GB2529311A

GB2529311A - Submersible fluid storage

Info

Publication number: GB2529311A
Application number: GB1513101.4A
Authority: GB
Inventors: David Collard
Original assignee: MONOBUOY Co Ltd
Current assignee: MONOBUOY Co Ltd
Priority date: 2012-02-28
Filing date: 2012-02-28
Publication date: 2016-02-17
Also published as: GB201513101D0

Abstract

A container for storing fluid which is less dense than water has a chamber 101, 201 which collapses under water pressure and is negatively buoyant so that it sinks even when the chamber is full. Ideally the chamber is in the form of a flexible bladder. The weight may be provided as a concrete plinth 202 attached by flexible lines integral to the chamber; or as a housing surrounding the bladder. The housing may include an aperture covered by a grill so that seawater pressure can act on the bladder. The bladder may fully surround its interior or part of the chamber may be formed by a wall of the housing. A buoyant concave cover 220 may be tethered to the bladder to capture any leaked fluids. Multiple bladders could be connected via a manifold to the surface by a single conduit. The fluid may come from a source below sea level such as an oil well. The bladder may be placed by falling under gravity.

Description

SUMBERSIBLE FLUID STORAGE

The present invention relates to the storage of fluids having a lower density than that of water. The invention has been specifically developed for the storage of hydrocarbons but has more general application in the storage of various fluids. The fluids will usually be in liquid form.

The following describes container assemblies for the storage of fluids having a lower density than water, and systems comprising the same. Also described is a method of storing fluids.

The assemblies to be described in the following were devised in order to satisfy a need to provide additional diesel storage at oil producing platforms. The objective was to design and install a practical, safe and cost efficient method of increasing hydrocarbon storage capacity of the platforms. One option that was considered was to install a subsea storage tank (steel fabricated pressure vessel) with a subsea pump close to the platform.

However this would be difficult an expensive to install.

In the following there is provided a submersible container assembly for underwater storage of fluid having a lower density than that of water, the assembly comprising a chamber for the fluid that is collapsible under water pressure and the assembly having a mass such that with the collapsible chamber filled with the fluid the container assembly is negatively buoyant. Thus the assembly is negatively buoyant even when the chamber is full of fluid.

The chamber may be wholly or partially defined by a flexible partition, which may be in the form of a bladder.

An underwater storage system may be provided comprising multiple container assemblies. The multiple assemblies may be connected, for example by flexible hoses or hard piping.

Depending on the suitability of the crude oil, the bladders or other suitable collapsible chambers could be employed at subsea locations to receive hydrocarbon directly from a subsea well, negating the need for expensive manned platforms and floating production storage & offloading (FPSOs) on the surface. They would be particularly useful for older wells nearing the end of their production lives where output has slowed to an extent that they are no longer commercially viable due to the cost of maintaining a permanent facility.

The bladders or other suitable collapsible chambers could be emptied regularly by a shuttle tanker that moored up to an unmanned offshore terminal installed close to the bladders (for example a catenery anchor leg mooring (CALM) or single anchor leg mooring (SALM) buoy).

For such applications the bladders or other collapsible chambers would require dedicated tilling and discharge lines as well as a vent line to a surface structure like a CALM buoy.

The bladders or other collapsible chambers may each be contained in a housing. For some implementations it may be useful to provide multiple collapsible containers in a single housing.

Alternatively the bladders or other collapsible chambers may simply be weighted, tor example using a plinth or gravity base. Usually one weight would be provided for each chamber but in some implementations it may be useful to tether multiple chambers to a single base or other weight.

There is also provided in the following a method ot underwater storage of fluid having a lower density than that of water, the method comprising providing a chamber for the fluid which is collapsible under pressure of water at the desired storage location, weighting the chamber so that with the collapsible chamber filled with the fluid the container assembly is negatively buoyant, positioning the weighted chamber and filling the chamber with the fluid.

Various embodiments of container assembly as described above will now be described with reference to the accompanying drawings in which: Figure 1(a) is a perspective view of a first container; Figure 1(b) is a top plan view of the container of figure 1(a); Figure 1(c) is a side elevation of the container of figure 1(a); Figure 1(d) is an end elevation of the container of figure 1(a); Figure 1(e) is a bottom elevation the container of figure 1 (a): Figure 1(f) is a cross section of the container of figure 1(a) along the line A-A shown in figure 1(c); Figure 2(a) is a perspective view from above and one side of a second container; and Figure 2(b) is a perspective view from below and the other side of the container shown in figure 2(a): Figure 3 is a schematic view of a possible implementation of a container at the base of a platform, in which the container is in fluid communication with a storage tank above sea level; Figure 4 is a schematic diagram showing piping interconnecting multiple containers with a discharge tank on an oil platform; and Figure 5 is a schematic view similar to figure 3 in which a container is arranged to be filled attheseabed.

Referring firstly to figures 1 (a) to (f), these figures show a container assembly comprising a housing indicated by reference numeral 100. Within the housing 100 is a bladder 101.

The bladder 101 has a port or opening 102 via which fluid can enter or exit the bladder.

The opening 102 is provided with a fitment 103 to which one or more pipelines can be connected. In the illustration of figure 1, fitment 103 provides connections to two pipelines 104 and 105. Pipeline 104 is used to fill the bladder 101 with fluid and pipeline 105 is used to vent fluid in the event that the bladder 101 overflows.

The fitment 103 permitting access to the interior of the bladder 101 is secured in an opening in the housing 100, and in use the pipelines 104 and 105 are connected to the fitment 103. Suitable valves 106 and 107 are provided for closing the pipelines.

In the container assembly of figure 1 the collapsible chamber for fluid is contained within the housing 100 and comprises the bladder 101. The housing should therefore be constructed so as to be able to accommodate loads imparted by the buoyancy of the filled chamber. For example, preferably the walls of the housing 100 should not bulge when the chamber is full, i.e. the chamber is at maximum volume. For some possible uses of the container assembly it should be possible to lift it when full in which case the housing should be able to withstand the mass of the filled chamber. The housing should also be able to accommodate loads incurred during transportation and installation.

The housing 100 has one or more apertures 108 arranged to allow entry of water in use.

The illustrated housing 100 is cuboidal in shape and has rectangular top bottom and side walls. The end walls may be square or rectangular. The housing 100 shown in the figures has two apertures 108 in its top wall and three apertures 108 in each of its side walls.

Each of the apertures is covered by a retractable grill or mesh 109. Thus each aperture 108 permits the ingress of water when the container is submerged. The grill prevents the bladder 101 from bulging out of the apertures 108. It will be appreciated that the same effect could be achieved without the use of grills. For example each of the apertures 108 could be replaced by multiple smaller apertures.

The illustrated container has reinforcing ribs extending transversely across its walls, some of which are indicated by reference 110.

Sacrificial anodes, some of which are indicated by reference numerals 111, are secured to the outside of the housing 100, for example by means of bolts. These are provided to limit corrosion and may for example be fabricated from aluminium or zinc.

The bladder 101 is secured to the housing 100 in the region of its opening 102. It may be fixed at additional points or regions so as to prevent it from flapping loosely in the housing and to control its shape as it is filled with fluid.

The container assembly will usually be provided in an empty condition. In this condition, the bladder 101 is preferably completely flat so that it contains no air, but this is not essential. The space in the housing 100 around the bladderl 01 will be filled with air.

In preparation for use the container assembly is submerged either by lowering it into water or simply dropping it, depending on how it is to be used. Water then enters the space around the bladder through the apertures 108.

The bladder 101 can be filled with fluid under pressure once the container assembly is in position. As the bladder 101 is filled, its volume increases and water is forced out of the apertures 108. The weight of the container assembly is such that when the bladder 101 is full it is still negatively buoyant. The necessary weight to counter the buoyancy of the filled fluid chamber can be provided entirely by the housing 100. Alternatively an additional weight can be provided in the housing 100. It will be appreciated that the necessary weight will depend on the relative densities of the water and the fluid to be stored. For example, a greater weight will be required to submerge the same volume of fluid in sea water rather than river water. The container will be designed for its intended purpose.

With the container assembly submerged and the bladder containing fluid, fluid can be removed from the bladder 101 very simply without the need for a pump. On opening of either of the valves to pipelines 104 or 105, the pressure of water on the bladder 101 or other collapsible chamber will force water out of the chamber.

It will be appreciated that the bladder 101 could be replaced by any collapsible chamber capable of holding fluid. The walls of the chamber need not be entirely flexible as is usually the case with a bladder. For example the bladder could be replaced with a chamber having one or more rigid walls onto which one or more flexible walls collapse.

The collapsible chamber could have collapsible walls in the form of a concertina.

An ideal construction for a container assembly for the storage of hydrocarbons such as crude oil or fuel such as diesel comprises a housing 100 in the form of a steel fabricated box structure. Such a structure can have the same dimensions as a standard shipping container (12.1 92m x 2.438m x 2.591 m). This will allow each container assembly to be easily transported and installed.

As noted above each container assembly should be heavy enough to overcome the buoyancy of the bladder (when full). For the purpose of storing hydrocarbons the container assembly should preferably not exceed the currently allowable shipping weight of a standard container (when empty) in order to allow easy transportation, and installation, of the containers. The maximum allowable weight of a full container for shipping is presently 30,480 kg.

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It may be advantageous for the housing to have no apertures 108 in its top wall. The top wall and surrounding portions of the side and end walls may then act as a trap for any fluid that may leak from the fluid chamber.

The additional weight, if required, may take the form of concrete ballast and may be added to the bottom of the housing. This may improve stability. With a standard shipping container used as the housing, approximately 200mm of concrete will be required in the base of the container for the container to be suitable for diesel storage.

The housing preferably has sufficient lifting points on its top wall to allow for easy installation. They are preferably fined with standard twistlock' fittings, e.g. at the corners in the manner of a normal shipping container, to allow for cheap and easy transportation

from the fabrication site to the field.

The housing may contain marine inhibitors to prevent marine growth from growing on the insides of the housing. This may be attached to the walls of the housing, for example as a coating.

A suitable coating, e.g. polyurethane, may be applied to the inside of the housing 100 which reduces the effect of friction as the bladder 101 expands and contracts inside the housing 100. Any other friction reducing material may be used.

The container assembly may be fitted with an overfill cut-off system to isolate the bladder 101 once it is full and prevent over pressurization of the filled volume. This may be achieved in several ways. Two possibilities are as follows: 1) A simple level switch inside a small overfill tank on the platform. The switch may be connected to a basic control system that controls a shut down valve on a subsea manifold, to be described further below. Once the chamber or chambers get too full the pressure will start to increase and fluid will start to flow out the open vent into the overfill tank. The level switch will then activate and the control system will close the shut down valve isolating the bladders.

2) A subsea cutoff device that is mechanically triggered by the expanding bladder -for example operating similarly to a toilet cistern. i.e. a simple lever in the bottom of the housing that is pushed down by the expanding bladder and isolates the bladder from the filling line. This may require independent filling and discharge lines.

The bladder 101 may be fabricated by a specialist rubber manufacturer, such as Dunlop or Michelin. Bladders suitable for the storage of hydrocarbon fuel have been manufactured in the past. One suitable combination of materials for the bladder comprises high tenacity woven nylon and nitrile rubber. It is preferred for the inside of the bladder 101 to be coated with nitrile rubber due to its resistance to petroleum products.

Each bladder 101 may have a capacity of 40 to 50 m3 and may be secured to the roof or top wall of the housing, as with the bladder shown in the figures which is secured to the top wall about its opening. The bladder 101 may be positioned to minimise any rubbing on the housing sides or so as to minimise any damage to the bladder 101 caused by such rubbing.

For use in hydrocarbon storage it is ideal for each bladder 101 to have a 2" (0.OSm) filling and discharge port and a 2" (0.05m) dedicated vent line port which are connected to respective pipelines 104 and 105.

The foregoing describes an example container assembly shown in figures 1(a) to 1(f) in which fluid is stored in the bladder 101 and water fills the space 115 between the housing and the bladder 101. The bladder 101 forms a flexible partition dividing the space inside the housing 100 into a first volume for fluid to be stored and a second volume for water. In an alternative embodiment of container assembly, the space 115 between the outside of the bladder 101 and the inside of the housing 100 may be made watertight.

The fluid may be then stored in this space 115. In this arrangement a chamber for storage of fluid is defined between the bladder 101 and the housing 100. The inside of the bladder 101 may then be open to water when submerged, e.g. via the opening 103, so that water inside the bladder 101 puts pressure on the fluid chamber. The discharge and vent pipelines 104 and 105 would then be arranged to communicate with the inside of the housing 100, outside the bladder 101.

The alternative container assembly described in the preceding paragraph has an advantages over a container assembly in which the fuel is contained in the bladder 101.

With the fuel being stored between the bladder 101 and the housing 100, there would be no possibility of marine growth on the inside of the housing 100 causing possible damage to the bladder 101. It will be appreciated that a standard shipping container would not

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then be suitable for use as the housing 100. The housing 100 would require to be designed, built and tested as per relevant industry rules and standards for pressure vessels, depending on the intended application, such as American Society of Mechanical Engineers (ASME) VIII.

In another possible arrangement, the bladder 101 could be replaced by a flexible partition dividing the housing 100 into two chambers each defined partially by one or more walls of the housing 100, one chamber being for the fluid to be stored and one being open to flow of water.

The container assembly as described above may be carried on an installation vessel to its required location. The design is such that it may be pre-tested on the installation vessel. A further advantage is that it can be installed without the need for divers.

The flexible connections, e.g. hoses, can be deployed by the installation vessel and connected up using a remotely operated vehicle (ROV). Where possible, rigid pipes may take the place of flexible hoses if preferred.

The assembly may be designed and fabricated in accordance with Bureau Veritas (BV) rules, where applicable.

The preferred system comprising multiple container assemblies is modular so that individual assemblies can be isolated and replaced if necessary without severely impacting the storage capacity of the system. Additional storage capacity can be added to the system in the future at relatively little cost.

The assemblies can be easily installed by a small installation vessel with a 25 T deck crane. No complex and expensive installation spread is required. As noted above, the installation of the system can be done without the need for divers, thus reducing cost and improving safety. The assemblies can be easily recovered from the seabed in the future.

The preferred assemblies are not buoyant and do not require additional ballasting. They can quickly and easily be lowered onto the seabed and connected up with suitable conduits such as flexible hoses or rigid pipes as appropriate.

The installation of the assemblies does not require a high degree of accuracy when being positioned on the seabed.

In the event of a leak fuel is prevented from escaping to the ocean as it is contained within the housing. Oily water detection instrumentation can be installed inside the housing to detect any leakage.

The bladders are protected from dropped objects and fishing trawls by the external steel container. The housings can be easily transported if they are standard shipping container sizes and within the allowable weight range.

The following are examples of suitable parameters for an assembly as illustrated in figures 1(a) to 1(0: Volume of Container: 77 m3 Bladder Volume: 50 m3 Volume Required: 300 m3 Number of Tanks: S Weight of Container (No Ballast) 12,000 kg Additional Concrete Ballast Required 11,500 kg Total Weight of Container (In Air): 23,500 kg SG Diesel: 0.81 SG Seawater: 1.025 Bladder Buoyancy: 10,750 kg Max Allowable weight of Container: 30,480 kg Water Depth (to top of container): 64 m Pressure 6.43 Bar Head of Diesel 81 m An alternative container assembly is now described with reference to figures 2(a) and 2(b). These figures show a container assembly indicated generally by reference numeral 200. In contrast to the assembly of figures 1(a) to (f), rather than having the bladder or other fluid chamber within a housing such as a container, a bladder or other collapsible chamber is tethered to a base other suitable weight. The weight of the base or other suitable weight is such that when the chamber is filled with fluid, the entire assembly 200 is negatively buoyant.

Figures 2 (a) and 2(b) show a bladder 201 tethered to a concrete plinth 202. The plinth 202 could have the size of a standard shipping container base. Tethers are provided for securing the bladder 201 to the plinth 202. Some of the tethers are indicated by reference 203. The tethers 203, which are preferably flexible, may be moulded into the bladder and thus nylon or polypropylene is a suitable material for the tethers. The purpose of the tethers is to secure the bladder 201 to the plinth or other gravity base and prevent it from floating to the surface of the water.

Depending on the construction of the bladder or other fluid chamber, multiple tethers 203 may be required to maintain the correct shape of the chamber when it is partially inflated.

Thus, figure 2 shows a generally elongate bladder 201 with tethers 203 spaced evenly along its length. The bladder may be similar in construction to the bladder 101 described in connection with figures 1(a) to (f).

As with the assembly of figures 1(a) to (f), the bladder has an opening provided with a fitment 205 to which filling and vent pipelines 206 and 207 are connected. Each pipeline has a suitable valve 208 and 209. In the illustration of figures 2(a) and 2(b) the fitment is provided at one end of the bladder 201.

The assembly of figures 2(a) and 2(b) is shown with an optional cover 220. The cover is positioned above the bladder 201 or other fluid chamber. It may be rigidly connected to the plinth or other weight. However in the preferred arrangement the cover is buoyant and is flexibly attached to the plinth by means of additional tethers, some of which are indicated at 221. The tethers 221 may be made from the same material as the tethers 203.

The cover 220 may protect the bladder from small dropped objects. It also assists in the positioning of the assembly by acting as a parachute when the assembly 200 is dropped or lowered into the water.

The cover shown in figures 2(a) and 2(b) has the shape of an inverted U and therefore defines an inverted channel positioned over the fluid chamber. The channel may be partially closed at its ends by end walls, one of which is indicated by reference 222. The cover 220 thus defines an inverted receptacle for capturing buoyant fluids or objects in the water above the bladder or chamber. For example, the cover may capture leakage of fluid from the chamber.

The deployment of the assembly shown in figures 2(a) and 2(b) may be similar to that of the assembly of figures 1(a) to 1(f). In particular, it is designed to be installed without the need for divers. The bladder, cover and tethers may be pre-connected on the installation vessel and secured to the base by securing straps. The base may then be lowered onto the seabed. Once the base is in place the securing straps may then be released remotely whereby the suspended bladder is deployed.

It may be necessary to ensure successful deployment of the bladder by visual ROV inspection.

The assembly of figures 2(a) and 2(b) has many of the same advantages as the assembly of figures 1(a) to 1(f).

The system comprising multiple assemblies is modular so that individual bladders can be isolated and replaced if necessary without severely impacting the storage capacity of the system. Additional storage capacity can be added to the system in the future at relatively little cost.

The container assemblies can be easily installed by a small installation vessel with a 25 T deck crane. No complex and expensive installation spread is required.

The installation of the system can be done without the need for divers, thus reducing cost and improving safety.

The bladders can be easily recovered from the seabed in the future.

The installation of the bladders does not require a high degree of accuracy when being positioned on the seabed.

The container assemblies can be easily transported as they are standard shipping container sizes and within the allowable weight range.

There is no risk of the bladders rubbing on any hard surfaces and suffering damage.

Figure 3 shows one possible implementation of a container assembly as described above for use in storing hydrocarbons, such as diesel or crude oil at the base of an oil platform.

Figure 3 shows an oil platform 300 supported on the sea bed 301 by legs 302. A storage tank 303 is provided on the platform for the storage of diesel fuel for power generation for the platform 300. A container assembly 310 of the kind illustrated in figures 1(a) to 1(f) is shown on the sea bed 301. The container assembly of figures 2(a) and 2(b) could equally well be used in this situation.

The tilling and discharge pipes shown generally at 311 extending trom the assembly 310 are connected to a manifold 312. Other assemblies, not shown, may be connected to the same manifold 312. The filing and discharge pipes are preterably but not necessarily flexible. The container assemblies are connected via the manifold 312 and further connecting pipes, preferably tlexible, to the storage tank 303 on the platform. The further connecting pipes form part of a riser bundle 313 that includes one or more control lines for the valves of the container assemblies. The riser bundle will in practice be attached to a platform leg 302 but is shown spaced from the leg for clarity.

The system is modular in that each container assembly can be isolated and replaced it necessary. Additional assemblies can be added to the system in the future provided there are enough spare connections on the manitold 312.

Suitable connections and manifolds for a system comprising multiple container assemblies, for use in subsea storage of hydrocarbons, will now be described with reference to figure 4. This shows the connection equipment at the sea bed and the platform respectively. The diagram of figure 4 is suitable tor use with the assemblies of figures 1 and 2 as well as other assemblies generally described in the toregoing.

Figure 4 shows multiple container assemblies 400 on the sea bed. Each ot these is connected via connections to a manifold 401 at the sea bed. The connections may be 2" (0.05m) flexible hoses. The manifold 401 is connected to equipment on the platform via 3" (0.075m) hoses torming part of one or more riser bundles attached to a plattorm leg.

An electro-hydraulic umbilical 402 carries one or more control lines from an electro-hydraulic control system 403 on the platform to the manitold 401.

More specifically, the chambers ot the assemblies 400 may be tilled through a 3" (0.075m) line 405, preferably flexible, that runs from a connection point on the platform to the manifold 401 on the sea bed and then to the individual subsea chambers via respective 2" (0.05m) flexible lines 406. Discharge of the containerized bladders to the platform storage tank may take place through the same line.

It is proposed that the assemblies be connected to the manifold 401 via 2" hydraulic lines 406, preferably flexible, and trom the manifold to the platform storage tank by a single 3" filling and discharge riser 405. The riser 405 is preterably fixed to the platform leg.

Although the discharge of fluid will be largely achieved by pressure of water on the collapsible chambers, a booster pump 407 may be provided on the filling/discharge line for use in discharging fluid from the chambers.

The connections between hoses and at the ends of the hoses can be remotely operated vehicle (ROV) compatible or quick couple hydraulic connections. For simple diesel storage the filling and discharging of the bladders will be done through the same line, although for alternative applications dedicated filling and discharge lines may be necessary.

In the proposed implementation, each container assembly 400 can be isolated at the manifold 401 by manual ball valves. The manifold 401 should ideally also contain an actuated fail closed' shutdown valve 407 that will prevent all loading and unloading of the chambers in the event of an emergency or catastrophic failure on the platform.

Each container assembly should also contain a 2" dedicated vent line 408 to prevent overpressure during filling. These lines 408 should be unrestricted and run back to a vent manifold forming pad of the manifold 401 which then goes to a single 3" riser 409 to the platform tank 303. The vent line 409 should preferably maintain a column of diesel rather than contain any pressure relief valves which would require subsea maintenance.

Overpressure will result in the diesel being forced back in to the platform diesel tank 303.

Each vent 408 can be closed by a manual ball valve for maintenance operations.

A level switch is provided to activate a shut down valve 410 at the storage tank 303 on detection of a high level reading.

Figure 5 shows an alternative implementation of one or more container assemblies arranged to be filled at the sea bed. Figure 5 shows a single mooring point platform 500 at the sea surface held in place by multiple anchor chains 501. Hydrocarbons are led directly from a well head 505 to a container assembly 506 via a first set of submarine hoses 508. A second set of submarine hoses 509 conducts fluid from the container assembly 506 to the platform. From here hydrocarbons may be led away via floating hoses 510.

The foregoing describes specific implementations of container assemblies for use in the storage of hydrocarbons but it will be appreciated that the container assemblies described above will have numerous other possible implementations for the storage of fluids.

The following table provides further information on the acronyms used in the foregoing

description:

ASME American Society of Mechanical Engineers, An organisation that publish codes and guidelines for the design, fabrication and testing of various mechanical components.

BV Bureau Veritas, A Classification Society based in France.

CALM Buoy Catenery Anchor Leg Mooring, An anchoring system with multiple anchor lines.

FPSO Floating Production Storage & Offloading, A floating installation that receives hydrocarbons directly from the subsea well, pre-processes it and stores for offloading to a shuttle tanker FSO Floating Storage and Offloading, a floating installation similar to an FPSO but does not perform any processing of the hydrocarbons.

PLEM Pipeline End Manifold, a subsea structure on the seabed for transferring hydrocarbons.

ROV Remote Operated Vehicle, a small submersible robot for performing subsea surveys and simple tasks.

SALM Buoy Single Anchor Leg Mooring, An anchoring system with a single anchor line SG Specific Gravity, Density of a liquid in relation to water.

SDV Shut Down Valve, An actuated valve that will always revert to the safe' position (In this case it will close) automatically in the event of a power or signal loss

Claims

Claims 1. A submersible container assembly for underwater storage of fluid having a lower density than that of water, the assembly comprising a chamber for the fluid that is collapsible under water pressure and the assembly having a mass such that with the collapsible chamber filled with the fluid the container assembly is negatively buoyant, wherein a housing of the submersible container assembly has dimensions the same as a standard shipping container or a plinth of the submersible container assembly has dimensions the same as a standard shipping container base.
2. An assembly as claimed in claim 1 comprising a flexible partition which wholly or partially defines the collapsible chamber.
3. An assembly as claimed in claim 2 in which the flexible partition is in the form of a bladder.
4. An assembly as claimed in any preceding claim wherein the collapsible chamber is formed within the housing.
5. An assembly as claimed in claim 4 in which the housing comprises one or more apertures arranged to allow water into the housing when the housing is submerged so as to apply pressure to the collapsible chamber.
6. An assembly as claimed in claim 5 in which at least one of the apertures is covered by a grille.
7. An assembly as claimed in claim 2 or 3 or any of claims 4 to 6 when dependent on claim 2 or 3 in which the flexible partition wholly contains the fluid in use.
8. An assembly as claimed in claim 2 or 3 or any of claims 4 to 6 when dependent on claim 2 or 3 in which the collapsible chamber is formed between the flexible partition and one or more walls of the housing.
9. An assembly as claimed in claim 8 in which the flexible partition defines a volume for receiving water and has an opening permitting the entry of water when the assembly is submerged.
10. An assembly as claimed in any preceding claim comprising one or more weights positioned outside the chamber to at least partially offset the buoyancy of the chamber in use.
11. An assembly as claimed in claim 10 wherein the plinth comprises a weight for the assembly.
12. An assembly as claimed in claim 10 or claim 11 in which the one or more weights are formed of concrete.
13. An assembly as claimed in claim 10, 11 or 12 in which the one ore more weights are attached to the chamber.
14. An assembly as claimed in any of claims 10 to 13 in which the one or more weights are attached to the chamber via one or more flexible lines.
15. An assembly as claimed in claim 14 in which the one or more flexible lines are integrally formed with the chamber.
16. An assembly as claimed in any of claims 4 to 9 in which the housing has negative buoyancy sufficient to overcome the positive buoyancy of the chamber when filled with the fluid.
17. An assembly as claimed in any preceding claim comprising a cover positioned over the collapsible chamber in use.
18. An assembly as claimed in claim 17 in which the cover is buoyant in water.
19. As assembly as claimed in claim 17 or 18 in which the cover defines an inverted receptacle for capturing buoyant fluids or objects in the water above the chamber.
20. An assembly as claimed in claim 17, 18 or 19 in which the cover is tethered to the collapsible chamber.
21. An underwater storage system comprising multiple assemblies each as claimed in any preceding claim.
22. A system as claimed in claim 21 in which each assembly has an outlet from the collapsible chamber connected to a manifold and the system further comprises a conduit for fluid from the manifold to a location at the water surface.
23. A method of underwater storage of fluid having a lower density than that of water, the method comprising providing a chamber for the fluid which is collapsible under pressure of water at the desired storage location, weighting the chamber so that with the collapsible chamber filled with the fluid the container assembly is negatively buoyant, positioning the weighted chamber and filling the chamber with the fluid, wherein the chamber is provided as a submersible container assembly and a housing of the submersible container assembly has dimensions the same as a standard shipping container or a plinth of the submersible container assembly has dimensions the same as a standard shipping container base.
24. A method as claimed in claim 23 in which the positioning of the chamber comprises allowing the weighted chamber to fall to the desired position under gravity.
25. A method as claimed in claim 23 or 24 comprising providing multiple weighted collapsible chambers in fluid communication with a source of the fluid.
26. A method as claimed in claim 23, 24 or 25 in which the source of the fluid is below sea level.
27. A method as claimed in any of claims 23 to 26 comprising providing a fluid line extending upwardly from the chamber whereby fluid may be forced towards the water surface under pressure of water on the chamber.
28. A method as claimed in any of claims 23 to 27 in which the chamber is part of an assembly as claimed in any of claims 1 to 20.
29. Any of the assemblies substantially as hereinbefore described with reference to the accompanying drawings.
30. A method of underwater storage of fluid substantially as hereinbefore described with reference to the accompanying drawings.