inner surface of the apparatus comprises a material 101 configured to reflect optical wireless communication signals. In an embodiment, the material 101 covers the entire inner surface of the apparatus. The reflective material 101 comprises one or more of a metal foil, a light-coloured paint, a glossy paint, and a glossy and/or light-coloured polymer material. Of course, the material 101 may be any material suitable for reflecting optical wireless communication signals. In an embodiment the material 101 is a coating on the inner side of an innermost sheet of the apparatus, e.g. a coating on the inner side of the first sheet. In an alternative embodiment, an innermost sheet of the apparatus is made from the reflective material.

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 equipment 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, a template, 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 apparatus, via 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

35624574-1 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 equipment 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 apparatus can be moved to provide access to an ROV 120 or diver, or other device, for repair and/or maintenance of the subsea equipment 106. Further details are set out in relation to Figure 5. 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 equipment) 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 equipment.

Figure 2 shows a side elevation schematic view of an apparatus 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 2 the apparatus 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 apparatus, 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 equipment 206. The ROV may also be used to add fluid 204, either by accessing the internal volume of the apparatus 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. An inner surface of the apparatus comprises the material 201 configured to reflect optical wireless communication signals. In the embodiment shown in Figure 2, the inner surface of the apparatus is the inner surface of the first sheet.

35624574-1 Figure 2 shows a gap between a bottom edge of the first sheet and the seabed (or fixed structure to which the apparatus is attached). However, in an embodiment the apparatus is installed flush with the seabed, or flush with the fixed structure to which the apparatus is attached (or the edges of the first sheet are sealed to the seabed or the fixed structure), to provide a closed internal volume under the apparatus. This closed volume is beneficial for containing e.g. fresh water (or another liquid that will prevent or mitigate corrosion of the underwater equipment, in line with the description below in relation to Figure 4), and for minimising the intensity of ambient light and the presence of debris in the volume under the apparatus. Surrounding the underwater equipment with fresh water (or another corrosion-preventing liquid) instead of salt water will minimise the risk of corrosion of the underwater equipment, and will also minimise the amount of biological growth on the underwater equipment.

Figure 3 shows a side elevation schematic view of a system in accordance with the invention, including apparatus 310 and underwater equipment 306. Figure 3 shows a particular configuration of the apparatus 310, but the apparatus of any one of the embodiments shown in Figures 1 , 2, 4, 5, 6A and 6B can be combined with any of the other features of the embodiment shown in Figure 3.

In the embodiment of Figure 3, the apparatus includes a second sheet 303 that is joined to the first sheet 302 and forms a closed volume 304 between the first sheet 302 and the second sheet 303. The closed volume contains the first fluid 304, which is buoyant relative to the surrounding seawater 310 and which provides the buoyancy force that provides the shape and structure of the apparatus. The first sheet, and hence the apparatus, is attached to the seabed via attachment points 312. In fact, in this embodiment, the first sheet is attached to a template, which is itself located on the seabed 308. The apparatus is installed flush with the template, or is sealed to the template, so that the volume covered by the apparatus 310 is a closed, sealed volume. In an embodiment the closed volume contains fresh water 309 (or another liquid that will prevent or mitigate corrosion of the underwater equipment, in line with the description below in relation to Figure 4), which provides benefits in reducing corrosion of the underwater equipment and marine growth.

35624574-1 The underwater equipment 306 includes a plurality of underwater equipment units, wherein each unit includes a transmitter, receiver, or transceiver for optical wireless communication. The following description of an example embodiment refers specifically to transceivers. In this embodiment the plurality of equipment units includes a first Christmas tree having a first transceiver 31 1 c; a second Christmas tree having a second transceiver 31 1 b, a manifold including a third transceiver 31 1 a, and an intervention drone 319 including a fourth transceiver. It is envisaged that the optical wireless communication will be achieved using the transmission of LED light, e.g. blue LED light, or laser light, between the transceivers of the equipment units. Of course, any suitable system for optical wireless communication, and any suitable type of light, may be used.

In the embodiment shown in Figure 3, the inner surface of the apparatus is the inner surface of the second sheet 303. That is, the inner surface of the second sheet 303 includes the material 301 configured to reflect optical wireless communication signals 315. In an embodiment, the material 301 covers the entire inner surface of the second sheet 405. It may be preferable to contain the first buoyant fluid in a closed volume, as is this case in this embodiment, so that the optical wireless communication signals are necessarily transmitted within a continuous medium (rather than potentially encountering an interface between two different fluids, as will be the case in the embodiment shown in Figure 2).

The optical wireless communication signals 315 derive from, for example, electrical signals sent via a cable from an onshore facility or a platform. The electrical signals are converted to optical wireless communication signals, e.g. at the manifold, and are transmitted between the transceivers of the underwater equipment units. The reflective, dome-like structure provided by the apparatus reflects incident optical wireless communication signals 315 back into the volume covered by the apparatus, and thereby increases signal coverage and/or signal intensity in the volume. Examples of optical wireless communication signals are e.g. control signals for controlling the intervention drone, control signals for controlling operations of the Christmas tree, or measurement data from downhole sensors transmitted from the Christmas tree.

In an embodiment, the system includes one or more reflectors 318 configured to reflect optical wireless communication signals, to thereby increase signal coverage in the

35624574-1 volume covered by the apparatus. The one or more reflectors are e.g. mirrors, or another suitable reflective material. In an embodiment, the one or more reflectors are parabolic reflectors. The reflectors may be set up to reflect optical wireless communication signals in a particular direction, e.g. into an area in which signal intensity and/or coverage is low. This could be useful to improve signal coverage in an area where e.g. it is necessary for drone 319 to perform operations, and where such operations would not otherwise be possible due to the low signal coverage.

The apparatus further includes a gas leak detection sensor 321 and one or more ports 313 for providing fluid communication between the internal volume of the apparatus, i.e. the volume covered by the apparatus, and the environment surrounding the apparatus. In one embodiment each ports 313 includes a valve to reversibly open and close the port. In the case that the port does not include a valve and the port is permanently open, any gas leaking from the underwater equipment will escape from the apparatus through the port. If the port includes a valve, the valve may be closed as a default, and may open automatically (or may be opened manually) in response to the sensor detecting leaked gas collecting inside the apparatus.

A port 314 through the template provides access to the volume under the apparatus for e.g. a drone configured to perform repair or maintenance operations on the underwater equipment 306.

Figure 4 shows an alternative embodiment in which the apparatus includes a second sheet 403 joined to the first sheet 402 to form a closed first volume containing the first fluid 404. Figure 4 also shows a third sheet 405 joined to the first sheet 402 and/or second sheet 403 to form a closed second volume between the second and third sheets that contains a second fluid 407. A third fluid 409 is contained in an open third volume under the third sheet. In this embodiment, the inner surface of the apparatus comprises the inner surface of the third sheet 405. That is, the inner surface of the third sheet 405 includes the material 401 configured to reflect optical wireless communication signals. In an embodiment, the material 401 covers the entire inner surface of the third sheet 405, and optionally covers at least part of the inner surface of the first sheet 402. This is an illustrative example, and the apparatus may include only a first sheet and a second sheet (and not a third sheet), in which case the inner surface of the apparatus comprises the inner surface of the second sheet 403, the inner

35624574-1 surface of the second sheet 403 includes the reflective material 401 , and the material 401 optionally covers the entire inner surface of the second sheet 403, and optionally covers at least part of the inner surface of the first sheet 402.

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 apparatus. The fluid is the third volume may be buoyant or non-buoyant. The apparatus optionally has more than three sheets, forming a multi-layer structure with multiple‘tanks’ containing fluid. Preferably, each closed volume in the apparatus 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 equipment is stored in the lowermost, open volume (in Figure 4 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 equipment will be surrounded by fresh water instead of salt water, which may mitigate corrosion due to salt water. The fresh water, or any other suitable non-buoyant fluid, may be negatively buoyant relative to the salt water, and may therefore leak out through any gaps between the apparatus and the seabed. To prevent this, in one embodiment the apparatus is installed flush with the seabed, or flush with the fixed structure to which the apparatus is attached, 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 5 illustrates an embodiment in which the apparatus has been uncoupled at one or more of its corners to provide access to the subsea equipment 506 for an ROV 520 or diver, or for heavier inspection, maintenance and repair (IMR) procedures. In particular, the first sheet 502 has been reversibly detached from one or more of the attachment points 512 while remaining attached at at least one attachment point, and the apparatus has moved upward under the influence of the buoyancy force provided by the first fluid 504. In Figure 5 the apparatus still partially covers the subsea equipment 506. In an alternative embodiment the apparatus is configured to provide vertical intervention access to the subsea equipment 506 when the apparatus is

35624574-1 reversibly detached from one or more (but not all) of the attachment points 512. In one embodiment the apparatus is re-attached to the attachment points 512 using an ROV 520 to pull the first sheet 502 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 6A and Figure 6B illustrate alternative ways of adding fluid to, and/or removing fluid from, the apparatus.

Figure 6A shows a surface vessel 660 adding a first fluid 604 to a first volume beneath the first sheet 602 of the apparatus, via a fluid line 662 that is attached to fluid port 616. Alternatively, as set out above in relation to Figure 2, the fluid is added using an ROV. In Figure 6A a fluid line is attached to a second fluid port 617 and the first fluid 604 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 616 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 6B 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 670.

In addition to facilitating optical wireless communication between subsea equipment 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,

35624574-1 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. However, 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, 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 apparatus of the invention is cheaper, lighter and easier to install, and is also less likely to result in snagging of e.g. trawling equipment.

Figure 7 shows a high-level flow diagram describing a method in accordance with the invention. In step S702 underwater equipment is covered with an apparatus comprising a flexible, non-permeable first sheet, wherein an inner surface of the apparatus comprises a material configured to reflect optical wireless communication signals. In step S704, the first sheet is retained in position at a plurality of points around the underwater equipment. In step S706, 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 apparatus and exerts a buoyancy force on the apparatus.

Whilst the description above relates to an oil and/or gas production system, the apparatus of the invention could of course be applied to facilitate optical wireless communication between any suitable underwater equipment.

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.

35624574-1