GB2578890A

GB2578890A - Method and apparatus for management of water in an underwater storage tank

Info

Publication number: GB2578890A
Application number: GB1818365.7A
Authority: GB
Inventors: Bowie Malcolm
Original assignee: SLLP 134 Ltd
Current assignee: SLLP 134 Ltd
Priority date: 2018-11-12
Filing date: 2018-11-12
Publication date: 2020-06-03
Anticipated expiration: 2038-11-12
Also published as: NO20210565A1; GB2578890B; WO2020099840A1; US20210395003A1; GB201818365D0

Abstract

Apparatus for managing water in underwater oil storage tank 14 comprises an inflatable bag 16 coupled to tank to capture water displaced from the tank. The bag may have a first port 40 for entry and exit of the water to and from the tank, and may have a second port (41, fig. 1) being closed when the bag is attached to the tank. The bag may be adjacent the tank. There may be a plurality of bags adjacent the tank. There may be a supply line 36 between the first port and a water exit port at the bottom of the tank. The supply line may have a valve and may have a branch line to divert water to production facilities. The supply may include an intermediate tank used to collect and separate oil or sludge in the supply line. The intermediate tank may have a take-off port to direct the separated oil or sludge to the oil tank. There may be two take-off ports at a top and a bottom of the intermediate tank, with the supply line arranged to draw from a point between the top and bottom of the intermediate tank.

Description

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

The present invention relates to underwater storage tanks and in particular, though not exclusively, to the means of managing the water in a rigid underwater storage tank, principally 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 3.0 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 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 below 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 s 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 treatments systems in terms of 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 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 density over steel plus 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.

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.

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

* Recover of the bags if damaged, or for inspection, needs to have a system to ensure the bag can be recovered without 113 damage or oil release, especially if there is a large quantity of solids or highly viscous oils in the bottom.

* Onshore procedures and methods will require to be developed to safely dispose of the bag contents and clean the insides.

* 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 may be caused to the bag by deposition of solids.

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 process for the material extensive to ensure chemical compatibility.

It is therefore an object of the present invention to provide apparatus for managing the water in an underwater oil storage tank 25 which obviates or mitigates at least some of the disadvantages of the prior art.

It is a further object of 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.

According to a first aspect of the present invention there is provided apparatus for managing the water in an underwater oil storage tank, comprising an inflatable bag coupled to the underwater oil storage tank to capture water displaced from the underwater oil s storage tank.

In this way, the 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 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 bags plus allow development of a discard criteria.

Preferably, the 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.

zo Preferably also, the inflatable bag has a second port, the second port being closed when the inflatable bag is attached to the underwater oil storage tank. In this way, the second port is used to allow flushing of the bag contents in-situ and onshore for maintenance and/or disposal.

Preferably, the inflatable bag is arranged adjacent the underwater oil storage tank. In this way, the 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 inflatable bags arranged adjacent the underwater oil storage tank. In this way, selected bags can be disconnected and towed into a safe location for inspection and testing. Additionally multiple bags give redundancy in the event of a s blockage or leakage.

Preferably, the apparatus further comprises a supply line between the single port and a water exit port at the bottom of the underwater oil 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 offloaded. 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 the water in an underwater oil storage tank, comprising capturing the water displaced from the underwater oil storage tank in an inflatable bag.

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 inflatable bag and a water exit port at the bottom of the underwater oil storage tank; (b) with the 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 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 inflatable bag.

In this way, the inflatable bag is used to hold the displaced water which reduces the risks inherent if the inflatable bag held oil.

Preferably, 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 inflatable bags. In this way, additional bags are available for redundancy.

Preferably, the method includes the step of removing an isolated 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, 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.

3.0 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 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 inflatable bag. In this way, the risk of contamination of the inflatable bags is prevented.

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 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.

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 113 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; 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 10 water in an underwater oil storage tank according to another embodiment of the present invention and 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.

as 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 inflatable bag 16 is coupled to the tank 14 to capture water displaced from zo 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 bottom 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 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 20 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 25 the production facility 18 for water 12/produced water 30 for cleaning, discharge or reinjection 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.

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.

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 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. 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.

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 s 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.

If sea water is used in the bag 16, as the formation water 30 mixes more it dilutes the sea water reducing 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.

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.

This method also has a large 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 cooling quickest and hence becoming more viscous towards the end of offloading.

This also helps maintain an efficient thermal balance in the tank 14 15 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, 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 inflatable bags 16a-d. Though four bags 16a-d 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-d. Bag 16d is disconnected to illustrate that the bags 16a-d 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 inflatable bag 16 in a continuous oil export scenario. Like parts to those in the earlier Figures have been used to aid clarity.

The oil is exported continuously through the pipeline 60 to a suitable export route, allowing the level in the tank to be maintained relatively constant at a level 62 in the subsea tank 14 which most benefits the separation performance.

This reduces the volume requirements for the bag 16, but still uses the bag for automatic management via hydrostatic pressure of not only the volume but pressure in the system. This is particularly 10 important during production upsets such as start-up and shutdown.

The principle advantage of the present invention is that it provides an 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, avoiding the need 25 for complex filtering systems for incoming seawater.

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 replace the seawater, reducing any scale inhibition requirements.

The use of multiple bags for loading of the displaced water allows selected bags to be disconnected and towed into 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 s the claims.

Claims

CLAIMS1. Apparatus for managing the water in an underwater oil storage tank, comprising an inflatable bag coupled to the s underwater oil storage tank to capture water displaced from the underwater oil storage tank.
2. Apparatus according to claim 1 wherein the inflatable bag has a first port for the entry and exit of water, to and from the underwater oil storage tank.
3. Apparatus according to claim 2 wherein the inflatable bag has a second port, the second port being closed when the inflatable bag is attached to the underwater oil storage tank.
4. Apparatus according to any preceding claim wherein the inflatable bag is arranged adjacent the underwater oil storage tank.
5. Apparatus according to claim 4 wherein there are a plurality of inflatable bags arranged adjacent the underwater oil storage tank.
6. Apparatus according to any one of claims 2 to 5 wherein the apparatus further comprises a supply line between the first port and a water exit port at the bottom of the underwater oil storage tank.
7. Apparatus according to claim 6 wherein the supply line includes one or more valves.
8. Apparatus according to claim 6 or claim 7 wherein the supply line includes a branch line to divert water to production facilities associated with the underwater oil storage tank.
9. Apparatus according to any one of claims 6 to 8 wherein the supply line includes 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 offloaded.
10. Apparatus according to claim 9 wherein the intermediate tank has at least one take-off port to direct the separated oil/sludge back to the underwater oil storage tank.
11. Apparatus according to claim 10 wherein there are two takeoff 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.
12. 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 inflatable bag.
13. A method according to claim 12 including the steps of: (a) connecting a supply line between a single port of the inflatable bag and a water exit port at the bottom of the underwater oil storage tank; (b) with the 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 inflatable bag and storing the displaced water in the 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 inflatable bag.A method according to claim 13 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 inflatable bags.A method according to claim 14 wherein the method includes the step of removing an isolated inflatable bag from the supply line.A method according to any one of claims 13 to 15 wherein at step (d) the underwater oil storage tank is initially filled with fresh water.A method according to any one of claims 13 to 16 wherein at step (c) the oil is loaded into the underwater oil storage tank from a production facility.A method according to any one of claims 13 to 17 wherein the method includes the step of returning gas in the underwater oil storage tank to the production facility.A method according to any one of claims 13 to 18 wherein the method includes the step of returning displaced water from the underwater oil storage tank via the supply line to a production facility.
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23.A method according to any one of claims 13 to 19 wherein the method includes the step of pressure monitoring.A method according to any one of claims 13 to 20 wherein the method includes passing the displaced water from the underwater oil storage tank through a separator on the supply line.A method according to any one of claims 13 to 21 wherein the method includes, at step (e), providing continuous export of oil while separating the oil and water in the underwater oil storage tank.A method according to any one of claims 13 to 21 wherein the method includes the step of discharging water from the underwater oil storage tank through an oil export line.