WO2021235940A1

WO2021235940A1 - Underwater vehicle for transporting cargo

Info

Publication number: WO2021235940A1
Application number: PCT/NO2021/050114
Authority: WO
Inventors: Kjell E ELLINGSEN; Ola Ravndal; Erling MYHRE; Lorents REINÅS
Original assignee: Equinor Energy As
Priority date: 2020-05-22
Filing date: 2021-05-05
Publication date: 2021-11-25
Also published as: GB2585758A; WO2021235945A1; GB2595321B; GB2595321A; GB202018077D0; GB202007678D0; GB2585758B

Abstract

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.

Description

Underwater Vehicle for Transporting Cargo

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 gas, liquid or objects.

Background

In conventional subsea field developments, produced fluids are transported from a well, through a pipeline and up a riser to an offshore installation, e.g a floating production unit (FPU) or floating production and storage unit (FPSU), or to an onshore facility. At the offshore installation the fluids are offloaded to a tanker (surface vessel). In the case of injecting carbon dioxide (C02) into a well, the direction of flow is reversed, so that the C02 flows from the tanker to the FPU, down the riser and through the subsea pipelines to the well. In either case, these offloading processes are weather-dependent and cannot be performed in severe weather conditions, such as strong winds and large waves.

The following publication discloses an alternative system of transport based on subsea shuttles: Research Disclosure database number 662093, published digitally on 20 May 2019. The subsea shuttle is intended to operate submerged under the sea surface, meaning it can operate under any type of weather conditions. The inventors have realised shortcomings in the technology described in this publication, in particular related to the storage of fluids within the shuttle, and have developed improvements as described in more detail below.

Statement of invention

According to a first aspect of the invention there is provided 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. Within each internal vessel, the pressure compensation system may comprise 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.

The first portion may contain the fluid cargo and the second portion may contain seawater.

The pressure compensation system may be arranged to control the pressure of the fluid cargo based on the pressure within the internal volume. Optionally, the pressure compensation system is arranged to maintain the pressure of the fluid cargo equal to or above the pressure within the internal volume.

Alternatively, the pressure compensation system may be arranged to control the pressure of the fluid cargo based on the pressure exterior to the outer hull. The pressure compensation system may be arranged to maintain the pressure of the fluid cargo equal to or above the pressure exterior to the outer hull, and the pressure compensation system may further comprise a second pressure communication channel between the second portion and the exterior of the outer hull.

The pressure compensation system may comprise 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.

The barrier may comprise 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.

The cargo in each of the one or more internal vessels may comprise one or more of the following: C02; production fluids; injection fluids; MEG; methanol; freshwater; seawater; toxic waste; sand; septic/bio-mass; mud; brine; drill cuttings; and bio fuel.

The internal volume may be filled with seawater. The underwater vehicle may further comprise 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. Optionally, the evaporated gas may be selectively vented from the outer hull to decrease the buoyancy; or wherein the vehicle may further comprise a compressor for compressing the evaporated gas into a liquefied state.

The liquefied gas may be the fluid cargo itself.

The underwater vehicle may further comprise a tank for holding the liquefied gas.

Each of the one or more inner vessels may have a substantially cylindrical shape and the outer hull may have a hydrodynamic shape.

According to a second aspect of the invention there is provided 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.

Loading the one or more internal pressure vessels with the fluid cargo may comprise: 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.

The method may further comprise: 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.

Alternatively, the method may further comprise: 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.

The method may further comprise: 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.

The method may further comprise flooding an internal volume between the outer hull and the one or more vessels with seawater.

The method may further comprise 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. Optionally, the method may further comprise 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.

Evaporating a liquefied gas may comprise evaporating some of the cargo.

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 2B 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.

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 2B, 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 2B. 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 T1 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 C02, 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 C02 (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 C02 is then in the liquid phase for typical subsea temperatures e.g. between 0 and 10 °C. Transport of C02 in liquid state is beneficial as a larger mass of C02 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 C02 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 C02 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 C02 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 C02 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 3B. 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, C02, 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 C02, the source of liquefied buoyancy gas may be the cargo liquid itself. Typically, the amount of C02 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 C02 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 6C 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 (S1) 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.

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. 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.

2. The underwater vehicle of claim 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.

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

4. The underwater vehicle of claim 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.

5. The underwater vehicle of claim 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.

6. The underwater vehicle of claim 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.

7. The underwater vehicle of claim 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.

8. The underwater vehicle of any of claims 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.

9. The underwater vehicle of any of claims 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.

10. The underwater vehicle of any preceding claim, wherein the cargo in each of the one or more internal vessels comprises one or more of the following:

C02; production fluids; injection fluids; MEG; methanol; freshwater; seawater; toxic waste; sand; septic/bio-mass; mud; brine; drill cuttings; and bio fuel.

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

12. The underwater vehicle of any preceding claim, 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.

13. The underwater vehicle of claim 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.

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

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

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

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.

18. The method of claim 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.

19. The method of claim 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.

20. The method of claim 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.

21 . The method of claim 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.

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

23. The method of any preceding claim, 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.

24. The method of claim 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.

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