This invention relates to the storage of fluids underwater, for example for the purposes of the subsea oil and gas industry. These fluids may include produced hydrocarbons or other fluids used to support hydrocarbon production, such as chemicals or fresh water for subsea processing of hydrocarbons or for injection into a subsea well.

Offshore exploration for oil and gas is being performed in increasingly challenging locations and deeper waters. To recover hydrocarbons from such locations and depths, the designers of riser and offloading systems face various technical challenges. While addressing those challenges, it is also necessary to minimise the cost of production and related capital investment and to simplify the installation and operation of the necessary subsea infrastructure.

A typical subsea oil production system comprises production wells each with a wellhead; pipelines running on the seabed; subsea structures to support valves and connectors; subsea manifolds; and risers to bring hydrocarbons to the surface. At the surface, a topside installation, which can be a platform or a vessel, receives the hydrocarbons before their onward transportation.

One approach to reduce the cost of production is to simplify subsea equipment as much as possible, for example by using a long pipeline extending from a wellhead and minimal additional equipment subsea. Another approach is to transfer at least some conventionally-topside production and storage functions to a subsea location for intermittent export of hydrocarbons by shuttle tanker vessels.

There is a requirement not only to store oil and gas subsea but also the various types of chemicals that are used for subsea production and processing. Conventionally, such fluids are supplied from a nearby surface facility and fed by a hose, pipes, a pipe bundle or umbilical to the various subsea consumers. In principle, storage tanks for such fluids could be positioned at the seabed but re-filling operations are cumbersome and costly, especially in deep-water applications.

In deep-water applications, or where there is a lack of infrastructure nearby, the need for periodically refilling tanks must be part of the strategy for operating and maintaining the integrity of an oil or gas field. That aside, storing large quantities of fluids underwater presents various technical challenges. One approach is to install storage tanks upon pre-installed subsea foundations, as exemplified by WO 2004/037681 . However, the necessary subsea foundations are bulky and costly and may be impractical to construct or install in deep water.

As robustness is essential, a subsea storage tank itself may also be costly and complex. A key challenge is the need for mechanical strength so as to withstand any pressure differential between external hydrostatic pressure and variable internal pressure. Thus, a tank requires rigid walls to withstand hydrostatic pressure if it is designed to store fluids at atmospheric or otherwise sub-ambient pressure. Alternatively, pressure compensation may be employed so that the storage pressure is equivalent to hydrostatic pressure. For this purpose, the walls of the tank may be flexible, such as a concertina arrangement. In all cases, storage tanks must be capable of handling a variable internal volume as they are filled with, and emptied of, fluids.

Thus, a conventional approach to the problem of differential pressure is to use a bladder or a deformable membrane so that internal and external pressures are balanced. However, such a bladder or membrane may be fragile and requires fine pressure management to avoid bursting. Another approach is to make a rigid storage tank strong enough to withstand an expected pressure differential. However, in deep water especially, the need to withstand hydrostatic pressure makes rigid tanks very expensive. For example, in a water depth of around 2000m, the hydrostatic pressure will be about 200 bar. This would necessitate an impractically large and heavy tank that would also be extremely difficult to install.

Floating storage tanks of various designs are also known. For example, spar storage and production units, exemplified by EP 0256177, combine a floating above-surface production facility with underwater rigid storage. However, as a spar structure extends partially above the water surface, it is more vulnerable to damage than a wholly submerged storage structure. Also, anchoring a spar structure can be complex. Additionally, rigid walls limit the depth at which fluid storage is possible, as do practical limits on the overall height of the spar structure. WO 2015/022476 combines a spar buoy with a distinct floating storage unit. The storage unit is a tank with an integral ballast system for managing buoyancy. Such a system is not readily scalable and the storage volume of the unit is highly dependent on the size of the spar buoy, in addition to the abovementioned disadvantages of spar solutions.

WO 2006/090102 adopts a more modular approach, in which rigid tanks can be attached to a column. Apart from the abovementioned challenges of rigid tanks, each tank must have its own buoyancy system as otherwise the assembly could collapse under its own weight. US 5117914 also discloses a cluster or array of rigid tanks arranged vertically along a mooring system.

GB 1275987 discloses a vertical column comprising a lower oil-filled compartment and an upper air-filled compartment for buoyancy. The walls of the compartments are rigid and therefore are not well suited to use in deeper water.

In KR 20160099943, floating storage units are arranged vertically. Each unit comprises a buoyant rigid support, a bladder and a piston that pushes against the bladder. However, stacking several of those units to increase capacity may be difficult, in particular because of the mobile piston.

Risers comprising a buoyancy tank are also known, as exemplified by WO 2008/056185. However, as its purpose is to ensure a constant buoyancy force, the buoyancy tank of a riser has rigid walls to withstand hydrostatic pressure and not to collapse. Thus, a buoyancy tank is not designed for, and would not be suitable for, storing a fluid underwater, especially where the volume of such a fluid is variable.

WO 2019/007975 discloses a riser pipe that extends upwardly from a subsea facility to a hose hanging down from a tanker at the surface. The riser pipe is kept under tension by a buoy at its upper end.

CH 354721 discloses an underwater storage tank for a liquid that is less dense than water, which is weighed down by a ballast. A pair of cables extend through the middle of the tank and between top and bottom plates thereof. The tank includes a duct and a pipe to allow communication between the tank and external sources or consumers of the stored liquid. US 2003/226489 discloses a subsea storage pod, which is attached to an anchor on the seabed by a mooring line. A hose provides fluid communication between the pod and a surface vessel. The pod supports itself via its own buoyancy and by an optional base and cap, which can be weighted or buoyant depending on the density of the substance stored in the pod.

GB 1407497 discloses an underwater oil storage system comprising an oil storage reservoir with a buoyancy control tank disposed around the tank. The storage system can be moored to the seabed using lines and weights, while respective lines allow oil and water into or out of the storage reservoir and water into or out of the buoyancy tank such that buoyancy is maintained at all times.

WO 2016/014841 relates to a subsea fluid storage system that automatically compensates for subsea pressure changes by means of a deformable bladder contained within a vessel. The bladder has an opening which is in fluid communication with a port of the vessel.

Against this background, the invention resides in a subsea fluid storage system comprising: a seabed foundation; a tensile member extending upwardly from the foundation; a subsea buoy applying buoyant upthrust to support the tensile member in an upright orientation; and at least one fluid storage container or reservoir disposed between the foundation and the buoy and mounted to the tensile member. The container has at least one inlet or outlet for fluid communication with a source or a consumer of fluid to be stored in the container. The tensile member extends from the foundation to the buoy through the container

The container may comprise a plurality of interengaged modules, each defining a respective storage compartment. The buoy could be separate from, or integrated with, the container.

The tensile member suitably comprises at least one of a pipe, a wire or a chain. For example, where the tensile member is a pipe, it may serve as a riser pipe for conveying hydrocarbon fluid from a seabed source. More generally, at least one upward fluid flow path from a subsea source may communicate with the container. Alternatively, or additionally, at least one upward fluid flow path from a subsea source may extend through, but not necessarily communicate with, the container and could extend to or through the buoy.

Fluid communication between the container and the source or the consumer may be effected via at least one flow path at a level beneath the container, beside or within the tensile member. For example, the tensile member could contain one or more fluid lines that define the or each flow path.

The container may have a first compartment that communicates with at least one upward fluid flow path and a second compartment that communicates with at least one downward fluid flow path toward a subsea consumer.

The container suitably has at least one filling port that is arranged for subsea connection with a flexible hose for filling with fluid from, or offloading fluid to, a surface vessel. Alternatively, the container may be in fluid communication with a flexible hose that extends to the surface for filling with fluid from, or offloading fluid to, a surface vessel.

In embodiments to be described, the container comprises an outer, flooded, rigidwalled housing and at least one inner chamber within the housing, the inner chamber being defined by a flexible-walled envelope. The housing may have a closed top and/or bottom end that is shaped to define a trap volume for capturing fluid leaking from the inner chamber. The housing may nevertheless comprise at least one port for inward or outward flow of seawater.

The housing may have an access opening through which the envelope of the inner chamber can be inserted, subsea, into the housing. A movable or removable closure is suitably arranged to close the access opening. Elegantly, the structure of the closure may be attached to or may support the envelope of the inner chamber so that the envelope is inserted into the housing when the closure closes the access opening. In that case, the closure may conveniently support at least one fluid coupling that communicates with the envelope to fill the envelope with fluid or to empty fluid from the envelope.

There could be two or more inner chambers disposed end-to-end within a common outer housing. Advantageously, for balance, the or each inner chamber may be substantially symmetrical about a central longitudinal axis of the housing. The envelope of the or each of the inner chambers is preferably extensible or collapsible longitudinally within the housing, which also facilitates balance. For example, the envelope may be disposed between end plates, at least one of which is movable longitudinally within the housing to guide extension or contraction of the envelope. At least one of the end plates may comprise material with positive buoyancy to facilitate subsea handling.

The envelope may comprise a slot for accommodating the tensile member when the envelope is within the housing. Such a slot suitably extends from a centre of the envelope to open to an outer edge of the envelope. The slot may follow a path that is curved from the centre to the outer edge of the envelope, reflecting that the envelope may following a similarly curved or swinging insertion path into the housing.

The inventive concept embraces a subsea installation that comprises at least one subsea fluid storage system of the invention. The installation may comprise at least one subsea consumer of fluid stored by the system, in fluid communication with the container of the system for supply of that stored fluid to the consumer. The installation may also, or alternatively, comprise at least one subsea source of hydrocarbon fluid, in fluid communication with the container of the system or with a riser serving as the tensile member of the system.

Correspondingly, the inventive concept also embraces a method of storing a fluid underwater. The method comprises: conveying the fluid into at least one subsea storage container mounted to a tensile member that extends between a seabed foundation and a subsea buoy, the buoy supporting the tensile member in an upright orientation; and dispensing the fluid from the container to a subsea consumer or offloading the fluid from the container to a surface vessel. The fluid may, for example, be a hydrocarbon fluid that is conveyed into the container from a subsea source, or a chemical for subsea processing or for injection into a subsea well that is conveyed into the container from a surface vessel or underwater vehicle. For example, the tensile member could be a riser up which hydrocarbon fluid flows toward a surface vessel without entering the container. More generally, fluid may be conveyed into the container or may be dispensed from the container via a flow path beside or within the tensile member. A fluid connection may be effected, subsea, between a flexible hose of a surface vessel and a port of the container, followed by filling the container with fluid or offloading fluid from the container through the hose. Alternatively, a fluid connection may be effected, at or near the surface, between a surface vessel and a flexible hose in fluid communication with the container, followed by filling the container with fluid or offloading fluid from the container through the hose.

The fluid may be held in an inner chamber that is defined by a flexible-walled envelope within an outer, flooded, rigid-walled housing of the container. The envelope may be inserted into the housing through an access opening of the housing, when underwater, for example after lowering the envelope from the surface to the access opening.

The access opening can be closed with a closure that supports the envelope in such a way that the envelope is inserted into the housing when the closure closes the access opening. In that case, the envelope and the closure may be lowered from the surface together before attaching the closure to the housing and then inserting the envelope into the housing by closing the closure across the access opening. The tensile member may be accommodated within a slot of the envelope when the envelope is inserted into the housing. The envelope may be extended and contracted longitudinally within the housing in accordance with changes in the volume of fluid held within the envelope.

In summary, the invention contemplates a modular flexible storage unit for subsea storage of injection chemicals or other fluids such as hydrocarbons. It recognises the benefit of raising storage tanks close to the surface to simplify access by regular supply or offloading vessels and work-class ROV systems.

Tank sizes may be tailored so that all tanks can be refilled or emptied every time a vessel is nearby for the purposes of resupply or offloading. The sizes of tanks can be adjusted or determined to suit reasonable refill or offloading intervals.

The invention makes it possible to eliminate the need for long umbilical tiebacks. In addition to large reductions in power consumption, remaining power and communication cables become simpler and cheaper.

The subsea fluid storage system of the invention can provide a double barrier against leakage of fluids into the sea. The system is capable of storing fluids with higher or lower density or specific gravity than the surrounding seawater. Preferred embodiments of the invention even provide for removal and replacement of an inner tank subsea.

Whilst enabling refilling or offloading from a subsea tank at a fixed, shallow water depth, the system of the invention is capable of use in ultra-deep-water applications. The system is suitable to feed chemicals to consumers, or to receive fluids from production systems, in all water depths.

This invention proposes a new and safe way of storing injection or processing fluid close to a subsea consumer. A typical application of the invention is in a small-pool field where the distance to the nearest host is too great or demands a long tie-back pipeline and so then becomes unprofitable.

The inventive concept embodies a modular design philosophy. This modularity enables cost-effective production methods; for example, each of the main parts of the storage unit may be produced in series using a single mould.

Embodiments of the invention provide a buoyant subsea storage device for fluids. The device comprises: a substantially vertical storage column; a foundation such as a suction pile connected to a lower end of the column; and a buoy such as a buoyancy tank connected to an upper end of the column.

The storage column may comprise a substantially vertical tension member and a rigid storage container. The rigid storage container may comprise at least one storage bladder, which may be in fluid communication with a port through the wall of the storage container. Storage bladders may be grouped in one or more compartments of the storage container.

The storage container may comprise a central shaft that may be integral with, or in series with, the vertical tension member of the storage column. The central shaft may be hollow and could carry fluid. For example, the central shaft may contain at least one riser pipe or may serve as a riser pipe.

More generally, the storage container may comprise ports comprising at least an outlet port and an inlet port; the outlet port may be located at the bottom, whereas the inlet port may be located at the top or at the bottom, with a hose running to the top. Each port can be connected underwater to a hose for emptying or filling the or each storage bladder of the storage container.

The volume between the storage bladder and the storage container can be filled with seawater. For example, the storage container may comprise a port or opening for seawater ingress through its wall. More generally, seawater can be admitted or expelled through a port. The top of the storage container may further comprise a venting port.

The top of the foundation may define a pivot axis to allow the column to tilt. The foundation may be combined with a subsea structure that comprises at least one accessory for managing flow through its piping.

Buoyancy can be adjusted to keep the storage column tensioned between the buoy and the foundation, for example by exchanging a fluid between a buoyancy tank and the storage column.

The tension member of the storage column may be, or may comprise, one of a pipeline, a chain or a wire. For example, a pipeline serving as a tension member may be a riser pipeline and may be made of rigid pipe.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a schematic part-sectioned side view of a subsea fluid storage system of the invention, providing for storage and output of injection or processing chemicals;

Figure 2 corresponds to Figure 1 but shows a variant of the system that further provides for export of hydrocarbon fluid;

Figure 3 is a schematic part-sectioned side view of a further variant of the system that provides for storage of injection or processing chemicals and outputs those chemicals from the system along a tubular spine; Figure 4 corresponds to Figure 3 but shows a further variant of the system that provides for storage of hydrocarbon fluid that enters the system along the tubular spine;

Figure 5 corresponds to Figure 2 but shows a further variant that provides for storage of hydrocarbon fluid that enters the system along a tubular spine and further providing for storage and output of injection or processing chemicals;

Figure 6 is a schematic part-sectioned side view of a further variant of the system in which a buoyancy tank is integrated with a storage structure;

Figure 7 is a perspective view of a field layout comprising a storage system of the invention;

Figure 8 is a perspective view of a marginal field layout that integrates an offloading riser with a chemical storage system of the invention;

Figure 9 is an enlarged detail view of the storage system shown in Figure 8;

Figure 10 is a selection of perspective views that show a storage system of the invention and internal and external details of a storage container of that system;

Figure 11 is a detail perspective view of an umbilical bundle of the storage system of Figure 10;

Figure 12 is a detail perspective view of a tension buoy of the storage system of Figure 10;

Figure 13 is a perspective view of a bladder of the storage system in collapsed and expanded states;

Figure 14 is a perspective view of a bladder adapted for connection to, and fluid communication with, a neighbouring bladder of a stack of such bladders;

Figure 15 is a perspective view of a storage module of the invention; Figure 16 is an enlarged detail view of a hinge for a hatch closure of the module of Figure 15;

Figure 17 is an enlarged detail view of a latch for the hatch closure of the module of Figure 15;

Figure 18 is a perspective view that shows the module of Figure 15 with the hatch open;

Figure 19 corresponds to Figure 18 but shows a stack of bladders in an expanded state;

Figure 20a to 20c are a sequence of perspective views that show an ROV mounting a new stack of bladders on a housing of the storage system;

Figure 21 is a perspective view from above of a variant of the invention;

Figure 22 is a perspective view of the variant of Figure 20 from beneath;

Figure 23 is a perspective view of a storage system of the invention in longitudinal section to show the operation of a leakage barrier when storing a fluid that is denser than seawater;

Figure 24 corresponds to Figure 23 but shows the operation of a leakage barrier of the system when storing a fluid that is less dense than seawater;

Figure 25 is another perspective view of the arrangement shown in Figure 24; and

Figures 26 and 27 are perspective views of further variants of the invention.

Referring firstly to Figures 1 to 6, these schematic drawings show various ways in which columnar storage systems 10 of the invention can be arranged. Like numerals are used for like features. Ancillary equipment such as valves has been omitted for simplicity. In each case, a buoy 12 applies tension to an upright elongate tensile member 14 that is anchored to a foundation 16 in the seabed 18, such as a pile foundation. The tensile member 14 may be, or may comprise, a hollow member such as a tube or a pipe or a flexible, solid or articulated member such as a wire or a chain, either individually or combined in series. The tensile member 14 is nominally vertical but may depart from the vertical by bending along its length and/or by pivoting at its connection to the foundation 16.

The tensile member 14 extends through an upright elongate storage container 20 that surrounds, and is supported by, the tensile member 14 between the buoy 12 and the foundation 16. In these examples, the container 20 is a cylinder that is rotationally symmetrical about the tensile member 14, which therefore lies on a central longitudinal axis of the container 20.

In the system 10 shown in Figure 1 , the tensile member 14 is a wire extending longitudinally through the container 20 from the foundation 16 to the buoy 12. A fluid such as a chemical for injection or processing is introduced into the container 20 through an inlet 22 and expelled from the container 20 through an outlet 24, when required by a subsea consumer 26 such as a wellhead or a processing unit.

Figure 2 shows a variant of the system 10 of Figure 1 , in which the tensile member 14 is a rigid riser pipe, for example of steel or polymer composite material, in fluid communication with a subsea pipeline 28 via the foundation 16. The pipeline 28 receives hydrocarbon fluids from a subsea source such as a buffer storage tank of a processing system, not shown.

In this example, the riser pipe serving as the tensile member 14 extends to, and longitudinally through, the buoy 12 to terminate at its upper end in an upwardly-facing port 30. The port 30 facilitates offloading the hydrocarbon fluids to a visiting shuttle tanker that lowers a hose and couples the hose to the port 30. In other arrangements, a permanently-connected flexible line can extend from the buoy 12 to the surface to be picked up by, and coupled to, a shuttle tanker when required.

The system 10 of Figure 3 is akin to that of Figure 1 but in this case, fluids from the container 20 are expelled through a pipe that forms a lower part of the tensile member 14 and communicates with a subsea consumer 26 via the foundation 16. In this example, a wire in series with the pipe forms an upper part of the tensile member 14 extending between the container 20 and the buoy 12. In principle, however, the pipe could instead extend all of the way to the buoy 12.

In Figure 4, the arrangement of the system 10 reverses that shown in Figure 3. Thus, in this case, the container 20 receives hydrocarbons from a pipe that forms a lower part of the tensile member 14 and that communicates with a subsea pipeline 28 via the foundation 16. Unlike the arrangement shown in Figure 2, hydrocarbons are stored temporarily in, and offloaded periodically from, the container 20.

In the system 10 of Figure 5, the arrangement is similar to that of Figure 4 in that hydrocarbons flow up a pipe of the tensile member 14 to be stored temporarily in, and offloaded periodically from, the container 20. However, Figure 5 exemplifies how the container 20 can be partitioned into compartments to store different fluids at different locations or levels within the container 20. In this example, like the arrangement of Figure 1 , a lower portion of the container 20 receives a chemical for injection or processing through an inlet 22. That chemical is expelled from the container 20 through an outlet 24 when required by a subsea consumer 26 such as a wellhead or a processing unit.

The system 10 of Figure 6 is functionally identical to that of Figure 1 in terms of flow paths and storage functionality but in this example, the buoy 12 is integrated with the top of the container 20, with no longitudinal gap between them.

In the examples shown in Figures 1 , 2, 5 and 6, chemicals flow along a path that is parallel to and separate from the tensile member 14. However, in other variants, the tensile member 14 may define or contain multiple parallel flow paths, for example concentric or bundled paths, through which hydrocarbons flow upwardly from the foundation along one flow path and chemicals or other fluids flow downwardly toward the foundation along another flow path.

As the remaining embodiments will make clear, the container 20, or each compartment of the container 20, could house an inner chamber or tank that holds the respective fluids, thus defining a double barrier to leakage. Also, the inner tank could be flexiblewalled to handle a variable volume of fluid and to compensate for hydrostatic pressure. Moving on now to Figures 7 to 9, these field layout drawings show storage systems 10 of the invention in practical contexts. These examples demonstrate that the storage system 10 can be provided in different configurations, either as a standalone chemical storage system or piggybacked on, clamped onto, or otherwise supported by, a hydrocarbon offloading system.

In Figure 7, for example, a servicing vessel 30 is shown on the surface 32 for supplying one or more injection or processing chemicals to the storage system 10. The depth of the water D1 from the surface 32 to the seabed 18 may, for example, range from about 200m to 3500m. Conversely, the top of the buoy 12 of the storage system 10 may be fixed at a depth D2 of about 75m to 100m, which is deep enough to be protected from wave action at the surface 32 but shallow enough to be readily accessible for refilling and maintenance, for example with an ROV.

Figure 8 shows the storage system 10 in the context of a subsea processing and storage operation (SPSO). Here, hydrocarbons from wells 34 on the seabed 18 are processed in a subsea processing system 36 and stored in buffer tanks 38 before being offloaded periodically up a riser 40 to the surface 32. In this case, bundled lines for chemical supply can be combined with the riser 40 and can thereby be integrated with the offloading system, with the riser 40 replacing the tensile member 14 of the preceding drawings and being supported by the buoy 12. The riser 40 communicates with a flexible hose 42 that extends above the buoy 12 to pick-up buoys 44 floating at the surface 32, as best seen in Figure 9.

Figure 8 shows a shuttle tanker 46 for offloading hydrocarbons via the hose 42, in addition to a separate servicing vessel 30 for supplying chemicals to the storage system 10. In principle, however, a single vessel could perform both functions.

Chemicals could be pumped down to the storage system 10 in one or more flow paths parallel with the hose 42. Alternatively, a connection to the storage system 10 could be made separately from the hose 42. Chemicals can then be pumped down from the storage system 10 to the various consumers on the seabed 18 in one or more flow paths parallel with the riser 40.

Figures 10 to 12 show features of the storage system 10 in more detail. For example, Figure 10 shows how, as an alternative to compartments, the storage container 20 can be subdivided into a longitudinal series of cylindrical modules 48 that are joined together end-to-end around the tensile member 14. The modules 48 may be fixed to the buoy 12 and/or to the tensile member 14.

Each module 48 has a rigid tubular housing 50 that contains and protects at least one flexible hollow bladder 52 defining an inner chamber within a flexible-walled envelope. In these examples, references to a bladder 52 encompass a stack of conjoined or intercommunicating elements of the bladder 52. Collectively, a stack of such elements of the bladder 52 have a concertina or bellows configuration. References to a bladder 52 also encompass one such element of a bladder 52.

The modules 48 and hence the bladders 52 are separated from each other by transverse partitions or bulkheads 54. The housings 50 of the modules 48 are flooded with seawater. Thus, the bladders 52 are exposed to external hydrostatic pressure whereas hydrostatic pressure on the housings 50 is balanced internally and externally. The housings 50 can therefore be thin and formed of inexpensive, easy-to-manufacture materials such as glass-reinforced plastics (GRP).

The typical fluid storage capacity of each bladder 52 or element of the bladder 52 is from 20m ³ to 60m ³ , based on the surrounding housing 50 having an outer diameter of between about 5m and 8m.

Each bladder 52 encloses a respective volume of chemicals at a pressure matching the ambient hydrostatic pressure of the corresponding depth. The bladder 52 of each module 48 is devoted to a particular chemical that may be required for injection into a well 34 or for the purposes of the subsea processing system 36. The number and sizes of the modules 48 and hence the bladders 52 can be varied to configure the container 20 for various applications, depending on the storage volume and the chemicals required.

The stacked elements of each bladder 52 are fluidly interconnected such that the stack forms a unitary storage volume. That volume is longitudinally collapsible and extensible within the housing 50 of a module 48, in the manner of a bellows or concertina, to compensate for external hydrostatic pressure and for changes in the volume of chemicals stored within. In principle, it would be possible for a module 48 to contain two or more storage volumes exemplified by bladders 52 disposed end-to-end, hence one above another, either in mutual abutment or separated by an intermediate partition or bulkhead.

Figure 11 shows umbilicals 56 that extend in parallel within a tubular tensile member 14 beneath the container 20. Each umbilical 56 is in fluid communication with the bladder 52 of a respective module 48 and with a respective consumer on the seabed 18, such as the wells 34 and the processing system 36. Pumps at the seabed 18 pump the chemicals from the bladders 52 to the consumers.

Figure 12 shows the interior of the buoy 12, whose purpose is to maintain tension in the tensile member 14 and to stabilise the storage system 10. The buoy 12 could be built of steel or of a composite material such as GRP.

The buoy 12 contains multiple chambers 58 that can be filled with a pressurised gas such as nitrogen to establish a desired degree of buoyant upthrust, depending upon the extent to which seawater is displaced from or floods the chambers 58. Where chambers 58 are offset laterally from the central longitudinal axis, this provides a possibility to balance the storage system 10.

Turning next to Figures 13 and 14, these show the design of a bladder 52, or one of the elements of a larger bladder 52, in more detail. The bladder 52 is a hollow disc of flexible sheet material shown deflated or collapsed on the left in Figure 13 and inflated or expanded on the right in Figure 13. The bladder 52 could, for example, be made of textile, coated with an impervious layer on one or both sides, or polyester woven yarn coated with an impervious layer on both sides.

A slot 60 penetrates the bladder 52 from top to bottom and follows a curved path from the centre to the periphery of the bladder 52, where the slot 60 is open at its outer end. Thus, the inflated bladder 52 is doughnut-shaped or toroidal, save for the slot 60 cutting through one side of the bladder 52 from a central opening to the radially-outer side of the bladder 52. Apart from the slot 60, the bladder 60 is generally circular in plan view apart from a shallow notch 62 in its periphery, close to the open outer end of the slot 60.

As noted above, the bladders 52 may be stacked and in fluid communication with each other to form elements of a larger storage volume. In this respect, Figure 14 shows an element of a bladder 52 with a cut-out 64 at its interface with a similarly-equipped neighbouring element of a stack for fluid communication between those elements. The cut-out is surrounded by an interface contact area 66 where the elements can be glued or vulcanized together when forming a stack. The elements of the stack are all oriented the same way so that their slots 60 align.

The design principle of the storage container 20, or where applicable a module 48 of the storage container 20, may depend upon whether the fluid to be stored has a greater or lesser specific gravity than seawater, hence being heavier or lighter than seawater for a given volume.

The module 48 shown in Figure 15 is designed for storing a fluid whose specific gravity or density is higher than that of seawater. The housing 50 of the module 48 is penetrated by a seawater inlet/outlet 68, which may be fitted with a filter to prevent egress of contaminants into the environment and to prevent ingress of sea life or floating debris into the interior of the module 48. The housing 50 is also penetrated by a hot stab connector 70 for fluid filling and by an auxiliary hot stab connector 72, which may be used for flushing if needed.

The hot stab connector 70 is positioned on a door or hatch 74 of the housing 50 that extends around about half of the circumference of the housing 50 near its bottom end and so has semi-tubular curvature. The hatch 74, shown closed in Figure 15, is pivotable relative to the housing 50 about a pivot axis that is parallel to the tensile member 14 extending centrally through the module 48.

The pivot axis is defined by an ROV-removable hinge pin 76 on one side of the housing 50, as shown in more detail Figure 16. The hatch 66 is held closed by ROV-operable latches or clamps 78 on the opposite side of the housing 50 that engage an array of straps 80 as shown in more detail in Figure 17.

An outlet hose 82 in communication with the collapsible bladder 52 of stacked elements within the module 48 emerges from the hatch 74 and extends up the side wall of the housing 50 onto the top of the housing 50. There, the hose 82 joins an umbilical within the tensile member 14, eventually for communication with a subsea consumer of the fluid that is stored in the bladder 52. To allow the hatch 74 to open, a hose coupling 84 allows the lower part of the hose 82 on the hatch 74 to be disconnected from the upper part of the hose 82 on the housing 50. The hose coupling 84 may, for example, comprise an ROV-operable flange clamp connection. The hose coupling 84 is also shown in more detail in Figure 16.

Figures 18 and 19 show the hatch 74 open, revealing a correspondingly-shaped opening 86 in the housing 50 surrounded by a gasket 88 that the hatch 74 compresses when closed. The gasket 88 can be thick and soft as it does not have to resist differential hydrostatic pressure. Moreover, Figures 18 and 19 show that the hatch 74 supports, or is joined to, a generally circular horizontal bottom plate 90 that serves as a base, support or cradle for the stacked elements of the bladder 52. The bottom element of the stack is fixed, for example with clamps, to the bottom plate 90 and hence in this example to the hatch 74, which may be made of steel or GRP.

Thus supported, the bladder 52 of stacked elements can be inserted into, or removed from, the housing 50 through the opening 86 in a side wall of the housing 50, in a direction transverse to the longitudinal axis of the housing 50. When the hatch 74 is closed, the bottom plate 90 rests on a circumferential flange 92 within the housing 50.

The bladder 52 is shown collapsed in Figure 18 and expanded, hence longitudinally extended, in Figure 19. It should be appreciated that the bladder 52 is shown extended outside the housing 50 in Figure 19 only for the purpose of illustration. In practice, the bladder 52 will not be extended when outside the housing 50, only when inside the housing 50. In this respect, it will be apparent that the bladder 52 has to be collapsed to enter the housing 50 through the shallow opening 86.

The stacked elements of the bladder 52 are sandwiched between the bottom plate 90 and a parallel, similarly-shaped top plate 94. The bottom plate 90 and the top plate 94 each have a curved slot 60 that is shaped like, and aligned with, the slots 60 of the elements of the bladder 52. The notches 62 in the periphery of each element of the bladder 52 are similarly mirrored by notches in the bottom plate 90 and the top plate 94 to accommodate the hinge that is completed by the hinge pin 76. As the hatch 74 is closed to carry the bladder 52 into the housing 50, the aligned slots 60 accommodate the tensile member 14 that extends centrally within the housing. To follow the swinging movement of the hatch 74 and with it the stack, the curvature of the aligned slots 60 is centred on the pivot axis defined by the hinge pin 76. The top plate 94 travels up and down within the housing 50 in accordance with the degree to which the bladder 52 is extended and hence in accordance with the volume of fluid that is held within the stack at a given point in time. The top plate 94 is a sliding fit within the housing 50 to serve as a guide or spacer that aligns the stacked elements of the bladder 52 during this vertical movement and prevents them from rubbing against the interior of the housing 50 or against the tensile member 14. However, there is sufficient radial clearance between the top plate 94 and the housing 50 that the top plate 94 will not jam against longitudinal movement.

As the stacked elements of the bladder 52 are filled with fluid from a surface vessel, typically with the assistance of an ROV, the bladder 52 will extend within the surrounding housing 50 of the module 48. In view of the resulting piston effect, seawater will be expelled from the seawater inlet/outlet 68 in the housing 50, shown in Figure 15. Conversely, as fluid is discharged from the bladder 52 and so the bladder 52 contracts, the resulting piston effect draws seawater into the housing 50 through the seawater inlet/outlet 68, via the associated filter.

Figures 20c to 20c show how the bladder 52 together with the hatch 74, the bottom plate 90 and the top plate 94 can be removed from the housing 50 and replaced together as a unit or assembly 96 with the assistance of an ROV 98.

In Figure 20a, the assembly 96 of the hatch 74, the bladder 52, the bottom plate 90 and the top plate 94 is shown suspended from a lifting wire 100 that terminates in a lifting jig 102 conveniently engaged in the aligned slots 60. The ROV 98 guides the suspended assembly 96 toward the opening 86 in the housing 50 of the module 48. Figure 20b shows the ROV 98 having linked the assembly 96 with the housing 50 via the hinge pin 76. The lifting wire 100 can then be released from the slot 60, as shown in Figure 20c, before the ROV 98 closes the hatch 74 to swing the bladder 52 into the housing 50 through the opening 86.

The ROV 98 then engages the clamps 78 with the straps 80 to hold the hatch 74 closed and to compress the gasket 88. Finally, the ROV 98 connects the lower part of the outlet hose 82 to the upper part of the outlet hose 82 via the hose coupling 84 shown in Figures 15 and 16. Conveniently, this is the only fluid connection required subsea and it can be performed by the ROV 98 at an easily-accessible location external to the housing 50.

Thus, the assembly 96 shown in Figures 20a to 20c serves as a replaceable cassette or cartridge that enables the bladder 52 to be collapsed, removed from the housing 50 and conveniently replaced underwater in the event of wear, damage or leakage. It would also be possible for the hatch 74 and the bottom plate 90 to remain in situ to receive and support a separate cartridge comprising one or more bladders 52.

Figures 21 and 22 show a variant of the assembly 96 in which the top plate 94 is made of, or comprises, buoyancy material such as syntactic foam. This reduces the apparent weight of the assembly 96 underwater to facilitate manoeuvring, supporting and lifting the assembly 96. Figure 24 also shows how the hot stab connector 70 and the lower part of the outlet hose 82 connect to the stacked elements of the bladder 52 of the assembly 96 behind the hatch 74 and beneath the bottom plate 90. As noted above, the hot stab connector 70 is for fluid filling and the outlet hose 82 is for fluid discharge.

Moving on to Figures 23 and 24, these drawings illustrate the environmental barrier philosophy of the invention. Specifically, there are two barriers for preventing leakage of stored fluid into the surrounding sea, the first barrier being the bladders 52 and the second barrier being the housing 50 around the bladders 52.

If one of the bladders 52 starts to leak, the gasket 88 around the opening 86 between the hatch 74 and the associated housing 50 will prevent the leaked fluid from reaching the surrounding sea. Depending on its density relative to the seawater within the housing 50, the leaked fluid will either float or sink and so will be captured at one of the closed ends of the housing 50. Figure 23 shows relatively heavy leaked fluid 104 trapped and accumulated at the bottom of the housing 50 whereas Figure 24 shows relatively light leaked fluid 104 trapped and accumulated at the top of the housing 50.

Where fluid with a higher density than seawater is being stored, as in Figures 15 and 23, the seawater inlet/outlet 68 is located near the top of the housing 50 so that leaked fluid cannot take that path into the surrounding sea. Conversely, for the same reason, the seawater inlet/outlet 68 is located near the bottom of the housing 50 if fluid with a lower density than seawater is being stored. In that case, however, the auxiliary hot stab connector 72 used for flushing is instead located near the top of the housing 50 so as to flush leaked fluid gathered at that upper level.

A leak detector positioned appropriately within the housing 50 can provide an alert or shutdown signal to a control system when a leak occurs. The defective bladder 52 can then be isolated by the control system before being replaced. At that time, a service vessel with an ROV will arrive at the site and will start by discharging the leaked fluid and any residual stored fluid into the vessel’s slops tank, for example via the auxiliary hot stab connector 72. This collapses the bladder 52. It may also be possible to transfer leaked and stored fluid into a different part of the storage system if space is available.

Next, the ROV disconnects the upper part of the outlet pipe 82 from the lower part of the outlet pipe 82 by releasing the coupling 84. The ROV then disconnects the clamps 78 from the straps 80 to release the hatch 74 and opens the hatch 74 to swing the collapsed bladder 52 out of the housing 50. After removing the hinge pin 76, the assembly 96 comprising the hatch 74 and the bladder 52 can be lifted up to the vessel, for example on a lifting line 100, and swapped with a replacement assembly 96.

The variant of the module 48 shown in Figure 24 is also shown in Figure 25. As noted above, the seawater inlet/outlet 68 is located near the bottom of the housing 50 and the auxiliary hot stab connector 72 is located near the top of the housing 50 to allow for the relatively low density of the stored fluid. In this variant, the seawater inlet/outlet 68 is located in the bottom end plate of the housing 50 and communicates with a pipe that emerges around the side of the housing 50.

Figures 26 and 27 show further variants of the module 48 to demonstrate that the hatch 74 need not necessarily be located near the bottom of the housing 50 as in preceding embodiments. For example, in Figure 26, the opening 86 and the hatch 74 are located near the top of the housing 50. Thus, the bladder 52 expands downwardly within the housing 50 when filled with fluid. In this case, therefore, the stack of elements of the bladder 52 is suspended from the top plate 94 whereas the bottom plate 90 moves up and down within the housing 50 to guide the position and movement of those elements. Again, this embodiment can be configured for storing fluids with greater or lesser density than seawater by swapping the levels of the seawater inlet/outlet 68 and the auxiliary hot stab connector 72 Finally, Figure 27 shows a variant of the module 48 in which the housing 50 contains two bladders 52 of stacked elements longitudinally spaced along the housing 50, each bladder 52 therefore being at a respective level in the housing 50. In order that the bladders 52 can be installed and replaced individually, each bladder 52 is supported via a respective hatch 74, the hatches 74 being spaced longitudinally along the housing 50 as appropriate. Ideally, the fluids stored in two or more bladders 52 within a common housing 50 should be of either greater or lesser density than seawater so as to accumulate more predictably and controllably at an end of the housing 50 in the event of a leak. However, those fluids need not be identical: it would be possible to store two or more different fluids in respective bladders 52 within a common housing 50.

In the example shown in Figure 27, the bladders 52 expand upwardly when filled but they could instead expand downwardly when filled, as in the variant shown in Figure 26. More than two bladders could be accommodated if the housing is long enough.

It will be apparent that, advantageously, differences in fluid volume within the bladders 52 of Figure 27 will not imbalance the system because the bladders 52 are disposed end-to-end along the longitudinal axis of the housing 50. This is also an advantage of previously-described arrangements with a single bladder 52 within a housing 50. In each case, the system remains balanced irrespective of the volume of fluid within the bladder 52 because the bladder 52 is substantially symmetrical about the longitudinal axis of the housing 50 and expands and contracts along that axis.