SUBSEA STORAGE TANK

Abstract

 The invention relates to a subsea storage tank (1) comprising a frame (2) and at least one container (3) for storing a liquid mounted in the frame (2), wherein the container (3) has a flexible wall, preferably a bellows (11A, 11B), comprising at least two liquid tight layers (20, 21, 22) and a space (25) between the layers (20, 21, 22). The space (25) is in liquid communication with a passive negative pressure generator (30) that is configured to maintain, at least during storage of a liquid in the container when the container is submerged, the pressure in the liquid in the space (25) below the pressure in the liquid in the container (3).

Description

[0001] The invention relates to a subsea storage tank comprising a frame and at least one container for storing a liquid mounted in the frame, wherein the container has a flexible wall, preferably a bellows, comprising at least two liquid tight layers and a space between the layers.

[0002] As explained in EP 3 292 055, many subsea petroleum production activities require the use of chemicals or mud to be added to the active operation to properly operate. Historically, these chemical provisions have been provided through hoses, tubes or pipes bundled into "umbilicals" to supply the chemicals from nearby surface facilities to the respective points of injection. Longer offsets, remote locations and deeper water depths contribute to making umbilical solutions expensive.

[0003] Existing subsea chemical storage tanks in use today may be used for short-term single purpose use and have relatively small volumes. For example, a number of bladder style chemical storage tanks have been developed for this purpose. Existing subsea chemical storage assemblies may include single wall flexible tanks or bladders that are exposed directly to seawater, which may be contained within some cage or frame for protection and transportation.

[0004] US 9,540,169 relates to a subsea storage tank for the storage of bulk fluids which is adapted for being set on the sea floor, and is adapted for supplying to local subsea oil and gas production facilities a wide variety of production support fluids. The subsea storage tank includes an inner and outer shell, the shells being adapted so that a barrier fluid can be inserted into and between the shells. An interior barrier separates the interior stored fluid from sea water that is free to enter into the tank, on the opposite side of the barrier, to compensate to the varying volume of stored fluid. An identical tandem barrier is formed nearly identical to the first barrier, and is formed and positioned so that the two barriers act together, and so that the space between them can contain fluid as well, and acts like a bladder.

[0005] It is an object of the present invention to provide an improved subsea storage tank.

[0006] To this end, the subsea storage tank according to the present invention is characterised in that the space between the layers is in liquid communication with a passive negative pressure generator that is configured to maintain, at least during storage of a liquid in the container when the container is submerged, the pressure in the liquid in the space below the pressure in the liquid in the container, preferably below the pressure in the surrounding water.

[0007] In an embodiment, the generator is configured to maintain the pressure in the liquid in the space in a range from 0.01 to 1 bar, preferably in a range from 0.1 to 0.9 bar, preferably in a range from 0.2 to 0.8 bar, below the pressure in the surrounding water.

[0008] In another embodiment, the passive negative pressure generator comprises a spring or a weight for generating the negative pressure.

[0009] The present invention provides a robust system suitable for prolonged use that detects leakage into or from the tank.

[0010] In an embodiment, the passive negative pressure generator comprises an auxiliary container for receiving liquid from the space.

[0011] Thus, it becomes possible to measure the flowrate and/or amount of the leakage and store the leaked liquid, in particular to prevent contamination of the liquid in the container and the surrounding water.

[0012] In a refinement, the negative pressure generator comprises an indicator or sensor for establishing the level of liquid in the auxiliary container.

[0013] In another refinement, the auxiliary container has a volume of at least 5%, preferably at least 10% of the volume of the container and/or of at least 50 liters, preferably at least 100 liters.

[0014] In another embodiment, the space and/or the auxiliary container contains an inert liquid.

[0015] In another embodiment, the layers and/or at least one element present in the space between the layers is/are configured to prevent the space from collapsing.

[0016] E.g., the layers comprise protrusions, depressions, and/or a porous material forming or having flow paths in the space. In an embodiment, the space contains a porous layer or a granular medium.

[0017] In an embodiment, the passive negative pressure generator is positioned underneath the container and/or within the flexible wall, e.g. in a compartment located in the dead volume of the container.

[0018] In an embodiment, the tank preferably comprises two or more containers for storing a liquid mounted in the frame. The containers can be connected in series or in parallel and/or have a net volume in a range from 0,5 to 40 cubic meter per container, e.g. per bellow stack, preferably in a range from 1 to 20 cubic meter per container.

[0019] The invention further relates to a subsea storage tank comprising a container, e.g. a bellows, for storing a liquid, a line for supplying liquid to and/or taking liquid from the storage container, and an overpressure protection mechanism that is in liquid communication with the line, directly or via a conduit or storage container or mechanical or electrically operated valve.

[0020] In an embodiment, the overpressure protection mechanism comprises a container having a variable volume and in communication with the storage container or the line.

[0021] In an embodiment, the container is urged towards a small volume by passive means, such as a biased spring or a weight.

[0022] The overpressure protection system according to the present invention provides a buffer for excess liquid in case of an overpressure event and can be configured to limit the pressure in the storage container and/or the line to a pre-set value by allowing liquid to flow into the buffer.

[0023] In an embodiment, the overpressure protection mechanism comprises a valve and a lever for opening the valve when the buffer is filled beyond a pre-set value.

[0024] US 2011/203379 relates to a pressure compensator configured to compensate for volume variations of an insulation medium of a subsea installation. The volume of the insulation medium will vary due to temperature variation of the surrounding, due to load variation also causing a variation of the oil temperature and due to hydrostatic pressure of the sea water. The pressure compensator includes a first bellows chamber in flow connection with an insulation medium chamber of the subsea installation. The first bellows chamber is surrounded by a second bellows chamber configured to form a closed intermediate space around the first bellows chamber.

[0025] US 2013/167962 relates to a pressure compensator for a subsea device for performing a pressure compensation between an ambient medium surrounding the subsea device and a liquid medium filling a volume of the subsea device. The pressure compensator has at least one outer bellow and a first chamber enclosed by the outer bellow. It further has at least one inner bellow which is arranged inside the first chamber, and a second chamber enclosed by the inner bellow. Between the outer bellow and the inner bellow, a compensation volume is confined, which is provided with a fluid connection to the volume of the subsea device.

[0026] Within the framework of the present invention, the word "passive" defines that the negative pressure generator does not require energy and/or control input from an external source. Instead, the generator preferably relies on mechanical components, preferably exclusively on mechanical components, e.g. does not comprise an electronic controller.

[0027] The invention will now be explained in more detail with reference to the Figures, which show a preferred embodiment of the subsea storage tank and overpressure protection system according to the present invention.

Figure 1 is a perspective view of a subsea storage tank comprising four storage elements in a frame.

Figure 2 is a perspective view of a storage element as used in the storage tank shown in Figure 1.

Figure 3 is a perspective view of a cross-section of a lower part of a storage element.

Figures 4 and 5 show two examples of a negative pressure generator according to the invention, connected to a storage element.

Figure 6 is a cross-section of a preferred embodiment of a storage element.

Figures 7 to 9 show a first overpressure protection system in perspective view and a second overpressure protection system in side view and in an 'open' and a closed' positions respectively.

[0028] It is noted that the Figures are schematic in nature and that details, which are not necessary for understanding the present invention, may have been omitted.

[0029] Figure 1 shows a subsea storage tank 1 comprising a carrier frame 2 accommodating e.g. four storage elements 3. The carrier frame is adapted to be lowered onto the seabed and to be fixed at the desired location, e.g. on a template (not shown). In the example shown in Figure 1, the frame comprises lifting eyes 5, one at each corner, and tubular guides 6 for pins on the template. The storage tank further comprises one or more connectors for connecting the tank e.g. to a header for filling the tank with fluid or taking fluid from the tank.

[0030] Figure 2 shows a storage element 3 comprising a (sub)frame 10 and a container 11 for storing a liquid mounted in the frame. The frame is provided with means, e.g. a ruler 12 extending along the container, to indicate the volume of the liquid stored in the container.

[0031] The container in Figure 2 is formed of a single bellows or a plurality of bellows, e.g. two bellows 11A, 11B, stacked on top of each other and connected via an intermediate construction, e.g. one or more rings 13, between and the upper rim of the lower bellows 11B and the lower rim of the upper bellows 11A. The bellows are sealed by top and bottom plates 14, 15, and guide elements, e.g. guide shoes 16, engaging guide rails 17 extending along the container thus allowing vertical movement of the top plate and the bellows when the container being filled or emptied.

[0032] As shown in Figure 3, the bellows 11A, 11B comprises at least two liquid tight walls (each in fact being bellows) layers and a space between the layers. More specifically, each bellows comprises a corrugated or pleated outer layer 20, which provides structural strength and which is made of a flexible material that is resistant to seawater and currents, such as fibre-reinforced rubber or PTFE (polytetrafluoroethylene), and corrugated or pleated middle and inner layers 21, 22, made of a flexible material that is resistant to the fluid that is to be stored in the container. In another example (not shown) the bellows comprises just a corrugated or pleated inner layer 22 and no middle layer. Suitable materials for the middle and inner layers include PVDF (polyvinylidene fluoride), polyamide, e.g. PA11, PTFE, and LDPE (low density polyethylene). The rims of the layers 20, 21, 22, are flanged and clamped between bolted rings 23, 24 and the bottom plate 15 of the container.

[0033] The inner and middle layers 21, 22 define a space 25 that is filled with a liquid, e.g. a mixture of glycol and water, and that is, as shown in Figures 4 and 5, in liquid communication with a passive negative pressure generator 30 via a conduit 31. The negative pressure generator comprising an auxiliary container, e.g. cylinder 32 and a piston 33 reciprocatingly accommodated in the cylinder, for receiving liquid from the space e.g. when a leak occurs in the bellows 11 / 20, 21, 21. In the example shown in Figure 4, the piston is located below the liquid and is provided with a weight 34 that pulls the piston downwards and that thus reduces the pressure in the liquid above the piston and in the space between the layers of the bellows.

[0034] In the example shown in Figure 5, the piston is located above the liquid. A spring 36 is provided that urges, i.e. pushes (Figure 5) or pulls, the piston upwards thus reducing the pressure in the liquid below the piston and in the space between the layers of the bellows.

[0035] The negative pressure generator 30 comprises an indicator, e.g. a ruler 37, and/or sensor for establishing the level of liquid in the auxiliary container. In the example shown, the generator is provided with a stroke sensor or a proximity sensor to remotely observe the indicator.

[0036] When a leak occurs, seawater or liquid leaking from containers is sucked into the auxiliary container and leakage is detected and signalled.

[0037] Figure 6 shows an embodiment comprising an compartment 40 in the bottom part of the bellows 11. The volume (height) of the compartment corresponds to the dead volume of the bellows. The negative pressure generator can be located inside the compartment, yielding a compact construction.

[0038] In another example, the subsea storage tank is provided with a overpressure protection system 45, shown in detail in Figure 7. The overpressure protection system is positioned in communication with a line, e.g. a header 46, for supplying liquid to and taking liquid from the tank. In this example, the header 46 is provided with a first branch 47 terminating in a tap 48 for connection to the consumers of the stored liquid and the overpressure protection system is positioned on top of the branch. The system comprises a vertical pipe 49 and a bellows 50 mounted on top of the pipe 49. A clump weight 51 of e.g. 100 kilograms is mounted on top of the bellows. The header 46 is provided with a second branch 52 which in turn is provided with a shut-off valve 53 and may be connected to a further container (not shown) or to a discharge opening. A mechanism 54 of levers is connected to the clump weight and to the shut-off valve, such that when the clump weight is lifted as a result of an overpressure event in the header, the levers open the shut-off valve and liquid is allowed to flow to the further container or the discharge, providing pressure relief. The clump weight and the bellows resisting expansion together determine the threshold at which the valve opens in case of an overpressure event. While the pressure is over the threshold value the bellows will be filled and the volume of the bellows determines how long the threshold pressure may persist before the relief valve is opened.

[0039] The invention is not restricted to the embodiment described above and can be varied in numerous ways within the scope of the claims. E.g., in another embodiment, shown in Figure 8, the overpressure protection system 45 with its mechanism 54 of levers comprises a shut-off valve 53 that is located directly in the header 46. When the clump weight 51 is lifted as a result of an overpressure event the mechanism 54 of levers turns the valve 53 to the closed position, shown in Figure 9, thus preventing further filling of the subsea storage tank.

Claims

1. Subsea storage tank (1) comprising a frame (2) and at least one container (3) for storing a liquid mounted in the frame (2), wherein the container (3) has a flexible wall, preferably a bellows (11A, 11B), comprising at least two liquid tight layers (20, 21, 22) and a space (25) between the layers (20, 21, 22), characterised in that the space (25) is in liquid communication with a passive negative pressure generator (30) that is configured to maintain, at least during storage of a liquid in the container when the container is submerged, the pressure in the liquid in the space (25) below the pressure in the liquid in the container (3).

2. Subsea storage tank (1) according to claim 1, wherein the passive negative pressure generator (30) is configured to maintain, at least during storage of a liquid in the container, the pressure in the liquid in the space (25) below the pressure in the surrounding water.

3. Subsea storage tank (1) according to claim 1 or 2, wherein the generator (30) is configured to maintain the pressure in the liquid in the space (25) in a range from 0.1 to 1 bar, preferably in a range from 0.2 to 0.8 bar, below the pressure in the surrounding water.

4. Subsea storage tank (1) according to any one of the preceding claims, wherein the passive negative pressure generator (30) comprises a spring (36) or a weight (34) for generating the negative pressure.

5. Subsea storage tank (1) according to any one of the preceding claims, wherein the passive negative pressure generator (30) comprises an auxiliary container (32) for receiving liquid from the space (25).

6. Subsea storage tank (1) according to claim 5, wherein the negative pressure generator (30) comprises an indicator (37) or sensor for establishing the level of liquid in the auxiliary container (32).

7. Subsea storage tank (1) according to claim 5 to 6, wherein the auxiliary container (32) has a volume of at least 5%, preferably at least 10% of the volume of the container and/or of at least 50 liters, preferably at least 100 liters.

8. Subsea storage tank (1) according to any one of the preceding claims, wherein the space (25) and/or the auxiliary container (32) contains an inert liquid. Bellows met dubbele lagen

9. Subsea storage tank (1) according to any one of the preceding claims, wherein the layers (20, 21, 22) and/or at least one element present in the space between the layers (20, 21, 22) is/are configured to prevent the space (25) from collapsing.

10. Subsea storage tank (1) according to any one of the preceding claims, wherein the passive negative pressure generator (30) is positioned underneath the container (3) and/or within the flexible wall (11A, 11B).

11. Subsea storage tank (1) according to the preamble of claim 1 or to any one of the preceding claims, comprising a container (3) for storing a liquid, a line (46) for supplying liquid to and/or taking liquid from the storage container (3), and an overpressure protection mechanism (45) that is in liquid communication with the storage container or the line (46).

12. Subsea storage tank (1) according to claim 11, wherein the overpressure protection mechanism (45) comprises a container, preferably comprising a bellows (50), having a variable volume and in communication with the storage container or the line (46).

13. Subsea storage tank (1) according to claim 11 or 12, wherein the container (50) is urged towards a small volume by passive means (51).

14. Subsea storage tank (1) according to claim 14, wherein the overpressure protection mechanism (45) comprises a further (larger) container or discharge opening, a valve (53), and a lever (54) for opening the valve (53) to establish fluid communication between the line (46) and the further container or discharge opening, when the pressure in the line (46) has risen above a pre-set threshold.

15. Subsea storage tank (1) according to claim 14, wherein the overpressure protection mechanism (45) comprises a valve (53), and a lever (54) for closing the valve (53) to shut-off fluid communication in the line (46), when the pressure in the line (46) has risen above a pre-set threshold.

Drawing