GB2566551A

GB2566551A - Subsea storage of crude oil

Info

Publication number: GB2566551A
Application number: GB1715096.2A
Authority: GB
Inventors: Daasvatn Sigbjørn; Stangeland Øivind
Original assignee: Subsea 7 Norway AS
Current assignee: Subsea 7 Norway AS
Priority date: 2017-09-19
Filing date: 2017-09-19
Publication date: 2019-03-20
Anticipated expiration: 2037-09-19
Also published as: GB2566551B; GB201715096D0

Abstract

A method of storing crude oil subsea for offloading comprising introducing oil 94 containing gas into a subsea storage tank 22, floating the oil above a variable-volume column of seawater 96 to trap the oil between the seawater and a top wall of the tank, the seawater being under hydrostatic pressure and applying that pressure to the oil, trapping gas emerging from the oil in a pocket between the oil and the top wall and venting a portion of said gas, offloading a portion of the oil, and causing seawater to flow into and out of the tank in response to venting the gas and introducing or offloading the oil. A second independent claim of a subsea storage tank 22 for storing crude oil comprising an external wall enclosing an internal volume, a seawater inlet 76 and outlet 78, an oil inlet 80 and outlet 82, a gas vent 86, wherein the oil inlet and outlet open to the internal volume at a level beneath where the gas vent opens and above a level at which the seawater inlet or outlet opens.

Description

This invention relates to the storage of crude oil subsea.

The background to the invention is the challenge of developing marginal subsea oil fields, including small, remote or inaccessible fields. To address that challenge, it is necessary to minimise the cost of production and related capital investment and to simplify the installation and operation of the necessary subsea infrastructure.

Offshore exploration for oil and gas is being performed in ever more challenging waters, with fields now being developed in water depths of 3000 metres or even more. To recover hydrocarbons from such depths, the designers of riser and offloading systems face various technical challenges. Metocean characteristics and relatively low reservoir temperatures compound those challenges.

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 production fluids to the surface. At the surface, a topside installation, which can be a platform or a vessel, receives the production fluids before their onward transportation.

Crude oil is a multiphase fluid. Specifically, a wellstream generally contains a mixture of sand, oil, water and gas. Also, the wellstream is hot at the outlet of the wellhead, typically around 200°C. If its temperature decreases below a certain threshold, at a given pressure, components of the wellstream may react together or individually to coagulate or precipitate as solid wax, asphaltenes or hydrates. For example, wax will typically appear in oil at a temperature of around 30°C. An accumulation of such solids could eventually plug a pipeline.

A blockage in a subsea pipeline is extremely disruptive and expensive to rectify. It is therefore a common objective to maintain the oil temperature above the critical threshold until the oil has been delivered to a topside installation. There, the oil can be treated to allow the treated oil to be transported at ambient temperature in tankers or in pipelines.

To reduce the cost of producing oil from marginal subsea fields, one approach 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. A challenge of that approach is that pipeline cost becomes a large element of development cost where fields are isolated or remote.

In this respect, conventional solutions to maintain oil temperature employ wet thermal insulation, which involves covering the pipeline with thermally-insulating materials. The pipeline may also be heated by electrical heating or by heat transfer from hot fluids. However, as some pipelines may be very long, in some cases longer than 100km, such solutions can become inordinately expensive.

Another approach adopts an opposite tactic, namely to transfer at least some conventionally-topside production and storage functions to a subsea location for intermittent export of oil by tanker vessels. By displacing at least some oil processing steps from topside to the seabed, there is less need for thermal insulation or heating of subsea pipelines. The present invention arises from this second approach, which involves subsea storage of produced oil.

Existing and proposed subsea units for storing hydrocarbons are extremely expensive. They employ various tank solutions in steel, in concrete or in GRP (glass-fibre reinforced plastics), typically with an internal expandable bag or bladder. There is a need for an alternative solution to reduce the cost of such units as far as possible.

Subsea storage units for hydrocarbons face various technical challenges. A key challenge is the need for mechanical strength so as to withstand the pressure differential between external hydrostatic pressure and variable internal pressure. Such units must also handle a variable internal volume as they are filled with, and emptied of, hydrocarbons. They must also deal with a substantial temperature difference between their contents and the surrounding seawater, which is uniformly at about 4°C at depths in excess of 1000m.

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 requires fine pressure management to avoid bursting.

EP 1554197 and WO 2016/116625 disclose typical subsea storage tanks in which a storage bag is located within a rigid support frame. Such a storage system requires additional pumps for managing differential pressure. US 2016/319652 discloses another design of subsea storage tank with a flexible bladder or bag within a protective rigid structure. It is noted that the presence of the gas phase, and not only the liquid phase, makes the pressure/volume problem more critical to solve.

Another conventional approach is to make a rigid storage tank strong enough to withstand the expected pressure differential. WO 2014/032107 discloses such a rigid tank with double, concentric hulls, the space between the hulls being used for ballasting during installation. The tank has side walls that are about 1m thick and so are presumably of concrete.

Similarly, WO 2015/022476 discloses a rigid storage tank that also employs a specific ballasting system. In addition, the tank is stabilised by docking it onto a separate base. This allows a somewhat lighter tank but it requires the base to be installed in a separate operation.

In deep water, 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 difficult to install.

FR 2776274 combines both of the above approaches by replacing a bladder with a mobile shell or floating roof that can travel up and down inside a rigid storage tank like a piston. This therefore defines a variable storage volume beneath the mobile shell and so allows internal and external pressures to be balanced while retaining the mechanical advantages of a rigid structure. However, it is a challenge to ensure tight sealing of the storage volume in a way that allows the shell to move. It is proposed to use polymer seals or a flexible membrane for this purpose. Neither solution is optimally reliable as both solutions rely upon relatively-movable or deformable parts.

CN 105151580 discloses a twin-walled subsea oil storage tank in which an upper compartment contains the oil to be stored and a lower compartment contains seawater. A mobile plate floats on the seawater in the lower compartment to separate the compartments from each other. An annulus between the walls of the tank holds additional seawater or air for ballasting or floating the tank.

There is no provision in CN 105151580 for storing a multiphase fluid that comprises liquid and gaseous phases, like crude oil. In particular, no provision is made to manage gas that may be dissolved or entrained in the oil before the oil enters the tank.

US 7735506 discloses an underwater gas storage tank that uses pressure variation to store a column of compressed natural gas above a column of water, which applies hydrostatic pressure to reduce the volume of the gas. Gas is introduced and drawn off through a port at the top of the tank. Water flows in and out of the tank in accordance with the changing gas volume. Again, there is no provision for storing a multiphase fluid comprising liquid and gaseous phases.

A floating membrane or flexible bag is provided in US 7735506 at the interface between the gas and the water to separate those two fluids. Whilst the physical separation provided by such a barrier is stated in US 7735506 as being optional, the skilled reader will understand that such separation is essential in practice if storing a hydrocarbon gas deep underwater. This is because if such a gas diffuses or dissolves in water under great hydrostatic pressure and at low temperature, solid hydrates will tend to form at the interface. Additionally, physical separation provided by a discrete rigid or flexible barrier is required in order to manage disturbance of the interface whenever gas is drawn off or introduced and hence water flows rapidly in and out of the tank.

Against this background, the invention may be expressed as a method of storing crude oil subsea for offloading. The method comprises: introducing the oil, containing gas, into a subsea storage tank; floating the oil above a variable-volume column of seawater within the tank to trap the oil between the seawater and a top wall of the tank, the seawater being under hydrostatic pressure and applying that pressure to the oil; trapping gas emerging from the oil in a pocket between the oil and the top wall; venting a portion of the trapped gas from the tank; offloading a portion of the oil from the tank; and causing seawater to flow into and out of the tank in response to venting the gas and introducing or offloading the oil, hence causing an interface between the oil and the seawater to move vertically within the tank.

Advantageously, water also contained in the introduced oil may be allowed to migrate into the seawater in the tank beneath the oil. Thus, the oil may be exposed directly to the seawater on which that oil floats within the tank. In that case, a fluid transition layer may be maintained at the interface between the oil and the seawater. Such a fluid transition layer may be a layer of emulsion.

Preferably, trapped gas is vented in response to an increase in volume of the pocket of trapped gas. For example, trapped gas may be vented in response to downward displacement of a top surface of the oil beneath the trapped gas.

In a preferred embodiment, trapped gas is vented when the top surface of the oil moves beyond a threshold level of displacement down the tank. For example, trapped gas may be vented by downward movement of a valve element that floats on the oil within the tank.

Preferably, oil is introduced into the tank through an oil inlet immersed beneath a top surface of the oil in the tank. Advantageously, the oil inlet may be beneath the threshold level of displacement of the top surface of the oil.

Oil may be conveyed through the seawater in the tank and the oil in the tank before being introduced through the oil inlet. Conversely, when offloading oil from the tank, oil may be conveyed from an oil outlet through the oil in the tank and the seawater in the tank.

Conveniently, both seawater and oil may flow into and out of the tank through a side wall of the tank, at a level beneath the interface between the oil and the seawater.

As the invention is apt to be used in the context of subsea processing of oil, the method of the invention suitably comprises the preliminary step of processing the oil subsea before introducing the oil into the storage tank.

The inventive concept also embraces a subsea storage tank for storing crude oil. The tank comprises: an external wall enclosing an internal volume; a seawater inlet and a seawater outlet communicating with the internal volume; an oil inlet and an oil outlet communicating with the internal volume; and a gas vent communicating with the internal volume; wherein the oil inlet and the oil outlet open to the internal volume at a level beneath where the gas vent opens to the internal volume and above a level at which the seawater inlet or the seawater outlet open to the internal volume.

The oil inlet and the oil outlet are preferably defined by respective conduits that extend within the internal volume to a level adjacent to the seawater inlet or the seawater outlet. The conduits that define the oil inlet and the oil outlet suitably penetrate the external wall.

The seawater inlet and the seawater outlet may be defined by separate conduits or by the same conduit.

The gas vent is preferably at an uppermost apex of the external wall.

The tank may further comprise a valve element that is movable between a lower open position and an upper closed position to open and close the gas vent, the open position of the valve element being above the level at which the oil inlet and the oil outlet open to the internal volume of the tank. Conveniently, the valve element may be positively buoyant in crude oil.

As internal and external pressures are balanced, the external wall of the tank may be made of fibre-reinforced polymer composite. For example, the external wall may be of sandwich construction comprising a layer of syntactic foam.

Preferably, the external wall is anchored by a ballast weight system that comprises ballast weights supported by a frame attached to the wall. For example, the external wall may conveniently be attached to a base that extends outwardly beyond the wall to define a platform for the ballast weights.

In use, the internal volume of the storage tank may contain: a variable-volume column of seawater; a variable volume of oil floating above the column of seawater; an interface between the column of seawater and the volume of oil; and a variable-volume pocket of gas trapped above a top surface of the volume of oil; wherein the oil inlet and the oil outlet open to the internal volume at a level above the interface; the seawater inlet or the seawater outlet open to the internal volume at a level beneath the interface; and the gas vent opens to the internal volume at a level above the top surface of the volume of oil.

Advantageously, the oil inlet and the oil outlet may open to the internal volume at a level between the interface and the top surface of the volume of oil.

The column of seawater and the volume of oil may be in mutual contact at the interface.

The inventive concept also extends to a subsea installation comprising a storage tank in accordance with the invention.

The invention reverses the logic of most prior art storage tanks, in which water pressure is applied above the stored hydrocarbon volume. An advantage is that in case of failure of the sealing system, the hydrocarbon volume comprising oil will, by virtue of its relatively low density or specific gravity, remain above the mobile boundary between oil and water.

The hydrocarbon volume also comprises gas that is entrained or dissolved in the oil. Residual phase separation takes place within that hydrocarbon volume during storage, as by virtue of its relatively low density or specific gravity, the gas phase rises through the liquid-phase oil. The gas thereby accumulates at the top of the tank, from where it can be evacuated conveniently.

By virtue of the invention, no separation plate, membrane, bladder or other physical barrier is normally required at the boundary or interface between the hydrocarbon volume and the volume of seawater beneath. However, the presence of such a barrier is not excluded by the invention. If such a barrier is present, its density should be between that of seawater and the oil being stored. This enables the barrier to be buoyantly self-supporting, floating at the interface between the two layers or volumes.

No pump is normally required to extract the oil because hydrostatic pressure exerted by the water underneath the hydrocarbon volume will push the oil up. However if some boosting is needed, this is not excluded by the invention.

In summary, therefore, the invention puts the stored fluid above seawater, which removes the need for a containment bladder or a specific design for resisting hydrostatic pressure. The invention also allows for the omission of an interface barrier. Crude oil, which is less dense than seawater and is immiscible with seawater, creates a slow-moving, floating, viscous interface comprising an unstable emulsion.

If such an interface can be created and maintained during operation of the storage tank, there is no longer a need for a solid interface or for sealing. Thus, an advantage of the invention is to allow the absence of a physical barrier at the interface. The difference in density between seawater and the oil stored above is sufficient to create an interface layer that will not break up as the volumes of oil and water fluctuate in use of the tank. This is because motion at the interface is slow due to the viscosity of the crude oil held in the upper volume.

Embodiments of the invention provide a subsea storage tank for a multiphase fluid that is lighter than and immiscible with seawater, such as crude oil. The storage tank comprises an external wall enclosing an internal volume. The wall may comprise a double shell, at least one of which shells may be of GRP. A mobile internal interface splits the internal volume into two variable volumes, namely an upper storage volume containing the multiphase fluid and a lower storage volume containing seawater. Fluid communication within the external wall allows seawater to enter or exit the lower storage volume, depending upon pressure differentials.

The interface is suitably made of a fluid transition layer between the multiphase fluid and seawater. The upper storage volume may also comprise outlets for separately evacuating the phases of the multiphase fluid. For example, the outlet for the gas phase may comprise a non-return vent leading to the surrounding sea. Efficient gas removal allows slow variation of the liquid volume and hence slow motion of the interface.

An inlet suitably introduces incoming multiphase fluid into the upper storage volume at a location near the top of the upper storage volume. Conversely, at least one tube, pipe or other passageway may be near the bottom of the lower storage volume in direct or indirect communication with the surrounding sea. This provides an inlet or outlet for seawater flowing into or out of the lower storage volume as the volume of the multiphase fluid in the upper storage volume fluctuates in use of the tank.

In preferred embodiments to be described below, crude oil is stored inside a subsea storage tank comprising a dual-barrier insulated hull. Due to gravity differences between the oil and seawater, oil may be offloaded from the tank to a transport tanker at the surface using a subsea offloading system that may, for example, draw the oil from the tank using a feed pump on the tanker.

The low differential-pressure oil storage tank of the invention is apt to be manufactured from bolted GRP elements. A standard buoyancy material used in the offshore industry such as syntactic foam may be disposed inside the hull of the tank. This provides buoyancy, strengthens the tank and eases its installation. This also provides thermal insulation that reduces heat loss from, and hence temperature reduction of, the oil in use of the tank.

Thus, the invention is used in the context of subsea storage of crude oil exported from a subsea processing plant. The main structure of the tank is preferably of a composite material. This is a convenient way of creating a towable structure that can be relocated to a new location. Stability of the tank during installation and use may be provided by attaching external weights, which are preferably of steel. Standardised tie-in connections may be used to connect the tank to a subsea processing plant upon installation.

In use, crude oil flows into the tank at a pressure slightly higher than the ambient hydrostatic pressure of the surrounding seawater. Pipes preferably extend within the tank to provide for loading and offloading the oil.

Due to differences of specific gravity, oil gathers at the top of the tank, floating over a layer or column of water at the bottom of the tank. Gas entrained in the oil and previously-dissolved gas that comes out of solution in the oil accumulates at the top of the tank and is released, thus stabilising the oil. The stabilised oil will also be separated to a very high quality because residual water left in the oil following subsea separation processes will migrate down to the bottom of the tank.

In preferred embodiments, the subsea storage system of the invention employs a simple, composite, double-hull, insulated structure and a direct interface between seawater and oil. A lower safety barrier ensures that no oil particles will be entrained in water flowing between the internal water layer and the surrounding sea.

Thus, in accordance with the invention, crude oil is stored in a subsea storage tank in which the oil floats trapped above a variable-volume column of seawater. The seawater is under hydrostatic pressure and so applies that pressure to the oil. Gas emerging from the oil is trapped in a pocket above the oil to be vented from the tank.

Venting gas, or loading or offloading oil, causes seawater to flow into and out of the tank to equalise internal and external pressure, as an interface between the oil and the seawater moves vertically within the tank. Water also contained in the oil can migrate into the seawater in the tank beneath the oil. No barrier or partition is necessary at the interface between the oil and the seawater.

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 side view of a subsea oil production and processing installation comprising a subsea storage tank in accordance with the invention, with a shuttle tanker shown here on the surface in the process of unloading crude oil from the storage tank;

Figure 2 is a perspective view of a subsea storage tank in accordance with the invention;

Figure 3 is an enlarged partial perspective view of the storage tank of Figure 2, sectioned in a vertical central plane;

Figure 4 is an enlarged part-sectioned detail perspective view showing inlet and outlet provisions of the storage tank shown in Figures 2 and 3;

Figure 5 is an enlarged detail view corresponding to Detail V shown in Figure 6;

Figure 6 is a perspective view of the storage tank shown in Figures 2 to 5, again sectioned in a vertical central plane and now showing fluids within the storage tank; and

Figure 7 is an enlarged sectional detail view of a gas outlet of the storage tank shown in Figures 2 to 6.

Referring firstly to Figure 1 of the drawings to put the invention into context, a subsea oil production and processing installation 10 comprises a wellhead 12 from which crude oil flows along a short subsea pipe 14 to a subsea processing unit 16 placed on the seabed 18 nearby. After being processed in the processing unit 16, the oil flows along another short subsea pipe 20 to a storage unit placed on the seabed 18 nearby, that storage unit being a subsea storage tank 22 in accordance with the invention.

The processing unit 16 may, for example, separate gas, water and sand from the oil to enable the oil to be stored subsea in the tank 22 before occasional offloading to the surface 24. However, some residual water and hydrocarbon gas, in particular, will inevitably remain in the oil that flows into the tank 22. For example, water may be mixed with the oil as an emulsion or may be suspended in the oil as droplets. Similarly, gas may be dissolved in the oil or may be entrained in the oil as small bubbles.

For completeness, Figure 1 shows a shuttle tanker 26 on the surface 24 in the process of offloading oil from the subsea storage tank 22. For this purpose, the tanker couples itself temporarily to an offloading system 28 that is in fluid communication with the tank 22.

The offloading system 28 may take many forms. In this much-simplified example, the offloading system 28 comprises a surface buoy 30 connected to a foundation or mooring 32 on the seabed 18. An offloading pipe 34 extends from the tank 22 to the mooring 32 and from the mooring 32 to the buoy 30.

A flexible offloading line 36 couples the tanker 26 temporarily to the buoy 30 to effect fluid communication between the tanker 26 and the subsea storage tank 22 via the offloading system 28.

To simplify subsea infrastructure as much as possible, the tanker 26 may carry a pump 38, as shown here, to draw the oil from the tank 22 and through the offloading system

28. However, it would be possible instead, or additionally, to provide a pump on the seabed 18 beside the tank 22. It would also be possible to add a flow booster system such as a gas lift system so as to ease the flow of oil from the seabed 18 toward the surface 24. In any event, hydrostatic pressure acting on the oil within the tank 22 may assist offloading of oil from the seabed 18 to the surface 24.

Referring next to Figures 2 and 3, the subsea storage tank 22 comprises a hollow, substantially rigid body 40 atop a generally planar base 42 that supports the tank 22 on the underlying seabed 18.

In this example, the body 40 and the base 42 are both generally circular in plan view, being rotationally symmetrical about a central vertical axis 44 as seen in Figure 3.

The body 40 comprises an upright continuous tubular side wall 46. In this example, the side wall 46 is of vertical orientation and of axially-uniform cross-section, hence extending parallel to the central vertical axis 44.

The body 40 further comprises a domed top 48 that surmounts and closes the otherwise open top of the side wall 46. The uppermost extremity of the domed top 48 is an apex 50 that lies on the central vertical axis 44.

The side wall 46 and the domed top 48 of the body 40 are joined integrally at their mutual boundary by a smoothly-radiused circumferential shoulder 52. Beneath the shoulder 52, the side wall 46 extends downwardly from the domed top 48 like a skirt.

A bottom edge of the side wall 46 rests upon the base 42 inboard of the outer peripheral edge of the base 42. Thus, the base 42 projects radially beyond the side wall 46 of the body 40. This projection defines a radially-extending circumferential shelf or flange 54 of the base 42 around the bottom periphery of the tank 22.

As best appreciated in the enlarged detail view of Figure 4, the flange 54 of the base 42 supports an annular frusto-conical frame 56 comprising an angularly-spaced array of triangulated buttresses 58. Each buttress 58 lies in a respective radial plane with respect to the central vertical axis 44.

Each buttress 58 comprises an axially-extending pillar 60, a radially-extending outrigger 62 and an inclined strut 64 that extends upwardly and radially inwardly from an outer end of the outrigger 62 to join the top of the pillar 60. Here, the struts 64 of the buttresses 58 together support a continuous ring member 66 that joins the buttresses 58. The ring member 66 encircles, and is attached to, the side wall 46 of the body 40, hence holding the body 40 onto the base 42 against any buoyant upthrust that acts on the body 40 when submerged.

The flange 54 of the base 42 also supports an annular weight system that, in this example, comprises a circumferential succession of clump weights 68 disposed around the side wall 46 of the body 40. When used together, the clump weights 68 overcome any buoyant upthrust acting on the tank 22 to hold the tank 22 stably on the seabed 18.

The clump weights 68 are distributed angularly about the central vertical axis 44, between, and in alternation with, the buttresses 58. The clump weights 68 are of steel in this example but they could be of another dense material, such as concrete.

In this example, the clump weights 68 are shaped to match the curvatures of the side wall 46 and of the frame 56 defined by the buttresses 58. Thus, the curvature of an outer face of each clump weight 68 matches the frusto-conical contour of the frame 56 whereas the curvature of an inner face of each clump weight 68 matches the external radius of curvature of the side wall 46.

Conveniently, at least some of the clump weights 68 are removably attached to the base 42. This allows the number of clump weights 68 supported on the flange 54 of the base 42 to be varied to adjust the buoyancy of the tank 22 during installation, removal or repositioning.

For example, the tank 22 may be towed during installation, removal or repositioning. Mid-water towing is preferred so that the tank 22 is not subject to surface wave dynamics but remains safely above the seabed contours until installation. However, depending upon the size of the tank 22, it would, in principle, be possible instead to carry the tank 22 on a surface vessel such as a barge, and to lower the tank 22 from there into the water using a crane.

Figures 3 and 4 show that the base 42 comprises a continuous circular bottom panel 70 under a base frame 72 comprising a lattice of intersecting beams. One such beam is a ring beam 74 that extends continuously under, and in contact with, the bottom edge of the side wall 46 of the body 40. The ring beam 74 joins the buttresses 58 at the bottom of their upright pillars 60 and inboard of their radial outriggers 62.

The bottom edge of the side wall 46 that rests upon the ring beam 74 of the base 42 may be sealed to the top of the ring beam 74. However, the design of the tank 22 does not make it essential to have perfect sealing of the body 40 to the base 42 as it provides for inflow and outflow of seawater at the lower end of the tank 22.

Figure 3 best shows the provisions for allowing seawater and oil to flow into and out of the tank 22 and to allow accumulated gas to escape from the tank 22.

A water inlet pipe 76 is shown in Figure 3 and a water outlet pipe 78 is shown additionally in Figure 4 beside the water inlet pipe 76, where the side wall 46 of the body 40 is cut away. The water inlet pipe 76 and the water outlet pipe 78 penetrate the side wall 46 at a low level very close to the bottom edge of the side wall 46. In principle, however, a single combined water inlet/outlet pipe could instead provide for the inflow and outflow of water from and to the surrounding sea.

Figures 3 and 4 also show an oil inlet pipe 80 and an oil outlet pipe 82. Outside the tank 22, the oil inlet pipe 80 communicates with the subsea processing unit 16 via the subsea pipe 20 and the oil outlet pipe 82 communicates with the buoy via the offloading pipe of the offloading system 28.

The oil inlet pipe 80 and the oil outlet pipe 82 enter the tank 22 by penetrating the side wall 46 of the body 40 beside the water inlet pipe 76 and the water outlet pipe 78, hence also at a low level very close to the bottom edge of the side wall 46. From there, the oil inlet pipe 80 and the oil outlet pipe 82 extend upwardly within the hollow body 40, substantially in parallel.

Initially, the oil inlet pipe 80 and the oil outlet pipe 82 follow the inner surface of the side wall 46 and then, after bending within the shoulder 52, they follow the inner surface of the domed top 48. The oil inlet pipe 80 and the oil outlet pipe 82 terminate at their upper ends in downturned end pieces, which are located between the apex 50 of the domed top 48 and the shoulder 52 of the body 40 but are closer to the apex 50 than the shoulder 52.

Positioning such a substantial length of the oil inlet pipe 80 and the oil outlet pipe 82 within the hollow body 40 has various benefits. It protects those pipes 80, 82 against damage during transport and installation. It also exposes those pipes 80, 82 to the warmth of oil stored within the tank 22. This helps to ease and to assure flow of oil along the pipes 80, 82 by minimising the viscosity of the oil that they carry and by avoiding or mitigating the formation of solid deposits such as waxes and asphaltenes.

Figure 4 shows that the water inlet pipe 76, the water outlet pipe 78, the oil inlet pipe 80 and the oil outlet pipe 82 all, conveniently, protrude in parallel from the wall of the body 40 at a gap in the frame 56 where one of the clump weights 68 is omitted between buttresses 58. This provides ready access to those pipes 76, 78, 80, 82 while positioning them as low as possible with respect to the body 40 of the tank 22.

Gas coming out of solution or entrainment in oil within the tank 22 will gradually rise above the oil to accumulate within the domed top 48 of the tank 22. Conveniently, the shape of the domed top 48 will gather the gas centrally toward and around the apex 50.

This makes the apex 50 a convenient location for a gas vent 84 that penetrates the domed top 48.

As will be explained later with reference to Figure 7, the gas vent 84 is normally closed by an outlet valve 86 within the tank 22. However the outlet valve 86 opens to release some gas through the vent 84 into the sea when a sufficient volume of gas has accumulated under the domed top 48 of the tank 22. Hence, a build-up of excessive gas pressure is avoided.

Figure 5 is an enlarged detail view of a region around the shoulder 52 between the domed top 48 and the side wall 46 of the body 40. The oil inlet pipe 80 and the oil outlet pipe 82 are shown within the body 40, closely following its internal contours.

Figure 5 shows, in cross-section, that the domed top 48 and the side wall 46 of the body 40 are integrally formed of twin walls 88 of GRP. In the space between them, the walls 88 sandwich a rigid core 90 of syntactic foam. In addition to buoyancy, the core 90 confers stiffness and thermal insulation upon the body 40, thus helping to retain heat in oil stored within the tank 22. Figure 5 also shows that the body 40 is apt to be assembled from panels or sections that are joined along mutually abutting edges, exemplified here by a flanged interface 92 between adjoining sections.

To illustrate the subsea storage tank 22 of the invention in use, Figure 6 shows the tank 22 now containing a variable volume of oil 94. The volume of oil 94 is capped by a much smaller, but also variable, volume of hydrocarbon gas 96 that floats upon the oil 94 and that is trapped under the domed top 48 of the body 40. These volumes of oil 94 and gas 96 float, in turn, upon a volume of seawater 98 that fills the remaining internal volume of the tank 22 and that applies hydrostatic pressure to the underside of the oil 94.

The volume of seawater 98 is itself variable in accordance with fluctuations in the volume of oil 94 and gas 96. As more oil 94 is introduced into the tank 22, seawater 98 is displaced downwardly within the tank 22 and some of that seawater 98 exits the tank 22 through the water outlet pipe 78. Conversely, as oil 94 is drawn from the tank 22, seawater 98 enters the tank 22 through the water inlet pipe 76 under hydrostatic pressure and rises within the tank 22 to compensate for the reduced volume of oil 94. It will be apparent that these flows of seawater 98 maintain a balance of pressure between the interior and the exterior of the tank 22, enabling the tank 22 to have a thinwalled structure that is both lightweight and inexpensive.

The level of seawater 98 within the tank 22 is always kept well above the low level of the water inlet pipe 76 and the water outlet pipe 78. This ensures that oil 94 floating above the seawater 98 cannot escape the tank 22 through the water outlet pipe 78 and be released into the sea. It also ensures that the interface or transition zone 100 between the seawater 98 and the oil 94 is not disturbed unduly by seawater 98 flowing into and out of the tank 22 in response to fluctuations in the volume of oil 94.

Oil 94 is added to the volume of oil 94 within the tank 22 through the oil inlet pipe 80 that leads from the subsea processing unit 16. The incoming oil 94 flows within that pipe 80 first through the seawater 98 under the volume of oil 94 and then through the volume of oil 94 itself. The incoming oil 94 is dispensed into the tank 22 through the downturned end piece at the upper end of the oil inlet pipe 80, which remains immersed in the oil 94 just under the top surface 102 of the volume of oil 94. This minimises disturbance of the volume of oil 94 as oil 94 flows into the tank 22. It especially minimises disturbance of the transition zone 100 between the seawater 98 and the oil 94.

The downturned end piece of the oil outlet pipe 82, through which oil 94 is drawn off into the offloading system 28, is also immersed in the oil 94 just under the top surface 102 of the volume of oil 94. This also minimises disturbance of the volume of oil 94 as oil 94 flows out, and hence also minimises disturbance of the transition zone 100 between the seawater 98 and the oil 94.

As oil 94 is drawn off and hence the volume of oil 94 in the tank 22 reduces, the inflow of seawater 98 through the water inlet pipe 76 under hydrostatic pressure continues to push the oil 94 up toward the top of the tank 22. This ensures that the upper ends of the oil inlet pipe 80 and the oil outlet pipe 82 remain immersed under the top surface 102 of the volume of oil 94.

It will be apparent that the volume of gas 96 under the domed top 48 of the tank 22 should also be controlled. Otherwise, the gas 96 will accumulate over time to the extent that the volume of oil 94 is pushed down too far inside the tank 22.

Left unchecked, an accumulation of gas 96 above the oil 94 would eventually push the top surface 102 of the volume of oil 94 below the upper ends of the oil inlet pipe 80 and the oil outlet pipe 82. This would increase the risk of disturbing the transition zone 100 between the seawater 98 and the oil 94 when loading oil 94 into the tank 22. It would also prevent the oil outlet pipe 82 from picking up oil 94 from the tank 22 during offloading. In the extreme, the volume of gas 96 could increase to the extent that there would be insufficient depth to accommodate the desired volume of oil 94 above the level of the water inlet pipe 76 and the water outlet pipe 78.

It is for this reason that the aforementioned gas vent 84 is provided at the apex 50 of the domed top 48 of the tank 22. As noted above, the gas vent 84 is normally closed by the outlet valve 86. This retains enough gas 96 under the domed top 48 to ensure that the top surface 102 of the volume of oil 94 never reaches the gas vent 84 and hence ensures that no oil 94 can escape to the surrounding sea through the gas vent 84. However the outlet valve 86 must open intermittently to release some gas 96 through the gas vent 84 into the sea before the volume of gas 96 within the tank 22 becomes excessive.

Turning finally to Figure 7, this shows an example of how the outlet valve 86 may be configured to perform the above duties automatically.

Figure 7 shows the domed top 48 of the tank 22 penetrated by the gas vent 84 at its apex 50. In this enlarged view, the sandwich construction of the domed top 48 is also evident, comprising syntactic foam 90 between twin walls 88.

The outlet valve 86 has a housing comprising a tubular upper portion 104 that fits into the gas vent 84 and a tubular cage 106 that is suspended from the upper portion 104 and so hangs from the domed top 48 inside the tank 22. The cage 106 is partially immersed in the volume of oil 94, hence dipping under the top surface 102. An internal flange defining an annular valve seat 108 is at the boundary between the upper portion 104 and the cage 106. The valve seat 108 surrounds a restricted orifice that, when open, allows fluid communication through and along the upper portion 104 of the housing.

The cage 106 retains a valve element in the form of a floating ball 110. The cage 106 is perforated with holes 112 to admit gas 96 and oil 94 into the cage 106. The buoyancy of the ball 110 is selected to float on the oil 94 and to sink in the gas 96.

Thus, when the level of the top surface 102 of the volume of oil 94 moves up or down within the height or length of the cage 106, the ball 110 can move up and down within the cage 106 to follow the changing level of that surface 102.

If the top surface 102 of the volume of oil 94 moves down below a threshold level, this indicates that the volume of gas 96 is becoming excessive and so some gas 96 must be vented from the tank 22. Thus, as the ball 110 follows the downward movement of the surface 102, it drops within the cage 106 and out of engagement with the valve seat 108 as shown in Figure 7. This opens the orifice to allows gas 96 to flow through the cage 106 and along the upper portion 104 of the housing, to escape as bubbles 114 into the sea 116 above the tank 22.

When the top surface 102 of the volume of oil 94 rises again as the volume of gas 96 decreases, the outlet valve 86 must close to prevent leakage of oil 94 through the gas vent 84. So, as the top surface 102 moves up, the ball 110 follows that upward movement to be pressed into engagement with the valve seat 108 at the top of the cage 106. This prevents the oil 94 from entering the upper part 104 of the housing and therefore escaping through the gas vent 84.

Many variations are possible within the inventive concept. For example, it would be possible, additionally or alternatively, to pin the base of the tank to the seabed using pin piles or other anchors. It would also be possible to attach the base of the tank to a pre-installed foundation, such as one or more suction piles embedded in the seabed.

Heating may be applied to the volume of oil in the tank to increase or to maintain its temperature prior to offloading.

Claims

1. A method of storing crude oil subsea for offloading, the method comprising:

introducing the oil, containing gas, into a subsea storage tank;

floating the oil above a variable-volume column of seawater within the tank to trap the oil between the seawater and a top wall of the tank, the seawater being under hydrostatic pressure and applying that pressure to the oil;

trapping gas emerging from the oil in a pocket between the oil and the top wall;

venting a portion of the trapped gas from the tank;

offloading a portion of the oil from the tank; and causing seawater to flow into and out of the tank in response to venting the gas and introducing or offloading the oil, hence causing an interface between the oil and the seawater to move vertically within the tank.

2. The method of Claim 1, further comprising allowing water also contained in the introduced oil to migrate into the seawater in the tank beneath the oil.

3. The method of Claim 1 or Claim 2, comprising exposing the oil directly to the seawater on which that oil floats within the tank.

4. The method of Claim 3, comprising maintaining a fluid transition layer at the interface between the oil and the seawater.

5. The method of Claim 4, wherein the fluid transition layer is a layer of emulsion.

6. The method of any preceding claim, comprising venting the trapped gas in response to an increase in volume of the pocket of trapped gas.

7. The method of Claim 6, comprising venting the trapped gas in response to downward displacement of a top surface of the oil beneath the trapped gas.

8. The method of Claim 7, comprising venting the trapped gas when the top surface of the oil moves beyond a threshold level of displacement down the tank.

9. The method of Claim 8, comprising venting the trapped gas by downward movement of a valve element that floats on the oil within the tank.

10. The method of any preceding claim, comprising introducing oil into the tank through an oil inlet immersed beneath a top surface of the oil in the tank.

11. The method of Claim 10 when dependent upon Claim 8 or Claim 9, wherein the oil inlet is beneath the threshold level of displacement of the top surface of the oil.

12. The method of Claim 10 or Claim 11, comprising conveying oil through the seawater in the tank and the oil in the tank before introducing that oil through the oil inlet.

13. The method of any preceding claim, comprising conveying oil from an oil outlet through the oil in the tank and the seawater in the tank when offloading oil from the tank.

14. The method of any preceding claim, comprising causing seawater and oil to flow into and out of the tank through a side wall of the tank at a level beneath the interface between the oil and the seawater.

15. The method of any preceding claim, preceded by processing the oil subsea before introducing the oil into the storage tank.

16. A subsea storage tank for storing crude oil, the tank comprising:

an external wall enclosing an internal volume;

a seawater inlet and a seawater outlet communicating with the internal volume;

an oil inlet and an oil outlet communicating with the internal volume; and a gas vent communicating with the internal volume;

wherein the oil inlet and the oil outlet open to the internal volume at a level beneath where the gas vent opens to the internal volume and above a level at which the seawater inlet or the seawater outlet open to the internal volume.

17. The storage tank of Claim 16, wherein the oil inlet and the oil outlet are defined by respective conduits that extend within the internal volume to a level adjacent to the seawater inlet or the seawater outlet.

18. The storage tank of Claim 17, wherein the conduits that define the oil inlet and the oil outlet penetrate the external wall.

19. The storage tank of any of Claims 16 to 18, wherein the seawater inlet and the seawater outlet are defined by separate conduits.

20. The storage tank of any of Claims 16 to 19, wherein the gas vent is at an uppermost apex of the external wall.

21. The storage tank of any of Claims 16 to 20, further comprising a valve element that is movable between a lower open position and an upper closed position to open and close the gas vent, the open position of the valve element being above the level at which the oil inlet and the oil outlet open to the internal volume of the tank.

22. The storage tank of Claim 21, wherein the valve element is positively buoyant in crude oil.

23. The storage tank of any of Claims 16 to 22, wherein the external wall is of fibrereinforced polymer composite.

24. The storage tank of any of Claims 16 to 23, wherein the external wall is of sandwich construction comprising a layer of syntactic foam.

25. The storage tank of any of Claims 16 to 24, wherein the external wall is anchored by a ballast weight system comprising ballast weights supported by a frame attached to the wall.

26. The storage tank of Claim 25, wherein the external wall is attached to a base that extends outwardly beyond the wall to define a platform for the ballast weights.

27. The storage tank of any of Claims 16 to 26, when the internal volume contains:

a variable-volume column of seawater;

a variable volume of oil floating above the column of seawater;

an interface between the column of seawater and the volume of oil; and a variable-volume pocket of gas trapped above a top surface of the volume of oil;

wherein the oil inlet and the oil outlet open to the internal volume at a level above the interface; the seawater inlet or the seawater outlet open to the internal volume at a level beneath the interface; and the gas vent opens to the internal volume at a level above the top surface of the volume of oil.

28. The storage tank of Claim 27, wherein the oil inlet and the oil outlet open to the internal volume at a level between the interface and the top surface of the volume of oil.

29.

29. The storage tank of Claim 27 or Claim 28, wherein the column of seawater and the volume of oil are in mutual contact at the interface.

30. A subsea installation comprising a storage tank as defined in any of Claims 16 to