GB2595321A

GB2595321A - Refuelling and storage system

Info

Publication number: GB2595321A
Application number: GB2018077.4A
Authority: GB
Inventors: Einar Ellingsen Kjell
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2020-05-22
Filing date: 2020-11-17
Publication date: 2021-11-24
Anticipated expiration: 2040-11-17
Also published as: GB2585758B; GB2585758A; WO2021235945A1; GB2595321B; GB202018077D0; GB202007678D0; WO2021235940A1

Abstract

A method of refuelling a surface vessel 808 from an unmanned underwater vehicle 802 where the underwater vehicle comprises one or more internal vessels (12, Fig 1) for containing fuel. The method comprises positioning a surface vessel and the unmanned underwater vehicle relative to each other at a distance smaller than the length of a fuel hose 804, connecting the fuel hose between the unmanned underwater vehicle and the surface vessel, transferring the fuel from the unmanned underwater vehicle to the surface vessel and releasing the hose. A fuel hose for use in the transfer method is also disclosed as is an unmanned underwater vehicle with an outer and an inner hull, a pressure compensation system and an arm (1204, Fig 12) protruding from the outer hull and suitable for connection to a fuel hose.

Description

Refuelling and storage system

Field of the invention

The invention relates to underwater vehicles and the operation thereof, and more specifically the utilisation of autonomous underwater vehicles for the transporting of gas or liquid for a refuelling operation of a surface vessel.

Background

The following video on YouTube (a registered trademark), which is accessible using the following URL https://m.youtube.com/watch?v=gC5RwghHUFg shows a method for refuelling ships on shipping lanes using a fleet of refuelling vessels. The video also describes a distribution and storage network, which could be powered by renewable energy sources, such as wind and solar.

The following publication discloses a subsea shuttle system: Research Disclosure database number 677082 published digitally on 21 August 2020. The subsea shuttle described is used as a materials handling system and adapted to receive a cargo tank in order to deliver a payload to an unmanned platform. The subsea shuttle navigates the seabed using a plurality of acoustic or electromagnetic beacons and corresponding receivers on the shuttle.

The use of fossil-fuel based fuels continues to contribute to climate change. At present, the shipping industry relies on Heavy Fuel Oil (HFO). Unfortunately, HFO has a higher volumetric energy density than renewable fuels and therefore the substitution of HFO with greener fuels, which would require more frequent refuelling, has not yet been made. As the world continues to globalise, the shipping industry is only going to increase in scale and the consumption of HFO continue to increase.

Statement of invention

The invention provides a method, a fuel hose and an unmanned underwater vehicle, for refuelling a surface vessel as set out in the accompanying claims.

Figures Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an underwater vehicle containing a pressure vessel; Figure 2A is a schematic view of a pressure vessel at a first depth underwater; Figure 28 illustrates the pressure vessel of Figure 2A having ascended to a shallower depth; Figure 2C illustrates the pressure vessel of Figure 2A having descended to a lower depth; Figures 3A and 3B are schematic views showing parts of a pressure vessel having a pressure compensation system; Figures 4A to 4C show parts of example pressure compensation systems; Figure 5 is a schematic view of an underwater vehicle having a gas pocket in the outer hull; Figure 6A is a schematic view of an underwater vehicle having a plurality of pressure vessels; Figures 6B and 6C are perspective views the underwater vehicle interior, having a plurality of pressure vessels; and Figure 7 is a flow chart of a method.

Figure 8 is a schematic view of an underwater vehicle refuelling a surface vessel.

Figure 9 is a schematic view of a refuelling hose, deployed from a surface vessel.

Figure 10 is a schematic view of steering means on the refuelling hose.

Figure 11 is a schematic view of steering means on the refuelling hose.

Figure 12 is a schematic view of a refuelling connection.

Figure 13 is a schematic view of refilling stations.

Figure 14 is a flow chart of a method.

Detailed description

The inventors have realised that the subsea transportation of a fluid cargo (i.e. liquid or gas) can be improved by providing of a pressure compensating system to maintain or control the internal pressure of the fluid based on the external hydrostatic pressure. In one embodiment of the shuttle a double hull is used. Further, internal pressure within the hull of an AUV but outside the pressure vessel holding the fluid is also maintained and controlled at or near external hydrostatic pressure. In this way, it is ensured that the internal pressure of the fluids remains the same as or similar to the external hydrostatic pressure, meaning that the vessel (containing the fluid) is not at risk of a collapse (or burst) failure.

Although the word 'sea' and 'seawater' are used throughout, these may equally be understood as 'lake' and 'freshwater', respectively, and the invention is envisaged to be used in any large body of water. Similarly, when the words 'seabed' or 'sea surface' are used, this is not intended to be limited to a sea in a strict sense but should also be understood to cover 'ocean bed' or 'ocean surface', or similar terms for any large body of water.

Figure 1 illustrates a schematic underwater vehicle according to an embodiment of the invention. The underwater vehicle (or 'shuttle') may be an autonomous underwater vehicle (AUV), or a remotely operated underwater vehicle (ROV). The vehicle comprises an outer hull 10, having a hydrodynamic shape to reduce drag. An elliptical outer hull 10 is shown in Figure 1, but other hydrodynamic shapes known in the art are suitable. Within the outer hull 10, an internal pressure vessel is provided 12. The pressure vessel 12 may be fixed within the outer hull 10 using a frame or other rigid supports (not shown). In this way, a space between the outer hull 10 and pressure vessel 12 is formed. The outer hull 10 has a channel 14, which is in fluid communication, or pressure communication, with the surrounding water when the vehicle is submerged. When the vehicle is submerged, the space between the outer hull 10 and inner pressure 12 vessel at least partially fills with seawater, depending on the net buoyancy requirements. In some embodiments, a part of the outer hull 10 volume is occupied by one or more compartments for containing gas (e.g. ballast tanks), as discussed in more detail below. In this way, the gas remains separate to the seawater, meaning that sloshing is avoided. In some embodiments, the channel 14 is selectively closed (e.g. via operation of a valve) to allow or block fluid communication through the channel.

At the stern (back end) of the vehicle, a propeller 16 is provided. The propeller is coupled to a power source and control unit (not shown) to enable autonomous and/or remote operation of the vehicle. An electric power source is preferably used. In some embodiments, the vehicle includes a docking station 18 provided on the bottom of the outer hull 10 for docking with an external platform or other equipment. The docking station 18 is provided within or largely within the outer hull 10 to reduce the drag. The outer hull 10 may include a hinged or detachable section (not shown) for accessing the internal pressure vessel 12.

The vehicle structure shown in Figure 1 is similar to a double hull structure used in some conventional submarines. However, a difference in the present invention is that instead of having a pressure hull maintained at or near to atmospheric pressure, the inner structure includes instead a pressure vessel 12 maintained at a pressure similar to the external hydrostatic pressure. Advantageously, this means that the pressure vessel 12 can be designed with a much lower collapse pressure capacity than a standard pressure hull, which results in significant weight savings (e.g. by reducing the need for stiffeners). By "similar' to the external hydrostatic pressure, it is meant that the internal pressure is kept suitably close to the external pressure so that the pressure differential dP (i.e. overpressure or under-pressure) is not too large. If the overpressure is too large, the pressure vessel may undergo a burst failure. Conversely, if the under-pressure is too large, the pressure vessel is at risk of a collapse failure.

The collapse pressure threshold is sensitive to geometric imperfections in the pressure vessel 12 shape, meaning that a smooth, curved shape is preferable. As shown in Figure 1, the pressure hull 12 has a substantially cylindrical shape with hemispherical ends.

While Figure 1 only shows a single cylinder, the underwater vehicle may include a plurality of cylindrical pressure vessels, preferably aligned parallel to each other, as discussed in more detail below.

Figures 2A-2C illustrate part of a pressure compensation system, where the internal pressure is equalised with the external pressure. The outer hull is not shown for clarity. As shown in Figure 2A, the internal vessel 22 contains a cargo, in this example a fluid F. The fluid F has an initial pressure P1. A pressure relief 26 is provided to selectively allow seawater SW in and out of the pressure vessel 22. The pressure relief unit 26 may also be connected to one or more of the other vessels and in pressure communication or fluid communication with that other vessel. Inside the pressure vessel 22, a barrier 24 is provided to separate the fluid F and the seawater SW. In this way, the pressure vessel is divided into a first portion 28a and a second portion 28b, containing the fluid F and seawater SW, respectively. The barrier 24 prevents fluids from being transferred across the barrier, while is moveable to allow pressure communication. (While the barrier is illustrated like a piston in Figures 2A-2C, further examples of barriers are discussed below.) If the vehicle changes depth, the external hydrostatic pressure Pe will change. As shown in Figure 28, if the vehicle ascends, the external pressure Pe decreases (i.e. Pe < P1). To compensate for this change, the pressure relief unit 26 opens to allow water SW to be expelled from the second portion 28b of the pressure vessel 22, as indicated by the arrow 29 in Figure 28. The barrier 24 moves to the right as the fluid F decompresses, and the pressure in the first and second portions 28a, 28b is equalised with Pe. The fluid F may be a liquid, meaning that the fluid F will always have some degree of compressibility.

Figure 2C illustrates the reverse case, where the vehicle descends. The external pressure Pe increases (i.e. Pe > P1). The pressure relief unit 26 opens to allow seawater SW to flow into the pressure vessel 22 and the barrier 24 moves to the left, compressing the fluid F until the pressure is equalised. The pressure relief unit 26 then closes. Advantageously, as the internal pressure remains equalised with the external pressure, stress on the pressure vessel is effectively eliminated.

As the vehicle may transport the fluid F over long distances, the surrounding seawater temperature Tsw may change from one location to the next during transit. If the seawater temperature Tsw is increased but the volume of fluid remains fixed, the fluid pressure will increase, with the potential for a burst failure of the vessel. Conversely, if temperature is decreased for a constant volume of fluid, the pressure will decrease, with the potential for a collapse failure of the vessel. Advantageously, the pressure compensation system described herein may also compensate for such thermal volume changes. As the vehicle is loaded with cargo fluid, the incoming fluid may have a temperature Ti that is different to the seawater temperature Tsw. The vehicle is submerged while transporting the fluid to a destination, meaning that the fluid temperature will change and become equal or close to equal to Tsw due to heat being exchanged with the surrounding seawater over time. As the temperature within the first portion 28a equalises with the seawater temperature Tsw, the fluid volume will adjust correspondingly, because fluid volumes are temperature dependent, It can be seen, therefore that the pressure compensating/equalising system proposed herein has the added advantage of compensating for thermal volume changes.

Alternatively, in some embodiments, the pressure vessel is thermally insulated to reduce the heat exchange process. However, insulation will add cost and weight to the shuttle.

Additionally, in some embodiments, the vehicle further comprises a heater system to add heat to the pressure vessel to compensate for heat exchange to the surroundings.

Alternatively the fluid may be temperature managed, e.g. by use of a heat exchanger as a part of the fluid loading system, to match the seawater temperature Tsw at the time of loading to the shuttle.

While Figures 2A-2C show a single valve to represent pressure relief unit 26, other suitable arrangements may be used. For instance, in some embodiments the pressure relief unit 26 includes a pair of one-way relief valves, with the valves having opposite operational directions. If the external pressure increases so that dP<0 (i.e. under-pressure), the first valve opens to allow water into the pressure vessel. Likewise, when dP>0 (i.e. overpressure), the second valve opens to allow water out of the pressure vessel, thereby equalising the pressure. In some embodiments, the valves are pressure relief valves which open when dP exceeds pre-set thresholds. In some embodiments the dP thresholds are adjustable, and may be controlled by a control system.

In some embodiments, the pressure compensation system maintains the internal pressure at a slight overpressure (dP>0), rather than equalising the pressure as above. Generally, the burst pressure threshold is higher than the collapse pressure threshold, meaning that keeping the internal pressure vessel in an overpressure regime may be preferable. In this case, the overpressure is maintained at a target value suitably far removed from the burst pressure threshold for the vessel, so that the pressure vessels are not at risk of bursting. At the same time, the slight overpressure provides a buffer such that if the external pressure were to increase suddenly, the risk of a collapse failure is reduced.

In this embodiment, the pressure compensation system includes a pump unit and a pressure relief unit. The pump unit is configured to pump seawater into the pressure vessel to create an overpressure (dP > 0) within the pressure vessel. The pressure relief unit is configured to relieve the surplus pressure when the overpressure exceeds a pre-set threshold (i.e. when dP = dPmax, where dPmax is the maximum overpressure permitted), e.g. by opening of a pressure relief valve. The pump unit and relief unit may work against each other if they are two separate systems, e.g. a pressure relief valve will need to have relief settings carefully determined and possibly controllable if combined with a pump system for pressure increase.

The pump unit and relief unit may be configured to disable/enable based on operational parameters, measurements or an autonomous control system. This will ensure that the pressure vessel never will experience a detrimental collapse pressure.

In some embodiments, the vehicle includes a secondary pressure compensation system for safety and increased reliability, configured to engage in the case the primary pressure compensation system fails.

In some embodiments, the pressure vessel includes one or more weak link burst plates or disks as a safety measure, should the pressure relief unit fail or stop working.

By way of an illustrative use case, assume that the fluid F cargo is CO2, and that the underwater vehicle travels at an optimal depth of 200 m. This operation depth is deep enough to avoid wave motion at the surface, but shallow enough to avoid any contact with the seabed topology. At 200 m, the hydrostatic pressure is approximately 20 bar. Assuming that the CO2 (by way of the pressure compensating system and the external hydrostatic pressure combined) is pressurised to an absolute pressure of 40 bar (i.e. having a 20 bar overpressure at the 200 m depth), the CO2 is then in the liquid phase for typical subsea temperatures e.g. between 0 and 10 °C. Transport of CO2 in liquid state is beneficial as a larger mass of CO2 can be moved in a single shipment.

If the vehicle ascends, e.g. to a depth of 100m, the external hydrostatic pressure is reduced by approximately 10 bar (i.e. from 20 bar to 10 bar). The pressure relief unit is not operated, thereby maintaining the CO2 absolute pressure of 40 bar inside the pressure vessel. This implies that the pressure vessel is able to withstand an overpressure of 30 bar (i.e. dPmax is greater than 30 bar in this example).

Alternatively, if the pressure vessel internal pressure limitation is reached (i.e. dP reaches dPmax), the pressure relief valve opens. In this way, it is ensured that that the CO2 absolute pressure is reduced to avoid a structural failure (e.g. a burst failure) while the external hydrostatic pressure is reduced by the vehicle ascending.

Conversely, if the vehicle descends e.g. to a depth of 500m, the relief unit is operated (e.g. pressure relief valve opened) to allow the seawater hydrostatic pressure to communicate into the pressure vessel, thereby equalising the internal pressure with the surroundings. This will prevent the pressure vessel from failing structurally (e.g. collapsing) as the external hydrostatic pressure is increased by the vehicle descending.

Alternatively, for a descending vehicle, the pump unit may be operated to increase the absolute pressure of the CO2 to a pressure that is the same or above the external hydrostatic pressure, but not higher than the threshold dPmax. This will prevent the pressure vessel from failing structurally (e.g. collapsing) as the external hydrostatic pressure is increased by the vehicle descending.

In some embodiments, the outer hull is not free-flooding, but is selectively opened to the surrounding seawater via one or more pressure communication channels. In this case, the outer hull may also be configured with a pressure compensation system. The pressure of both the internal vessel and the outer hull are compensated separately. For instance, the outer hull may be maintained at a slight overpressure, when compared to the external hydrostatic pressure. Advantageously, this allows the outer hull to be made of a lightweight flexible or semi-rigid material, while maintaining a hydrodynamic shape (i.e. analogous to an airship). Moreover, by over-pressuring the internal volume, the difference between the internal vessel pressure and the external hydrostatic pressure can be staggered. In this way, the pressure differential across the internal pressure vessel may be reduced, reducing the risk of a burst failure of the internal pressure vessel.

Pressure compensation of the outer hull may also be beneficial for safety reasons, i.e. to avoid a potential pressure build-up in the internal volume between the pressure vessels and the outer hull in the event of a failure. In other words, the pressure compensation system of the outer hull may be used a safety device for automatic bleed off (e.g. of escaped gas) in case something goes wrong. In this way, the vehicle will sink to the bottom rather than rise to the surface, which is generally preferable.

Another illustrative example relates to the offloading of CO2 as the cargo. Initially, the vehicle is stationary and connected to a receiving client for delivering of the cargo out of the internal pressure vessel. To enable transfer of the cargo fluid, the pressure compensation system operates the pump unit to maintain an overpressure (dP > 0) as the volume of the cargo is gradually reduced during offloading to the receiving client. The pump unit controls the absolute pressure of the cargo at a pressure above the external hydrostatic pressure and/or the inlet pressure threshold of the receiving client. Alternatively, or in addition, the receiving client may be equipped with a pump that drains the cargo fluid, with the risk of imposing a negative overpressure (dP < 0) and causing a structural failure (e.g. collapse) of the pressure vessel. Advantageously, by using the pressure compensating system to generate an overpressure (dP > 0), the cargo flows into the receiving client, without the risk of a collapse failure during cargo offloading. Moreover, the design of the receiving client may be simplified i.e. by reducing/eliminating operation of the external pump, or eliminating the need for an external pump altogether.

For the complementary case where the vehicle is receiving (loading) cargo, the pressure compensation system is operated to ensure that the internal pressure does not exceed dPmax as the cargo fluid is input from a supply client. If the input pressure exceeds the allowable pressure structural failure (e.g. bursts) of the pressure vessel may be the result. In this case, operation of the pressure relief unit provides an acceptable and safe overpressure dP inside the cargo fluid in the pressure vessel during filling. Operation of the pump unit (in the reverse direction compared to the above offloading example) may provide a desired dP inside the cargo fluid inside the pressure vessel during filling. In some embodiments, the pump unit and relief valve are both be operated simultaneously to achieve a higher filling volume rate. The pump may be operated to further generate an absolute pressure inside the pressure vessel less than the pressure at the supply client, such that the cargo fluid will flow naturally into the pressure vessel. This may only be possible within the structural limitations of the pressure vessel, e.g the absolute pressure inside the pressure vessel cannot be lower that the collapse strength of the pressure vessel.

Figures 3A and 3B show an example pressure vessel 300 in more detail. The vessel 300 has a cylindrically body 302, with a hemispherical end section 304 bolted to the end of the body 302 via a flange 306. As shown in the zoomed-in portion of Figure 3A, the end section 304 includes a first one-way valve 308 as an inlet, and a second one-way valve 310 as an outlet. The end section 304 may be unbolted for access to the vessel interior.

Inside the end section 304, a reel 312 is provided. The reel 312 may be spring energised. On the reel 312, a cable 314 is coiled. At the far end of the cable 314, a piston assembly 316 is connected, as shown in Figure 33. The piston assembly 316 includes a rigid outer frame 318. A pair of inflatable sealing rings 320a, 320b are provided at the front and rear end of the outer frame. The sealing rings 320a, 320b are inflated with a gas (e.g. air). The sealing rings 320a, 320b are connected to an umbilical 322 for pressure adjustment and condition monitoring. When inflated, the sealing rings 320a, 320b each form a ring-shaped seal between the frame 318 and the inner surface of pressure vessel body 302, thereby defining a closed volume 324 between the rings 320a, 230b. The closed volume 324 is filled with a barrier substance. The barrier substance is a non-reactive liquid, oil or gel (e.g. silicone grease). In some embodiments, the barrier substance is maintained at a slight overpressure compared to the neighbouring fluids.

The piston assembly also comprises buoyancy chambers 326a, 326b, at the front and rear end, respectively. The buoyancy chambers make the piston neutrally buoyant within the surrounding fluids.

A positioning device 328 is provided on the far end of the piston assembly, for detecting the longitudinal position of the piston assembly within the pressure vessel. The positioning device 328 is calibrated relative to a reference point within the vessel body 302. Pressure and temperature sensors 330 are provided on both end faces of the outer frame 318. Further pressure and temperature sensors 330 may also be provided the inside buoyancy chambers 326a, 326b and inner volume 324.

In some embodiments, the cable 314 is a bundle comprising the following: a feed line umbilical for supply of the fluids (e.g. the barrier substance, and gas for sealing ring inflation); power cables; and cables for acoustic and/or digital communication. Power is supplied by an internal power supply (e.g. a battery or accumulator, not shown), which may be provided on the reel 312 or within the piston assembly 316.

While Figures 2A-C and 3A-B show a piston-like movable barrier, other types of barrier are suitable for separating the fluid F and seawater SW in the present invention. For example, Figure 4A (not to scale) illustrates an example movable piston 40, which resembles a batching pig used in the pipeline industry. Typically batching pigs are used in pipelines as a moving seal to separate two different products so that they can both be transported using the same pipeline. Batching pigs typically employ polyurethane (PU) disc seals. The inventors have realised that a batching pig (scaled up to the diameter required for the pressure vessel) may be used as the moveable barrier, as illustrated in the Figure.

Figure 4B shows an alternative embodiment, wherein the barrier is a balloon-type piston. A balloon 42 is connected to an external pressure supply and control system (not shown). The balloon 42 is inflated by the external pressure supply such that it seals in the transverse direction, but can move in the longitudinal direction under the pressure of the seawater SW or fluid F. The balloon 42 may be filled with air or any other suitable compressible gas.

Alternatively, as shown in Figure 4C, the barrier may be formed by a bladder 44. The bladder 44 is elastic (i.e. a balloon) or inelastic, and is situated inside the pressure vessel to divides at the fluids from mixing. In some embodiments, the bladder 44 has a maximum volume that exceeds that of the pressure vessel, meaning it will receive protection from overpressuring from the walls of the pressure vessel. In a partly filled state, the bladder 34 will float in the seawater SW (or vice versa, depending on the density of the fluid F).

In some embodiments, the movable barrier is a layer of a third substance which divides the two fluids. The third substance may be an emulsion or a chemical matter that does not resolve in any of the other fluids, but will coalesce enough to ensure that the first and second fluid does not have direct contact. The third substance may be an immiscible liquid. In some embodiments, the pressure vessel may be oriented vertically, relative to the overall longitudinal axis of the vehicle, so that the third substance layer floats on the seawater SW but sinks beneath the fluid F, or vice versa dependant on the fluid F density.

In some embodiments, the space between the outer hull and internal pressure vessel is filled partially with gas instead of being completely water-filled, as shown in Figure 5. In Figure 5, the space between the outer hull 50 and pressure vessel(s) 52 is partially filled with gas G. Partially filling the outer hull with gas G provides a method to adjust the resulting buoyancy forces of the vehicle, and also adjust the location of the buoyancy centroid relative to the gravitational forces and centroid. In this way, the gas may improve the stability of the vehicle, as gas will be at top. Further, it can provide weight benefit and remove some or all of the need for ballast tanks.

In some embodiments, the underwater vehicle is configured to have variable buoyancy so that that the equilibrium depth of the vehicle can be controlled. For instance, in a docking approach where the vehicle proceeds to lay on the seabed, the vehicle will need to become negatively buoyant and sink to the bottom. In reverse, the vehicle needs to leave the seabed by becoming more buoyant, e.g. after having offloaded or loaded with a cargo, again affecting the total weight. Further, it is known that salinity of seawater may have minute differences over time at one ocean location or as one moves from one ocean location to another. During transport of cargo at a desired constant water depth the equilibrium depth may change due to minute salinity changes (i.e. the water density is changed) and buoyancy is adjusted accordingly to maintain a desired water depth during transit. During operation, it may be necessary needed to move the vehicle between water depth levels, which is also made possible by adjustments to the buoyancy.

In some embodiments, the gas is evaporated from a liquid gas tank, located within or attached to the outer hull. The gas is evaporated and allowed directly into the fluid-filled volume (typically seawater) between the outer hull and the internal pressure vessel(s). The in-situ water is allowed to evacuate through a pressure relief system, thereby allowing for some of the seawater to be displaced by the gas. The gas locates itself at the top of the water inside the outer hull. The gas may be air, CO2, H2 etc. Coolant fluids known from use in heating, ventilation and air conditioning (HVAC) applications may be used. To decrease the buoyancy, the gas is evacuated from the top of the outer hull e.g. by venting into the surrounding environment exterior to the vehicle. This will imply that the gas is environmental friendly.

Alternatively, the gas may be allowed into a bladder to contain the gas and expand in volume, displacing the seawater in the same way as before. Advantageously, the bladder can be located and secured in a desirable location inside the outer hull. As above, the gas is released from the bladder into the surrounding seawater exterior to the vehicle to decrease the buoyancy. This may be more easily achieved when the gas is contained within the bladder and not free inside the hull, since if the gas is free inside the hull, its location relative to vent exit(s) would be dependent of the orientation of the vehicle.

In some embodiments, the gas G is evaporated into a rigid tank (i.e. a tank which has a fixed volume). The tank is originally liquid filled, and the evaporated gas displaces the liquid (e.g. seawater) out of the tank. In this way, the tank becomes increasingly and finally positively buoyant. By securing the tank to the outer hull, the buoyant forces act on to the entire shuttle. Further, the tank may be equipped with a technology that isolates the gas and the fluid (seawater) e.g. a bladder or movable piston as described above in connection with the pressure vessels In some embodiments, the gas is circulated between a pressurised tank holding gas in liquefied state and another tank (the variable buoyancy tank) where gas is in gas state, using vaporization/condensing phase change recirculating refrigeration technology (i.e. an evaporator and compressor system). In this variable buoyancy tank, the process of displacing seawater by gas produces a change to the tanks buoyant forces. A control system is provided to control the operation of the evaporator pump vs. the compressor, allowing the accumulation of gas in the variable buoyancy tank to be controlled to achieve the desired buoyancy. If the compressor rate is increased and/or the evaporation process is reduced, the gas will accumulate in the liquid holding tank, allowing the buoyancy to decrease.

Beneficially, in the case a refrigeration/heat pump system, the power required to operate the evaporator s typically low, and the cooling effect of the surrounding seawater can be used for the compressor and the condensing phase change. The control system may be operated automatically and/or remotely. Again, the buoyant tank may be equipped with a technology that isolates the gas and the fluid (seawater) e.g. a bladder or movable piston as described above in connection with the pressure vessels.

Considering the example use case where the shuttle is transporting liquid 002, the source of liquefied buoyancy gas may be the cargo liquid itself. Typically, the amount of CO2 required for buoyancy control is negligible compared to the bulk volume under transportation. The benefit of this approach is that no additional gas source is required.

A plurality of bladders and/or rigid tanks as described above may be provided along the length and breadth of the vehicle to enable buoyancy and trimming of stability. The CO2 gas that has served its buoyant purpose may be released to the surrounding water, with negligible negative environmental consequences.

Although Figures 1 and 5 show a vehicle with a single internal pressure vessel, the underwater vehicle may include a plurality of pressure vessels. In some embodiments, the plurality of pressure vessels are arranged substantially parallel to each other, as shown in Figures 6A-6C. In this example, the pressure vessels 62 are arranged horizontally, in line with the axis of the outer hull 64. In other embodiments, the vessels are arranged vertically, or at an angle in between the horizontal and vertical. The vessels 62 are held in place by a plurality of supports 66 provided along the length of the vessels 62. Figure 60 illustrates the end of the pressure vessels at which the cargo fluid is loaded/unloaded. The pressure vessels 62 are mutually connected to a conduit system 68 for loading/unloading the fluid from the vessels 62.

In some embodiments, approximately half of the pressure vessels contain the fluid cargo at the front end, and the remaining half contain the fluid cargo at the rear end. In this way, the vehicle remains balanced. Further, such embodiments are suitable for management of the vehicle trim angle (the longitudinal axis being changed from horizontal, including all the way to a vertical or near to a vertical position).

The present invention can be used for storage and then the transportation of a variety of fluids for various applications. The underwater vehicle can be used to transport fluids to a well or from a well. Examples of suitable fluids are: CO2 (for injection into a well); chemical supply (MEG, Methanol etc); oil & gas; freshwater; toxic waste; separation of oil, gas, water and sand; septic/bio-mass; and rig fluids such as mud, brine, drill cuttings etc. In other words, the 'fluid' cargo throughout this specification may comprise a liquid with solids suspended or contained within the liquid.

The vehicle can further be used for energy storage and transportation: the internal volume can be filled with bio fuel such as ethanol; diesel, heli-fuel or ammonia; or the internal volume can be filled with a battery bank for temporary storage and/or transport of electrical energy. The internal volume could even be used to storage and transport of live seafood, e.g fish.

The general method described above is illustrated in Figure 7, and includes the steps of (Si) loading the one or more vessels with the fluid cargo; (S2) balancing the pressure of the internal volume with the pressure exterior to the underwater vehicle; and (S3) controlling the pressure of the fluid cargo based on a pressure outside of the internal vessels.

Examples of the invention are described below in the following numbered paragraphs: Paragraph 1: An underwater vehicle for transporting a fluid cargo, comprising: an outer hull defining an internal volume; a pressure communication channel between the internal volume and the exterior of the outer hull, arranged control the pressure in the internal volume based on the exterior pressure; one or more internal vessels for containing the fluid cargo; and a pressure compensation system arranged to control the pressure of the cargo based on a pressure outside of the interval vessels.

Paragraph 2: The underwater vehicle of paragraph 1, wherein the pressure compensation system comprises: within each internal vessel, a barrier for separating a first portion of the vessel from a second portion of the vessel, the barrier permitting pressure communication between the first and second portions.

Paragraph 3: The underwater vehicle of paragraph 2, wherein the first portion contains the fluid cargo and wherein the second portion contains seawater.

Paragraph 4: The underwater vehicle of paragraph 2 or 3, wherein the pressure compensation system is arranged to control the pressure of the fluid cargo based on the pressure within the internal volume Paragraph 5: The underwater vehicle of paragraph 4, wherein the pressure compensation system is arranged to maintain the pressure of the fluid cargo equal to or above the pressure within the internal volume.

Paragraph 6: The underwater vehicle of paragraph 2 or 3, wherein the pressure compensation system is arranged to control the pressure of the fluid cargo based on the pressure exterior to the outer hull.

Paragraph 7: The underwater vehicle of paragraph 6, wherein the pressure compensation system is arranged to maintain the pressure of the fluid cargo equal to or above the pressure exterior to the outer hull, and wherein the pressure compensation system further comprises: a second pressure communication channel between the second portion and the exterior of the outer hull.

Paragraph 8: The underwater vehicle of any of paragraph 4 to 7, wherein the pressure compensation system comprises: a pump unit for pumping seawater into the second portion of the vessel to create an overpressure in the vessel; and a relief unit for relieving pressure when the overpressure exceeds a maximum overpressure value and for increasing the pressure the when the underpressure exceeds a maximum underpressure value.

Paragraph 9: The underwater vehicle of any of paragraph 2 to 8, wherein the barrier comprises one of the following: a mechanical piston; a batching pig; an expandable balloon; or a layer of a third substance separating the fluid cargo and seawater.

Paragraph 10: The underwater vehicle of any preceding paragraph, wherein the cargo in each of the one or more internal vessels comprises one or more of the following: CO2; production fluids; injection fluids; MEG; methanol; freshwater; seawater; toxic waste; sand; septic/bio-mass; mud; brine; drill cuttings; and bio fuel.

Paragraph 11: The underwater vehicle of any preceding paragraph, wherein the internal volume is filled with seawater.

Paragraph 12: The underwater vehicle of any preceding paragraph, further comprising an evaporator for evaporating a liquefied gas, the evaporated gas arranged to fill at least one of: a top portion of the outer hull; an expandable bladder in the top portion of the outer hull; and a rigid tank in the top portion of the outer hull.

Paragraph 13: The underwater vehicle of paragraph 12, wherein the evaporated gas is selectively vented from the outer hull to decrease the buoyancy; or wherein the vehicle further comprises a compressor for compressing the evaporated gas into a liquefied state.

Paragraph 14: The underwater vehicle of paragraph 12 or 13, wherein the liquefied gas is the fluid cargo.

Paragraph 15: The underwater vehicle of any of paragraph 12 to 14, further comprising a tank for holding the liquefied gas.

Paragraph 16: The underwater vehicle of any preceding paragraph, wherein each of the one or more inner vessels has a substantially cylindrical shape, and wherein the outer hull has a hydrodynamic shape.

Paragraph 17: A method for transporting a fluid cargo in an underwater vehicle, the underwater vehicle having an outer hull and one or more internal vessels arranged within the outer hull, the outer hull defining an internal volume, the method comprising: loading the one or more vessels with the fluid cargo; balancing the pressure of the internal volume with the pressure exterior to the underwater vehicle; and controlling the pressure of the fluid cargo based on a pressure outside of the internal vessels.

Paragraph 18: The method of paragraph 17, wherein loading the one or more internal pressure vessels with the fluid cargo comprises: filling a first portion of the pressure vessel with the fluid cargo, wherein the first portion is separated from a second portion of the pressure vessel by a barrier, the barrier permitting pressure communication between the first and second portions.

Paragraph 19: The method of paragraph 18, further comprising: when the pressure of the internal volume changes, opening the second portion to the pressure of the internal volume, causing the moveable barrier to move to equalise the pressure of the fluid cargo in the first portion with the pressure of the internal volume.

Paragraph 20: The method of paragraph 18, further comprising: when the pressure external to the outer hull changes, opening the second portion to the pressure external to the outer hull, causing the moveable barrier to move to equalise the pressure of the fluid cargo in the first portion with the pressure external to the outer hull.

Paragraph 21: The method of paragraph 18, further comprising: pumping seawater into the second portion of the vessel to create an overpressure; relieving surplus pressure if the internal overpressure exceeds a maximum overpressure value; and increasing the pressure if the internal underpressure exceeds a maximum underpressure value.

Paragraph 22: The method of any preceding paragraph, further comprising: flooding an internal volume between the outer hull and the one or more vessels with seawater.

Paragraph 23:The method of any preceding paragraph, further comprising: evaporating a liquefied gas to fill at least one of: a top portion of the outer hull, an expandable bladder in the top portion of the outer hull; and a rigid tank in the top portion of the outer hull.

Paragraph 25: The method of paragraph 23, further comprising: venting the evaporated gas from the outer hull to decrease the buoyancy; or compressing the evaporated gas into a liquefied state to decrease the buoyancy.

Paragraph 25: The underwater vehicle of paragraph 23 or 24, wherein evaporating a liquefied gas comprises evaporating some of the cargo.

Referring to Figure 8, the underwater vehicle 802 transports fuel to a tanker 808 (surface vessel) for in-situ refuelling. The tanker or other vessel may be at rest or may be moving.

Typically, the vessel slows down from its normal transport speed but keeps moving during the refuelling operation. The tanker 808 comprises a refuelling hose 804, which is deployed from a reel, spool or other storage device on the tanker 808, to a water depth below the wave zone. The refuelling hose 804 is also referred to herein as fuel hose 804. The wave zone may be defined as the volume of water proximal to the surface of the water 810, which is affected by surface weather conditions. Below the wave zone, the effects of surface water conditions, such as wind and waves, are negligible. In some examples, the wave zone is approximately 100 metres in depth from the mean surface water level 810. The end of the refuelling hose, which terminates in the water, comprises a head 806. The head 806 may comprise a connecting means, stabilising means and a communicating means.

The underwater vehicle 802 can transport the fuel to the head of the refuelling hose 806. The underwater vehicle 802 comprises a connector. The connector may be built into the underwater vehicle 802, or, it may be disposed on the end of an arm 1208, which protrudes from the underwater vehicle. The arm can be withdrawn into a cavity of the underwater vehicle 802 during transit and can be extended in response to locating the head of the refuelling hose. For example, the arm can be extended when the position of the head at a distance smaller than the length of the arm from the underwater vehicle. In some examples, the arm is rigid. The arm is able to transport fuel from the underwater vehicle 802 to the refuelling hose 804 via the connector, when in connection with the connector of the refuelling hose.

The connecting means of the refuelling hose may comprise a mechanical connector, adapted in shape to form a hermetic seal with the underwater vehicle 802 such that fuel from the underwater vehicle 802 can be delivered to the moving tanker 808 via the refuelling hose 804. In some examples, the connecting means further comprises an electrical connector. The electrical connector and the connector of the underwater vehicle may comprise an inductor element for transferring electrical power. In another example, the electrical connector may comprise a male or female end, which is respectively configured to fit or receive a female or male end of the connector of the underwater vehicle 802. In this way, electrical power can be transferred from the tanker 808 to the underwater vehicle 802.

As the tanker 808 is moving, the effect of water resistance causes the head of the refuelling hose 806 to trail behind the tanker 808 in a substantially horizontal fashion, as shown in Figure 8. In some examples, the head of the refuelling hose 806 comprises a "shuttlecock-shaped" end, which increases water resistance and therefore ensures that this substantially horizontal configuration is set up. Substantially horizontal means the head of the refuelling hose 806 is disposed at an angle of less than 10 degrees relative to the water surface 810, more preferably 5 degrees. This horizontal configuration ensures that the connecting means of the refuelling hose can easily fit or receive the corresponding connector on the underwater vehicle. As the skilled reader would appreciate, there are a number of different shapes similar to a shuttlecock, such as parachutes and funnels, which generate the same intended effect. All of these variations are within the knowledge of the skilled reader.

In some examples, one or more weights 902 are attached to the refuelling hose 804. As described above, it is preferable that the head of the refuelling hose 806 is in a substantially horizontal configuration as this simplifies the geometry of the connecting process. Figure 9A shows a refuelling hose 804 without a weight. The refuelling hose 804 is attached to a moving tanker 808. If the velocity or the drag on the refuelling hose 804, which acts, to a first approximation, in a horizontal direction, is not large enough, the head of the refuelling hose 806 may not reach the horizontal, as depicted in Figure 8. However, as shown in Figure 9B, if a weight 902 is added to the refuelling hose 804, the shape of refuelling hose in the water can be modified. In an example, the weight is added to the midpoint of the refuelling hose.

In some examples, one or more weights 902 can be added at one or more different points on the refuelling hose 804. For the same drag, the end of the refuelling hose 806 can reach closer to the horizontal because the refuelling hose 804 is able to bend around the point in which the weight is disposed.

In some examples, the "shuttlecock-shaped" end may be the stabilising means. In some examples, the stabilising means of the refuelling hose further comprises one or more elements 1002, 1104. The one or more elements 1002, 1104 may act as a steering means to stabilise the head of the refuelling hose 806. In some examples, the head of the refuelling hose 806 comprises two wing elements 1002. Referring to Figures 10A to 100, exemplary wing-shaped steering elements 1002 are shown in different configurations and views. In Figure 9A, a projected view of the head of the refuelling hose 806 and two wing elements 1002 is shown along the direction of water drag. In Figure 9B, a side-view of the head of the refuelling hose 806 and wing elements 1002 are shown. The direction of water drag is denoted by the arrow. Turning to Figure 10C and 10D, two configurations of the wing elements 1002 are shown. In Figure 10C, the wing 1002 is in a "negative y" configuration. In the "negative y" configuration, both the wing elements 1002 are inclined at a "negative" angle to the direction of water drag, thereby causing a net force on the wing element 1002 in the negative y direction. Similarly, in the "positive y" configuration (shown in Figure 100), the wing element 1002 is in a "positive y" configuration. In the "positive y" configuration, both the wing elements 1002 are inclined at a "positive" angle to the direction of water drag, thereby causing a net force on the wing element 1002 in the positive y direction. In this way, by rotating the wings along their longitudinal axis, the position of the head of the refuelling hose 806 can be controlled in the up and down sense. Specifically referring back to Figures 10C and 10D, the wings 1002 are respectively rotated in an anticlockwise and clockwise fashion along the longitudinal axis of the wing element. In some examples, the wing elements 1002 may not be configured to rotate but are fixed instead. In such examples, the water, which flows above and below the wing elements 1002 as the tanker moves, acts to increase the rotational inertia of the refuelling hose 804, thereby stabilising it. Accordingly, in the un-rotated configuration shown in Figure 10B, the wing elements 1002 act to stabilise the head of the refuelling hose 806, which aids the connection process. In addition, the position of the head 806 may be controlled by unreeling the refuelling hose 804 from the surface vessel 808.

In some examples, the head of the refuelling hose 806 comprises four wing elements 1002, 1104. Referring to Figures 11A to 110, exemplary wing-shaped steering elements 1002, 1104 are shown in different configurations and views. In Figure 11A, a projected view of the head of the refuelling hose 806 and four wing elements 1002, 1104 are shown along the direction of water drag. In Figure 11B, a topside view of the head of the refuelling hose 806 and wing elements 1104 is shown. The direction of water drag is denoted by the arrow. Turning to Figure 11C and 110, two configurations of the wing elements 1104 are shown. In Figure 11C, the wing 1104 is in a "positive x" configuration. In the "positive x" configuration, both the wing elements 1104 are inclined at a "positive" angle to the direction of water drag, thereby causing a net force on the wing element 1104 to move in the positive x direction.

Similarly, in the "negative x" configuration (shown in Figure 110), both the wing elements 1104 are inclined at a "negative" angle to the direction of water drag, thereby causing a net force on the wing element in the negative x direction. In this way, by rotating the wings along their longitudinal axis, the position of the head of the refuelling hose 806 can be controlled in the x direction. Specifically referring back to Figures 11C and 11D, the wings 1104 are respectively rotated in an anticlockwise and clockwise fashion along the longitudinal axis of the wing element. The other two wing elements 1002 can be used to control the position of the head of the refuelling hose 806 in the y direction, thereby facilitating greater control of the refuelling hose 804. In addition, the position of the head 806 may be controlled by unreeling the refuelling hose 804 from the surface vessel 808.

In an example, the actuation of the steering elements is powered by power lines connected to the surface vessel. The controller may also be located on the surface vessel.

In other examples, the head of the refuelling hose 806 may comprise one or more propulsion elements. The propulsion elements may be configured to control the position of the head. The propulsion elements may be propellers, thrusters or other elements known to the skilled person and may be controlled from the vessel via an umbilical or wireless communication means. In some examples, the head of the refuelling hose 806 may comprise one or more buoyancy elements.

The communicating means between the head of the refuelling hose and the subsea vehicle may comprise an acoustic receiver and/or transmitter. The acoustic system can be either active or passive. In an active acoustic system, an acoustic transmitter generates a sound, which in turn, may cause an echo. The echo can be analysed to determine the position of the object that caused the echo. In this way, the head of the refuelling hose 806 may comprise an acoustic receiver and acoustic transmitter to locate the connector of the underwater vehicle 802. In addition, the connector of the underwater vehicle 802 may also comprise an acoustic receiver and/or transmitter. Therefore, the underwater vehicle may also be configurable to locate the connector of the refuelling hose 804 in the manner described above. In such a system, the connector of the underwater vehicle 802 and the acoustic receiver may further be communicatively coupled. In an example, they may be communicatively coupled by wireless communication means. By communicating between the two acoustic systems, the relative position of each connector can be more finely tuned.

For example, the relative positions of each result can be compared to generate a moving average. In some examples, the acoustic transmitters and receivers may comprise an array, centred on the connector of the refuelling hose or connector of the underwater vehicle, in order that the results can be triangulated to improve accuracy. In other examples, the acoustic system may be passive. In a passive acoustic system, the connecting means may comprise a transmitter or a receiver and the connector of the underwater vehicle comprises a corresponding receiver or transmitter. In such examples, a transmitter emits a sound from the head of the refuelling hose 806, which is detected by the receiver. Generally, the "stationary" object comprises a transmitter and the component comprising the receiver element moves relative to that object. Therefore, either the connector of the underwater vehicle can be actively positioned onto the connecting means of refuelling hose, or, the connecting means of the refuelling hose can be actively positioned onto the connector of the underwater vehicle.

In some examples, the underwater vehicle 802 will have a large inertia. Accordingly, fine positional control of the underwater vehicle 802 may be difficult to achieve in practice. For this reason, the underwater vehicle 802 may be used to position the fuel proximal to the connecting means of the refuelling hose 806. In some examples, proximal means within a ten-metre radius of the connecting means of the refuelling hose 806, more preferably 1 metre. In other examples, it may include positioning the underwater vehicle within the range of the arm 1208, or the range of motion of the refuelling hose. Thereafter, either the position of the arm 1208, from which the connector of the underwater vehicle is disposed, or, the position of the head of refuelling hose 806 is adjusted so that the connectors can engage to form a seal. The method of refuelling the tanker 808 may therefore comprise two stages: i) positioning of the underwater vehicle 802 into range of the arm 1208 of the underwater vehicle 802 and/or range of the head of the refuelling hose 806; and ii) fine positioning of the arm 1208 of the underwater vehicle 802 or fine positioning of the head of the refuelling hose 806. In some examples, the connector of the underwater vehicle 802 and the connector of the refuelling hose 806 may each comprise a magnetic element. The magnetic elements of each connector may be configured to attract to ensure the connectors align and engage correctly.

In some examples, the underwater vehicle 802 and the head of the refuelling hose 806 may be connected to a surface buoy via a line. The line is preferably slack to decouple subsea components from the wave zone. Each surface buoy may include one or more positioning devices, such as a GPS tracker, to monitor the position of the surface buoys. As a first approximation, the respective buoys can be used to determine the location of the underwater vehicle and the head of the refuelling hose. However, as the line is slack, the buoys may not be directly above the underwater vehicle 802 or head of the refuelling hose 806 from which they are connected. The buoys may therefore serve as an indicator of the position, rather than denoting an accurate location.

In some examples, the communicating means may comprise an optical system, comprising an optical transmitter and/or receiver in an analogous way to the acoustic system described above.

After the connectors are engaged to form a seal, the seal can be locked into place using a locking mechanism. After the locking mechanism is activated, a valve separating the fuel in the underwater vehicle 802 from the arm 1208 may be opened. In some examples, the fuel may be stored under high pressure and the pressure difference between the refuelling hose 804 and internal vessel of the underwater vehicle urges the fuel, from the internal vessel of the underwater vehicle, through the arm 1208 via the connection, into the refuelling hose 804 and finally into the tanker 808. However, in some examples, the overpressure may not be large enough to urge the fuel into the tanker 808 because the refuelling hose 804 is at a reasonable depth (to avoid the wave zone). Therefore, preferably, although not necessarily, one or more pumps are used to deliver the fuel from the underwater vehicle 802 to the tanker 808.

In some examples, the one or more pumps are located within the refuelling hose 804. Power cables may connect the pump directly to a power source on the tanker 808. The pumps may be disposed in series along the refuelling hose 804 to ensure rapid refuelling. In other examples, a pump may be located in the underwater vehicle 802. In such examples, the pump may be powered by a local energy storage unit on the underwater vehicle 802, or, by a power source on the tanker 808. The local energy storage unit may comprise a hydrogen fuel cell, battery or a nuclear power source. For a battery, the connecting means of the refuelling hose 806 and the connector of the underwater vehicle 802 may comprise an inductor element. Turning to Figure 12, the inductor elements may comprise a coil 1202, 1206 and the plane of each coil may be adjacent to the connection/seal between each connector to ensure efficient energy transfer. The coil 1202, 1206 may be located in an annulus 806, 1212 of the refuelling hose and arm 1208. Power cables 1204, 1210 for connecting the power source on the tanker 808 and the local energy storage unit on the underwater vehicle may also be located in the annulus 806, 1212 of the refuelling hose and arm 1208. In this way, power can be delivered to the local energy storage unit for recharging, or, directly to the pump.

In some examples, the fuel is ammonia and stored in a liquid state at a pressure of around 10 bar. The fuel may also be other types of fuel, including biofuels, liquid oxygen or liquefied petroleum gas, as would be appreciated by the skilled reader.

Preferably, but not necessarily, refuelling occurs just below the wave zone. That is, at a water depth of approximately 100 metres. At this depth, the temperature and pressure conditions are that ammonia may be in a liquid state.

After the refuelling process is complete, the locking mechanism is unlocked and the connectors are disengaged. The refuelling hose 804 may then be reeled back onto the reel on the tanker 808. In some examples, the valve, separating the internal vessel containing the fuel from the refuelling hose 804, may be closed before the pump is stopped to ensure that any fuel inside the refuelling hose 804 has been delivered to the tanker 808 before disengagement.

In a different example, the surface vessel does not release a refuelling hose, but instead the underwater vehicle releases a refuelling hose with a positive buoyancy element in the vicinity of the surface vessel. The refuelling hose is also referred to herein as fuel hose. The vicinity of the surface vessel means within a distance between the surface vessel (or tanker) which is less than the length of the refuelling hose. Preferably, but not necessarily, the refuelling hose is released along a shipping lane so that the surface vessel does not need to deviate substantially from its path. The refuelling hose may be stored in a cavity of the underwater vehicle and can be released when it is detected that the underwater vehicle is in the vicinity of the surface vessel. The positive buoyancy element will bring the head of the hose to the water surface where it can be located and picked up by the surface vessel and connected to the surface vessel for refuelling. This arrangement does not require any specialised equipment at the surface vessel, but equipment conventionally present at the surface vessel can be used in this operation. The positive buoyancy element may include an acoustic or optical beacon or simply a brightly coloured buoy to facilitate locating the element at the water surface.

Referring to Figure 13, the underwater vehicle 802 may be refilled with fuel at a designated refilling station 1302. The refilling station 1302 is preferably located on the seabed in shallow water. Shallow water means water to a depth of less than 500 metres, preferably less than 300 metres. The refilling station 1302 comprises one or more fuel storage tanks for refilling the underwater vehicle 802. The refilling station may also be connected to an onshore storage or production facility with pipelines. In some examples, the same connector used in the refuelling process of the tanker 808 may be used with the one or more storage tanks during refilling.

If the water depth is much greater 500 metres, the refilling station 1302 may be located on a floating platform 1310, which is anchored to the seabed using one or more wires 1306. The floating platform 1310 may comprise one or more buoyancy elements 1304 configured to ensure that the platform 1310 and the one or more storage tanks 1308 have a combined positive buoyancy. The floating platform 1310 may comprise a platform where one or more underwater vehicles 802 can park for refilling. The depth of the platform where the underwater vehicles park is preferably less than 500 metres.

The general method described above is illustrated in Figure 14, and includes the steps of (S1402) positioning the surface vessel and the unmanned underwater vehicle relative to each other at a distance smaller than the length of a fuel hose; (S1404) connecting the fuel hose between the unmanned underwater vehicle and the surface vessel; (S1406) transferring the fuel from the unmanned underwater vehicle to the surface vessel; and (S1408) releasing the fuel hose.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure, which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: 1. A method of refuelling a surface vessel from an unmanned underwater vehicle, wherein the underwater vehicle comprises one or more internal vessels for containing fuel, the method comprising: positioning the surface vessel and the unmanned underwater vehicle relative to each other at a distance smaller than the length of a fuel hose; connecting the fuel hose between the unmanned underwater vehicle and the surface vessel; transferring the fuel from the unmanned underwater vehicle to the surface vessel-and releasing the fuel hose.
2. The method according to claim 1, wherein the surface vessel and the unmanned underwater vehicle are moving.
3. The method according to any preceding claim, wherein the fuel hose is released from the surface vessel before connecting to the unmanned underwater vehicle.
4. The method according to claim 3, wherein the fuel hose is stored on a spool on the surface vessel and wherein releasing the fuel hose comprises unreeling the fuel hose from the spool.
5. The method according to any of claims 3 to 4, wherein one or more weights is disposed at one or more points on the fuel hose.
6. The method according to any of claims 3 to 5, wherein the fuel hose terminates in a head and the head comprises a connector.
7. The method according to claim 6, wherein the head further comprises one or more of: a stabiliser; one or more steering elements; and one or more communication elements.
8. The method according to claim 7, wherein the stabiliser has a conical shape and the vertex of the cone points in the opposite direction to the end of the fuel hose.
9. The method according to claim 7, wherein the one or more steering elements are wing-shaped.
10. The method according to claim 9, wherein the one or more steering elements are rotatable about their longitudinal axes.
11. The method according to claim 7, wherein the one or more communication elements comprise one or more acoustic transmitters and/or one or more acoustic receivers.
12. The method according to claim 7, wherein the one or more communication elements comprise one or more optical beacons and/or one or more optical sensors.
13. The method according to claim 7, wherein the one or more communication elements further comprise an umbilical or wireless communication receiver and transmitter.
14. The method according to any preceding claim, wherein connecting the fuel hose between the unmanned underwater vehicle and the surface vessel comprises: locating a connector of the unmanned underwater vehicle using the communication elements; and positioning the head of the fuel hose by unreeling the fuel hose from the spool and rotating the one or more steering elements.
15. The method according to claim 14, wherein the connector of the unmanned underwater vehicle is disposed on the end of an arm, which protrudes from the underwater vehicle.
16. The method according to claims 1 or 2, wherein the fuel hose is released from the unmanned underwater vehicle before connecting to the surface vessel.
17. The method according to claim 16, wherein the fuel hose comprises one or more positive buoyancy elements
18. The method according to claim 17, wherein at least part of the fuel hose floats to the water surface and wherein connecting the fuel hose between the unmanned underwater vehicle and the surface vessel comprises locating the fuel hose and connecting the fuel hose to a connector for refuelling the surface vessel
19. The method according to claim 18, wherein the buoyancy elements comprise one or more optical or acoustic beacons and locating the fuel hose comprises locating the optical or acoustic beacon.
20. The method according to any preceding claim, wherein the unmanned underwater vehicle comprises: an outer hull defining an internal volume; a pressure communication channel between the internal volume and the exterior of the outer hull, arranged control the pressure in the internal volume based on the exterior pressure; said one or more internal vessels for containing the fuel; and a pressure compensation system arranged to control the pressure of the fuel based on a pressure outside of the interval vessels.
21. The method according to claim 20, wherein the pressure compensation system comprises: within each internal vessel, a barrier for separating a first portion of the vessel from a second portion of the vessel, the barrier permitting pressure communication between the first and second portions.
22. The method according to claim 21, wherein the first portion contains the fuel and wherein the second portion contains seawater.
23. The method according to claim 21 or 22, wherein the pressure compensation system is arranged to control the pressure of the fuel based on the pressure within the internal volume
24 The method according to claim 23, wherein the pressure compensation system is arranged to maintain the pressure of the fuel equal to or above the pressure within the internal volume.
25. The method according to claim 21 or 22, wherein the pressure compensation system is arranged to control the pressure of the fuel based on the pressure exterior to the outer hull.
26. The method according to claim 25, wherein the pressure compensation system is arranged to maintain the pressure of the fuel equal to or above the pressure exterior to the outer hull, and wherein the pressure compensation system further comprises: a second pressure communication channel between the second portion and the exterior of the outer hull.
27. The method according to any of claims 20 to 26, wherein the barrier comprises one of the following: a mechanical piston; a batching pig; an expandable balloon; or a layer of a third substance separating the fuel and seawater.
28. The method according to any preceding claim, wherein the fuel is ammonia.
29. A fuel hose comprising: a hose configured to transfer fuel from an unmanned underwater vehicle to a surface vessel, wherein the hose terminates at an end, which defines a head; and a connector, disposed at the head of the fuel hose, configured to connect between the unmanned underwater vehicle and the surface vessel.
30. The fuel hose according to claim 29, wherein the head of the fuel hose comprises a stabiliser, wherein the stabiliser has a conical shape and the vertex of the cone points in the opposite direction to the end of the fuel hose.
31. The fuel hose according to claims 29 or 30, wherein the head of the fuel hose comprises one or more steering elements are wing-shaped and rotatable about their longitudinal axes.
32. The fuel hose according to claims 29 to 31, wherein the fuel hose is stored on a spool on the surface vessel.
33. The fuel hose according to claims 29 to 32, wherein the head of the fuel hose comprises one or more acoustic transmitters and/or receivers for locating a corresponding connector on the underwater vehicle.
34. The fuel hose according to claims 29 to 32, wherein the head of the fuel hose comprises one or more optical transmitters and/or receivers for locating a corresponding connector on the underwater vehicle.
35. The fuel hose according to any of claims 29 to 34, wherein the connector of the fuel hose is adapted in shape to receive or fit the corresponding connector on the underwater vehicle.
36. The fuel hose according to claim 29, wherein the fuel hose is stored in a cavity of the underwater vehicle.
37. The fuel hose according to claim 36, wherein the head of the fuel hose comprises one or more buoyancy elements and the fuel hose, when released from the cavity of the underwater vehicle, floats to the water surface.
38. The fuel hose according to claim 37, wherein the buoyancy elements comprise one or more optical or acoustic beacons.
39. An unmanned underwater vehicle comprising: an outer hull defining an internal volume; a pressure communication channel between the internal volume and the exterior of the outer hull, arranged control the pressure in the internal volume based on the exterior pressure; one or more internal vessels for containing the fuel; a pressure compensation system arranged to control the pressure of the fuel based on a pressure outside of the interval vessels; an arm protruding from the outer hull and suitable for coupling to the connector of the fuel hose according to claim 29.