Field of the invention

The present disclosure relates to a method of offloading cargo fluid from an unmanned subsea vehicle, and in particular to controlling the buoyancy of the unmanned subsea vehicle during or after the offloading.

Background

In recent years, it has been proposed that underwater vehicles can be used to transport fluid cargo. Research Disclosure 662093 (published 20 May 2019) describes a subsea shuttle system, using autonomous subsea shuttles for transportation and storage purposes. Research Disclosure 677082 (published 21 August 2020) provides further detail regarding possible shuttle structure and support, applications, e.g., on/offloading of a payload, and the propulsion system of the subsea shuttle. Typical underwater vehicles for this purpose include a cargo tank and a buoyancy system for controlling buoyancy. Conventional buoyancy systems work by controlling the density of the underwater vehicle by pumping water or air from gas canisters into ballast tanks.

Statement of the Invention

According to a first aspect of the invention, there is provided a method for offloading cargo fluid from a container in an underwater vehicle, the method comprising: displacing cargo fluid from the container with a backfill liquid at a first pressure; depressurising the backfill liquid to a second pressure, wherein the second pressure is lower than the first pressure; and discharging at least part of the backfill liquid from the container.

Optionally, the at least part of the backfill liquid discharged from the container comprises a volume, which is less than, or equal to the volume of cargo fluid in the container before offloading multiplied by: one minus the ratio between the density of the cargo fluid and the density of the backfill liquid, whereby, discharging the volume of backfill liquid from the container reduces, or eliminates the net buoyancy change after offloading. In an example, the backfill fluid is denser than the cargo fluid and there is a net, negative buoyancy change during cargo fluid offloading, which can be reduced or eliminated by discharging the volume (defined above) of backfill fluid from the container.

The second pressure may be the ambient water pressure immediately surrounding the underwater vehicle.

The second pressure may be less than, or equal to the vapour pressure of the backfill liquid at ambient temperature.

Optionally, displacing cargo fluid from the container with backfill liquid comprises removing substantially all of the cargo fluid from the container and replacing the cargo fluid with the backfill liquid.

Optionally, displacing cargo fluid from the container with backfill liquid comprises removing a volume fraction of the cargo fluid from the container and replacing the volume fraction of cargo fluid in the container with the backfill liquid, thereby retaining a volume of cargo fluid in the container.

The volume of cargo fluid retained in the container may be less than, or equal to

P wherein, is the pressure of the backfill liquid in the container; V ₂ is the volume of backfill liquid discharged from the container; p ₃ is the pressure of the volume of cargo fluid retained in the container. Thus, discharging the at least part of backfill liquid from the container may comprise displacing the volume of backfill liquid discharged from the container by vaporisation and/or expansion of the volume of cargo fluid retained in the container.

Optionally, removing cargo fluid from the container may comprise fluidly connecting one or more pumps to the cargo fluid in the container; and pumping the cargo fluid from the container using the one or more pumps.

Optionally, replacing the cargo fluid with backfill liquid may comprise fluidly connecting a backfill liquid source to the container via one or more pumps; and pumping the backfill liquid from the source to the container at a third pressure. The third pressure may be substantially equal, or equal to the first pressure. Alternatively, discharging the at least part of backfill liquid from the container may comprise displacing the volume of backfill liquid discharged from the container by vaporisation of the backfill liquid. The backfill liquid may be vaporised by heating the at least part of backfill liquid discharged from the container using one or more heating elements and the method may further comprise feeding the resulting vaporised backfill liquid back into the container. The backfill liquid may also vaporise in the container via the pressure drop produced by the one or more pumps.

Optionally, discharging the at least part of backfill liquid from the container may comprise fluidly connecting one or more pumps to the backfill liquid in the container; and pumping the backfill liquid from the container using the one or more pumps.

Optionally, discharging the at least part of backfill liquid from the container may comprise fluidly connecting water, immediately surrounding the underwater vehicle, to the backfill liquid in the container, wherein the ambient water pressure is less than the third pressure.

The container may comprise a moveable barrier, which defines a cargo side and a backfill side of the container.

Preferably, but not necessarily, the cargo fluid is arranged on the cargo side of the container and backfill liquid is arranged on the backfill side of the container, such that the cargo fluid and backfill liquid are separated by the moveable barrier and, wherein the cargo fluid and backfill fluid are in pressure communication with another via the moveable barrier.

The moveable barrier may be any one of a piston, a batching pig, a membrane or an expandable balloon.

The method for offloading may further comprise feeding back vaporised backfill liquid into the backfill side of the container via a return line, for example, produced by heating the at least part of backfill liquid discharged from the container using the one or more heating elements. In some examples, the moveable barrier may move towards the backfill side of the container as the volume of cargo fluid retained in the container vaporises and/or expands.

The cargo side of the container may be closed after the step of displacing cargo fluid from the container.

The method may comprise a plurality of containers, which are operable to be fluidly connected with one another in series or in parallel. Alternatively, the plurality of containers may be fluidly isolated from one another.

The cargo fluid may comprise any one of a hydrocarbon, ammonia, carbon dioxide or hydrogen.

The backfill liquid may comprise any one of seawater, water, or liquid carbon dioxide.

The method may further comprise displacing substantially all fluids, retained in the container after the method of offloading, from the container with cargo fluid at a fourth pressure; and pressurising the cargo fluid to the first pressure by fluidly connecting one or more pumps to the backfill liquid source and pumping the backfill liquid into the container using the one or more pumps, wherein the fourth pressure is less than the first pressure, and wherein the cargo fluid comprises a gas or a gas and liquid mixture. In this way, cargo fluid may be loaded (or restored) into the container and the method for offloading set out in detail above may be repeated.

Alternatively, the method may further comprise displacing at least part of the fluids, retained in the container after the method of offloading, from the container with cargo fluid at the first pressure, such that cargo fluid is loaded (or restored) into the container and the method for offloading set out in detail above may be repeated.

According to a second aspect of the invention, there is provided an underwater vehicle configured to perform the method for offloading as set out above, the underwater vehicle comprising: one or more containers, the one or more containers each comprising at least one fluid inlet, at least one fluid outlet; and one or more pumps. Brief Description of the Drawings

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 vertical cross section through a schematic illustration of an underwater vehicle.

Figure 2 is a vertical cross section through a schematic illustration of an underwater vehicle.

Figure 3 is a cross section through a schematic illustration of a cargo tank during different stages of a cargo offloading process

Figure 4 is a schematic diagram of a pressure-temperature phase diagram.

Figure 5 is a cross section through a schematic illustration of a cargo tank.

Figure 6 is a cross section through a schematic illustration of a cargo tank during different stages of a cargo offloading process.

Figure 7 is a cross section through a schematic illustration of a plurality of cargo tanks during different stages of a cargo offloading process.

Figure 8 is a method diagram.

Detailed Description of Preferred Embodiments

FIG. 1 shows a schematic illustration of an underwater vehicle 100. 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, with a hydrodynamic shape to reduce drag. The shape of the outer hull in FIG. 1 should not be viewed as limiting. The underwater vehicle 100 comprises compartments 102, 104 located at the bow and stern of the vehicle for storing machinery (e.g., a drive system) for operating the underwater vehicle 100. The central portion of the underwater vehicle 106 comprises a cargo assembly 108. The cargo assembly 108 comprises one or more cargo tanks arranged on a frame or rack and configured to contain a cargo fluid. The cargo tanks 110 may be arranged either vertically or horizontally with respect to the main longitudinal axis of the shuttle. In some examples, the underwater vehicle nominal cargo capacity is around 15,000,000 litres. As the skilled reader would appreciate, larger or smaller volumes are possible. This disclosure sets out a number of illustrative examples, which refer to a cargo tank 110 with a volume capacity of 10,000 litres for simplification, but this cargo tank volume capacity should not be viewed as limiting. The underwater vehicle 100 may further comprise one or more air tanks, one or more forward and aft trim tanks and one or more compartments for storing batteries. One or more pumps may be used to admit or discharge fluid (e.g., water or air) from the trim tanks to control buoyancy.

In some examples, the cargo fluid in the cargo tanks 110 is a fuel, for example, liquid or pressurised gaseous ammonia, a hydrocarbon or hydrogen. However, the cargo fluid may be liquid carbon dioxide, pressurised gaseous carbon dioxide, or any other fluid, as the skilled reader would envisage. In some examples, carbon dioxide is present in the cargo tank 110 at a pressure greater than the saturation pressure for carbon dioxide. In other examples, the fuel is present in the cargo tank 110 at a pressure substantially equal to fuel source pressure, e.g., wellhead pressure.

Referring to FIG. 2, the cargo tanks 110 in the underwater vehicle 100 may comprise a barrier 112, 114, which defines two, spatially separated volumes 116, 118 (e.g., a cargo side and a backfill side) in the cargo tank. The barrier 112, 114 is moveable such that fluids contained in respective volumes 116, 118 are in pressure communication with one another. The barrier may comprise a piston 112, a batching pig, a membrane 114 or an expandable balloon. The membrane 114 may be affixed to the cargo tank wall and expandable, such that it can inflated to fill the entire volume of the cargo tank 110. The expandable balloon may be disposed within the cargo tank 110, defining a volume enclosed by the balloon for fluid and a volume not enclosed by the balloon, but contained by the cargo tank 110, for containing fluid.

Turning to FIG. 3, a schematic illustration of a cargo fluid offloading process from a cargo tank 110 is shown. In an initial state 301 , the cargo tank 110 comprises a cargo fluid 304 on a cargo side 312 of the cargo tank and a backfill fluid 306 on a backfill side 314 of the cargo tank 110 at an initial pressure, Pi. The two volumes 312, 314 are spatially separated (but in pressure communication) via a barrier 310. The barrier 310 may be any one of the barriers previously described. In practice, the volume of backfill fluid 306 in the initial state is low (i.e., the backfill side 314 of the cargo tank 110 is initially empty), but as the skilled person would appreciate, the initial volume of backfill fluid 306 is arbitrary. In a first step 303, the cargo fluid 304 is removed from the cargo side 312 of the cargo tank 110 and replaced by the backfill fluid 306, which is introduced into the backfill side 314 of the cargo tank 110. Preferably, but not necessarily, the pressure of the backfill fluid 306 is the same, or close to, the initial pressure (Pi) of the cargo fluid 304 in order to maintain the pressure on the cargo side 312 of the cargo tank 110. As cargo fluid 304 is removed from the cargo tank, the barrier 310 separating the cargo side 312 from the backfill side 314 moves.

After all or substantially all of the cargo fluid 304 is removed (state 305 in FIG. 3) from the cargo tank 110, the cargo side 312 of the cargo tank 110 is (approximately) empty and the backfill side 314 of the cargo tank 110 is full, comprising the backfill liquid 106 at a third pressure P3. Preferably, but not necessarily, the third pressure P3 is substantially equal to the initial pressure, Pi.

Typically, the backfill fluid 306 and the cargo fluid 304 have a different density. Therefore, displacement of the cargo fluid 304 with backfill fluid 306 leads to a net change in weight of the cargo tank 110, whereas the volume of the cargo tank 110 remains the same. Hence, cargo fluid offloading leads to a net change in buoyancy. In practice, the backfill fluid (e.g., seawater) is denser than the cargo fluid (e.g., hydrocarbon), which leads to a net, negative change in buoyancy during cargo fluid offloading. In conventional underwater vehicles, the change in buoyancy is accommodated using a ballast system; however, for large cargo tanks, the change in buoyancy is correspondingly large and complex ballast systems are required. It is therefore desirable to reduce the net change in buoyancy during cargo fluid offloading to reduce the burden on these systems.

Referring back to FIG. 3, in a second step 307, the backfill fluid is depressurised to a second pressure, P2 _, wherein the second pressure is below or equal to the vapour pressure of the backfill fluid 306 at ambient temperature. In the examples described herein, the backfill fluid 306 is a liquid, which, to a first approximation, is incompressible. The backfill liquid may be depressurised to the second pressure by pumping out the backfill liquid from the cargo tank 110 using one or more pumps. The backfill fluid and backfill liquid are referred to interchangeably in all examples described herein. As backfill liquid 306 is pumped out from the backfill side 314 of the cargo tank 110, a low-pressure region develops momentarily on the backfill side 314, which causes the backfill liquid 306 to vaporise, thereby forming a backfill fluid vapour 308. More specifically, the pressure in the backfill side 314 of the cargo tank drops below the vapour pressure of the backfill liquid 306, causing vaporisation. Backfill liquid 306 is therefore displaced in the cargo tank 110 by vaporisation of the backfill liquid. As set out in further detail below, vaporised backfill liquid 308 may also be fed back into the backfill side 314 of the cargo tank, using a return line.

Initially during pumping, the barrier 310 moves a little (in the “downwards” sense when viewing state 307 of FIG. 3) to try to accommodate the volume of backfill liquid 306 removed from the cargo tank 110. However, the barrier movement is limited because the volume of fluid cargo 304 left in the cargo side 312 of the cargo tank 110 is minimal and therefore cargo side 312 cannot expand significantly before reaching the pressure of the backfill side 314. The port 504 of the cargo side 312 is closed, and will therefore not take in any fluid, when the pressure is reduced.

Turning to FIG. 4, an exemplary pressure-temperature phase diagram of a backfill fluid is shown. The phase diagram comprises a solid state 402, liquid state 404 and gas state 406 phase regions. The boundaries between these states 402, 404, 406 shows the pressure at which the respective states 402, 404, 406 are in equilibrium with one another at a particular temperature. In the context of this disclosure, an expected temperature during offloading is around 278K to 283K, shown as Ti in FIG. 4. At these temperatures, the backfill fluid is a liquid at the initial pressure Pi and at the third pressure P3 (not shown) but has a tendency to vaporise below a pressure P2. Hence, when one or more pumps are used to reduce the pressure on the backfill side 314 of the cargo tank 110 to the second pressure (or lower), the backfill liquid 306 has a tendency to vaporise. The vaporisation may lead to cooling, but such effects are not considered in further detail in this disclosure because the surroundings (i.e., seawater) function as an infinitely large heat sink and such effects are only transient. Eventually, the backfill liquid 306 and vaporised backfill liquid (i.e., gaseous backfill fluid) 308 reach equilibrium at the vapour pressure (as can be read from the phase diagram at a particular temperature). As the backfill liquid 306 is replaced with gas 308, the mass of fluid in the cargo tank reduces (compared to state 305). The difference in mass between: (a) the initial mass of the cargo tank in state 301 (i.e., the mass of the cargo fluid 304, plus the mass of any back fill fluid 306) and (b) the final mass of backfill fluid 306, 308 in the cargo tank 110; is therefore less than the difference in mass between: (c) the initial mass of the cargo tank in the initial state 301 and (d) the mass of backfill fluid 306 in the cargo tank 110 in the state 305. Hence, the net, negative change in buoyancy is reduced. In this way, the burden on the ballast system can be reduced (e.g., smaller ballast tanks, smaller gas canisters).

The buoyancy change incurred by offloading can be eliminated altogether if the initial mass (state 301) of the fluid cargo 304 equals the final mass of the backfill fluid 306, 308 (state 307). Assuming the density of the backfill gas 308 is much smaller than the density of backfill liquid 306 and the density of the fluid cargo 304 and cargo tank is initially full with cargo fluid 304, then, this is satisfied, when: the ratio between the volume of backfill liquid 306 and the initial volume of fluid cargo 304 in the cargo tank (i.e., the volume of cargo fluid in the cargo tank before offloading) equals the ratio between the density of the fluid cargo 304 and the density of the backfill fluid 306.

In FIG. 3, the cargo tank 110 is shown as a closed container for simplicity. However, the cargo tank 110 comprises ports 502, 504 disposed on the cargo 312 and backfill sides 314 of the cargo tank, as shown in FIG. 5. Port 502 is configured to admit or discharge the backfill fluid 306 and controlled by the opening and closing of one or more valves 510, 512. Port 504 is configured to admit or discharge cargo fluid 304 from the cargo tank 110. Port 504 may also comprise the valves 510, 512, although this is not shown in FIG. 5 for simplicity.

In an example, the backfill fluid 306 is seawater (or water) and the fluid cargo 304 is hydrocarbon-based, e.g., an oil and gas mixture from a wellhead. Seawater has a density of 1.04g/cm ³ and the hydrocarbon has an average density of 0.42g/cm ³ . In practice, the cargo side 312 of the cargo tank 110 is filled directly from a wellhead and the hydrocarbon comprises a mixture of compounds with differing densities. Over time, the mixture of compounds in the hydrocarbon may segregate in the cargo tank 110, generating spatial variations in density. However, for these effects are ignored in the following illustrative example, which is only concerned with initial and final weights of the cargo tank 110. The initial pressure of the hydrocarbon may be approximately 60bar. The seawater, provided into the backfill side 314 of the cargo tank 100 during offloading, may also be approximately 60bar. More generally, the initial pressure of the hydrocarbon may be substantially equal to the wellhead pressure.

In an example, the capacity of the cargo tank 110 is approximately 10,000 litres. The initial mass of hydrocarbon (state 301) in such a cargo tank (assuming it is initially full with hydrocarbon) would then be approximately 4.2 tonnes. The mass of fluid (i.e., seawater) in the cargo tank after offloading all the fluid cargo 304 (state 305) would be approximately 10.04 tonnes. There would therefore be a net change in mass of approximately 5.84 tonnes from displacing the hydrocarbon with seawater. The net, negative buoyancy change would be around 60kN. Conventional underwater vehicles would need to accommodate this change in buoyancy using a dedicated ballast system, which would be a disadvantage. However, in the present disclosure, the seawater (backfill fluid) is depressurised, removed from the cargo tank and replaced with water vapour in order to reduce this net, negative buoyancy change. As has already been outlined, the change in buoyancy can be eliminated if the change in mass during offloading is eliminated. Assuming the cargo tank is initially full with hydrocarbon, the cargo tank, in the final state 307, would have the same mass if approximately 40% seawater is left on the backfill side after pumping (the remainder being water vapour).

The vapour pressure of seawater is approximately 10mbar at a temperature of around 278K to 283K. As such, the one or more pumps used to the remove the backfill fluid 306 in FIG. 3 are configured in power to produce this pressure drop (i.e., 60 bar to 10mbar). In practice, this places considerable burden on the pumps, which, among other things, may be subject to cavitation that may affect the useful life of the pumps.

Turning back to FIG. 5, port 502 disposed on the backfill side 314 of the cargo tank 110 is connected to one or more pumps 508. When the backfill fluid 306 is pumped out from the cargo tank 110, the backfill fluid may vaporise. In an example, the backfill fluid 306 removed from the cargo tank 110 is heated using one or more heating elements. A gas line 506, connected to downstream of one or more of the pumps 508, may be used to collect the vaporised liquid 308. The gas line can then feed the backfill vapour 308 back into the backfill side 314 of the cargo tank 110. Preferably, but not necessarily, the gas line 506 is also heated using one or more heating elements. The heating increases the temperature, which according to FIG. 4, increases the vapour pressure of the backfill liquid 306 and, in turn, increases the rate of vaporisation. In this way, the gas line 506 is able to feed back more backfill gas 308 into the cargo tank 110, reducing the low-pressure region (i.e., removing the vacuum above the backfill fluid 306) that develops during backfill pumping. The power burden on the pumps is thereby reduced because, during pumping, the backfill gas 308 is able to expand to displace the backfill liquid 306 removed from the cargo tank 110. The pressure reduction from pumping is therefore more gradual (compared to pumping only a liquid, which cannot expand) and the burden on the pumps is reduced. The use of a gas return line 506 is particularly beneficial when using seawater as the backfill liquid 306 because the vapour pressure of seawater at 278 to 283K is particularly low (~10mbar). However, the arrangement would benefit any backfill liquid 306, including liquid carbon dioxide.

In FIG. 5, each valve is a three-way valve, but such a configuration is merely shown as an illustrative examples, and should not be viewed as limiting. A three-way valve is able to control the flow of fluid along three fluid lines connecting to the valve. The valves 510, 512 are operable to allow the backfill fluid 306 (e.g., stored at location 516) to pass through the valves 510, 512 into the cargo tank via port 502. The valves 510, 512 are further operable to allow the backfill fluid 306 to pass through the valves 510, 512 via the one or more pumps 508.

The valves 510, 512 are operable to fluidly connect a source of backfill fluid 516 to the backfill fluid, stored in the cargo tank 110, via the one or more pumps 508. The one or more pumps are configured to pump the backfill fluid from the backfill fluid source 516 to the backfill side of the cargo tank at the third pressure, P3 _. The backfill fluid 306 can therefore be depressurised to the pressure of the backfill fluid source 516. In an example, the source of backfill fluid 306 may be ambient seawater 516, with a pressure of 20 bar (i.e., a depth of 200m) and the one or more pumps 508 are used to increase the pressure (e.g., to the pressure of the fluid cargo).

The valves 510, 512 are operable to fluidly connect the backfill fluid 306 stored in the cargo tank with the one or more pumps 508, thereby allowing depressurisation of the backfill fluid 306 to lower pressures by appropriate pumping of the one or more pumps 508. Valve 514 is operable to open and close in order to feed back vaporised backfill fluid 308, where appropriate.

The valve arrangement in FIG. 5 is only shown with respect to port 502. However, a similar valve arrangement is also connected to the other port 504 (on the cargo side). The valves connected to the other port 504 are operable to fluidly connect one or more (other) pumps to the cargo fluid 304 stored in the cargo tank 110, such that pumping of the one or more (other) pumps removes cargo fluid from the cargo tank 110.

In a further example, the backfill fluid 306 is liquid carbon dioxide and the fluid cargo 304 is hydrocarbon-based, e.g., an oil and gas mixture from a wellhead. Liquid carbon dioxide has a density of 0.89g/cm ³ at 60bar, and the hydrocarbon has an average density of 0.42g/cm ³ . The initial pressure of the hydrocarbon may be approximately 60bar. The liquid carbon dioxide, provided into the backfill side 314 of the cargo tank 100 during offloading, may also be approximately 60bar. As the skilled reader would appreciate, the thermodynamically stable state is given by the carbon dioxide P-T phase diagram. At 278 to 283K (the approximate ambient temperature of seawater), carbon dioxide is in the liquid state above approximately 40 to 45 bar.

In an example, the capacity of the cargo tank 110 is approximately 10,000 litres. The initial mass of hydrocarbon (state 301) in such a cargo tank (assuming it is initially full with hydrocarbon) would then be approximately 4.2 tonnes. The mass of fluid (i.e., liquid carbon dioxide) in the cargo tank after offloading all the fluid cargo 304 (state 305) would be approximately 8.9 tonnes. There would therefore be a net change in mass of approximately 4.7 tonnes from displacing the hydrocarbon with liquid carbon dioxide at 60bar. The net, negative buoyancy change would be around 50kN. Conventional underwater vehicles would need to accommodate this change in buoyancy using a dedicated ballast system, which would be a disadvantage. However, in the present disclosure, the liquid carbon dioxide (backfill fluid) is depressurised, removed from the cargo tank and replaced with carbon dioxide vapour in order to reduce this net, negative buoyancy change. As has already been outlined, the change in buoyancy can be eliminated if the change in mass during offloading is eliminated. Assuming the cargo tank is initially full with hydrocarbon, the cargo tank, in the final state 307, would have the same mass if approximately 47% liquid carbon dioxide is left on the backfill side after pumping (the remainder being carbon dioxide gas). The vapour pressure of liquid carbon dioxide is approximately 40bar at a temperature of around 278K to 283K. As such, the one or more pumps used to the remove the backfill fluid 306 in FIG. 3 are configured in power to produce this pressure drop (i.e., 60 bar to 40 bar). Comparing this example to the example with seawater, it is clear that less pumping power is required if liquid carbon dioxide is used. Generally speaking, the required pumping power reduces if the vapour pressure of the backfill fluid increases.

FIG. 6 shows a further schematic illustration of an offloading process of a cargo tank 110 comprising a barrier 310, defining a backfill side 314 of the cargo tank and a cargo side 312 of the cargo tank (as in FIG. 3). The barrier 310 may be any one of the barriers previously described. Each side 312, 314 comprises one or more valves that controls fluid flow into and out from each respective side 312, 314. The valves may either be in an open or in a closed state.

In an initial state 601 before offloading, the cargo side 312 of the cargo tank 110 comprises cargo fluid 604 and a backfill fluid 606 on the backfill side 314 of the cargo tank at an initial pressure, Pi _. In practice, the volume of backfill fluid 606 in the initial state 601 is low (i.e., the backfill side 314 of the cargo tank is initially empty), but as the skilled person would appreciate, the initial volume of backfill fluid 606 is arbitrary.

In a first step 603, the cargo fluid 604 is removed from the cargo side 312 of the cargo tank 110 and displaced by the backfill fluid 606, which is introduced into the backfill side 314 of the cargo tank 110. Preferably, but not necessarily, the pressure of the backfill fluid 606 (the third pressure, P3) is substantially equal to the initial pressure (Pi) of the cargo fluid 604 in order to maintain the hydrostatic pressure within the cargo side 312 of the cargo tank 110. As fluid cargo 604 is removed from the cargo tank, the barrier 310 separating the cargo side 312 from the backfill side 314 moves.

However, in contrast to FIG. 3, where the fluid cargo 304 is substantially, or completely removed from the cargo side 312 of the cargo tank 110, in FIG. 6, a volume of fluid cargo 610 is retained on the cargo side 312 of the cargo tank 110 (state 605). As such, a volume fraction of cargo fluid is removed from the cargo side 312 of the cargo tank 110 and replaced with the backfill liquid on the backfill side 314 of the cargo tank 110. In state 605, the backfill fluid 606 and the fluid cargo 604 are at a third pressure, P3, which may be equal to the initial pressure, Pi.

In a second step 607, backfill fluid 606 is depressurised to a second pressure, P2, and removed from the cargo tank 110. As the backfill fluid 606 is depressurised, the barrier 310 moves towards the backfill side 314 of the cargo tank and the volume of fluid cargo 610 remaining on the cargo side 312 of the cargo tank 110 expands, effectively displacing the backfill fluid 606 as it is removed. The expansion of the cargo fluid 604 leads to liquid vaporisation and/or gas expansion of the cargo fluid 604, depending on the composition of cargo fluid 610. If only gas is present in the cargo fluid, there will be no vaporisation but only gas expansion. As the barrier 310 moves, powerful pumps are not required (although they may still be used) to remove the backfill fluid 606 from the cargo tank. For example, a valve, exposing the backfill fluid 606 to ambient seawater pressure, may be opened, resulting in depressurisation of the backfill fluid 606. As the backfill fluid 606 depressurises to ambient pressure, the cargo fluid 610 (which was at the same initial higher pressure as the backfill fluid 306) expands, thereby driving the barrier 310 towards the backfill side 314 of the cargo tank 110 and expelling backfill fluid 606 from the cargo tank. In this way, as backfill liquid 606 is discharged from the cargo tank 110, the backfill liquid 606 is displaced by vaporisation and/or expansion of the volume of cargo fluid 610 retained in the cargo tank.

As the backfill fluid 606 is replaced with cargo gas 608, the mass of fluid in the cargo tank reduces (compared to state 605). The difference in mass between:

(a) the initial mass of the cargo tank in state 601 (i.e., the mass of the cargo fluid 604, plus the mass of any backfill fluid 606) and (b) the final mass of backfill fluid 606 and cargo gas 608 in the cargo tank 110; is therefore less than the difference in mass between:

(c) the initial mass of the cargo tank in state 601 and (d) the mass of backfill fluid 306 and cargo fluid 610 in the cargo tank 110 in the state 605.

Hence, the net, negative change in buoyancy is reduced. In this way, the burden on the ballast system can be reduced (e.g., smaller ballast tanks, smaller gas canisters).

The buoyancy change incurred by offloading can be eliminated altogether if the initial mass (state 601) of the fluid cargo 604 equals the final mass of the backfill fluid 606, 608 (state 607). Assuming the density of the cargo gas 608 is much smaller than the density of backfill liquid 606 and the density of the fluid cargo 604 and cargo tank is initially full (state 601) with cargo fluid 604, then, this is satisfied, when: the ratio between the volume of backfill liquid 606 and the initial volume of fluid cargo 604 in the cargo tank equals the ratio between the density of the fluid cargo 604 and the density of the backfill fluid 606.

It should be appreciated that any volume 610 of fluid cargo left behind in the cargo side 312 of the cargo tank 110 helps to drive some of the backfill fluid 606 from the cargo tank 110 and/or reduce the power required by pumping (i.e., ensures that the barrier 310 moves and a low pressure region on the backfill side 314 is avoided). However, the barrier 310 in FIG. 6 only moves if there are enough moles of cargo gas 608 to fill the space vacated in the cargo tank 110 at the second pressure, P2. As such, to eliminate a net change in buoyancy, the volume 610 of fluid cargo retained in the cargo tank preferably contains a sufficient number of moles to fill a volume equal to:

Equation wherein, V, is the initial volume of cargo fluid 604 (or the volume of the cargo tank) and Pcargo gas 608 is density for cargo gas 608 at state 607.

As a first approximation, the cargo gas 608 acts as an ideal gas and therefore the number of moles, n, of cargo gas required to fill this volume, V, at a pressure, P and temperature, T, is:

Equation

To calculate an upper bound for the volume 610 of fluid cargo required to be retained in the cargo tank 110, it is assumed that the fluid 610 is in the gaseous state. However, it should be appreciated that the fluid 610 may be in the liquid state, or a mixture of the gas and liquid state, depending on the pressure, temperature and cargo fluid being used. Taking the fluid 610 to be a gas serves as an upper bound because liquids are denser than gases and therefore the volume 610 of “n” moles of liquid cargo is less than the volume 610 of “n” moles of gaseous cargo. Assuming the fluid 610 also acts as an ideal gas:

Equation 1.3: Wherein, V, denotes the volume of fluid i, and p, denotes the pressure of fluid i.

In general, the volume 610 retained in the cargo tank 110, to accommodate the volume 608 by expansion and/or vaporisation, is less than the value provided by Equation 1.3 because the volume 610 may be a liquid or comprise liquid cargo.

In an example, the backfill fluid 606 is seawater (or water) and the fluid cargo 604 is hydrocarbon-based, e.g., an oil and gas mixture from a wellhead. Seawater has a density of 1.04g/cm ³ and the hydrocarbon has an average density of 0.42g/cm ³ . The initial pressure of the hydrocarbon may be approximately 60 bar. The seawater, provided into the backfill side 314 of the cargo tank 100 during offloading, may also be approximately 60 bar.

In an example, the capacity of the cargo tank 110 is approximately 10,000 litres. The initial mass of hydrocarbon (state 601) in such a cargo tank (assuming it is initially full with hydrocarbon) would then be approximately 4.2 tonnes. The mass of fluid (i.e., seawater) in the cargo tank after (partially) offloading the fluid cargo 604 (state 605) would be greater and there would therefore be a net change in mass from displacing the hydrocarbon with seawater. This would generate a net, negative buoyancy change in the underwater vehicle. Conventional underwater vehicles would need to accommodate this change in buoyancy using a dedicated ballast system, which would be a disadvantage. However, in the present disclosure, the seawater (backfill fluid) is depressurised from 60 bar to 20bar and removed from the cargo tank 110 by expanding the volume 610 of fluid cargo retained in the cargo tank 110 after offloading. The depressurisation to 20 bar could be achieved using the surrounding seawater pressure (i.e., the second pressure is the ambient water pressure immediately surrounding the underwater vehicle 100). For example, at a depth of 200m, the surrounding seawater pressure is approximately 20bar. In this way, pumps may be not be necessary (although they may still be used). The valve arrangement shown in FIG. 5 is operable to fluidly connect seawater, immediately surrounding the underwater vehicle, with the backfill liquid in the cargo tank 110.

As has already been outlined, the change in buoyancy can be eliminated if the change in mass during offloading is eliminated. Assuming the cargo tank is initially full with hydrocarbon, the cargo tank, in the final state 607, would have the same mass if approximately 40% seawater is left on the backfill side after pumping (the remainder being cargo gas 608). The upper bound for the volume 610 required to accommodate the remaining volume of the cargo tank (i.e., approximately 60% of the cargo tank) is approximately 20%, when taking the pressure of the cargo gas 608 as 20 bar.

In FIGs. 3, 5 and 6, fluids 304, 306; 306, 308; 604, 606; 606, 608 are shown as being spatially separated in the cargo tank 110 via a barrier 310. However, in some examples, it may not be necessary for there to be a physical barrier. For example, the fluids 304, 306; 306, 308; 604, 606; 606, 608 may be separated (without a barrier) because they are immiscible with one another or they separate naturally because of their differing densities. In some examples, mixing of fluids 304, 306; 306, 308; 604, 606; 606, 608 may be tolerated. Hence, the barriers 310 of the present disclosure are preferable, but not essential.

Although the cargo tank 110 shown in FIGs. 3, 5 and 6 is shown as being a single container, it should be appreciated that the buoyancy control method described in this disclosure also applies when more than one cargo tank 110 is used.

Turning now to FIG. 7, an offloading process according to FIG. 3 is shown, wherein the cargo tank 110 comprises an arrangement of cargo tanks 700, rather than a single cargo tank 110. In FIG. 7, the number of cargo tanks, “n”, is equal to four; however, as the skilled person would appreciate any number of cargo tanks can be used and the cargo tanks are not necessarily of identical size. In the initial state, all the cargo tanks 700 are filled with fluid cargo 304. In the first step, the fluid cargo 304 is removed from each of the cargo tanks 110 in the cargo tank arrangement 700. The fluid cargo 304 may either be removed from each of the cargo tanks independently according to a particular sequence (as shown). Alternatively, the cargo tanks 110 in the arrangement 700 may be fluidly connected with one another and the cargo tanks 110 may be emptied of fluid cargo 304 concurrently. In some examples, the cargo tanks 110 in the arrangement 700 are operable to be fluidly connected, or fluidly isolated from one another using one or more valves (in series or in parallel). In an example, one or more valves are disposed in a manifold design to allow fluid connection or fluid isolation between any of the cargo tanks 110 in the arrangement 700. In a specific example, immediately adjacent cargo tanks 110 in the arrangement 700 are fluidly connected in series with one another. At state 305, the fluid cargo 304 has been removed from the cargo tanks 700 and in the second step 307; the backfill fluid 306 is removed from the cargo tanks 700. As has been outlined above in a specific example, the net negative buoyancy change when using seawater as a backfill fluid 306 and hydrocarbon as the fluid cargo 304 can be eliminated if the cargo tanks in the arrangement 700 are approximately 40% full of backfill fluid after offloading is complete. In such an example, only 60% of the total backfill fluid in all the cargo tanks in the arrangement 700 would need removing, which can be from any of the cargo tank(s) 110 in the arrangement 700. It should be appreciated that there are a plurality of ways to distribute the backfill fluid 306 in the cargo tank arrangement 700 (filling arrangements) to satisfy this condition. For example, one filling arrangement would be to displace 60% of backfill fluid 306 in each cargo tank 110 with backfill vapour 308. Another filling arrangement (as shown in FIG. 7) would be to completely displace backfill fluid 306 with backfill gas 308 in two cargo tanks 110, keep one cargo tank 110 completely full with backfill fluid 306, and displace the around 40% of the backfill fluid 306 in the remaining cargo tank 110 with backfill gas 308. Other arrangements are possible.

As has been outlined above, the one or more cargo tanks in the arrangement 700 may be fluidly connected, fluidly isolated from one another, or operable to be, using appropriate valve control. That is, backfill fluid 306 may also be removed either concurrently from one or more cargo tanks, or consecutively according to a particular sequence. If the cargo tanks 110 are fluidly isolated from one another, the gas feedback line 506 shown in FIG. 5 may be arranged to return backfill gas 308 to the cargo tank 110 from which backfill fluid 306 is being removed. Alternatively, if the cargo tanks 110 are fluidly connected with one another, the gas feedback line 506 may be arranged to return backfill gas 308 to any cargo tank in fluid communication with the cargo tank 110 from which backfill fluid 306 is being removed.

Generally speaking, to achieve an arrangement of cargo tanks 700 that reduces or eliminates the net, negative buoyancy associated with offloading, backfill liquid 306 may either be: completely displaced with backfill gas 308 (e.g., water vapour) in one or more cargo tanks 110; partially displaced with backfill gas 308 in one or more cargo tanks 110; or left in one or more cargo tanks. The distribution of backfill fluid 306 in the arrangement of cargo tanks 700 is, to an extent, based on the stability of the underwater vehicle 100. Referring back to FIG. 7, the cargo tank 110 comprising a mix of backfill gas 308 and backfill fluid 306 could leave to poor trim stability of the underwater vehicle 100. For example, if, in transit, the underwater vehicle was inclined at an angle relative to the vertical, then the heavier backfill fluid 306 may move towards either the stern or bow of the underwater vehicle 100. This shift in fluid would generate a net moment on the underwater vehicle, tending to rotate it into a vertical configuration (i.e., an unstable configuration). This instability is particularly a concern when the cargo tanks 110 are arranged horizontally in the underwater vehicle 100 and the backfill fluid 306 is able to move relative to the backfill gas 308. Put differently, the instability arises when the centre of gravity (from which weight acts) and the centre of buoyancy (from which the buoyancy force acts) are not vertically aligned. The choice of filling arrangement is therefore chosen to avoid this type of instability. Vertically arranged cargo tanks 110 may be useful in minimising this effect because they reduce the distance that the backfill fluid 306 is able to move in the horizontal sense in the preferred orientation for underwater vehicle movement. The net moment produced is therefore smaller compared to a horizontally oriented cargo tank 110. Another rule of thumb is to avoid filling a cargo tank 110 approximately half with backfill liquid 306 and half backfill gas 308 because a half filled cargo tank is able to produce the largest net moment (destabilizing force) on the underwater vehicle 100.

It should be appreciated that the offloading process illustrated in FIG. 6 may also comprise one or more cargo tanks 110 in an arrangement similar to that shown in FIG. 7. However, in the example shown in FIG. 6, the backfill side 314 of the cargo tank 110 comprises a single phase (i.e., a liquid) and in pressure communication with the cargo side 312 via the barrier 310. As each side of the cargo tank 312, 314 comprises a single phase, the fluids 606, 608 are not able to segregate out by weight. Furthermore, the fluids 606, 608 are unable to segregate out by weight with one another because of the barrier 310 physically separated them. As such, trim stability problems is less of a concern for the offloading process shown in FIG. 6. However, it should be noted that any filling arrangement is still possible with this process.

FIG. 8 summarises the disclosure described above. The method for offloading cargo fluid from a container in an underwater vehicle comprises: displacing 802 cargo fluid from the container with a backfill liquid at a first pressure; depressurising 804 the backfill liquid to a second pressure, wherein the second pressure is lower than the first pressure; and discharging 806 at least part of the backfill liquid from the container. The method may comprise one or more containers, such as one or more cargo tanks.

A method for offloading a cargo fluid from a container, which reduces the net, negative buoyancy change associated with offloading, has been described. However, similar principles apply during the cargo fluid loading process. In practice, a loading process may follow the offloading process. Referring back to FIG. 3 and FIG. 6, cargo fluid 304, 604 may be added to the cargo tank 110 after the offloading process is complete (i.e., to a cargo tank in the final state 307, 607), such that the initial state 301, 601 of the cargo tank 110 is restored.

In the same way that offloading cargo fluid generates a net, negative buoyancy change, loading cargo fluid generates a net, positive buoyancy change. To reduce this net, positive buoyancy either:

• Fluids 306, 308; 606, 608 are partially displaced with the cargo fluid 304, 604 at the first pressure, by partially removing fluids 306, 308; 606, 608 from the backfill side 314, 614 of the cargo tank 110 and replacing the volume with cargo fluid 304, 604 on the cargo side 312, 612 of the cargo tank 110. In this approach, a volume of fluid 306, 308; 606, 608 is retained in the cargo tank 110 after cargo fluid loading, thereby reducing the net, positive buoyancy during cargo fluid loading; or

• A process comprising: i) displacing substantially all fluids 306, 308; 606, 608 from the cargo tank 110 with cargo fluid 304, 604 at the fourth pressure, by removing substantially all fluids 306, 308; 606, 608 from the backfill side 314, 614 of the cargo tank 110 and replacing the volume with cargo fluid 304, 604 on the cargo side 312, 612 of the cargo tank 110; and i) fluidly connecting one or more pumps to a backfill liquid source and pumping the backfill liquid at a pressure greater than the fourth pressure (e.g., the first pressure), thereby compressing the cargo fluid 304, 604 in the cargo tank 110. In this approach, the density of the cargo fluid 304, 604 is increased, thereby reducing the net, positive buoyancy change during cargo fluid loading.

In the latter approach, the cargo fluid 304, 604 comprises a gas, or a gas and liquid mixture, such that it is compressible. In an example, the cargo fluid 304, 604 is a hydrocarbon released from a wellhead with a non-zero gas to oil ratio (GOR).

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.