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

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 CO2, 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.

Claims

1. An 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. The 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. The 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. The 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. The apparatus according to claim 1 wherein the dis-connectable inflatable bag is arranged adjacent the underwater storage tank.

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

7. The apparatus according to claim 2 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. The apparatus according to claim 7 wherein the supply line includes one or more valves.

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

10. The apparatus according to claim 7 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. The 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. The 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 draw from a point between the top and the bottom of the intermediate tank.

13. The apparatus according to claim 1 wherein there is a holding tank to capture any fluids lighter than the first fluid which have entered the dis-connectable inflatable bags, using the natural buoyancy of those compounds.

14. 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.

15. The 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 a dis-connectable inflatable bag.

16. The method according to claim 15 including 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 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.

17. The 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. (canceled)

19. The method according to claim 16 wherein at step (d) the underwater oil storage tank is initially filled with fresh water.

20. (canceled)

21. (canceled)

22. (canceled)

23. (canceled)

24. (canceled)

25. (canceled)

26. The method according to claim 16 wherein the method includes the step of discharging water from the underwater oil storage tank through an oil export line.

27. The method according to claim 14 wherein the first fluid is water and the second fluid is liquefied CO2 to provide a method for managing the water in an underwater CO2 storage tank, comprising capturing the water displaced from the underwater CO2 storage tank in a dis-connectable inflatable bag.

Description

[0073] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, of which:

[0074] FIG. 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;

[0075] FIG. 2 is the apparatus of FIG. 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;

[0076] FIG. 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;

[0077] FIG. 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;

[0078] FIG. 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

[0079] FIG. 6 is a schematic illustration of apparatus for managing the water or other ballasting fluid in an underwater liquefied CO2 storage tank according to another embodiment of the present invention.

[0080] Reference is initially made to FIG. 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] By this means the piping 42 and 44 can control the water volume within the closed loop, independently of the production facilities.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] Alternatively the flushing process can be used to displace solids from the tank 14 into the bag 16 for recovery and disposal onshore.

[0092] 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.

[0093] FIG. 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.

[0094] 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 equaling 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] Reference is now made to FIG. 2 of the drawings which shows the apparatus of FIG. 1 with the tank 14 now full of stabilised crude oil 20 and the bag 16 containing a majority of the water 12.

[0100] 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.

[0101] 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 whereby oil export will be stopped.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] Reference is now made to FIG. 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.

[0107] 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.

[0108] 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 FIG. 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.

[0109] This provides an additional level of operational efficiency and flexibility to the apparatus 10 though with correct monitoring the tank 50 is not required.

[0110] Reference is now made to FIG. 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.

[0111] 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.

[0112] 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.

[0113] Reference is now made to FIG. 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] Reference is now made to FIG. 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 CO2 underwater storage scenario. Like parts to those in the earlier Figures have been used to aid clarity. The CO2 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 CO2 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 CO2 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 CO2 can be recovered from the tank and injected through piping 72.

[0119] In this way, an aspect is in circulating the water to allow the CO2 in solution to escape, but hydrostatic head keeps the pressure down below.

[0120] 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.

[0121] 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.

[0122] Further advantages of the present invention are realised as:

[0123] 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.

[0124] 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.

[0125] 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 for complex filtering systems for incoming seawater, which has clean environmental benefits.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.