WO2012155957A1

WO2012155957A1 - Method of replacing a flexible container provided inside a subsea storage tank, replaceable flexible container and method of installing a storage container on the seabed

Info

Publication number: WO2012155957A1
Application number: PCT/EP2011/057865
Authority: WO
Inventors: Ola Ravndal; Eirik Strøm UTHAUG; Erlend Hovland
Original assignee: Statoil Petroleum As
Priority date: 2011-05-16
Filing date: 2011-05-16
Publication date: 2012-11-22

Abstract

The present invention relates to a method of replacing a flexible container (24) provided inside a subsea storage tank, wherein the flexible container defines a space (30) separated from a remaining storage space (32) of the subsea storage tank, and wherein the flexible container is connected via one opening (26) thereof to a first inlet/outlet (16) of the subsea storage tank, for allowing seawater exclusively into and out from the flexible container. The method comprises: filling seawater into the remaining storage space of the subsea storage tank from a second inlet/outlet (20) of the subsea storage tank; disconnecting and/or removing the old flexible container; and inserting a new flexible container (24') through the first inlet/outlet. The present invention also relates to a replaceable, flexible container for a subsea storage tank. The present invention also relates to a method of installing a storage tank on the seabed.

Description

METHOD OF REPLACING A FLEXIBLE CONTAINER PROVIDED INSIDE A SUBSEA STORAGE TANK, REPLACEABLE FLEXIBLE CONTAINER AND METHOD OF INSTALLING

A STORAGE CONTAINER ON THE SEABED

The present invention relates to a method of replacing a flexible container provided inside a subsea storage tank. The present invention also relates to a replaceable, flexible container for a subsea storage tank. The present invention also relates to a method of installing a storage tank on the seabed.

Subsea storage tanks may be used for storing oil or other hydrocarbons from oil production offshore. The oil is stored in the tanks until it can be exported using e.g. shuttle tankers. There are typically two ways of storing this oil production:

1. In subsea storage cells, which depend on the difference in density between seawater and the produced oil. Here the oil is pumped into the top of the cell and likewise drained from the same spot.

2. In a floating storage. The floating storage can be incorporated in the production unit (FPSO - floating production, storage and offloading), or it can be a dedicated storage vessel (FSU - floating storage unit).

However, the use of FSUs may be very costly. Furthermore, FSUs, although reinforced, have a tendency to suffer fatigue cracking in its internal structure arising from wave action on the hull over long periods. In order to repair such damage, a conventional storage vessel will normally have to be brought ashore for repair at a suitable facility. With the usual storage vessel away, a temporary replacement vessel must be sought in order to avoid shut down in the production rate.

Further, WO2004037681 Al discloses a seabed located storage for crude oil or other fluid, which storage comprises a storage section in form of an oil and waterproof cloth formed as a flexible balloon that can be filled with, store and emptied for a storage fluid, a structure section formed as an external casing over the storage section. All equipment that can have demand for maintenance or replacement, such as valves, instruments, pumps and the storage section, can be disconnected from the storage and brought to the surface, either together or as modules of equipment. However,

WO2004037681 Al is rather silent about how such replacements are carried out in practice.

WO2004037681 Al also discloses that deployment of the storage preferably takes place by use of a crane vessel or at least a crane means. An appropriate volume of air is discharged through hatches, pipes or other outlets such that a weak positive buoyancy is achieved, while the storage or an assembly of several storages are handled stably of the crane. By beginning deployment air is discharged in a controlled way until a weak negative buoyancy is achieved. As the storage is deployed into the sea the air will be compressed and the buoyancy will become more and more negative, which has to be handled entirely by the crane vessel or similar. This may limit the actual water depths for which such a storage may be placed.

It is an object of the present invention to provide an improved method of replacing a flexible container provided inside a subsea storage tank. It is another object to provide an improved method of installing a storage tank on the seabed.

These objects, and other objects that will be apparent from the following description, are achieved by the present invention as defined in the appended independent claims. Embodiments are set forth in the appended dependent claims.

According to an aspect of the present invention, there is provided a method of replacing a flexible container provided inside a subsea storage tank, wherein the flexible container defines a space separated from a remaining storage space of the subsea storage tank, and wherein the flexible container is connected via one opening thereof to a first inlet/outlet of the subsea storage tank, for allowing seawater exclusively into and out from the flexible container, which method comprises:

- filling seawater into the remaining storage space of the subsea storage tank from a second inlet/outlet of the subsea storage tank, such that any seawater in the flexible container is (mostly or completely) discharged through the first inlet/outlet;

- disconnecting and/or removing the flexible container; and

- inserting a new flexible container through the first inlet/outlet.

Disconnecting and/or removing the flexible container may include one of the alternatives a)-c):

a) introducing a cutting means through the first inlet/outlet, cutting loose the flexible container from the first inlet/outlet by means of the cutting means, and leaving the cut away flexible container in the subsea storage tank,

b) while the flexible container is inside the subsea storage tank, disconnecting a mounting flange via which the flexible container is connected to the first inlet/outlet, and then removing the mounting flange from the first inlet/outlet such that the flexible container still affixed to its mounting flange is pulled out from the storage tank through the first inlet/outlet, or

c) turning the flexible container at least partly inside out through the first inlet/outlet by means of a pulling member having one part or end attached to the inside of the flexible container remote from the first inlet/outlet and another part or end accessible via the first inlet/outlet.

It is appreciated that the alternatives a)-c) are of a similar nature. The replacement of the flexible container according to the present method is relatively simple. Also, the flexible container can be replaced while the (remaining) storage tank is maintained on the seabed. Hence, costly and time-consuming surfacing of the storage tank is avoided. Also, the risk for oil contamination may be removed or at least reduced. Also, with alternative c), "snagging" at the first inlet/outlet is avoided, i.e. that the flexible container unintentionally attaches itself to other parts of the structure along the path it is being removed.

In one embodiment, inserting the new flexible container through the first inlet/outlet includes: providing the new flexible container pre-packed in an elongated shape; inserting the pre-packed new flexible container into the subsea storage tank through the first inlet/outlet; and allowing the inserted, pre-packed new flexible container to unfold or develop inside the subsea storage tank. The new flexible container may be pre-packed using a fluid-soluble packing means adapted to dissolve after a specific time (e.g. 24 hours) of exposure to fluid. The packaging means may for instance be water-soluble, and the fluid may be seawater. Further, allowing the inserted, pre-packed new flexible container to unfold or develop inside the subsea storage tank may include: extracting seawater from the remaining storage space of the subsea storage tank, and allowing seawater into the new flexible container.

In another embodiment, inserting the new flexible container through the first inlet/outlet includes: if still in place, removing a mounting flange from the first inlet/outlet; inserting a new mounting flange including a rolled-up new flexible container into the first inlet/outlet; and extracting seawater from the remaining storage space of the subsea storage tank, and allowing seawater into the new flexible container, causing the new flexible container to unroll in the storage tank.

The present method may further comprise: storing hydrocarbons (e.g. oil) in the remaining storage space of the subsea storage tank.

Further, the flexible container may be provided below the remaining storage space of the subsea storage tank, the first inlet/outlet may be provided in a side wall at the bottom of the storage tank, and the second inlet/outlet may be provided in the top of the storage tank.

According to another aspect of the invention, there is provided a replaceable, flexible container for a subsea storage tank, which container is made of a water- and oil- proof material and has one opening for connection to an inlet/outlet of the subsea storage tank, wherein the container is pre-packed in an elongated shape and provided with a water-soluble packing means keeping the container in the elongated shape. The packing means may be adapted to dissolve after a specific time of exposure to water. Further, the container can be made of a polymeric material, such as - but not limited to - polyethylene. This aspect may exhibit the same or similar technical effects and features as the previously described aspect of the invention, and vice versa.

According to yet another aspect of the invention, there is provided a method of installing a storage tank on the seabed, wherein a flexible container is provided inside the storage tank, the flexible container defining a lower space separated from a remaining upper storage space of the subsea storage tank, wherein the method comprises: filling an incompressible hydrocarbon liquid which is lighter than surrounding seawater into the remaining upper storage space, such that the storage tank gets a positive net buoyancy; positioning the storage tank in the sea above or nearby an installation location; removing at least a fraction of the hydrocarbon liquid from the remaining storage space, and allowing seawater into the flexible container, such that the storage tank gets a negative net buoyancy; and allowing the storage tank to immerse down to the installation location. This aspect may exhibit the same or similar technical effects and features as the previously described aspects of the invention. This method may further comprise: adjusting the amount of hydrocarbon liquid in the remaining storage space container for controlling the net buoyancy of the storage tank. The hydrocarbon liquid may for example be pentane.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred

embodiments of the invention.

Fig. 1 is a perspective view partly in cross section of a subsea storage tank for use in aspects of the present invention.

Fig. 2 is schematic view of a tank farm comprising a plurality of subsea storage tanks of the type shown in fig. 1.

Figs. 3a-3d schematically illustrate a method of installing a storage tank on the seabed.

Figs. 4a-4h schematically show a sequence of operations for replacing a flexible container according one or more embodiments of the present invention.

Fig. 1 shows a subsea storage tank 10. The tank 10 has the general shape of a hemisphere or torispherically domed cylindrical shape 12 mounted on a rigid base 14. The whole construction, which constitutes the tank 10, is arranged such that it may be placed on the sea floor or seabed in way of, or in connection with, offshore oil production facilities.

The tank 10 is provided with a first inlet/outlet 16 in or near the base 14, and a first filling and emptying line 18 for seawater is connected to the first inlet/outlet 16. The tank 10 is also provided with a second inlet/outlet 20 at the top of the tank, and a second filling and emptying line 22 primarily for oil is connected to the second inlet/outlet 20.

Attached to the first inlet/outlet 16, there is a flexible container or bellows 24. Namely, the flexible container 24 is provided inside the tank 10, and the flexible container has one opening 26 which is connected to the first inlet/outlet 16, preferably via a mounting flange 28 of the flexible container 24, which mounting flange 28 is attached to a mating flange of the first inlet/outlet 16. The flexible container 24 defines a lower space 30 which is separated from the (remaining) upper storage space 32 of the tank 10. The lower space 30 may for instance be a seawater zone, and the upper space 32 may be an oil zone. The flexible container 24 is preferably made of a water- and oil- proof material, for instance a polymeric material, such as reinforced polyethylene.

By this arrangement, seawater coming into and out of the container 24 is prevented from direct contact with the oil contained in the space 32 of the tank 10, such that no release of oil or oil emulsion is ensured as oil is filled into or emptied from the tank 10. Further advantages of the tank 10 include inexpensive storage capacity, scalable capacity over time, no fatigue cracking, and no manning costs.

During regular or normal storage operations, a product like oil may be filled into the storage space 32 of the tank 10 via the second inlet/outlet 20. As oil is filled into the space 32, seawater is displaced out from flexible container 24 to make space for the oil. Likewise, when oil is pumped out from the storage space 32, seawater flows into the flexible container 24 to replace the oil thus pumped out from space 32.

Fig. 2 shows a tank farm comprising a plurality of tanks 10 of the type described above in relation to fig. 1. In the exemplary tank farm in fig. 2, three tanks lOa-lOc are provided. The top inlets/outlets 20 allow import of oil (or other hydrocarbons) from an oil production facility 34. They also allow export of oil, for instance to a tanker or a pipeline as indicated by reference sign 36. The top inlets/outlets 20 may also allow import/export of seawater from/to the sea 38 in connection with replacement of the flexible containers. Hence, also seawater could be provided into the upper space 32 of each tank 10, as will be explained further below. Seawater that has been in the spaces 32 is preferably passed through an Oily (water) Discharge Monitor (ODM) before being discharged back to the sea 38. The ODM monitors the oil content in the water and directs the flow towards a holding tank 40 if the oil level is above acceptable limits.

A method of installing the storage tank 10 on the seabed will now be explained in the following with reference to figs. 3a-3d. First, pentane P is filled into the remaining space 32, such that the storage tank 10 gets a positive net buoyancy (fig. 3a). Here, the entire tank 10/upper space 32 may be filled with pentane. Then, the tank 10 is positioned is the sea 38 above or near an installation location 42 on the seabed 44 (fig. 3b). The tank 10 may for instance be towed to this position. As the tank 10 is positioned in the sea 38 above or near the installation location 42, pentane P is pumped out from the upper space 32, allowing seawater W to flow into the flexible container 24 (fig. 3b). The relative amounts of seawater and pentane are selected such that the storage tank 10 gets a desired slightly negative net buoyancy or flotation, whereby the tank 10 may start to immerse towards the seabed 44 (fig. 3c). During the immersion towards the seabed, the tank 10 may for instance be guided laterally by means of a surface based crane. Also during the immersion, the relative amounts of pentane and seawater in the tank 10 may be adjusted, by pumping pentane P into or out of the upper space 32. This allows the submerged weight of the tank 10 to be carefully controlled at any water depth for safe installation. When the tank 10 has reached its installation location 42 on the seabed 44 (fig. 3d), all the pentane may be pumped out from the storage space 32, thereby allowing seawater to be filled into the flexible container 24, so that the storage tank 10 gets it final submerged weight after which the storage tank 10 can be safely secured to the seabed. The stiff base 14 of the tank 10 allows the tank 10 to be either piled to the seabed where it is positioned, or to be suctioned in place by a number of suction skirts underneath the base 14 of the tank 10, for example. Instead of pentane, other light hydrocarbon liquids could be used, for instance hexanes. The hydrocarbon liquid used here should be incompressible, and be lighter than the surrounding water in the sea 38. For instance, the hydrocarbon liquid used may have a relative density (with respect to water) of less than 0.75.

Methods for replacing the flexible container 24 provided inside the subsea storage tank 10 will now be described in the following. The container 24 used for containing seawater is likely to be made of a polymeric material, which has a tendency to lose its mechanical characteristics with age in combination with a number of bending cycles (fatigue). The container 24 may therefore have to be replaced with regular intervals, such as 3 to 5 years typically.

A sequence of operations for replacing the flexible container 24 is shown in figs. 4a-4h. The methods for replacing the flexible container 24 according to the present invention may be performed subsea, without having to bring the tank 10 to the surface.

First, any oil O and/or other hydrocarbon(s) contained in the upper space 32 of the tank 10 is pumped out or otherwise discharged from the space 32 (see fig. 4a) via the second inlet/outlet 20. Then, seawater Wl or another suitable fluid is filled into this space 32 via the second inlet/outlet 20 or via another inlet/outlet in fluid communication with the space 32, such that any seawater W2 in the flexible container is mostly or completely discharged through the first inlet/outlet 16 and the flexible container 24 is fully deflated/flattened (see fig. 4b). About 80-90 % of the tank's storage space 32 may be filled with seawater in this state, for instance. After that, there are several ways of disconnecting and/or removing the old flexible container 24. It should be noted that Old' not necessarily means that the flexible container 24 is very aged, but rather that it for some reason(s) is time to replace it.

In one embodiment (a) (not shown), a cutting means or tool is introduced through the first inlet/outlet 16 wherein the flexible container 24 is connected. The cutting tool may for instance be controlled by means of an ROV. Then, the flexible container 24 is cut loose from the first inlet/outlet 16 by using the cutting tool. The thus cut away container 24 may be left at the bottom of the storage tank 10.

In another embodiment (b) (not shown), the mounting flange 28 is disconnected from the first inlet/outlet 16 while the old flexible container 24 is still inside the storage tank 10. Then the mounting flange 28 is removed from the first inlet/outlet 16, whereby the old flexible container 24 still affixed to the mounting flange 28 is pulled out from the storage tank 10 through the first inlet/outlet 16.

In yet another embodiment (c) illustrated in figs. 4c-4e, the flexible container 24 is turned inside out through the first inlet/outlet 16 and its own mounting flange 28 by means of a pulling member 46. The pulling member 46 may for instance be a rope or a wire or the like. One end 48a of the pulling member 46 is attached to the inside of the flexible container 24 remote from the first inlet/outlet 16. Preferably, said end 48a is attached opposite the first inlet/outlet 16, as illustrated. The other end 48b of the pulling member 46 is accessible or protrudes through the first inlet/outlet 16. The pulling member 46 was preferably pre-installed in the flexible container 24 during its assembly/manufacture. In fig. 4c, an ROV 50 connects to the end 48b of the pulling member 46 at the first inlet/outlet 16. Then, when the ROV 50 pulls the member 46 out from the tank 10, the flexible container 24 is turned inside out (fig. 4d). Finally, the complete flexible container 24 has been pulled out through the first inlet/outlet 16 (fig. 4e) by means of the ROV 50. Once the container 24 has been extracted through the first inlet/outlet 16 in this manner, the flange 28 is disconnected from the inlet/outlet 16, and the entire container 24-flange 28 assembly is removed from the tank 10. This method has the advantage of avoiding contact with the oil infested outside of the old container 24 with that of the surrounding seawater in the tank's environment. This is so because the outside of the container 24 becomes the inside of same once it has been removed from the tank 10 in this manner. Also in embodiment c), the mounting flange 28 and the flexible container 24 can be removed when the flexible container is (only) partially pulled out. Once the old flexible container 24 has been disconnected and/or removed, there are several ways of inserting a new flexible container 24'. It should be noted that 'new' not necessarily means that the flexible container 24' is brand new or previously unused, but rather that it is another container compared to the original or previous "old" container 24, or that it is the previous container that has been mended.

In one embodiment illustrated in figs. 4f-4h, the new flexible container 24' is pre-packed in an elongated shape, like a solid bar-shaped body 52. It is hence suitable for insertion through the first inlet/outlet 16. The pre-packed new container 24' may for instance be packaged in a water-soluble packaging means 54. The packaging means 54 may for instance be a foil or tapes or a cover or the like wrapped around the new container 24' . The packaging means 54 is adapted to dissolve in water, preferably after a pre-defined time. The pre-defined time may for instance be approximately 24 hours. In fig. 4f, the new packed bar-shaped container 24' is provided on guide bars 56 outside the first inlet/outlet 16. Then, an ROV 50 pushes the packed container 24' into the tank 10 through in inlet/outlet 16 (fig. 4g). When the complete container 24' has been inserted, its opening 26 is connected to the first inlet/outlet 16. In this embodiment, any old mounting flange may be removed before the new container is inserted, and the new container may have a new mounting flange with becomes attached to a corresponding flange of the inlet/outlet 16.

The operation of inserting the pre-packed new container 24' shall take less time than it takes for the water-soluble foil or tape or cover 54 to dissolve. Hence, once the new flexible container 24' has been inserted into the tank 10, the foil or tape or cover 54 will dissolve in the seawater in the tank 10, "releasing" the new flexible container 24' and allowing it to unfold or develop inside the tank 10 (fig. 4h). The unfolding of the new flexible container 24' is effected by pumping seawater Wl from the storage space 32 until the new flexible container 24' is fully developed. As a consequence of extracting seawater Wl from the storage space 32, seawater W2 enters the new container 24' via the reconnected first filling and emptying line 18. Once the new flexible container 24' is fully developed, the tank 10 is yet again ready to operate as an oil storage facility. The seawater pumped out from the storage space 32 is preferably passed through an OMD before being discharged to the sea.

In another embodiment (not shown), the old mounting flange 28 is removed from the first inlet/outlet 16 (if it has not already been removed, cf. embodiments (b) and (c) described above wherein the flexible container is removed together with the mounting flange). Then, a new mounting flange including a rolled-up new flexible container is inserted into the first inlet/outlet 16, and the new mounting flange may be secured to a mating or corresponding flange of the inlet/outlet 16. Then, to assist the new flexible container to unroll, seawater is pumped out from the storage space 32, and seawater is allowed into the new container via the first inlet/outlet 16 and the reconnected first filling and emptying line 18. Thereafter, the tank 10 is yet again ready to operate as an oil storage facility. The seawater pumped out from the storage space 32 is preferably passed through an OMD before being discharged to the sea.

As mentioned above, the tank 10 may be used to store hydrocarbons, such as oil. However, it can also be used for storing other (imported) fluids, such as fresh water for increased oil recovery (IOR) purposes or other hydrocarbons for diluent purposes.

The person skilled in the art will realize that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

C l a i m s

1. A method of replacing a flexible container (24) provided inside a subsea storage tank (10), wherein the flexible container defines a space (30) separated from a remaining storage space (32) of the subsea storage tank, and wherein the flexible container is connected via one opening (26) thereof to a first inlet/outlet (16) of the subsea storage tank, for allowing seawater exclusively into and out from the flexible container, which method comprises:

filling seawater into the remaining storage space of the subsea storage tank from a second inlet/outlet (20) of the subsea storage tank, such that any seawater in the flexible container is discharged through the first inlet/outlet;

one of

b) while the flexible container is inside the subsea storage tank, disconnecting a mounting flange (28) via which the flexible container is connected to the first inlet/outlet, and then removing the mounting flange from the first inlet/outlet such that the flexible container still affixed to the mounting flange is pulled out from the storage tank through the first inlet/outlet, and

c) turning the flexible container at least partly inside out through the first inlet/outlet by means of a pulling member (46) having one part or end (48a) attached to the inside of the flexible container remote from the first inlet/outlet and another part or end (48b) accessible via the first inlet/outlet; and

inserting a new flexible container (24') through the first inlet/outlet.

2. A method according to claim 1, wherein inserting the new flexible container through the first inlet/outlet includes:

providing the new flexible container (24') pre-packed in an elongated shape

(52);

inserting the pre-packed new flexible container into the subsea storage tank through the first inlet/outlet; and

allowing the inserted, pre-packed new flexible container to unfold or develop inside the subsea storage tank.

3. A method according to claim 2, wherein the new flexible container is pre-packed using a fluid-soluble packing means (54) adapted to dissolve after a specific time of exposure to fluid.

4. A method according to claim 2 or 3, wherein allowing the inserted, pre-packed new flexible container to unfold or develop inside the subsea storage tank includes: extracting seawater from the remaining storage space of the subsea storage tank, and allowing seawater into the new flexible container.

5. A method according to claim 1, wherein inserting the new flexible container through the first inlet/outlet includes:

if still in place, removing a mounting flange (28) from the first inlet/outlet; inserting a new mounting flange including a rolled-up new flexible container into the first inlet/outlet; and

extracting seawater from the remaining storage space of the subsea storage tank, and allowing seawater into the new flexible container, causing the new flexible container to unroll in the storage tank.

6. A method according to any preceding claim, which method further comprises: storing hydrocarbons in the remaining storage space of the subsea storage tank.

7. A method according to any preceding claim, wherein the flexible container is provided below the remaining storage space of the subsea storage tank, the first inlet/outlet is provided in a side wall at the bottom of the storage tank, and the second inlet/outlet is provided in the top of the storage tank.

8. A replaceable, flexible container (24') for a subsea storage tank (10), which container is made of a water- and oil-proof material and has one opening (26) for connection to an inlet/outlet (16) of the subsea storage tank, wherein the container is pre-packed in an elongated shape (52) and provided with a water-soluble packing means (54) keeping the container in the elongated shape.

9. A container according to claim 8, wherein the packing means is adapted to dissolve after a specific time of exposure to water.

10. A container according to claim 8 or 9, wherein the container is made of a polymeric material.

11. A method of installing a storage tank (10) on the seabed (44), wherein a flexible container (24) is provided inside the storage tank, the flexible container defining a lower space separated (30) from a remaining upper storage space (32) of the subsea storage tank, wherein the method comprises:

filling an incompressible hydrocarbon liquid which is lighter than surrounding seawater (38) into the remaining upper storage space (32), such that the storage tank gets a positive net buoyancy;

positioning the storage tank in the sea (38) above or nearby an installation location (42);

removing at least a fraction of the light hydrocarbon liquid from the remaining storage space, and allowing seawater into the flexible container, such that the storage tank gets a net negative buoyancy; and

allowing the storage tank to immerse down to the installation location.

12. A method according to claim 11, which method further comprises: adjusting the amount of hydrocarbon liquid in the remaining storage space for controlling the net buoyancy of the storage tank.

13. A method according to claim 11 or 12, wherein said hydrocarbon liquid is pentane.