Storing and delivering fluids underwater

Abstract

A fluid (52) is delivered from a subsea fluid storage tank (44) by pumping seawater into a chamber (54) adjoining a container (50) that contains the fluid, the chamber being defined between the container and a housing (48). Increasing the pressure in the chamber by pumping the seawater increases the pressure of the fluid in the container, expanding the chamber to cause the container to contract. The fluid can thereby flow out of the container along an outlet channel for injection into a subsea structure or equipment. The flow of the fluid is driven by the increased pressure of the fluid in the container, without requiring additional pumping in the outlet channel.

Description

[0001] Storing and delivering fluids underwater

[0002] This invention relates to storage of fluids underwater to be delivered or dispensed underwater, for example for use in subsea oil and gas production or in subsea carbon capture.

[0003] It is common to inject chemicals into a well or into a well stream when producing or processing hydrocarbons offshore. For example, liquid chemicals can be added to production fluids to mitigate pipeline corrosion or formation of hydrates. It is also known to inject gases into subsea reservoirs, notably carbon dioxide in the context of carbon capture, utilisation and storage (CCLIS).

[0004] Traditionally, chemicals have been stored at the surface and pumped down to the seabed for injection into a wellhead or other subsea equipment. As this creates challenges arising from lengthy supply conduits and limited topside storage space, there have been various proposals more recently to store chemicals underwater, close to the point of injection, as part of a subsea liquid storage and delivery system.

[0005] Conventionally, subsea storage tanks used for subsea chemical storage and injection (SCSI) are pressure-compensated to handle hydrostatic pressure at depth. They comprise a rigid outer housing and at least one expandable and collapsible inner container within the outer housing for holding the, or each, chemical. The inner container is usually a bladder made of an elastomeric material. Examples of such storage tanks are disclosed in US 7448404 and US 9656801.

[0006] A pressure-balancing medium, typically seawater, is disposed between the inner container and the outer housing, for example in an annulus region surrounding the inner container. For this purpose, the interior of the outer housing is in fluid communication with the seawater surrounding the tank, typically through a check valve or an opening in a wall of the outer housing. The pressure-balancing medium acts on the inner container and the outer housing as the tank is lowered to the seabed, as the inner container is emptied in use, and as the tank is recovered from the seabed. Thus, the hydrostatic pressure of seawater outside the tank acts on the fluids within the tank and hence on the chemical stored in the inner container within the tank.US 9540169 discloses a subsea storage tank that features dual-barrier bladders within a shell structure. The shell structure and the barriers of the bladder can be subjected to elevated or reduced pressure to test their integrity. US 9470365 also discloses a dualbarrier bladder system. In that case, a recirculation loop includes a pump for circulating a liquid in the space between an inner bladder and an outer bladder to detect leakage.

[0007] In US 12060221, there is no bladder, membrane or other barrier; instead, a liquid separation layer is maintained between pressure-compensating seawater and a stored working fluid.

[0008] EP 3444427 discloses a subsea storage and processing system for produced fluids, featuring a primary storage volume and a secondary buffer volume. Fluid communication between the two volumes is controlled by a device that responds to pressure changes.

[0009] The abovementioned US 9656801 notes that there can be some instances where injection pressure is lower than the hydrostatic pressure at which a chemical is stored in a pressure-compensated tank. In that case, if the hydrostatic pressure is sufficient to drive the chemical from the tank to the point of injection, it is then simply a matter of controlling that unpumped flow using metering and valves. However, where the pressure required for injection is greater than the pressure at which the chemical is stored, a pump is required to draw the chemical from the tank and to raise the pressure of the chemical. In practice, for redundancy, two pumps for each type of chemical, and hence for each bladder, are installed beside the tank.

[0010] The requirement to pump chemicals from a subsea storage tank creates a problem, namely the need to use and to qualify a different pump for each chemical product being stored and injected. Also, chemical pumps are typically more complex and expensive than water pumps. Thus, the requirement to pump chemicals militates against standardisation of pumps and increases the cost of the pumps, both their capital cost and their operating cost.

[0011] GB 2607617 addresses the challenges of pumping chemicals from a subsea storage tank by proposing to eliminate such pumps entirely. Instead, it proposes to pressurise a storage tank using a compressed gas to maintain a liquid chemical above a thresholdpressure that enables the chemical to be injected into a high-pressure wellstream without raising the pressure of the chemical further.

[0012] Whilst a membrane could be provided as a barrier to isolate the liquid chemical from the pressurising gas, GB 2607617 prefers that no such barrier is provided and that the chemical and gas are therefore in contact with each other. Consequently, the gas is preferably nitrogen or is otherwise inert or unreactive in relation to the chemical being stored.

[0013] As the chemical flows from the tank of GB 2607617 for injection, the level of chemical inside the tank will fall and the pocket of gas above the chemical volume will expand to fill the additional space. Consequently, the gas will decrease in pressure as it expands. This requires the tank to be charged initially with an excess of gas so that the minimum pressure of the gas will keep any remaining chemical above the threshold pressure for injection.

[0014] Handling gas deep underwater and managing fluctuations of pressure and flow rate as the gas expands, as required by GB 2607617, can be challenging. Also, noting that the sealed tank of GB 2607617 cannot be pressure-compensated, there is the further challenge of handling a substantial overpressure of gas and chemical within the tank, relative to external hydrostatic pressure, especially when the tank is full and the gas is therefore highly compressed. As a result, the wall of the outer housing must be thick and strong, and therefore heavy and expensive.

[0015] To put the prior art into further context, Figure 1 is a piping and instrumentation diagram that represents an SCSI system 10 of the prior art for supplying a chemical to a subsea production system (SPS) 12. The SPS 12 comprises multiple wellheads corresponding to respective Christmas trees 14.

[0016] In this example, the SCSI system 10 comprises two subsea chemical storage modules 16, each containing a collapsible inner bladder within a rigid outer housing as disclosed in US 7448404 and US 9656801. The SCSI system 10 could comprise more or fewer such modules 16. The modules 16 may contain different chemicals or the same chemical, hence multiplying the storage capacity of a single module.Hydraulic flying leads 18 extend from each module 16 to permanent piping 20.

[0017] Additional hydraulic flying leads 22 extend from the piping 20 to convey chemical(s) from the modules 16 into respective outlet channels 24 of the SCSI system 10. Each outlet channel 24 comprises:

[0018] parallel filters 26 with pressure transmitters 28 situated upstream and downstream of the filters 26 to determine pressure drop across the filters 26;

[0019] a metered pump 30 under the control of an electronic subsea control module (eSCM) 32, itself controlled by a master control station (MCS) 34 of the SPS 12;

[0020] an accumulator 36 serving as a buffer to handle possible backpressure;

[0021] a flow meter 38; and

[0022] a further pressure transmitter 40.

[0023] An infield umbilical, or further hydraulic flying leads 42, convey chemical(s) from the outlet channels 24 of the SCSI system 10 to the Christmas trees 14 of the SPS 12 for injection into their associated wellheads.

[0024] NO 340075 and US 2014301790 are disclosures relating to subsea storage tanks comprised in the state of the art.

[0025] Against this background, the invention resides in a method of delivering a fluid from a subsea fluid storage tank. The method comprises: pumping seawater into a chamber adjoining a container that contains the fluid, the chamber being defined between the container and a housing; and by increasing pressure in the chamber due to said pumping of seawater, increasing pressure of the fluid in the container.

[0026] For example, expanding the chamber until internal pressure exerted by the pumped seawater can cause the container to contract. The pressurised fluid can then be caused or allowed to flow out of the container along an outlet channel. The fluid can then be injected into a subsea structure or equipment at a point of injection disposed at an end of the outlet channel but, advantageously, the outlet channel need not include a pump. The pressure of the fluid in the container or along the outlet channel can be controlledby controlling operation of an outlet valve in the outlet channel while the fluid is being delivered from the container.

[0027] As the fluid is being delivered from the container, the pressure of the seawater in the chamber can be controlled by controlling operation of a seawater pump in an inlet channel that communicates with the chamber, for example through a first seawater inlet in a wall of the housing.

[0028] Seawater can also be admitted into the chamber through a check valve that communicates with the chamber, for example through a second seawater inlet in a wall of the housing. Admission of seawater through the check valve can be driven by hydrostatic overpressure of seawater around the tank, for example while lowering the tank through a water column or while delivering fluid from the container. The pressure of the seawater in the chamber can also be controlled by controlling operation of the check valve while the fluid is being delivered from the container.

[0029] Seawater pumped into the chamber can also be vented from the chamber, for example through the check valve that can admit seawater into the chamber. The pressure of seawater in the chamber can also be controlled by controlling venting of the pumped seawater while the fluid is being delivered from the container.

[0030] Operation of the seawater pump and the check valve can be controlled and coordinated by a common control module.

[0031] Correspondingly, the inventive concept embraces a subsea fluid storage system for storing and delivering a fluid underwater. The system comprises: a container for holding the fluid; a housing; a chamber defined between the container and the housing and adjoining the container; and an inlet channel in fluid communication with the chamber, the inlet channel comprising a seawater pump arranged to pump seawater into the chamber to increase pressure of the seawater in the chamber and hence of the fluid in the container.

[0032] The chamber may be expandable in response to the increased pressure of the seawater in the chamber and the container may be contractable in response to such expansion of the chamber.Operation of the seawater pump may be controllable to control the pressure of the seawater in the chamber while the fluid is being delivered from the container along an outlet channel. The outlet channel can terminate at a point of injection for injecting the fluid into a subsea structure or equipment, and need not include a pump.

[0033] The container, the housing and the chamber can be implemented in a module that is separable from the inlet channel and / or from the outlet channel. The module may be connectable to the inlet channel and / or the outlet channel by at least one flying lead.

[0034] Thus, the invention embodies the principle of squeezing out the fluid content of an inner chamber or bladder using pressure elevated by accumulation of a different fluid in an outer chamber adjoining or surrounding the inner chamber.

[0035] Instead of using and qualifying different pumps for each chemical product as in the prior art, the invention contemplates using a simple pump that manages only seawater. A subsea storage tank is therefore equipped with a seawater entry point or inlet communicating with the pump. The pump thereby injects seawater into the gap or annulus of the storage tank defining an expandable outer chamber between the outer housing and the inner container or bladder to increase the pressure in the outer chamber defined by the annulus. This increased pressure is controlled to push the chemical product out of the bladder with the desired pressure and flow rate.

[0036] The pressure in the annulus can be controlled by controlling the pump. The pressure in the annulus can also, or instead, be controlled by a valve in the seawater inlet or in a seawater outlet communicating with the annulus.

[0037] For redundancy, an additional pump could be provided per storage tank or module or per type of chemical, or indeed for an entire SCSI station comprising multiple tanks or modules. In the latter case, a series of valves can adjust pressures in each tank or storage module, under the control of an electronic subsea control module. Sensors can monitor pressure in the annulus of each tank, in the chemical product lines and in associated valves. A buffer can be provided between the bladder and the injection point to manage backpressure from the well, pipe or other equipment or installation downstream of the injection point.Cassettes can be provided for flying leads, for example one cassette per storage module. Each cassette can be split into multiple sections with partitions, for example of glass-reinforced plastics (GRP), to separate the flying leads into groups. There may be three groups of flying leads per module, namely flying leads used to convey seawater, flying leads used to convey chemicals, and flying leads used for the purpose of monitoring. A single connector on an exit side of the cassette, such as a cobra-head connector, can enable one-step connection by an ROV of flying leads from each group.

[0038] In summary, the invention involves delivering or dispensing a fluid from underwater storage by pumping seawater into a chamber adjoining a container that contains the fluid, the chamber being defined between the container and a housing. Increasing the pressure in the chamber by pumping in the seawater increases the pressure of the fluid in the container by expanding the chamber to cause the container to contract. The fluid can thereby flow out of the container along an outlet channel for injection into a subsea structure or equipment. The flow of the fluid along the outlet channel to a point of injection is driven by the increased pressure of the fluid in the container, without requiring additional pumping in the outlet channel.

[0039] Reference has already been made to Figure 1 of the drawings, which is a piping and instrumentation diagram that represents an SCSI system of the prior art for supplying a chemical to a subsea production system. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

[0040] Figure 2 is a piping and instrumentation diagram including a subsea chemical storage module of the invention;

[0041] Figures 3a and 3b are schematic sectional views of the storage module of Figure 2 containing a collapsible bladder shown in full and partially emptied states respectively; and

[0042] Figures 4a and 4b correspond to Figures 3a and 3b but show a storage module of the invention containing more than one collapsible bladder.

[0043] Figure 2 shows a subsea chemical storage module 44 of the invention when submerged in seawater 46 and supported by a seabed or other underwater structure.The module 44 comprises a rigid outer canister or housing 48 containing an inner bladder 50 made of an elastomeric material. The bladder 50 serves as an expandable and collapsible inner container for holding a liquid chemical 52 other than seawater 46.

[0044] A chamber 54, such as an annulus, within the module 44 is disposed between the outer housing 48 and the bladder 50. The chamber 54 contains seawater 46 admitted through first and second seawater inlets 56, 58 that penetrate a wall of the outer housing 48. The wall of the outer housing 48 otherwise seals the chamber 54 from the seawater 46 surrounding the module 44.

[0045] The first seawater inlet 56 receives seawater 46 from an inlet channel 60, comprising a filter 62 upstream of a metered seawater pump 64, in fluid communication with the chamber 54. By using the seawater pump 64, seawater 46 in the chamber 54 can be pressurised to a pressure exceeding the ambient hydrostatic pressure of the seawater 46 surrounding the module 44.

[0046] The seawater pump 64 can be controlled by an electronic subsea control module (eSCM) 66 to control the pressure of seawater 46 in the chamber 54. As in the prior art system shown in Figure 1, the eSCM 66 can itself be controlled by a master control station of a subsea production system.

[0047] The second seawater inlet 58 is fitted with a check valve 68 that admits seawater 46 into the chamber 54, for example as the module 44 is lowered through the water column. For this purpose, the check valve 68 has at least one-way operation. However, optionally, the check valve 68 can have two-way operation to relieve a predetermined or controllable degree of overpressure of seawater 46 in the chamber 54 generated by the seawater pump 64. To this extent, the second seawater inlet 58 can also serve as an outlet for venting seawater 46 from the chamber 54. In addition to reducing any overpressure in the chamber 54 generated by the seawater pump 64, seawater 46 can be vented from the chamber 54 when the bladder 50 is refilled with the chemical 52.

[0048] Optionally, the check valve 68 can be controlled by the eSCM 66 to control the pressure of seawater 46 in the chamber 54. Thus, the hydrostatic pressure of seawater 46 in the chamber 54 can be controlled by the seawater pump 64 and / or by the check valve 68.In accordance with the invention, the hydrostatic pressure of the seawater 46 within the chamber 54 is controllable to exert a desired pressure on the flexible or movable wall of the bladder 50 and so to induce a desired hydrostatic pressure in the chemical 52 within the bladder 50.

[0049] The chemical 52 in the bladder 50 is brought to a pressure sufficient to drive the chemical 52 from the bladder 50 through an outlet 70 and along an outlet channel 72 to an injection point 74. The pressure of the chemical 52 at the injection point 74 exceeds the pressure prevailing in the wellhead, pipeline or other subsea equipment into which the chemical 52 is to be injected.

[0050] The outlet channel 72 comprises:

[0051] parallel filters 76 with pressure transmitters 78 situated upstream and downstream of the filters 76 to determine pressure drop across the filters 76;

[0052] a valve 80 providing further control to the pressure and flow rate of the chemical 52 along the outlet channel 72, optionally also under control of the eSCM 66;

[0053] an accumulator 82 serving as a buffer to handle backpressure;

[0054] a flow meter 84; and

[0055] a further pressure transmitter 86.

[0056] In advantageous contrast to the prior art, no specialised, specially-qualified pump is required in the outlet channel 72 to draw the chemical 52 from the bladder 50 and to drive the chemical 52 to the injection point 64. Instead, the seawater pump 64 is capable of elevating pressure in the bladder 50 by elevating pressure in the chamber 54 adjoining and acting on the bladder 50.

[0057] As the seawater 46 in the chamber 54 remains at a hydrostatic pressure close to or substantially equal to the ambient hydrostatic pressure of the seawater 46 surrounding the module 44, the wall of the outer housing 48 can be thin, lightweight and correspondingly inexpensive.Two or more of the modules 44 can be provided in a SCSI system of the invention to inject respectively different chemicals 52 or to provide additional storage volume for more of a given chemical 52.

[0058] Figures 3a, 3b, 4a and 4b show modules 44 that include an ROV panel 88 connected or connectable to the inlet channel 60 and the outlet channel 72. For example, an ROV can couple the inlet channel 60 and the outlet channel 72 to the ROV panel 88 via hydraulic flying leads, in which case at least part of the inlet channel 60 and / or the outlet channel 72 can be implemented outside the module 44.

[0059] A seawater line 90 connects the first seawater inlet 56 to the ROV panel 88 to receive seawater 46 from the inlet channel 60. Similarly, a chemical product line 92 connects the outlet 70 to the ROV panel 88 to convey chemical 52 into the outlet channel 72. A frame 94 supports the outer housing 48, the bladder 50, the ROV panel 88, the seawater line 90 and the chemical product line 92.

[0060] Figures 3a and 3b show the outer housing 48 containing a single bladder 50 in expanded and contracted states respectively. Conversely, Figures 4a and 4b show the outer housing 48 containing dual bladders 50, again in expanded and contracted states respectively. In that case, the bladders 50 can contain the same chemical 52 or can contain respectively different chemicals 52 to be expelled through respective outlet channels 72. In this respect, the valves 80 of the outlet channels 72, as shown in Figure 2, can be controlled independently to control the pressures and flow rates of the respective chemicals 52, even though the bladders 50 are subjected to the same pressure of seawater 46 in the chamber 54.

[0061] The bladders 50 shown in Figures 4a and 4b can be conjoined or abut as shown, or can be separated by a gap, or can be defined within a single envelope that is partitioned into compartments corresponding to the respective bladders 50. The bladders 50 are shown in Figures 4a and 4b to be substantially equal in volume but they could instead have different volumes. There could be more than two bladders within the outer housing.

[0062] Many other variations are possible within the inventive concept. For example, the inlet channel may comprise other components that act on or measure the incoming flow of seawater, such as one or more valves, flow meters or pressure transmitters.The first and second seawater inlets in the wall of the outer housing could be combined into a single inlet that can admit water driven by the seawater pump or by ambient hydrostatic pressure. A seawater outlet that relieves overpressure in the chamber could be provided separately from the first and second seawater inlets or could be combined with either of those inlets or with a combined single inlet.

[0063] The variable-volume inner compartments exemplified above by bladders with flexible walls could instead be defined by bellows or pistons, each with at least one rigid wall exposed to the pressure of seawater in an adjoining chamber within an outer housing.

[0064] It is preferred that the wall of the outer housing is rigid for protective purposes but it would be possible instead for at least part of the wall of the housing to be flexible but substantially inextensible.

[0065] Whilst the invention has been described above in the context of SCSI applications, the invention also has significant potential for injection of CO2 into subsea reservoirs in the context of CCLIS applications.

Claims

Claims1. A method of delivering a fluid from a subsea fluid storage tank, the method comprising:pumping seawater into a chamber adjoining a container that contains the fluid, the chamber being defined between the container and a housing;by increasing pressure in the chamber due to said pumping of seawater, increasing pressure of the fluid in the container; andventing, from the chamber, seawater pumped into the chamber.

2. The method of Claim 1, comprising allowing the fluid to flow out of the container along an outlet channel, driven by the increased pressure of the fluid in the container.

3. The method of Claim 2, comprising expanding the chamber to cause the container to contract, that contraction forcing the fluid out of the container along the outlet channel.

4. The method of Claim 2 or Claim 3, comprising injecting the fluid into a subsea structure or equipment at a point of injection disposed at an end of the outlet channel.

5. The method of any of Claims 2 to 4, wherein the outlet channel does not include a pump.

6. The method of any of Claims 2 to 5, comprising controlling the pressure of the fluid in the container by controlling operation of an outlet valve in the outlet channel while the fluid is being delivered from the container.

7. The method of any preceding claim, comprising controlling the pressure in the chamber, while the fluid is being delivered from the container, by controlling operation of a seawater pump in an inlet channel that communicates with the chamber.

8. The method of Claim 7, wherein the inlet channel communicates with the chamber through a first seawater inlet in a wall of the housing.

9. The method of any preceding claim, comprising admitting seawater into the chamber through a check valve that communicates with the chamber.

10. The method of Claim 9, comprising admitting seawater through the check valve driven by hydrostatic overpressure of seawater around the tank.

11. The method of Claim 10, comprising admitting seawater through the check valve while lowering the tank through a water column.

12. The method of any of Claims 9 to 11, comprising controlling the pressure in the chamber by controlling operation of the check valve while the fluid is being delivered from the container.

13. The method of any of Claims 9 to 12, wherein the check valve communicates with the chamber through a second seawater inlet in a wall of the housing.

14. The method of any preceding claim, comprising controlling the pressure in the chamber by controlling venting of the pumped seawater while the fluid is being delivered from the container.

15. The method of Claim 14 when dependent on any of Claims 9 to 13, comprising venting the pumped seawater through the check valve that can admit seawater into the chamber.

16. A subsea fluid storage system for storing and delivering a fluid underwater, the system comprising:a container for holding the fluid;a housing;a chamber defined between the container and the housing and adjoining the container;an inlet channel in fluid communication with the chamber, the inlet channel comprising a seawater pump arranged to pump seawater into the chamber toincrease pressure of the seawater in the chamber and of the fluid in the container; anda check valve that communicates with the chamber and an exterior of the housing through a wall of the housing, wherein the check valve is arranged to vent seawater from the chamber.

17. The system of Claim 16, wherein the chamber is expandable in response to the increased pressure of the seawater in the chamber and the container is contractable in response to the expansion of the chamber.

18. The system of Claim 16 or Claim 17, wherein operation of the seawater pump is controllable to control the pressure of the seawater in the chamber while the fluid is being delivered from the container along an outlet channel.

19. The system of Claim 18, wherein the outlet channel terminates at a point of injection for injecting the fluid into a subsea structure or equipment.

20. The system of Claim 18 or Claim 19, wherein the outlet channel does not include a pump.

21. The system of any of Claims 18 to 20, wherein the outlet channel comprises an outlet valve that is controllable to control the pressure of the fluid in the container while the fluid is being delivered from the container.

22. The system of any of Claims 16 to 21 , wherein the check valve is arranged to admit seawater into the chamber driven by hydrostatic overpressure of seawater around the housing.

23. The system of any of Claims 16 to 22, wherein the check valve is controllable to control the pressure of seawater in the chamber while the fluid is being delivered from the container.

24. The system of any of Claims 16 to 23 when dependent on Claim 18, further comprising a control module that is configured to control both the seawater pump and the check valve.

25. The system of any of Claims 16 to 24, wherein the container, the housing and the chamber are implemented in a module that is separable from the inlet channel.

26. The system of Claim 25, wherein the module is connectable to the inlet channel by at least one flying lead.

27. The system of Claim 25 or Claim 26 when dependent on any of Claims 18 to 21, wherein the module is also separable from the outlet channel.

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