Inlet-outlet system and method for subsea storage

Abstract

A system for emptying a gravity separated fluid including a flexible bag storing the fluid arranged within a protection structure, the protection structure including a bottom element forming the lower section of the protection structure, the flexible bag being connected to the protection structure, and includes at least one fluid conduit providing fluid communication between the flexible bag and outside the protection structure. A first outlet pipe is arranged inside the flexible bag near the bottom element and is connected to and in fluid communication with the at least one fluid conduit. A perforated tube surrounding the outlet pipe provides an annulus fluid passage, water is supplied to the structure volume, and the fluid is emptied through the at least one fluid conduit, through the perforations in the perforated tube, through the annulus fluid passage to the first outlet pipe.

Claims

1. Method for emptying a gravity separated fluid from a storage system comprising a flexible bag for storing the fluid, wherein the flexible bag is arranged within a protection structure with a structure volume, wherein the protection structure comprises a bottom element forming the lower section of the protection structure, the flexible bag is connected to the protection structure, the system comprises at least one fluid conduit providing fluid communication between a storage volume inside the flexible bag and a position outside the protection structure, wherein the method comprises providing a first outlet pipe with an opening arranged inside the flexible bag near the bottom element wherein the first outlet pipe is connected to and in fluid communication with the at least one fluid conduit, providing a perforated tube surrounding the outlet pipe providing an annulus fluid passage, supplying water to the structure volume, and emptying the fluid from the storage volume through the at least one fluid conduit by allowing the fluid to flow through the perforations in the perforated tube, through the annulus fluid passage to the opening of the first outlet pipe, thereby securing removal of a fluid phase gravity separated within the flexible bag.

2. Method according to claim 1, wherein the storage system comprises one or more second outlet pipe(s) arranged within the perforated tube, wherein the second outlet pipe comprises an opening at a different level than the opening of the first outlet pipe, and wherein the method comprises selecting the amount of fluid removed from each outlet pipe.

3. Method according to claim 2, wherein the method comprises mixing the fluid removed from each outlet pipe thereby controlling the composition of the mixed fluid.

4. Method according to claim 2, wherein the method comprises selective removal of the gravity separated fluid from the different levels.

5. Method according to claim 1, wherein the perforated tube comprises asymmetric perforations over the length of the tube providing a larger inflow area in the upper half of the tube compared to the lower half of the perforated tube, wherein the method comprises maintaining a high flow of a low density fluid out of the flexible bag.

6. Storage system comprising a flexible bag for storing a fluid, where the flexible bag is arranged within a protection structure with a structure volume, wherein the protection structure comprises a bottom element forming the lower section of the protection structure, the flexible bag is connected to the protection structure, the system comprises at least one fluid conduit providing fluid communication between a storage volume inside the flexible bag and a position outside the protection structure, wherein the storage system comprises a first outlet pipe with an opening arranged inside the flexible bag near the bottom element wherein the first outlet pipe is connected to and in fluid communication with the at least one fluid conduit and a perforated tube surrounding the outlet pipe providing an annulus fluid passage, such that fluid when being removed from the storage volume can flow through perforations in the perforated tube, through the annulus fluid passage to the opening of the first outlet pipe and through the outlet pipe to the at least one fluid conduit, wherein the perforated tube comprises asymmetric perforations over the length of the tube providing a larger inflow area in the upper half of the tube compared to the lower half of the perforated tube.

7. Storage system according to claim 6, wherein the protection structure comprises at least one opening either connectable to the open sea or to a pipeline for transferring fluid in and out of the structure volume.

8. Storage system according to claim 6, wherein the bottom element comprises a top surface facing the structure volume, and wherein the top surface is inclined towards the opening of the outlet pipe.

9. Storage system according to claim 6, wherein the flexible bag is connected to the perforated tube in proximity of the opening of the first outlet pipe.

10. Storage system according to claim 9, wherein the flexible bag comprises a connection element on the outside thereof for connecting the flexible bag to the bottom element.

11. Storage system according to claim 10, wherein the connection element comprises a fluid passage in fluid communication with the storage volume.

12. Storage system according to claim 6, wherein one or more second outlet pipe(s) is arranged within the perforated tube, wherein the second outlet pipe comprises an opening at a different level than the opening of the first outlet pipe.

13. Storage system according to claim 6, wherein the perforated tube and the first outlet pipe extend more than 50% of the diameter of the storage volume.

14. Storage system according to claim 13, wherein the perforated tube and the first outlet pipe extend more than 75% of the diameter of the storage volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in further detail with reference to the enclosed drawings illustrating embodiments thereof.

(2) FIG. 1 schematically illustrates a cross sectional view of an embodiment of the outlet system according to the present invention.

(3) FIG. 2 schematically illustrates a cross sectional view of the embodiment on FIG. 1 during emptying.

(4) FIG. 3 schematically illustrates a cross sectional view of another embodiment of the outlet system according to the present invention.

(5) FIG. 4 schematically illustrates a cross sectional view of a further embodiment of the outlet system according to the present invention.

(6) FIG. 5 schematically illustrates a cross sectional view of yet another embodiment of the outlet system according to the present invention.

PRINCIPAL DESCRIPTION OF THE INVENTION

(7) The present invention will now be described in further detail with reference to the enclosed figures. A person skilled in the art will understand the figures are schematic illustrations of the principles of the invention and that the invention can be utilized together with other storage solutions than the ones visible in the drawings. A person skilled in the art will also appreciate that the different embodiments of the present invention may be freely combined.

(8) FIG. 1 provides a cross sectional view of a first embodiment of a subsea storage system 1 comprising a flexible storage bag 3 arranged within a protection structure 2. The flexible bag 3 is connected to a flange 6 connected to the protection structure 2. The connection between the flexible bag and the flange is preferably fluid tight so that the storage volume within the flexible bag is not in fluid communication with a structure volume within the protection structure. The flange may form part of a hatch for accessing the internal side of the protection structure. The flange comprises a fluid conduit 10 connecting the storage volume of the flexible bag with a position outside the protection structure. The fluid conduit can be connected to a pipeline 11 leading to a topside location such as an on-shore installation, another subsea installation, a platform, a buoy or a vessel.

(9) Seawater surrounding the subsea storage system can in the illustrated embodiment enter and leave the protection structure freely trough the opening 4 in the protection structure. The present invention is focused on providing an outlet system for this storage system which secures that the flexible bag 3 can be fully emptied including any substances like sand, particles or water that may accumulate in the bottom of the flexible bag 3 when fluids is stored in the flexible bag over a period of time. The storage system comprises a first outlet pipe 12 connected to the fluid conduit 10 and expanding from the flange 6 into the flexible bag. The outlet pipe 12 comprises an opening 16 preferable arranged near or in proximity of a bottom element 8 of the protection structure. Surrounding the outlet pipe 12 a perforated tube 14 is arranged forming an annulus 17. In the illustrated embodiment the perforated tube 14 surrounds the free end of the outlet pipe 12. The arrows indicate the flow of fluid when the flexible bag is being emptied. The stored fluid flows through the perforations in the perforated tube into the annulus 17. In the annulus the fluid floes down to the opening 16 in the outlet pipe 12. Then the fluid flows up through the outlet pipe 12, through the fluid conduit 10 and further through the pipeline 11. This pipe in tube outlet system secures that any fluid accumulating in the bottom section of the flexible bag will be transported out of the flexible bag. Fluid positioned in higher levels will also flow in through the perforations and trough the annulus to the opening 16 in the lower section of the protection structure.

(10) FIG. 2 illustrates the situation wherein a part of the fluid stored in the flexible bag illustrated in FIG. 1 has been emptied out through the pipeline 11. The fluid stored in the flexible bag has a lower density than the surrounding seawater and can for instance be crude oil. Due to the lower density the remaining fluid in the flexible will occupy mainly the upper section of the flexible bag. The water pressure from the outside of the bag combined with the flow of fluid out of the flexible bag results in the bag being folded around the lower end of the perforated tube 14. The bag material when folded around part of the perforated tube is likely to at least partly block some of the perforations. However, as the tube 14 comprises perforations along at least large sections of its longitudinal extent the fluid will continue to be able to flow into the perforations in the sections not blocked by the bag material. The arrows 20 indicate the flow of stored fluid. Due to the positioning of the opening 16 in the lowest section of the flexible bag any high density fluid that may separate out from the stored fluid will by the help of gravity be transported to the lowest section of the flexible bag and from there be emptied out of the bag through the outlet pipe 12.

(11) FIG. 3 illustrates an alternative embodiment of the present invention wherein the inlet and outlet of fluid to the structure volume is controllable by the opening and closing of the valve 5. This embodiment can be combined with the embodiment illustrates on the FIGS. 1 and 2. Further in the illustrated embodiment the bottom element 8 comprises a top surface 7 towards the structure volume and the flexible bag 3. The top surface 7 is inclined towards the position of the opening 16 in the outlet pipe 12. Also illustrated on FIG. 3 is a connection element 18 fastened to the outside of the flexible bag 3 opposite the flange 6 and optionally fastened to the wall of the bag opposite a fastening of the perforated tube 4 to the inside of the bag near the opening 16 of the first outlet pipe. The bottom element 8 comprises reception means adapted to releasable receive and connect the connection element 18 to the bottom element 8. The top surface 7 is inclined towards the reception means.

(12) The connection element 18 can have any suitable form for establishing a preferably releasable connection between the flexible bag and the bottom element thereby providing for increased control of the positioning of the flexible bag within the protection structure and positioning of the opening in the outlet pipe with respect to the inclined top surface of the bottom element.

(13) FIG. 4 illustrates an alternative embodiment similar to the embodiment illustrated on FIG. 3 but comprising a second outlet pipe 112 in fluid communication with a second pipeline 11. The opening of the second outlet pipe 116 is at a different level in the flexible bag than the opening 16 of the first outlet pipe. The second outlet pipe is arranged in the annulus between the first outlet pipe 12 and the perforated tube 14. In the illustrated embodiment the second outlet pipe 112 surrounds a section of the first outlet pipe 12 but it is equally possible to arrange the two or more outlet pipes independently within the perforated tube 14. By including more than one outlet pipe it is possible to empty different fractions of the fluid separated out by gravity selectively. If for instance the fluid is crude oil that has been kept in the storage for sufficient time to result in an accumulation of water in the lower section the fraction removed through opening 16 will have a higher water content then the fraction removed trough opening 116 and as the fractions are obtained separately they can be treated separately if needed.

(14) FIG. 5 illustrates a further embodiment of the present invention wherein the perforated tube 114 in the upper section 115 closest to the flange 6 has an increased diameter thereby providing an increased surface area allowing for increasing the area of the perforations such that the flow into the annular passage can be maintained also when the flow through a larger fraction of the perforations in the lower section is blocked by sections of the collapsed flexible bag.

Aspects of the Present Invention

(15) 1. Method for emptying a gravity separated crude oil from a storage system comprising a flexible bag for storing the crude oil, wherein the flexible bag is arranged within a protection structure with a structure volume, wherein the protection structure comprises a bottom element forming the lower section of the protection structure, the flexible bag is connected to a flange, wherein the flange is connected to the protection structure, the flange comprises at least one fluid conduit providing fluid communication between a storage volume inside the flexible bag and a position outside the protection structure, characterised in that the method comprises providing a first outlet pipe with an opening arranged inside the flexible bag near the bottom element wherein the first outlet pipe is connected to and in fluid communication with the at least one fluid conduit, providing a perforated tube surrounding the outlet pipe providing an annulus fluid passage, supplying water to the structure volume, and emptying the crude oil from the storage volume through the at least one fluid conduit by allowing the crude oil to flow through the perforations in the perforated tube, through the annulus fluid passage to the opening of the first outlet pipe, thereby securing removal of a crude oil phase gravity separated within the flexible bag. 2. Method according to aspect 1, wherein the storage system comprises one or more second outlet pipe(s) is arranged within the perforated tube, wherein the second outlet pipe comprises an opening at a different level than the opening of the first outlet pipe, and wherein the method comprises selecting the amount of crude oil removed from each outlet pipe. 3. Method according to aspect 2, wherein the method comprises mixing the fluid removed from each outlet pipe thereby controlling the composition of the mixed fluid. 4. Method according to aspect 2, wherein the method comprises selective removal of the gravity separated crude oil from the different levels. 5. Method according to any one of the preceding aspects, wherein the perforated tube comprises asymmetric perforations over the length of the tube providing a larger inflow area in the upper half of the tube near the flange compared to the lower half of the perforated tube, wherein the method comprises maintaining a high flow of crude oil out of the flexible bag. 6. Storage system (1) comprising a flexible bag (3) for storing a fluid, where the flexible bag (3) is arranged within a protection structure (2) with a structure volume, wherein the protection structure (2) comprises a bottom element (8) forming the lower section of the protection structure, the flexible bag is connected to a flange (6), wherein the flange is connected to the protection structure (2), the flange comprises at least one fluid conduit (10) providing fluid communication between a storage volume inside the flexible bag (3) and a position outside the protection structure (2), characterised in that the storage system comprises a first outlet pipe (12) with an opening (16) arranged inside the flexible bag (3) near the bottom element (8) wherein the first outlet pipe (12) is connected to and in fluid communication with the at least one fluid conduit (10) and a perforated tube (14) surrounding the outlet pipe (12) providing an annulus fluid passage (17), such that fluid when being removed from the storage volume can flow through perforations in the perforated tube (14), through the annulus fluid passage (17) to the opening (16) of the first outlet pipe (12) and through the outlet pipe to the at least one fluid conduit (10), wherein the perforated tube (14) comprises asymmetric perforations over the length of the tube providing a larger inflow area in the upper half of the tube near the flange (6) compared to the lower half of the perforated tube. 7. Storage system according to aspect 6, wherein the protection structure comprises at least one opening (4) either connectable to the open sea or to a pipeline for transferring fluid in and out of the structure volume. 8. Storage system according to aspect 6 or 7, wherein the perforated tube (14) comprises asymmetric perforations over the length of the tube. 9. Storage system according to any one of the aspects 6 to 8, wherein the bottom element (8) comprises a top surface (7) facing the structure volume, and wherein the top surface (7) is inclined towards the opening (16) of the outlet pipe (12). 10. Storage system according to any one of the aspects 6 to 9, wherein the flexible bag (3) is connected to the perforated tube (14) in proximity of the opening (16) of the first outlet pipe (12). 11. Storage system according to aspect 10, wherein the flexible bag (3) comprises a connection element (18) on the outside thereof for connecting the flexible bag (3) to the bottom element (8). 12. Storage system according to aspect 11, wherein the connection element comprises a fluid passage in fluid communication with the storage volume. 13. Storage system according to any one of the aspects 6 to 12, wherein one or more second outlet pipe(s) (112) is arranged within the perforated tube (14), wherein the second outlet pipe (112) comprises an opening (116) at a different level than the opening (16) of the first outlet pipe (12). 14. Flexible bag element for a storage system, wherein the flexible bag element comprises a flexible bag (3) for storing a fluid and a flange (6), wherein the flexible bag (3) is connected to the flange (6), wherein the flange (6) is connectable to a protection structure (2) of a storage system, wherein the flange (6) comprises at least one fluid conduit (10) providing fluid communication between a storage volume inside the flexible bag (3) and a position outside the protection structure when the flange is connected thereto, characterised in that the flexible bag element comprises a first outlet pipe (12) with an opening (16) arranged inside the flexible bag wherein the first outlet pipe (12) is connected to and in fluid communication with the at least one fluid conduit (10) and a perforated tube (14) surrounding the outlet pipe (12) providing an annulus fluid passage (17), wherein the perforated tube (14) is connected to the flange (6) such that fluid when being removed from the storage volume can flow through perforations in the perforated tube (14), through the annulus fluid passage (17) to the opening (16) of the first outlet pipe (12) and through the outlet pipe to the at least one fluid conduit (10), wherein the perforated tube (14) comprises asymmetric perforations over the length of the tube providing a larger inflow area in the upper half of the tube near the flange (6) compared to the lower half of the perforated tube. 15. Flexible bag element according to aspect 14, wherein the perforated tube (14) at one end is connected to the flange (6) and at the opposite end is connected to the flexible bag (3). 16. Flexible bag element according to aspect 14 or 15, wherein the flexible bag (3) is folded around the perforated tube (14). 17. Flexible bag element according to any one of the aspects 14-16, wherein the flexible bag element comprises a connection element (18) on the outside of the flexible bag (3) for connecting the flexible bag to a bottom element (8) of a protection structure opposite the flange (6). 18. Flexible bag element according to any one of the aspects 14-17, wherein the flexible bag element comprises one or more second outlet pipe (112) arranged within the perforated tube (14), wherein the one or more second outlet pipe (112) comprises an opening (116) at a different position than the opening (16) of the first outlet pipe (12). 19. Flexible bag element according to any one of the aspects 14-18, wherein the perforated tube (14) and the first outlet pipe (12) extends across a considerable part of the diameter of the storage volume, preferably more than 50%, more preferably more than 75% of the diameter.