WO2020099840A1

WO2020099840A1 - Method and apparatus for management of fluids in an underwater storage tank

Info

Publication number: WO2020099840A1
Application number: PCT/GB2019/053180
Authority: WO
Inventors: Malcolm Bowie
Original assignee: Sllp 134 Limited
Priority date: 2018-11-12
Filing date: 2019-11-11
Publication date: 2020-05-22
Also published as: GB2578890B; GB2578890A; NO20210565A1; US20210395003A1; GB201818365D0

Abstract

Apparatus and method for managing water or other ballast fluid in an underwater storage tank (14), used for storing fluids such as oil or C02, where a dis-connectable inflatable bag (16) is coupled to the underwater storage tank to capture displaced ballast fluid from the underwater storage tank. A supply line (36) between the bag and tank provides a closed loop system. Embodiments are shown in which the tank is initially filled with fresh water and multiple dis-connectable inflatable bags are used.

Description

METHOD AND APPARATUS FOR MANAGEMENT OF FLUIDS IN AN UNDERWATER STORAGE TANK

The present invention relates to underwater storage tanks and in particular, though not exclusively, to means of managing fluids in a rigid underwater storage tank, principally managing water in a tank which stores oil and discharges oil.

Storage tanks have been used in the offshore oil industry for many decades to either store the oil itself or aid in the separation of the oil from other compounds found in reservoir fluids such as water and sand. Most of these storage tanks are either onshore or in the hull of floating facilities and hence the density of the oil being as low as 80% of the seawater density does not have any significant consequences. If the tank is placed under the sea, the buoyancy effect of this oil can be significant, plus the gap or void space in the tank above the oil is impractical to be a large volume gas, even under pressure, due to the very large buoyancy resulting. To replace the void space it is generally accepted that water may be used in lieu of gas. Thus we have a situation of water with oil on top and often a very small gas cap associated with any gas which is coming out of solution.

The other practical and environmental aspect is that the water in the tank must be displaced as oil is loaded. The principal options are re-injection into the wells or discharge to sea. Discharge to sea requires a degree of treatment and filtration to ensure the quality of the water released to the environment is at safe levels. Re-injection can be performed; however, there is usually formation water in the oil which will drop out in the tank and mix with the water in the tank. If this existing water is seawater then there is a risk of precipitation of scale from the reaction between seawater and formation water. Chemical inhibition of scale can be performed but the volumes involved are large in this case.

Additionally as the oil is offloaded, which can demand high offloading rates of 30,000 - 50,000 m³ per day, the water must be replaced in the tank. This places high demands on the seawater filtration and treatment systems to accommodate the throughput required.

Further the water that is in the tank may be heated and thus the discharge to sea will result in the loss of some of the thermal energy, even with heat exchangers being used . Conversely the incoming seawater to replace the oil being exported may cool down the tank contents which make the oil more viscous plus require heating to the target temperature. The energy required due to the very high specific heat capacity of water will be significant.

There are principally three methods currently used for storing oil or other fluids subsea : · A submerged heavy gravity structure tank with sufficient weight to counteract the buoyancy from a partially or fully empty tank containing air;

• A submerged tank, in which the water is exchanged with local seawater; and

· A submerged tank with an internal bag to store the oil.

The methodology of using a tank with sufficient weight to counteract the buoyancy forces of an empty tank, leads to a very large, heavy and expensive structure. In addition to float out such a structure, which is designed to rest on the seabed when empty, necessitates the addition of ballast chambers which further increase the size and cost. Concrete is often used for part of the majority of such structures with the attendant inefficiency of concrete having a lower density when compared to steel plus the environmental impact of its use.

The methodology of using a seawater exchange tank to store the oil places demands on the filtration system for the seawater entering and leaving the tank plus requires the use of chemical inhibition to control scale formation as a result of mixing seawater with formation water.

In addition the methodology of using a seawater exchange tank to keep C02 in liquid form in deep water (i.e. greater than 500m depth) places demands on the management of the seawater, which will become acidic by dissolution of C02 into the seawater in contact with the liquid C02.

A more recent methodology is to use an inflatable bag to store the oil, as described in W02015/110413 and W02004/037681. This provides seabed located storage for crude oil or other fluid, distinguished in that it is comprising a storage section in the form of an oil and waterproof cloth formed as a flexible balloon that can be filled with, store and emptied for a storage fluid, a structure section formed as an external casing over the storage section, which structure section is closed in the upper part such that an upward close volume of size at least corresponding to the volume of the storage section is formed, but with openings to the surroundings in the lower part, an anchoring section formed as a substructure between the structure section and the seabed, with means for anchoring to or stable placement on the seabed, and a transfer section comprising pipes and valves for loading and unloading of a storage fluid, arranged in substance exterior to the upper part of the storage. Such an arrangement has the following disadvantages.

• Inspection of the external surface of the bag for damage may be difficult and inspection of the internals almost impossible. · The bag will require a significant elasticity to accommodate the surface area change when loading.

• Any penetrating damage to the bag which results in the release of oil, will require a cap to capture the oil released, together with a monitoring system to identify such a release and shutdown the processing.

• The inside system of such a cap will require close fabrication scrutiny to remove any weld scars or edges which could damage the bag.

• Recover of the bag if damaged, or for inspection, needs to have a system to ensure the bag can be recovered without damage or oil release, especially if there is a large quantity of solids or highly viscous oils in the bottom, which may hinder or prevent bag removal.

• Onshore procedures and methods will require to be developed to safely dispose of the bag contents and clean the insides, minimising the hazards to people and the environment.

• Entry to the bag by personnel is unlikely to be allowed on safety grounds, hence access to inspect the bag without destruction of the bag will be difficult.

· Damage, loss of flexibility or stress concentrations may be caused to the bag by deposition of solids or thicker compounds attaching to the internals.

Given that oil from the process train(s) usually contains gas, solids, water and a range of different compounds of different pH and viscosity the integrity demands on an elastic bag to store the oil can be onerous and qualification testing processes for the material will have to be extensive to ensure chemical compatibility.

A related invention to W02015/110413 A1 is EP 3444427 Al, whereby a first elastic bag is used to store oil with connectivity to a second smaller elastic bag which is used to manage pressure variations in the first bag via fluid communication. However, the second bag does not have sufficient volume to contain all of the displaced water from the first bag and the first bag is subject to the considerations as stated earlier. The invention does not state that replacement of either bag is required or possible.

A further invention US4662386 describes a toroidal or cylindrical tank to which is attached a bladder arrangement. This bladder accepts the displaced water in a similar manner; however, the bladder is a permanent fixture, which places demands on the integrity of the material used for chemical and service compatibility and does not have removability at the core as a key feature of the invention. This places onerous demands on the bladder material, given the varying formation water and oil compositions in different reservoirs. The profile of the bladder is such that any gas particles coming out of solution will gather that the top of the bladder when it is more than 50% full (i.e. the upper surface of the bladder is convex) and migrate to the sides when less than 50% full (bladder upper surface is concave). Therefore, there is no means of escape for gas coming out of solution, which will lead to increasing buoyancy and stresses on the bladder membrane. Further, oil particles which are carried in with the water will also migrate to the top of the bladder with no means of exit or means of testing the fluid/gas mixture composition for quantity. The gas dissolution and oil particles will also interfere and reduce with the static external pressure on the bladder which would normally force the lighter products in the storage tanks to elevations above sea level during offloading operations. It is therefore an object of the present invention to provide apparatus for managing fluids in an underwater storage tank which obviates or mitigates at least some of the disadvantages of the prior art.

It is a further object of at least one embodiment of the present invention to provide apparatus for managing the water in an underwater oil storage tank which obviates or mitigates at least some of the disadvantages of the prior art.

It is an object of the present invention to provide a method for managing fluids in an underwater storage tank which obviates or mitigates at least some of the disadvantages of the prior art.

It is a further object of at least one embodiment the present invention to provide a method for managing the water in an underwater oil storage tank which obviates or mitigates at least some of the disadvantages of the prior art. It is a further object of at least one embodiment of the present invention to provide a method for managing the water or other displaced ballasting fluid in an underwater liquefied C02 storage tank which obviates or mitigates at least some of the disadvantages of the prior art. According to a first aspect of the present invention there is provided apparatus for managing a first fluid in an underwater storage tank for storing a second fluid, comprising a dis-connectable inflatable bag coupled to the underwater storage tank to capture the first fluid displaced from the underwater storage tank.

The first fluid may be a displaced ballasting fluid. More preferably the first fluid is a liquid. More preferably the second fluid is a liquid. Preferably, the first fluid is water. The second fluid may be liquified C02. In a preferred embodiment the second fluid is oil. There may be a plurality of dis-connectable inflatable bags.

In this way, the dis-connectable bags are used to contain the displaced formation/seawater instead of the oil, the contents of which will have a significantly lower range of trace compounds within. The risk of an oil-leak together with the consequences is greatly reduced, plus a major advantage is that the dis-connectable inflatable bags can be disconnected and towed to shore for inspection and testing. Such regular inspection will give the manufacturers higher confidence in the integrity and longevity of the dis-connectable inflatable bags plus allow development of a discard criteria.

Further the regular inspection of the replaced dis-connectable inflatable bags allows planning for the final decommissioning minimising hazards to humans and the environment, which otherwise would be uncertain from a permanently connected bag.

Preferably, the dis-connectable inflatable bag has a first port for the entry and exit of water. In this way, a closed loop system with the underwater oil storage tank can be made so that water in the underwater oil storage tank is not mixed with seawater outside of the underwater oil storage tank.

Preferably also, the dis-connectable inflatable bag has a second port, the second port being closed when the dis-connectable inflatable bag is attached to the underwater oil storage tank. In this way, the second port is used to remove gas which has come out of solution or lighter than water fluids, which would otherwise increase the buoyancy of the gas and on pass said compounds for further treatment. The port also allows flushing of the dis-connectable inflatable bag contents in-situ in preparation for disconnection and onshore for maintenance and/or disposal.

Preferably the dis-connectable inflatable bags are arranged such that there are top and bottom entrance ports, to allow removal of heavier than water compounds through one port and lighter than water compounds from the other.

Preferably the dis-connectable inflatable bags are aligned so that complete draining of lighter than water and heavier than water compounds can occur under the action of gravity through the ports.

In particular, a top port allows lighter than water fluids and gases to naturally rise by the action of buoyancy out of the dis- connectable inflatable bag for passing back into the process stream.

This port may, subject to the arrangement of the pipework, be open in service to allow any compounds to automatically rise to a holding tank, which can be periodically emptied into the process stream. The lower port is usually open to allow water to enter and exit the dis-connectable inflatable bag, but this can be used to reverse flush the dis-connectable inflatable bag contents to remove any solids. A solids trap may be incorporated within the pipework prior to entry to the dis-connectable inflatable bag to capture solids, improving the efficiency of solids management.

The plurality of ports provide a passageway between an inside and an outside of each dis-connectable inflatable bag. The provision of more than one port on the dis-connectable inflatable bag, gives the opportunity to flush the dis-connectable inflatable bags, removing any solids, gas, or fluids other than seawater/formation water.

Preferably, the dis-connectable inflatable bag is arranged adjacent the underwater storage tank. In this way, the dis-connectable inflatable bag allows a large amount of enthalpy in the system by allowing warmer displaced water to re-fill the tank, rather than colder seawater.

Preferably, there are a plurality of dis-connectable inflatable bags arranged adjacent the underwater storage tank. In this way, selected dis-connectable inflatable bags can be disconnected and towed into a safe location for inspection and testing. Additionally multiple dis-connectable inflatable bags give redundancy in the event of a blockage or leakage.

Preferably, the apparatus further comprises a supply line between a port and a water exit port at the bottom of the underwater storage tank. Preferably the supply line includes one or more valves. In this way, unplanned back flow can be prevented and piping branches can be isolated.

The supply line may include a branch line to divert water to production facilities associated with the underwater oil storage tank.

In this way, water and/or produced water from the underwater oil storage tank can be brought to the production facility for cleaning and discharge or injection.

The supply line may include an intermediate tank, the intermediate tank being used to collect and separate oil or sludge which may have entered the supply line as the water from the underwater oil storage tank is transferred to the dis-connectable inflatable bag. Preferably, the intermediate tank has at least one take-off port to direct the separated oil/sludge back to the underwater oil storage tank. More preferably there are two take-off ports, arranged at a top and a bottom of the intermediate tank, with the supply line arranged to draw from a point between the top and the bottom of the intermediate tank. According to a second aspect of the present invention there is provided a method for managing a first fluid in an underwater storage tank, used for storing a second fluid, comprising capturing the first fluid displaced from the underwater storage tank in a dis- connectable inflatable bag. The first fluid may be a displaced ballasting fluid. More preferably the first fluid is a liquid. More preferably the second fluid is a liquid. Preferably, the first fluid is water. The second fluid may be liquified C02. In a preferred embodiment the second fluid is oil. In this way the underwater storage tank is an underwater oil storage tank There may be a plurality of dis-connectable inflatable bags.

In this way, the mix of seawater and formation water is not discharged to sea as the oil is loaded into the underwater oil storage tank.

Preferably the method includes the steps of:

(a) connecting a supply line between a single port of the dis- connectable inflatable bag and a water exit port at the bottom of the underwater oil storage tank;

(b) with the dis-connectable inflatable bag deflated, initially filling the underwater oil storage tank with water;

(c) loading oil into the underwater oil storage tank to displace the water into the supply line;

(d) passing the displaced water into the dis-connectable inflatable bag and storing the displaced water in the bag; (e) discharging oil from the underwater oil storage tank for export while allowing the displaced water to return to the underwater oil storage tank from the dis-connectable inflatable bag. In this way, the dis-connectable inflatable bag is used to hold the displaced water which reduces the risks inherent if the dis- connectable inflatable bag held oil.

Preferably, at step (a) a plurality of dis-connectable inflatable bags are attached to a branched connection on the supply line, with each branch containing a valve and the method includes the step of isolating one or more dis-connectable inflatable bags. In this way, additional bags are available for redundancy.

Preferably, the method includes the step of removing an isolated dis-connectable inflatable bag from the supply line. In this way, the isolated bag can be inspected or replaced.

Preferably, at step (d) the underwater oil storage tank is initially filled with fresh water. In this way, when the fresh water mixes with formation water, introduced via the oil, there is a lower risk of scaling than would occur if seawater was used to displace the oil. Additionally, as the oil is offloaded in step (e), complex filtering systems are not required for incoming seawater.

Preferably, at step (c) the oil is loaded into the underwater oil storage tank from a production facility.

Preferably, the method includes the step of returning gas in the underwater oil storage tank to the production facility. In this way, the underwater storage tank is kept full of only oil and water.

The method may include the step of returning displaced water from the underwater oil storage tank via the supply line to a production facility. In this way, the displaced water which now has added formation water can be cleaned, discharged or used for injection.

The method may include the step of pressure monitoring. This can be performed at the dis-connectable inflatable bag(s), the tank or at any point in the pipelines. In this way, the inflation level in the bags and the tank can be determined.

The method may include passing the displaced water from the underwater oil storage tank through a separator on the supply line.

In this way, any oil or solids/sludge which may have passed into the supply line is prevented from entering the dis-connectable inflatable bag. In this way, the risk of contamination of the dis- connectable inflatable bags is minimised.

This method may also include, at step (e), providing continuous export of oil while separating the oil and water in the underwater oil storage tank. In this way, the tank is used as a long-residence time separator to improve the quality of the oil but also allowing continuous export. The dis-connectable inflatable bags may be used according to the same methodology to capture the produced water, but also provide a volumetric buffer and against process upsets allowing greater management of the process system. The storage requirements of the bags may then be significantly reduced.

The method may further include the step of discharging water from the underwater oil storage tank through an oil export line. In this way, the oil export line can be flushed .

The management of pressurised and liquefied C02 in a deep water storage tank, for carbon sequestration, faces similar challenges in that exchange of the seawater with the natural environmental is undesirably due to the resulting acidity from carbon dioxide entering the water in solution and reducing the ph, making it acidic.

By provision of the same closed loop system that ability to manage the water is provided by using the dis-connectable inflatable bags to contain the displaced water. In addition the provision of piping to the facility from the bags and the supply line, allows circulation of the water to topsides, where the C02 has an opportunity to come out of solution due to the near ambient pressure and be recovered for re-injection. Additionally the water can be filtered and treated before being returned to the tank in the same manner as formation water. The ability to disconnect the bags is vital in order that the effect of the acidification of the ballast fluid can be assessed on the bag material in the same manner as oil or formation water does.

Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope languages such as including, comprising, having, containing or involving and variations thereof is intended to be broad and encompass the subject matter listed thereafter, equivalents and additional subject matter not recited and is not intended to exclude other additives, components, integers or steps. Likewise, the term comprising, is considered synonymous with the terms including or containing for applicable legal purposes. Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters form part of the prior art based on a common general knowledge in the field relevant to the present invention. All numerical values in the disclosure are understood as being modified by "about". All singular forms of elements or any other components described herein are understood to include plural forms thereof and vice versa.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, of which:

Figure 1 is a schematic illustration of apparatus for managing the water in an underwater oil storage tank according to an embodiment of the present invention;

Figure 2 is the apparatus of Figure 1 shown with the underwater oil storage tank full of oil ready for export and the dis-connectable inflatable bags filled with the displaced water;

Figure 3 is a schematic illustration of apparatus for managing the water in an underwater oil storage tank according to another embodiment of the present invention; Figure 4 is a schematic illustration of apparatus for managing the water in an underwater oil storage tank according to another embodiment of the present invention;

Figure 5 is a schematic illustration of apparatus for managing the water in an underwater oil storage tank according to another embodiment of the present invention and

Figure 6 is a schematic illustration of apparatus for managing the water or other ballasting fluid in an underwater liquefied C02 storage tank according to another embodiment of the present invention. Reference is initially made to Figure 1 of the drawings which illustrates an apparatus, generally indicated by reference numeral 10, for managing the water 12 in an underwater oil storage tank 14 according to an embodiment of the present invention. An dis- connectable inflatable bag 16 is coupled to the tank 14 to capture water displaced from the tank 14.

The underwater oil storage tank 14 is a standard rigid construction typically located on the seabed below or in proximity to a production facility 18. Tank 14 is used to store oil 20 in the produced fluids 24 from the facility 18 which is transported to the tank 14 via piping 22. Within the tank 14, due to density differences, gas 26 will rise and is fed back via a line 28 to the facility 18. Produced water 30, sometimes termed formation water, will partially mix with the water in the tank and together collect in the lower portion 32 of the tank 14. An export line 34 is also provided from the tank 14 to remove the stabilized crude oil 20 when the tank 14 is full.

Initially the tank 14 is filled with water 12. In the prior art this would have been seawater and a complex arrangement of filtering would be required to treat the seawater in an attempt to mitigate the scaling that occurs when seawater comes into contact with produced water 30. In the present invention, water 12 is preferentially fresh water. A supply line 36 is arranged between a port 38 at the bottom 32 of the tank 14 and a first port 40 into and out of an dis-connectable inflatable bag 16. The bag 16 is of a flexible material to create a barrier between water 12 and the surrounding seawater. The bag 16 can be located within the vicinity of the tank 14 either located on top, on the side, underneath or on the local seabed. Optionally the bag 16 can be located within a structure which provides a degree of protection against dropped objects and/or stabilisation against the effect of wave and current.

The bag 16, supply line 36 and tank 14 therefore provide a closed loop system. In this way the water 12 is contained within the system. Further piping 42 can be provided for a water top-up line from the production facility 18 to the tank 14. If fresh water is not available to top-up the tank 14, then filtered seawater may be used.

Branch piping 44 can also be provided from the supply line 36 to the production facility 18 for water 12 and/or produced water 30 for cleaning, discharge, reinjection or storage in the hull as is known in the art.

By this means the piping 42 and 44 can control the water volume within the closed loop, independently of the production facilities. Valves 46 are arranged on the pipelines (not all shown) to control flow through and/or prevent unwanted back flow in the apparatus 10. Pressure sensors 48 may also be located through the apparatus 10 for monitoring purposes.

Dis-connectable inflatable bag 16 also has a second port 41. Port 41 allows the bag 16 to be flushed through via use of first port 40. This can be done to clean out the bag during maintenance or assist in emptying the bag for disposal. These actions can be done offshore in-situ or onshore.

The dis-connectable inflatable bag 16 has connectors 43 on each port, which allow the bag to be removed and re-connected, once the isolation valves 46 have both been closed.

The bag 16 is shown vertically in order that solids generally fall to the bottom of the bag 16 and lighter than water liquids and gases rise to the top port. By this arrangement undesirable liquids, solids or gases can be removed from the bag 16 in service by flushing through the second port 41. In particular gases and lighter than water liquids can be flushed back to the facility 18 via piping 45 where they can re-enter the process stream. Alternatively the flushing process can be used to displace solids from the tank 14 into the bag 16 for recovery and disposal onshore.

Also shown are the connection / disconnect arrangements 43 which allow the bag to be flushed, isolated and then removed for onshore inspection, replacement or prior to decommissioning of the tank 14.

Figure 1 shows the apparatus 10 in an initial configuration where the water 12 which is displaced from the tank 14, can be stored in dis-connectable inflatable bag 16. Thus a mix of seawater and formation water is therefore not discharged to sea as the oil is loaded into the main tank 14. If there is a significant quantity of formation water 30, some of the combined water can be passed back to the main facilities for either re-injection or cleaning to discharge. Generally, the quantities involved will be the same as the produced water 30 dropout rate. In use, produced fluids 24 are sent to the tank 14 via piping 22. On entering the tank 14, the gas 26 will rise and can be piped 28 back to the facility 18. The oil 20 will sit on top of the water 12, though there may be an emulsion line created therebetween, with any produced water 30 mixing with the water 30 in the bottom 42 of the tank 14. As the volume of oil 20 in the tank 14 increases, the water 12 is displaced from the tank 14 through the supply line 36 and into the bag 16. The bag 16 will inflate under the introduction of the water 12 with its volume equalling the volume of displaced water, minus any water which is sent to the production facility 18, via the branch line 44. In this way, only water 12 is contained in the bag 16 and it provides a variable storage volume in response to the amount of displaced water. Connectors 43 allow the bags 16 to be dis-connected and replaced with another bag at any time, by closure of the isolation valves 43. As the tank 14 fills, the pressure in the tank 14 will reduce due to the density of the oil 20 being less than water 12; however, hydrostatic pressure acts on the bag 16 keeping the pressure in the top of the tank 14 slightly above external pressure, due to the head of oil 20 inside, which increases as more oil 20 is loaded. The production facility 18 will therefore have to pass processed oil, in the form of produced fluids 24, into the tank 14 at or above the seabed hydrostatic pressure. Pressure monitoring 48 can be provided at various points in the apparatus 10, including the storage bag 16.

Valves 46 are provided as require to prevent unplanned backflow and isolate piping branches as required. For example, where part of the water 12 being displaced from the tank 14 is being filtered off to the production facilities 18 to provide re-injection water. The bag storage 16 thus provides a flexible buffer storage for the water to allow management of the treatment and rate of injection into wells and automatically by their presence provides a buffer against pressure surges or rapid changes due to process interruptions and valve opening/closures. If sea water is used in the bag 16, as the formation water 30 mixes more it dilutes the sea water reducing and potentially eliminating scale inhibitor requirements.

Reference is now made to Figure 2 of the drawings which shows the apparatus of Figure 1 with the tank 14 now full of stabilised crude oil 20 and the bag 16 containing a majority of the water 12.

In use, once the tank 14 is full, the oil 20 will be discharged to an export tanker or other export means via export line 34. As the oil 20 is evacuated from the tank 14 by the oil export pumps 58, the water 12 and formation water 30 stored in the bag 16 is passed back through the supply line 36 into the main tank 14 assisted by the external hydrostatic head of pressure acting on the bag 16 and the lower static pressure at the top of the oil in the tank.

The bag 16 will deflate as water 12 is displaced back to the tank 14 until the water 12 reaches a level in the tank 14 where-by oil export will be stopped.

The rate of offloading is therefore limited generally by the export rate of the oil, rather than the filtration of seawater to replace the oil, if the bag were not present. The rate of offloading can also be increases by a taller tank to maximise the head differential between the external water pressure and that at the top of the oil column.

This method also has a large technical, economic and environmental advantage in that the thermal energy of the water 12 is not discharged to sea and dissipated in the ocean, but passed straight back into the tank 14. Also as water 12 is passed back into the tank 14 the viscosity of the oil 20 is less affected, than if it was colder seawater, which would lead to the oil 20 closest to the water interface cooling quickest and hence becoming more viscous towards the end of offloading. This also helps maintain an efficient thermal balance in the tank 14 and minimises the temperatures changes which have some influence of the fatigue life of the tank structure.

If the quantity of stored water exceeds the oil volume exported the evacuation may continue to allow flushing of the oil export pipeline using the water. Any oily water received on the tanker is normally directly to slops tanks.

Reference is now made to Figure 3 of the drawings which illustrates a further embodiment of the present invention by the incorporation of a separating tank 50 in the supply line 36. An oil particulates return line 52 is provided from the top of the separation tank 50 to the tank 14 and a solids/sludge return line 54 is provided from the bottom of the separation tank 50 to the tank 14. This additional tank 50 is used if there is a risk that the solids/sludge level at the bottom 32 of the tank 14 reaches a level close to the water exit point, port 38, near the tank bottom 32 during offloading the water 12 into the supply line 36. In addition, if the oil 20 level gets close to the exit point, port 38 during loading, some oil 20 may accidently be drawn down into the supply line 36.

A tall intermediate tank 50 can be specified which will allow separation of the oil 20 and solids, prior to the water 12 the bag 16. Such a tank 50 is illustrated in Figure 3, with take-offs top and bottom, but the water 12 has to pass around a baffle plate, maximising its transit time through the separation tank 50.

This provides an additional level of operational efficiency and flexibility to the apparatus 10 though with correct monitoring the tank 50 is not required.

Reference is now made to Figure 4 of the drawings which shows a further embodiment of the present invention in the form of using multiple dis-connectable inflatable bags 16a-c. Though three bags 16a-c are illustrated, it will be appreciated that any number and size can be used to meet the requirements of the subsea tank 14 used. Supply line 36 from the tank 14 now has a manifold 56 for water 12 distribution to each of the bags 16a-c. Bag 16c is disconnected to illustrate that the bags 16a-c can be removed for inspection and replacement if needed. More than one bag 16 is used to provide redundancy, allow a greater use of smaller, more readily available designs, plus allow for disconnection and recovery for onshore inspection or replacement. Reference is now made to Figure 5 of the drawings which shows a further embodiment of the present invention in the form of using a holding tank 52 to capture lighter than water gases or liquids. Like parts to those in the earlier Figures have been used to aid clarity. The valves on the top port of the bags 50a and 50b are open to allow such fluids to rise by virtue of their buoyancy relative to water into the holding tank 52.

Valve 50c is shown as closed to prevent backflow into the bag 16c Valve 52 on the piping 45 to the production facility is shown closed, but can be opened to clear the contents of the tank 52 into the process stream.

A further embodiment of the present invention could use the dis- connectable inflatable bag in a continuous oil export scenario. The oil is exported continuously through a pipeline to a suitable export route, allowing the level in the tank to be maintained relatively constant at a level in the subsea tank which most benefits the separation performance.

This reduces the volume requirements for the bag, but still uses the bag for automatic management via hydrostatic pressure of not only the volume but pressure in the system. This is particularly important during production upsets such as start-up and shutdown.

In addition it allows greater regulation over the export rate of the oil and by adjustment of the residence time improve the quality of the exported oil. Reference is now made to Figure 6 of the drawings which shows a further embodiment of the present invention in the form of using dis-connectable inflatable bag 16 in a liquefied C02 underwater storage scenario. Like parts to those in the earlier Figures have been used to aid clarity. The C02 76 is injected and recovered in liquid form into the tank 14 through piping 70 and maintained under pressure using the hydrostatic head acting on the dis- connectable inflatable bags. As the C02 occupied more of the tank the bag inflate to accommodate the displaced water or other ballast fluid. The water can be treated and tested by cycling through the bag to the topsides facility 78 along the piping 45. The C02 can then be released due to the significantly lower pressure at the surface 78, then re-injected down the piping 44. Valving 74 is provided to control the flow. The C02 can be recovered from the tank and injected through piping 72.

In this way, an aspect is in circulating the water to allow the C02 in solution to escape, but hydrostatic head keeps the pressure down below.

The principle advantage of the present invention is that it provides a dis-connectable inflatable underwater bag or multiplicity of, which is used to capture ballast fluid displaced from an underwater fluid storage tank.

A further advantage of the present invention is that it provides a dis-connectable inflatable underwater bag or multiplicity of, which is used to capture the water displaced from an underwater oil storage tank.

Further advantages of the present invention are realised as:

The bag substantially forms a 'closed loop system' whereby the water in the tank is not mixed with the seawater outside of the tank, other than filtered seawater injected in to top-up any shortfall in the water available within the system.

This "closed loop system" can be used as a very large separator in a continuous export scenario, giving a higher quality crude product, better water management and a volumetric buffer against process upsets.

The offloading of oil can take place using the water in the bag to replace the oil as it is extracted from the tank, avoid ing the need for complex filtering systems for incoming seawater, which has clean environmental benefits.

The bag allows retention of a large proportion of the enthalpy of the system (principally heat energy) by allowing the warmer displaced water to re-fill the tank, rather than colder seawater, thus aiding energy efficiency and minimising environmental impact. In addition the reduced temperature changes has some beneficial influence on the fatigue life of the tank structure, plus maintain the oil at a higher temperature reducing its viscosity.

The bag also captures any formation water which separates from the oil in the storage tank and hence mixes with the water already in the tank. The 'closed loop system' allows management of the chemical interaction between the seawater in the tank and the formation water. In addition monitoring, testing and treatment of the large volume of water can be performed as part of the operations to re-inject the surplus water into wells. Additionally, the bag also allows for fresh water to initially be used in the tank during the installation phase which will mix with the formation water with significantly less risk of scale than seawater. Further, due to the repeated load/unload operations the formation water will eventually dilute and replace the seawater, reducing and potentially eliminating any scale inhibition requirements.

The use of multiple bags for loading of the displaced water allows selected bags to be disconnected and towed or lifted onto a vessel to a safe location for inspection and testing, thus allowing a greater confidence in the longevity and integrity of the selected bag system. Multiple bags also give redundancy in the event of a blockage or leak.

The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention herein intended with the invention being defined within the scope of the claims.

Claims

1. Apparatus for managing a first fluid in an underwater storage tank, used for storing a second fluid, comprising a dis- connectable inflatable bag coupled to the underwater storage tank to capture displaced first fluid from the underwater storage tank.

2. Apparatus according to claim 1 wherein the dis-connectable inflatable bag has a first port for the entry and exit of the first fluid, to and from the underwater storage tank.

3. Apparatus according to claim 2 wherein the dis-connectable inflatable bag has a second port, the second port being closed when the dis-connectable inflatable bag is attached to the underwater storage tank.

4. Apparatus according to claim 2 wherein the dis-connectable inflatable bag has a second port, the second port being open when the dis-connectable inflatable bag is attached to the underwater storage tank to allow fluids lighter than the first fluid to rise to a holding vessel.

5. Apparatus according to any preceding claim wherein the dis- connectable inflatable bag is arranged adjacent the underwater storage tank.

6. Apparatus according to claim 5 wherein there are a plurality of dis-connectable inflatable bags arranged adjacent the underwater storage tank.

7. Apparatus according to any one of claims 2 to 6 wherein the apparatus further comprises a supply line between the first port and a first fluid exit port at the bottom of the underwater storage tank.

8. Apparatus according to claim 7 wherein the supply line includes one or more valves.

9. Apparatus according to claim 5 or claim 6 wherein the supply line includes a branch line to divert the first fluid to production facilities associated with the underwater storage tank.

10. Apparatus according to any one of claims 7 to 9 wherein the supply line includes an intermediate tank, the intermediate tank being used to collect and separate the second fluid or sludge which may have entered the supply line as the first fluid from the underwater storage tank is offloaded.

11. Apparatus according to claim 10 wherein the intermediate tank has at least one take-off port to direct the separated second fluid/sludge back to the underwater storage tank.

12. Apparatus according to claim 11 wherein there are two take off ports, arranged at a top and a bottom of the intermediate tank, with the supply line arranged to d raw from a point between the top and the bottom of the intermediate tank.

13. Apparatus according to any preceding claim wherein there is a holding tank to capture any fluids lighter than the first fluid which have entered the d is-connectable inflatable bags, using the natural buoyancy of those compou nds.

A method for managing a first fluid in an underwater storage tank, used for storing a second fluid, comprising capturing the first fluid displaced from the underwater storage tank in a dis- connectable inflatable bag.

A method according to claim 14 wherein the first fluid is water and the second fluid is oil to provide a method for managing the water in an underwater oil storage tank, comprising capturing the water displaced from the underwater oil storage tank in an dis-connectable inflatable bag.

A method according to claim 15 including the steps of:

(d) passing the displaced water into the dis-connectable inflatable bag and storing the displaced water in the dis- connectable inflatable bag; and

(e) discharging oil from the underwater oil storage tank for export while allowing the displaced water to return to the underwater oil storage tank from the dis-connectable inflatable bag.

A method according to claim 16 wherein at step (a) a plurality of bags are attached to a branched connection on the supply line, with each branch containing a valve and the method includes the step of isolating one or more dis-connectable inflatable bags.

18. A method according to claim 16 wherein the method includes the step of removing an isolated dis-connectable inflatable bag from the supply line.

19. A method according to any one of claims 16 to 18 wherein at step (d) the underwater oil storage tank is initially filled with fresh water.

20. A method according to any one of claims 16 to 19 wherein at step (c) the oil is loaded into the underwater oil storage tank from a production facility.

21. A method according to any one of claims 16 to 20 wherein the method includes the step of returning gas in the underwater oil storage tank to the production facility.

22. A method according to any one of claims 16 to 20 wherein the method includes the step of returning displaced water from the underwater oil storage tank via the supply line to a production facility.

23. A method according to any one of claims 16 to 22 wherein the method includes the step of pressure monitoring.

24. A method according to any one of claims 16 to 23 wherein the method includes passing the displaced water from the underwater o il storage ta nk throug h a sepa rator on the su pply line.

25. A method according to any one of claims 16 to 24 wherein the method includes, at step (e), providing continuous export of oil while separating the oil and water in the underwater oil storage tank.

26. A method according to any one of claims 16 to 24 wherein the method includes the step of discharging water from the underwater oil storage tank through an oil export line.

27. A method according to claim 14 wherein the first fluid is water and the second fluid is liquified C02 to provide a method for managing the water in an underwater C02 storage tank, comprising capturing the water displaced from the underwater C02 storage tank in an dis-connectable inflatable bag.