AN UNDERWATER VEHICLE FOR TRANSPORTING FLUID

The present invention relates to an underwater vehicle for transporting fluid and to a method of transporting fluid using the underwater vehicle. The invention has particular application, but is not limited, to the field of hydrocarbon transportation.

Generally, fluid transportation from a subsea location, such as a wellhead, comprises the use of a pipeline and/or a surface vessel (e.g. tanker) to transport the fluid to a desired location. For instance, the transportation of a produced hydrocarbon fluid (e.g. oil, gas and/or a semi-stable hydrocarbon product) typically comprises the use of pipelines from the subsea wells and/or tankers which connect to the subsea well, either directly or indirectly, to transport fluids produced therefrom.

There are recognised drawbacks with pipeline transportation, such as the significant capital expenditure involved in constructing a pipeline and the operational expenditure associated with maintenance and upkeep. Moreover, transportation of certain fluids can be technically challenging to achieve using pipelines. Such fluids include, for example, semi-stable hydrocarbon products which are prone to hydrate formation within the pipelines, which may cause blockages.

There are also various recognised drawbacks associated with traditional surface vessel (e.g. tanker) transportation solutions. For instance, surface vessel transportation is dependent on weather and surface conditions. In particular, wind direction and rough seas can cause significant problems and delays.

As an alternative to these more typical approaches to fluid transportation the applicant has previously proposed an alternative means of fluid transportation. This alternative solution comprises the use of a subsea vehicle in the form a subsea shuttle to transport fluid. A detailed description of the applicant’s earlier proposed solution to fluid transportation can be found in Research Disclosures 662093 and 667082 published on 20 May 2019 and 21 August 2020, respectively. A video further detailing this concept was published on YouTube™ (https://www. youtube. com/watch?v=4b3R1vWN3F0) on 14 June 2020. Further details in connection with the inventor’s earlier concept for fluid transportation via subsea shuttle system can be found in an article entitled ‘Design Considerations of a Subsea Shuttle Tanker System for Liquid Carbon Dioxide Transportation’ submitted for the 39 ^th International Conference on Ocean, Offshore & Arctic Engineering (OMAE 2020-18070). Within these vessels, the former arrangement employs large pressure vessel tanks with thick steel walls for the fluid, whereas the latter uses an inflatable bag to hold the carbon dioxide.

WO 2015/059617 A1 discloses an underwater vessel comprising a plurality of storage tubes extending horizontally along the primary direction of travel of the underwater vessel. The storage tubes are configured to store fluid therein, e.g. gas, oil or water. The vessel is configured to be towed by one or two surface vessels such that fluid stored within the tubes in the vessel can be transported as desired.

These subsea vessels address various drawbacks associated with the more typical transportation solutions as discussed above. Capital expenditure would be reduced compared to that for a pipeline, and maintenance and upkeep would be reduced. Moreover, since the shuttle travels subsea it, at least in part, avoids the surface conditions that can make surface vessel transportation challenging. However, whilst advantageous, further improvements for fluid transportation using subsea transportation are desired.

In accordance with a first aspect of the present invention, there is provided an underwater vehicle for transporting fluid, wherein the underwater vehicle comprises: a prime mover configured to power propulsion of the underwater vehicle through the water; and a plurality of storage pipes for storing the fluid therein.

The term ‘storage pipe’ refers to a storage container formed from a length of pipe, which has been closed at each end, optionally by a hemispherical cap or dome that has, for example, been welded to the end of the pipe. Accordingly, the storage pipes are highly elongate, typically having a length-to-diameter ratio of at least 20.

Preferably, a large number of such storage pipes are provided within the vehicle, preferably arranged in parallel with each other as discussed further below. Any suitable number can be provided, depending on the storage capacity required.

The use of such pipe storage as compared to conventional tank storage (i.e. vessel storage) or storage using an inflatable bag as in the applicant’s own earlier proposed shuttle systems is advantageous since it is associated with a significantly lower capital and operational expenditure, particularly in the context of transporting fluids at elevated pressures as discussed in further detail below as an optional implementation of the vehicle of the first aspect. Typical ‘tank’ type storage solutions (e.g. the cylindrical tanks disclosed in RD 662093) require thick steel walled tanks. These tanks are expensive to provide (given the large amount of material typically required), and are also expensive to transport given their weight (again, given the large amount of material required). The requisite wall thickness (and hence weight of the tank) also limits the size of the tank that can be used, meaning that the volume of fluid that can be transported on a vehicle comprising tank storage is limited.

In contrast, pipe storage is relatively inexpensive to provide because standard, ‘off-the-shelf’ pipes can be used to manufacture them. Moreover, for a given volume of storage, pipe storage can have a comparatively smaller wall thickness. Thus, a given volume of fluid can be transported using a comparatively lower total weight of storage tank material using pipe storage and can be achieved at a lower capital expenditure.

Inflatable bag storage, e.g. of the type disclosed in RD 662093, is necessarily bespoke/tailored as it must closely match the dimensions and the requirements of the vessel/vehicle in which it is to be used. As such, inflatable bag storage is associated with a high capital expenditure. In contrast, as noted above, the pipe storage used in the context of the current invention is essentially an ‘off-the shelf’ component, and thus is associated with a significantly lower capital expenditure. Furthermore, in the event of failure or leakage, an inflatable bag storage solution results in the potential loss of the entire contents of fluid stored therein. In contrast, given a plurality of storage pipes are used in the context of the current invention, in the event of failure or leakage from one (or a subset) of the pipe storage units only a portion of the total contents of fluid stored on the vessel is at risk of being lost.

‘Pipe’ storage also better utilises the available space on a vessel as compared to at least ‘tank’ storage solutions (e.g. as compared to the cylindrical tank storage solutions disclosed in RD 662093). Since pipe storage has a regular shape, the pipes can be relatively tightly packed and easily arranged to efficiently fill a large portion of the available space on the vehicle. In contrast, storage tanks have an irregular shape, in part resulting from the flanges, nozzles and welded supports typically provided on the outside of said tanks. Thus, given their irregular shape, storage tanks inefficiently use the available volume on board a vehicle meaning that comparatively less fluid may be transported on said vessel. As such, vessels having tank storage provide poorer returns. The pipe used may have a nominal diameter of between 40-60 inches (1.0m - 1.5m). Preferably, the pipe may have a nominal size of 42 inches (1.1m) or 56 inches (1.4m), or may have any nominal size in the range of 42 inches (1.1m) to 56 inches (1.4m).

A vessel having a nominal diameter greater than about 56-60 inches (1.4 m -1.5m) would typically be considered by the skilled person as a conventional tank (or pressure vessel) that is distinct from a pipe. This consideration is also true in the context of the current invention, whereby any vessel having a nominal diameter of greater than about 56-60 inches (1 ,4m -1 ,5m) would not be considered as a pipe.

The pipe used may be an X45, X52, X56, X60, X65, X70 or X80 pipe in accordance with the API SPEC 5L specification.

As noted above, the storage pipes are highly elongate. Accordingly, the storage pipe may have a length of between 10 m to 30 m, for example 12 m, 24 m or 26 m.

The storage pipes may be formed from rolled pipes with, optionally, a single longitudinal seam. Such pipes are commonly available as ‘off-the-shelf’ type components and are typically inexpensive.

Like the known vehicles, the vehicle of the invention will typically have an elongate form with a longitudinal axis parallel to its intended direction of travel.

The storage pipes storage may be arranged horizontally in the vehicle. That is, the primary axes of the pipes may align, or substantially align, with the longitudinal axis of the subsea vehicle (i.e. parallel to the intended direction of travel of the subsea vehicle). This arrangement enables storage pipes with a large length to diameter ratio (i.e. having a large volume for a given diameter) to be used and packed efficiently within the vehicle.

However, it is possible for the storage pipes to be arranged vertically in the vehicle. That is, the longitudinal axes of the pipes may be normal (i.e. perpendicular), or substantially normal, to the longitudinal axis of the subsea vehicle. Such a vertical orientation of the storage pipes may enable separation of the fluid stored therein, which can be advantageous in the case of, e.g., semi-stable hydrocarbon product such that, for instance, water can be separated from the more valuable hydrocarbon components of the fluid. A vertical orientation is also advantageous since it avoids or minimises ‘sloshing’ of the fluid on-board the vehicle during transportation, which can cause instability and transportation difficulties and issues. The vertical orientation of the pipes also enables selective filling of the pipes about the vehicle to ensure appropriate buoyancy/ballast of the vehicle.

Alternatively, a first subset of the plurality of pipes may be arranged vertically in the vehicle whilst a second subset of the plurality of pipes may be arranged horizontally. In some cases, such an arrangement may allow for the greatest utilisation of the available space on the vehicle for storage purposes.

Any other suitable orientation of the storage pipes may also be used.

The subsea vehicle may comprise tens, hundreds or even thousands of storage pipes, e.g. 1000 or 2000 pipes. The number of storage pipes may be selected to maximise storage on the vehicle. A greater number of storage pipes on the vehicle provides a greater reliability in fluid transport in that if one (or even several) of the pipes is/are damaged or ruptured such that its/their contents of fluid leaks therefrom the fluid stored within the plurality of remaining pipes remains unaffected.

The vessel may comprise a plurality of cargo holds. Each cargo hold may have a sub-set of the plurality of storage pipes housed therein, e.g. each cargo hold may comprise up to 100 storage pipes. For example, each cargo hold may comprise at least 10, 20, 30, 40, 50, 60, 70, 80 or 90 storage pipes.

The pipes may be comprised within one, or several, cargo modules (e.g. pallets or crates) on the underwater vehicle. That is, the underwater vehicle may comprise one or more cargo module(s), each cargo module comprising some or all of the storage pipes. The cargo modules may be installable into and removable from the underwater vehicle. In that way, the storage pipes can be easily installed into and removed from the underwater vehicle in a batch approach.

The storage pipes on the vessel may be capable of storing 10,000 m ³ - 200,000 m ³ fluid therein. For example, the storage pipes may be capable of storing 50,000 m ³ fluid therein.

The storage pipes may be configured for storing fluid at an elevated pressure (i.e. at a pressure greater than the ambient water pressure in which the vehicle is to be situated). Storage at an elevated pressure may be required for certain fluids to be transported, e.g. certain semi-stable hydrocarbon products, ammonia and the like.

The storage pipes may be configured to store the fluid as a liquid. The storage pipes may be configured to store the fluid as a liquid at ambient temperature conditions. The ambient temperature conditions may be the temperature of the surrounding sea water. The skilled person will appreciate that storing certain fluids as liquids at ambient sea temperature conditions will require those certain fluids to be pressurised above the ambient sea pressure conditions with the exact pressure conditions being determined by the specific ambient sea temperatures at which the fluid is stored in the storage pipes. As such, where the storage pipes are configured for storing the fluid as a liquid at ambient sea temperature conditions the storage pipes may equally be configured for storing the fluid at an elevated pressure (i.e. at a pressure greater than the ambient water pressure in which the vehicle is to be situated). However, some fluids may naturally be in a liquid state at ambient sea temperature and sea pressure conditions and thus the skilled person will understand that these fluids do not require storage at elevated pressures within the storage pipes in order to be stored as a liquid.

The optional transportation of fluids having elevated pressures is particularly advantageous as compared to at least some of the prior art solutions. With regard to the tank and inflatable bag solutions discussed above, given the comparatively larger size of these vessels as compared to the pipes of the current invention, the cost to manufacture such vessels such that they are strong enough to safely and reliably store pressurised fluids therein is extremely high to the point of being commercially unviable. The technical demands for such vessels are also extremely high to the point of being impractical. Moreover, at least with respect to the tank solution discussed above, the degree of material necessary in the walls of such a vessel in order to maintain pressurised fluids therein results in an extremely large weight that results in inefficient transportation.

These same drawbacks are not shared by the pipe storage solution as discussed herein. As discussed above, the pipe storage of the current invention may be based on ‘off-the-shelf’ pipes which are capable of storing fluids at elevated pressures. Furthermore, the ratio of the volume of pressurised fluid that can be stored to the weight of the vessel is significantly higher for the pipe storage of the current invention as compared to the tank solution of the prior art, and this ratio only further increases with increasing fluid pressure. Thus, more efficient transportation of fluids having elevated pressures is achieved by the underwater vehicle of the current invention.

The storage pipes may be configured to store the fluid at between 5 bar - 400 bar, optionally 10 bar - 100 bar, optionally 20 bar - 80 bar, and further optionally 40-60 bar, for example 55-56 bar. The exact pressurised conditions may be selected dependent on the nature of the fluid to be stored (e.g. if the fluid is to be stored as a liquid at ambient sea temperature conditions), the tolerances of the storage pipe and/or the tolerances of the equipment used for loading and unloading the fluid into the storage pipes.

The elevated pressure of the fluid which is configured to be stored may be sufficient such that no pumps and/or compressors are required on the subsea vehicle for the loading operation of the fluid. That is, the pressure of the fluid may be sufficient for loading the fluid into the storage pipes under its own impetus. At the very least, the pressure of the fluid may reduce the demand on compressors and/or pumps for the loading operation.

The fluid to be transported may be oil, a hydrocarbon gas, MEG, CO2, ammonia, freshwater, seawater, biofuel, hydrogen, a semi-stable hydrocarbon oil product and/or chemicals used at offshore hydrocarbon sites (e.g. an offshore hydrocarbon production site). The fluid to be transported may comprise one or more different fluids.

There are particular synergies when the invention is used to transport a semi-stable fluid. The term “semi-stable” herein is used to describe a liquid hydrocarbon product that has been stabilised to a certain extent, but has not been fully stabilised. This means that, under certain pressure and temperature conditions (in this case, the conditions of storage within the pipes), it will remain in a single (liquid) phase, avoiding evaporation and precipitation (i.e. the precipitation of hydrates in the liquid). However, unlike a fully-stabilised liquid, it must be maintained at a pressure above atmospheric pressure to retain it in that state.

Optionally, the semi-stable hydrocarbon product would be outside of the “hydrate envelope” for the conditions under which it is stored and transported in the pipe storage on the vehicle. Hydrates are ice-like crystalline solids composed of water and gas, and hydrate deposition on the inside wall hydrocarbon pipelines, processing equipment, transportation vessels etc. is a serious problem in oil and gas production infrastructure. As discussed below with reference to Figure 1, for a given hydrocarbon fluid, hydrates form at higher pressures and lower temperatures. When warm hydrocarbon fluid containing water flows through equipment with cold surfaces, hydrates will precipitate and adhere to the surfaces of the equipment. This reduces the equipment cross-sectional area, which, without proper counter measures, will lead to a loss of pressure, function and ultimately to a complete blockage of the equipment. The processing and subsequent transportation of a gas-containing product therefore normally requires hydrate control.

A hydrocarbon product is semi-stabilised by part-processing, and such partprocessing typically involves the partial degassing of produced fluid and/or the separation of water from the produced fluid to a certain extent. Degassing of the produced fluid to a certain extent can be considered as a reduction in its gas composition (i.e. of fractions that are gaseous under atmospheric conditions), in particular a reduction of the gas composition as compared to produced fluid emanating from a reserve. Most typically, it is the lightest gas fractions (e.g. C1-C4) that are removed from the produced fluid in the formation of the semi-stable oil product. The extent of this processing may be dependent on the conditions at which the semi-stable hydrocarbon product will be held and transported in the storage pipes on the vehicle, such that it may be taken outside of the hydrate envelope, as noted above. As the fluid will cool whilst being stored on the vehicle in the pipe storage, and as its pressure will reduce whilst stored (due to imperfect storage), it may be necessary to consider conditions of storage, for example, the ambient temperature and expected length of the voyage. In this example, the ambient temperature may be the temperature of the sea water. A semi-stable oil product typically still comprises some gas fractions from the produced fluid combined with oil fractions and a small amount of water from the produced fluid in a single liquid phase, wherein the gas fractions remain entrained in the liquid product under pressurised conditions.

In line with the explanation above, the term “part-processing” herein is used to indicate that, whilst a produced fluid is processed, it is only processed to a limited extent and is not processed fully. Full processing would involve the preparation of an oil product and, optionally, a gas product into a fully stabilised state. In contrast, in the scenario of a semi-stable hydrocarbon product, the produced fluid is only “part-processed” to form a semi-stable hydrocarbon product.

The stability of a hydrocarbon product is often described by its true vapour pressure (TVP). The true vapour pressure of a fully stabilised oil product is typically around 0.97 bar, and such an oil product will be stable under atmospheric conditions. Part-processing of produced fluid to form a semi-stable hydrocarbon product may lower the TVP of the oil product to below the TVP of fluid in the reservoir from which it is produced, but the TVP of the semi-stable hydrocarbon product may remain above 0.97 bar, more usually above 1 bar, and more typically above 1.3 bar. Given that the TVP of a semi-stable hydrocarbon product is higher than that of a stabilised oil product, a semi-stable hydrocarbon product may be termed a high TVP (HTVP) oil product.

The underwater vehicle may be a shuttle type vehicle, e.g. aside from the fluid storage arrangements, of the general type described in RD 662093 and RD 677082.

The underwater vehicle may be configured to travel at a constant depth subsea (e.g. 100 metres to 500 metres below sea level, or 200 metres to 400 metres below sea level, for instance 300 meters below sea level).

As discussed above, the vehicle of the first aspect comprises a prime mover. This differs from, e.g., WO 2015/059617 A1 , where the subsea vessel is towed by one or two surface vessels and does not propel itself. The presence of a prime mover on/in the underwater vehicle is advantageous since it provides improved flexibility of movement and use over, e.g., the vessel of WO 2015/059617 A1. This is because the reliance on surface vessels can be avoided. The absence of tow lines connected to the surface vessel allows for improved flexibility of movement of the vehicle of the first aspect. In addition, surface conditions have no bearing on the use of the vehicle of the first aspect, and thus it can be used during a greater range of weather conditions which the vessel of, e.g., WO 2015/059617 A1 cannot.

The primer move may be powered by batteries, hydrogen fuel cells or a nuclear power source.

The prime mover may be an electric motor.

The underwater vehicle may comprise one or more batteries. The battery/batteries may provide power to the prime mover for propulsion. The battery/batteries may additionally or alternatively provide power supply for ancillary functionality on the underwater vehicle.

The underwater vehicle may comprise one or more propeller(s) in communication with the prime mover and provided at, e.g., the aft end of the underwater vehicle with respect to the intended direction of travel of the underwater vehicle. The one or more propeller(s) may provide propulsion to the subsea vehicle. Additional, and/or alternative propulsion means in communication with the prime mover may be comprised in the underwater vehicle. For example, a thruster may be provided at, e.g., the aft end of the subsea vehicle to provide propulsion. The underwater vehicle may comprise a rudder and/or fins extending from an external surface, the rudder and/or fins being configured to provide stability and/or steering to the underwater vehicle.

The underwater vehicle may be an autonomous underwater vehicle (AUV). Additionally and/or alternatively the underwater vehicle may be a remotely operated vehicle (ROV). In each case, the underwater vehicle would comprise the necessary software, hardware and, optionally, communication devices to allow for the vehicle to be implemented as an AUV and/or an ROV.

The underwater vehicle may comprise autonomous navigation means.

The underwater vehicle may comprise one or more ballast tank(s). The one or more ballast tanks may be configured to be selectively filled with water (e.g. seawater surrounding the vehicle in use) and/or air. The content of water and/or air may be configured to be adjusted in order to control the buoyancy of the vehicle as necessary, e.g. in response to the vehicle being loaded or unloaded with cargo fluid to ensure that the vehicle remains neutrally buoyant at the same depth within the water. Other fluids may be used in place of water and air to the same effect in the ballast tank(s).

Additionally and/or alternatively, one or more of the storage pipe(s) may be used as ballast tanks. This can be achieved by selectively filling the one or more storage pipe(s) about the underwater vehicle as required to provide suitable ballast/buoyancy to the underwater vehicle. As such, one or more storage pipe(s) may be configured to be selectively filled so as to provide ballast to the underwater vehicle.

Each of the storage pipes may be in communication with at least one valve that allows for the fluid to be selectively loaded/unloaded into the storage pipes. The fluid may be configured to be loaded into each storage pipe through a first valve and configured to be unloaded through a second, different valve. Optionally the fluid may be configured to be loaded and unloaded via the same valve. The valve(s) may be pressure relief valve(s).

The underwater vehicle may comprise an outer hull with one or more cargo holds housing the storage pipes situated therein. The outer hull may be free- flooding or configured to be selectively opened to the surrounding water, so that when the vehicle is submerged the space between the outer hull and the storage pipes (i.e. the portion of the cargo hold(s) not filled with storage pipes) is at least partially filled with water. One, several or each of the storage pipe(s) may comprise a pressure compensation system.

The pressure compensation system may comprise a membrane (e.g. in the form of a ‘bladder’ type bag) situated within the storage pipe(s). The membrane may divide the interior of each storage pipe into two separate, hermetically sealed compartments. They may be configured to expand/contract in response to a pressure differential provided across the membrane between the two compartments defined by the seal.

Alternatively, the pressure compensation system may comprise a pistonlike seal (piston-like member) dividing the interior of each storage pipe into two separate, hermetically sealed compartments. To provide the piston-like seal, the seal may have an outer diameter that corresponds with the dimensions of the interior surface of the storage pipe(s) in order to provide sealing contact therewith. The piston-like seal may be slidable along the interior of the storage pipe(s). The piston-like seal may move in response to a pressure differential provided across the piston-like seal between the two compartments defined by the seal.

One of the two compartments defined by the piston like seal/membrane may be in communication with a (the) valve that is configured to allow for the fluid to be selectively loaded into said compartment. The second of the two compartments may be configured to be in communication with the water in which the underwater vehicle is situated and/or the water that has been flooded into the cargo hold(s) via an opening in the storage pipe and, optionally, a valve (e.g. the pressure relief valve as discussed below). This arrangement provides for a pressure compensation system, arranged to control the pressure of the fluid based on a pressure outside of the storage pipes. The pressure compensation system ensures the pressure differential in the storage pipes is minimised or even zero meaning that the storage pipes are not at risk of bursting or failure.

The pressure compensation system may be configured to compensate for pressure changes due to a change in water temperature, cargo fluid temperature and/or depth or change in depth of the underwater vehicle.

The pressure compensation system may also be configured to act as a volume displacement system used during loading and offloading the fluid from the storage pipes. That is, the pressure compensation system may also be configured to assist in loading and unloading the fluid from the storage pipes. In such a scenario, the pressure compensation system may additionally comprise a pump unit. The pump unit may be configured to pump water into the second of the two compartments of the storage pipe in order to increase pressure in the second of the two compartments.

The pressure compensation system may additionally comprise a pressure relief unit. The pressure relief unit may be configured to relieve water pressure from the second of the two compartments of the storage pipe (e.g. by opening a pressure relief valve).

The pump unit and the pressure relief unit may be configured to create a pressure differential across the two compartments in the storage pipe, which in turn moves the piston-like seal or causes the volume defined by the membrane to expand or contract, and thereby enables the loading and unloading of the fluid to be transported into and out of the storage pipes.

The piston-like seal may include an indicator of its position within the storage pipe, e.g. a radio frequency transmitter. This may be used to determine the degree of loading or unloading of the storage pipe.

The piston-like seal may comprise a pipeline pig (e.g. a foam pig). Foam pigs in particular are readily available, ‘off the shelf’ components and thus maintain the low costs associated with the underwater vehicle of the invention.

In addition to the fluid, the underwater vehicle may be configured to transport other, non-fluid cargo. These non-fluid loads can be transported internally within the subsea vehicle (i.e. held within a cargo hold) and/or held externally to the subsea vehicle.

The underwater vehicle may be configured to connect to a subsea loading site to have the fluid loaded into the storage pipes. The subsea loading site may be provided in the vicinity of, for example, a hydrocarbon production and/or injection site, or a subsea landfall terminal. The subsea loading site may be provided by a conduit attached to a surface vessel. After loading of the fluid into the storage pipes, the vehicle may be configured to disconnect from the loading site and to travel, via propulsion provided for by the prime mover, to an unloading site in order to unload the fluid. The unloading site may be a hydrocarbon production and/or injection site, or a subsea landfall terminal. The subsea unloading site may be provided by a conduit attached to a surface vessel.

When loading and/or unloading the fluid, the vehicle may be configured to simultaneously connect to an umbilical at the loading and/or unloading site for the provision of power/fuel and/or data. In accordance with a second aspect of the invention, there is provided a method of transporting a fluid using the underwater vehicle of the first aspect, the method comprising loading a fluid into storage pipes of the underwater vehicle at a loading site, and transporting the fluid in the underwater vehicle by means of propulsion provided for the prime mover.

The method of the second aspect may comprise a subsequent step of offloading the fluid from storage pipes at an offloading site.

The loading and/or offloading step of the second aspect may be carried out in accordance with the loading and/or offloading discussed above in connection with the first aspect of the invention.

Any of the optional features of the first aspect of the invention that are also applicable to the second aspect of the invention may be employed therewith.

In a third aspect of the invention, there is provided a method of assembling an underwater vehicle for transporting fluid, the underwater vehicle comprising a prime mover configured to power propulsion of the underwater vehicle through the water, the method comprising providing a plurality of storage pipes on-board the underwater vehicle for storing the fluid therein.

The underwater vehicle assembled in the third aspect of the invention may be in accordance with the underwater vehicle of the first aspect of the invention, optionally incorporating any optional features thereof.

In a fourth aspect of the invention there is provided use of the underwater vehicle of the first aspect of the invention for transporting a fluid. Use of the underwater vehicle of the first aspect may include use of any optional features of the underwater vehicle of the first aspect.

Certain preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a generic hydrate-formation phase diagram for an oil product;

Figure 2 shows an underwater vehicle according to an embodiment of the invention;

Figure 3A shows schematic cross-sectional views of the vehicle of Figure 2 in a first alternative arrangement;

Figure 3B shows schematic cross-sectional views of the vehicle of Figure 2 in a second alternative arrangement; Figure 4 shows an underwater vehicle according to an embodiment of the invention;

Figure 5 shows an underwater vehicle according to an embodiment of the invention;

Figure 6 shows a loading operation of an underwater vehicle in accordance with an embodiment of the invention;

Figure 7 shows a plurality of underwater vehicles being used to transport produced hydrocarbon product from a subsea wellhead to a storage vessel;

Figure 8 shows a loading operation of an underwater vehicle in accordance with an embodiment of the invention;

Figure 9 shows a pipe storage and pressure compensation system used in an underwater vehicle in accordance with an embodiment of the invention; and

Figure 10 shows an alternative pipe storage and pressure compensation system used in an underwater vehicle in accordance with an embodiment of the invention.

Figure 1 shows a hydrate formation phase diagram of a typical oil product (which may contain oil, water and gas), with the temperature and pressure at which the oil product may be held shown on the x- and y-axes respectively. There is a hydrate free region 401 on the right-hand side of a hydrate dissociation curve 402, a hydrate stable region 403 (i.e. a region where hydrates have formed and are stable in the fluid) on the left-hand side of a hydrate formation curve 404 and a metastable region 405 in between the hydrate formation curve and the hydrate dissociation curve where there is a risk of hydrate formation.

An oil product held at low pressure and high temperature will reduce hydrate formation, whereas high pressures and low temperatures increase hydrate formation.

The degassing and separation of water from the product alters the location of the hydrate formation and dissociation curves. Typically, such processing will move the hydrate formation curve to the left of the figure such that the oil product can be held at higher pressures and lower temperatures without the formation of hydrates. Thus, the oil product is said to be more stable, or further stabilised, when gas and water is removed therefrom.

Figure 2 shows an underwater vehicle 1. The underwater vehicle 1 has an outer hull 3 of a generally cylindrical shape with a hemispherical bow 5 and a tapered stern 7. A number of fins 9 radial protrude from the tapered stern 7 of the vehicle 1. A propeller 11 is provided at the stern 7 and is configured to be driven by a prime mover (not shown) in order to propel the vehicle 1 through the water.

Figures 3A and 3B respectively show schematic cross-sectional views of the vehicle 1 of Figure 2 in two alternative embodiments as taken from an aft end and a side of the vehicle 1. In the embodiment of Figure 3A, a plurality of storage pipes 13 can be seen to be provided within the hull of the vehicle 1. The storage pipes 13 are provided horizontally within the vehicle 1. That is, the primary axis of each of the longitudinal pipes 13 is aligned with the longitudinal axis of the vehicle 1. As can be seen in Figure 3A, each pipe 13 has a length that is substantially the same as the length of the vehicle 1, itself having a length of hundreds of metres. Each of the storage pipes 13 has a nominal diameter of 54 inches (140 cm). Schematically shown, there are thirty-two storage pipes 13 provided on the vessel; however, in a real-life implementation of the vehicle 1 there would be hundreds of storage pipes 13 provided on the vehicle 1 in the horizontal orientation, for instance one hundred storage pipes 13.

Each of the storage pipes 13 is configured to be loaded with and store therein a volume of fluid (e.g. a hydrocarbon gas, oil, MEG, CO2, ammonia, freshwater, seawater, biofuel, hydrogen, a semi-stable hydrocarbon oil product and/or chemicals). Once the fluid is stored in the storage pipes 13, the vehicle 1 can propel itself by means of the prime mover and the propeller 11 to transport the fluid in the storage pipes 13 as desired.

The embodiment of the underwater vehicle 1 as shown in Figure 3B is substantially the same as that of the Figure 3A embodiment, except that the plurality of storage pipes 13 are provided vertically (rather than horizontally) within the vehicle 1. That is, the primary axis of each of the longitudinal pipes 13 is normal to the longitudinal axis of the vehicle 1. As can be seen in Figure 3B, each pipe 13 has a length that is substantially the same as the diameter of the vehicle 1 , itself having a diameter of tens of metres in magnitude. Each of the storage pipes 13 has a nominal diameter of 54 inches (140 cm). Schematically shown there are one hundred and forty-four storage pipes 13 provided on the vessel; however, in a real-life implementation of the vehicle 1 there would be several hundred storage pipes 13 provided on the vehicle in the vertical orientation, for instance seven hundred and seventy storage pipes.

Figure 4 shows another embodiment of the vehicle 1 with the storage pipes 13 being provided in a horizontal orientation. Structural details with respect to the outer hull 3 of the vehicle 1 can also be seen in Figure 4. As shown in the Figure, the cylindrical outer hull 3 is formed from an outer layer 3a supported by a plurality concentric supports 3b surrounding the storage pipes 13 provided within the vehicle 1. This arrangement allows for the support layer 3a of the outer hull 3 to be made of a lightweight flexible or semi-rigid material, which is advantageous in terms of weight of the vehicle 1, whilst the vehicle 1 can still maintain its cylindrical, hydrodynamic shape.

Figure 5 shows an alternative embodiment of an underwater vehicle 10. The underwater vehicle 10 in many ways corresponds to the underwater vehicle 1 described above. For instance, the underwater vehicle 10 comprises an outer hull 3 with a propeller 11 powered by a prime mover (not shown) and configured to propel the vehicle 10 positioned at a tapered aft end 9 of the underwater vehicle 10. Within the hull 3 of the underwater vehicle 10 there is comprised a plurality of vertically orientated storage pipes 13 for storing fluid therein such that the fluid can be transported by means of the vehicle 10.

Where the underwater vehicle 10 differs from the embodiments of the underwater vehicle 1 described above is that the outer hull 3 is substantially cuboidal (rather than cylindrical) in shape. This cuboidal shape of the outer hull 3 results from the nature in which the storage pipes 13 are loaded, stored and unloaded to, in and from the vehicle 10 as discussed below.

As shown, the plurality of storage pipes 13 are comprised within seven different cargo modules 13a on the underwater vehicle 10. That is, the underwater vehicle 10 comprises a plurality of cargo modules 13a, each cargo module 13a comprising a subset of the plurality of storage pipes 13. As shown in Figure 5, each cargo module 13a comprising a subset of storage pipes 13 has a cubic shape. The cross-sectional area of the cubic shape of each cargo module 13a corresponds to the cross-sectional area of the outer hull 3 such that there is a close conformity between the two once the cargo modules 13a are stowed within the vehicle 10.

The cargo modules 13a are loaded into and removable from the underwater vehicle 10. In that way, the storage pipes 13 can be easily loaded into and removed from the underwater vehicle 10 in a batch wise approach as is desired. This has several advantages in that the vehicle 10 can be loaded and unloaded with the storage pipes 13, and thus fluid therein, quickly and simply. Moreover, the modularity provided for by use of the cargo modules 13a allows for a selective distribution of the storage pipes 13 about the vehicle which can assist with ballast and balance of the vehicle 10. Furthermore, cargo modules 13a comprising storage pipes 13 suitable for or containing different types of fluid can be used in a single vehicle 10 thereby enabling the vehicle 10 to carry a plurality of different fluids as is desired.

Figure 6 shows an exemplary loading operation of an underwater vehicle 1 in accordance with an embodiment of the invention. The underwater vehicle 1 is, for example, in accordance with the underwater vehicle 1 discussed above in connection with Figures 1-4.

As shown, the vehicle 1 is situated on the seabed 21 proximate a subsea loading site 23. In this instance, the subsea loading site 23 is a subsea wellhead 23 from which a hydrocarbon product is produced. A conduit 25 is connected to the wellhead 23 at a first end and is connected at its other end to an inlet 2 of the underwater vehicle 1. The connection between the conduit 25 and the inlet 2 is reversible such that the vehicle 1 can attach and detach from the conduit as desired.

In use, the vehicle 1 propels itself, by virtue of its prime mover (not shown) and propeller 11 to the proximity of the wellhead 23. Once located as appropriate for loading, the vehicle 1 positions itself on the seabed 21.

Once situated on the seabed 21, the conduit 25 is connected to the inlet 2 of the vehicle 1. The hydrocarbon product is then transferred from the wellhead 23 to the vehicle 1 via the conduit 25, where it passed through the inlet 2 and is transferred into the plurality of storage pipes (not shown) within the vehicle 1.

Once the storage pipes are completely filled, or filled as desired, the loading of the hydrocarbon product onto the vehicle 1 is ceased. The conduit 25 is then detached from the inlet 2 of the vehicle 1. The vehicle 1 is then able to lift itself from the seabed 21 and propel itself, by means of its prime mover and propeller 11, to a desired location (e.g. a landfall terminal) where the hydrocarbon product stored within the storage pipes can be unloaded.

Figure 7 shows a plurality of underwater vehicles 1 used as shuttles to transport produced hydrocarbon product from a subsea wellhead 23 to a storage vessel 31.

Specifically, in Figure 7, three underwater vehicles 1a-1c are shown. A first underwater vehicle 1a is situated on the seabed 21 proximate the subsea wellhead 23 and is connected thereto via a conduit 25. The first underwater vehicle 1a is being loaded with hydrocarbon product from the subsea wellhead 23 via the conduit 25. This loading process corresponds to that described above with reference to Figure 6, and will not be described further here.

A second underwater vehicle 1b is travelling between the storage vessel 31 and the subsea wellhead 23. The storage pipes (not shown) within the underwater vehicle 1b will have no hydrocarbon product stored therein since this will have been offloaded to the storage vessel 31 (see description below with reference to vehicle 1c). The underwater vehicle 1b is travelling to the site of the subsea wellhead 23 so as to be loaded with more produced hydrocarbon product for subsequent transportation.

A third underwater vehicle 1c can be seen to be in the proximity of the storage vessel 31. Specifically, the underwater vehicle 1c is situated on the seabed 21 beneath the storage vessel 31, which is a surface vessel floating at sea level. The storage vessel 31 comprises a plurality of storage tanks 33 thereon which enable more permanent storage of the hydrocarbon product produced at the wellhead 23 and transported thereto by the underwater vehicles 1a-1c.

The third underwater vehicle 1c is attached at its inlet 2 to a first end of a riser 35 which at its second end connects to the storage vessel 31. The conduit enables the hydrocarbon product stored within the storage pipes of the underwater vehicle 1c to be passed to the storage tanks 33 on the storage vessel 31. The hydrocarbon product in the storage pipes of the underwater vehicle 1c is unloaded therefrom to the storage tanks 33 via the riser 35. Once this offloading is complete such that the storage pipes of the vehicle 1 comprise no hydrocarbon product, the conduit 35 is disconnected from the vehicle 1c. The vehicle 1c then propels itself from the seabed 21 toward the subsea wellhead 23 for collection of further hydrocarbon product (i.e. as is the case for underwater vehicle 1b).

The underwater vehicles 1a-1c thus work in cyclical shuttle rotation with one another to thereby allow for an effectively continuous transportation of hydrocarbon product from the subsea wellhead 23 to the storage vessel 31 for storage.

Figure 8 shows an alternative use for an underwater vehicle 1. In Figure 8, the underwater vehicle 1 is being loaded with ammonia for transportation from a surface vessel 41 via a conduit 43 attaching to an inlet 2 of the underwater vehicle 1. The ammonia is stored within storage pipes (not shown) on the underwater vehicle 1 once loaded thereon. The underwater vehicle 1 is loaded with ammonia whilst suspended subsea (i.e. not at the seabed) from the vessel 41. In that way, loading can be done nearer the surface and optionally whilst both vessel 41 and underwater vehicle 1 are on the move.

Figure 9 shows a piston-like seal 51 and how such a piston-like seal 51 is arranged within the interior of a storage pipe 13 of an underwater vehicle in order to provide a pressure compensation system therein. The piston-like seal 51 comprises a first 51a and a second 51b arcuate sealing surface that each are sized and configured to engage with an interior surface of the storage pipe 13.

The piston-like seal 51 divides the interior of the storage pipe 13 into two separate, hermetically sealed compartments when installed therein: a first compartment 14a and a second compartment 14b. The piston-like seal 51 is slidable along the interior of the storage pipe 13 and moves along the interior of the storage pipe 13 in response to a pressure differential provided across the pistonlike seal 51 between the two compartments 14a, 14b defined by the seal.

The first compartment 14a defined by the piston like seal 51a is in communication with a pressure relief valve (not shown) that is configured to allow for cargo fluid (e.g. hydrocarbon product) to be selectively loaded into the first compartment 14a of the storage pipe 13. The second compartment 14b is in communication with the water in which the underwater vehicle is situated via another pressure relief valve (not shown) in the storage pipe 13. This arrangement provides for a pressure compensation system arranged to control the pressure of the cargo fluid in the first compartment 14a. The pressure compensation system ensures the pressure differential in the storage pipe 13 is minimised or even zero meaning that the storage pipe 13 is not at risk of bursting or failure.

The pressure compensation system provided for by means of the piston-like seal 51 can be used, in combination with a pump unit (not shown), to act as a volume displacement system that is used to load and offload the cargo fluid into and out of the storage pipe 13.

The pump unit can be controlled to pump water into the second of the two compartments 14b of the storage pipe 13 in order to increase/decrease pressure in the second of the two compartments 14b. As such, the pressure in the second compartment 14b can be increased to above that of the pressure in the first compartment 14a (defined by the pressure relief valve in communication with the first compartment 14a), which in turn causes the piston-like seal 51 to slide so as to reduce the volume of the first compartment 14a and thereby force any cargo fluid out of the first compartment 14a via the pressure relief valve (not shown) associated with the first compartment 14a. In that way, the water in the second compartment 14b can act as a displacement fluid for unloading cargo fluid from the storage pipe 13. Cargo fluid can be loaded into the first compartment 14a and displace water in the second compartment 14b in the reverse manner to that which has been described.

Figure 10 shows an alternative pressure compensation system provided within a storage pipe 13. In this embodiment, the pressure compensation system comprises a membrane 61 situated within the storage pipe 13 in place of the pistonlike seal 51 of Figure 9. The membrane 61 divides the interior of the storage pipe 13 into two separate, hermetically sealed compartments 14a, 14b as is the case for the piston-like seal 51 as discussed above in relation to Figure 9. Functionality surrounding the use of the pressure compensation system comprising the membrane 61 substantially corresponds to the pressure compensation system comprising the piston-like seal 51 described above, with the expansion and contraction of the volume of the membrane 61 permitting the compartments 14a, 14b to alter their size to permit loading and unloading of cargo fluid.