[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.
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.