Offloading hydrocarbons from subsea fields

Patent metadata

Field	Value
Publication number	US-11248421-B2
Application number	US-201916980173-A
Country	US
Kind code	B2
Filing date	Mar 13, 2019
Priority date	Mar 14, 2018
Publication date	Feb 15, 2022
Grant date	Feb 15, 2022

Abstract

An offloading system for conveying hydrocarbons from a buoyancy-supported subsea riser to a surface tanker vessel comprises a flexible hose that hangs from the riser structure in a U-shape having first and second limbs. An upper end of the first limb communicates with the riser and an upper end of the second limb terminates in a pulling head for connecting the hose to the tanker. A clump weight acts on a lowermost bend of the hose between the limbs to maintain tension in the limbs. A subsurface holder fixed to the riser structure is arranged to hold the pulling head against the tension in the second limb of the hose when the system is in a standby state. The holder is offset laterally from a central longitudinal axis of the riser structure and a counterweight is positioned to a side of that axis opposed to the holder.

First claim

The invention claimed is:

1. A subsea riser structure fix offloading hydrocarbons, the structure comprising:
a riser column;
a subsurface buoy that supports the riser column; and
an offloading system for conveying hydrocarbons from the riser column to a surface tanker vessel, that system comprising:
a flexible hose hanging from thy riser structure in a U-shape having first and second limbs, an upper end of the first limb communicating fluidly with the riser column and an upper end of the second limb terminating in a pulling head for connecting the hose to the tanker;
at least one clump weight acting on a lowermost bend of the hose between the first and second limbs to maintain tension in the first and second limbs wherein the hose is movable along its length relative to the or each clump weight; and
a subsurface holder fixed to the riser structure, the holder being arranged to engage with and support the pulling head so as to hold the pulling head against said tension in the second limb of the hose when the system is in a standby state, the holder being further arranged to guide the second limb of the hose when the pulling head is disengaged from the holder.

2. The riser structure of claim 1 , wherein the holder is offset laterally from a central longitudinal axis of the riser column.

3. The riser structure of claim 2 , wherein the holder is cantilevered away from a side of the riser structure.

4. The riser structure of claim 2 , further comprising at least one counterweight positioned to a side of the central longitudinal axis opposed to the holder.

5. The riser structure of claim 1 , wherein the holder slidingly surrounds the second limb of the hose when the pulling head is disengaged from the holder.

6. The riser structure of claim 1 , wherein the first and second limbs of the hose lie in a substantially vertical plane.

7. The riser structure of claim 1 , wherein the first and second limbs of the hose are substantially parallel to each other.

8. The riser structure of claim 1 , wherein the first and second limbs of the hose are substantially parallel to the riser.

9. The riser structure of claim 1 , wherein the first and second limbs of the hose are substantially coplanar with the riser.

10. The riser structure of claim 1 , wherein the or each clump weight is supported by a cradle that embraces the lowermost bend of the hose, the cradle defining a path along which the hose may move relative to the cradle.

11. The riser structure of claim 1 , wherein the or each clump weight is hung from the hose via one or more rollers that lie on the lowermost bend of the hose.

12. The riser structure of claim 1 , wherein the clump weight acts on the hose via a bend restrictor that limits the bend radius of the lowermost bend of the hose.

13. The riser structure of claim 12 , wherein the bend restrictor has a limiting radius defined by an army of rollers on an upper side of the lowermost bend of the hose.

14. The riser structure of claim 1 , wherein the pulling head comprises a downwardly tapering engagement formation that complements a downwardly narrowing engagement formation of the holder.

15. The riser structure of claim 14 , wherein the downwardly tapering engagement formation of the pulling head is a bend stiffener that surrounds the hose.

16. The riser structure of claim 1 , wherein the pulling head comprises at least one buoyancy element that confers positive buoyancy on the pulling head.

17. The riser structure of claim 16 , wherein negative buoyancy of the second limb of the hose and of the or each clump weight acting on the pulling head exceeds the positive buoyancy of the pulling head.

18. The riser structure of claim 1 , wherein the subsurface buoy holds the riser upright and under tension.

19. The riser structure of claim 1 , wherein the holder is attached to the buoy of the riser structure.

20. The riser structure of claim 1 , wherein the holder is disposed at a level above a centre of buoyancy of the buoy.

21. The riser structure of claim 1 , wherein the holder is disposed at a level below a centre of buoyancy of the buoy.

22. A method of offloading hydrocarbons to a surface tanker vessel from a buoyantly supported subsea riser structure, the method comprising:
imparting tension in first and second limbs of a flexible hose that hangs from the riser structure in a U-shape, an upper end of the first limb fluidly communicating, with a riser column of the riser structure and an upper end of the second limb terminating in a pulling head for connecting the hose to the tanker;
hanging at least one clump weight from a lowermost bend of the U-shaped hose to impart said tension in said limbs of the hose;
engaging with and supporting the pulling head so as to hold the pulling head against said tension in the second limb of the hose, when the pulling head is subsurface in a standby state;
guiding upwards movement of the second limb of the hose against said tension when the pulling head is being lifted toward the tanker; and
moving the hose along its length relative to the or each clump weight.

23. The method of claim 22 , comprising holding the pulling head offset laterally from a central longitudinal axis of the riser column.

24. The method of claim 23 , comprising applying a counterbalancing moment to the riser structure to a side of the central longitudinal axis opposed to the pulling head.

25. The method of claim 22 , comprising limiting a bend radius of the hose at a lowermost bend of the U-shape.

26. The method of claim 22 , wherein buoyant upthrust of the pulling head is exceeded by said tension in the second limb of the hose.

27. The method of claim 22 , comprising engaging the pulling head with a holder fixed relative to the riser, said engagement being promoted by said tension in the second limb of the hose.

28. The method of claim 22 , comprising shortening the first limb of the hose and lengthening the second limb of the hose while lifting the pulling head toward the tanker.

29. The method of claim 22 , comprising shortening a U-shape portion of the hose hanging from the riser structure while lifting the pulling head toward the tanker.

30. The method of claim 29 , comprising maintaining a U-shaped portion of the hose hanging from the riser structure when the pulling head has been connected to the tanker.

31. The method of claim 30 , comprising damping movements of the hose driven by movement of the tanker.

Assignees

• Subsea 7 Norway As

Inventors

• Saeterdal Morten

Classifications

• B63B21/508

  connected to submerged buoy (mooring buoys per se B63B22/02) · CPC title

• E21B17/012

  with buoyancy elements (E21B17/015 takes precedence) · CPC title

• E21B43/01 Primary

  specially adapted for obtaining from underwater installations · CPC title

• E21B43/0107

  Connecting of flow lines to offshore structures (E21B43/013 takes precedence) · CPC title

• B63B2035/448

  Floating hydrocarbon production vessels, e.g. Floating Production Storage and Offloading vessels [FPSO] · CPC title