Top Connections of Subsea Risers

Abstract

A top connection arrangement for a subsea riser comprises a pivot or joint combination disposed between, and fluidly connecting, upper and lower sections of rigid pipe. The pivot combination comprises an upper ball joint about which the upper pipe section is pivotable. A lower joint, being a flexible joint or a tapered stress joint to which the lower pipe section is attached, is fixed to the ball joint in series.

A sleeve is fixed to the ball joint and surrounds the upper pipe section to permit limited pivotal movement of that pipe section about the ball joint. The sleeve may seat into the bellmouth of an I- or J-tube of a surface facility or may be omitted if the lower joint is seated in a hang-off formation.

A locking mechanism is capable of locking the ball joint and hence preventing pivotal movement of the upper pipe section.

Claims

1. A top connection arrangement for a subsea riser, the arrangement comprising a pivot combination disposed between, and fluidly connecting, upper and lower pipe sections of rigid pipe, wherein the pivot combination comprises: an upper ball joint to which the upper pipe section is attached and about which the upper pipe section is pivotable; a sleeve that is fixed to the ball joint and that surrounds the upper pipe section, wherein an annular gap between the sleeve and the upper pipe section permits pivotal movement of the upper pipe section about the ball joint; and a lower joint fixed to the ball joint in series, the lower joint being a flexible joint or a tapered stress joint to which the lower pipe section is attached.

2. (canceled)

3. The arrangement of claim 1, wherein the sleeve limits further pivotal movement of the upper pipe section about the ball joint.

4. The arrangement of claim 1, wherein an outer face of the sleeve comprises an upwardly-tapering mating formation.

5. The arrangement of claim 1, wherein the ball joint is attached directly to the lower joint.

6. The arrangement of claim 1, further comprising a flange adapter that interconnects respective flanges of the ball joint and of the lower joint.

7. The arrangement of claim 1, wherein the upper pipe section comprises a load-bearing formation spaced longitudinally from, and tapering toward, the ball joint.

8. The arrangement of claim 1, wherein the ball joint comprises a locking mechanism that is capable of locking the ball joint against movement of the upper pipe section relative to the ball joint.

9. The arrangement of claim 8, wherein the locking mechanism comprises a circumferential array of dogs disposed around a central pivot element, the dogs being movable radially inwardly to engage the pivot element to lock the ball joint.

10. The arrangement of claim 9, wherein the dogs are movable radially inwardly to respectively different extents.

11. The arrangement of claim 1, when supported by a supporting structure of a surface facility.

12. The arrangement of claim 11, wherein the supporting structure comprises a hang-off formation in which the lower joint is seated.

13. The arrangement of claim 11, wherein the lower joint is engaged with the supporting structure via the ball joint.

14. The arrangement of claim 1, when supported by a supporting structure of a surface facility, the supporting structure comprising a tubular support defining a bellmouth in which the sleeve is received.

15. The arrangement of claim 14, wherein tension in the upper pipe section retains the sleeve in the bellmouth.

16. The arrangement of claim 11, when supporting a catenary-type riser of rigid or flexible pipe.

17. The arrangement of claim 16, wherein tension in the upper pipe section supports the suspended weight of the riser.

18. The arrangement of claim 11, wherein the upper pipe section is in fluid communication with pipework of the surface facility without an intermediate flexible conduit.

19. A surface facility supporting the arrangement of claim 11.

20. A method of connecting a subsea riser to a surface facility, the method comprising: pulling in a top connection arrangement of the riser toward engagement with a supporting structure of the surface facility; while pulling in, pivoting an upper rigid pipe section of the arrangement about an upper ball joint of the arrangement; engaging a lower joint of the arrangement with the supporting structure, the lower joint being a flexible joint or a tapered stress joint fixed in series to the ball joint; suspending the riser from the surface facility via a lower rigid pipe section of the arrangement that is attached to the lower joint engaged with the supporting structure; and effecting fluid communication between the upper rigid pipe section and pipework aboard the surface facility.

21. The method of claim 20, further comprising locking the ball joint to restrain further pivoting of the upper rigid pipe section.

22. The method of claim 20, comprising also pivoting the lower rigid pipe section about the lower joint while pulling in the arrangement.

23. The method of claim 20, comprising engaging the lower joint with a hang-off formation of the supporting structure.

24. The method of claim 20, comprising pulling the upper rigid pipe section into a tube and engaging the lower joint with a bellmouth of the tube.

25. The method of claim 24, comprising engaging the lower joint with the bellmouth via the ball joint.

26. The method of claim 25, comprising engaging the lower joint with the bellmouth via a sleeve attached to the ball joint.

27. The method of claim 26, comprising using the sleeve to limit pivotal movement of the upper rigid pipe about the ball joint.

28. The method of claim 24, comprising holding the lower joint in engagement with the bellmouth by tension in the upper rigid pipe section.

29. The method of claim 20, comprising engaging the upper rigid pipe section with a hang-off formation of the supporting structure.

30. The method of claim 29, comprising suspending the weight of the riser through the upper rigid pipe section.

Description

[0072] Referring next, then, to FIGS. 2 to 4 of the drawings, a riser top arrangement 22 of the invention comprises a longitudinal series of elements, namely, progressing upwardly: a lower rigid pipe section 24; a flexible joint 26; a ball joint 28 or ball connector surmounting the flexible joint 26 in series; and an upper rigid pipe section 30 being a rigid spool pipe, which may be up to about 30 m long, welded to the ball joint 28. In this example, flanged connections are made between each of those elements and the neighbouring element(s) in the series.

[0073] The upper end of the upper pipe section 30 comprises a flange 32, typically to an API specification. When this embodiment of the invention'is positioned for use, a downwardly-tapering axial load-bearing formation 34 beneath the flange 32 engages with an upper balcony of a supporting structure to support the entire tension load of the catenary.

[0074] The lower end of the lower pipe section 24 is welded contiguously to the upper end of a steel catenary riser, effectively becoming integral with the riser which extends to, and includes, the flexible joint 26. An optional riser monitoring system 36 is shown in FIGS. 2 and 4.

[0075] A hollow sleeve 38 known in the art as a ‘bishop hat’ surrounds the upper pipe section 30 with substantial radial clearance defining an annular gap between them. As will be explained, the sleeve 38 is responsible for attaching the riser top arrangement 22 to the bellmouth 20, transferring bending moments to the upper pipe section 30.

[0076] The annular gap between the upper pipe section 30 and the sleeve 38 permits limited pivotal movement of the upper pipe section 30 about the ball joint 28. In this example, the upper pipe section 30 can pivot about the ball joint 28 by up to 15° from the central axis of the sleeve 38, hence being free to move relative to the other elements of the riser top arrangement 22 within an upwardly-diverging conical volume.

[0077] A skirt 40 at the bottom of the sleeve 38 is normally seated on top of the ball joint 28 as shown in FIG. 2. However, the sleeve 38 is shown lifted away from the ball joint 28 in FIG. 3 to show the weld 42 by which the upper pipe section 30 is attached to the ball joint 28. The skirt 40 of the sleeve 38 is surrounded by upwardly-tapering flanges that together impart a frusto-conical profile to the base of the sleeve 38.

[0078] Turning now to FIGS. 5 and 6, these drawings show the riser top arrangement 22 being pulled into and then engaged with the bellmouth 20 of an I-tube or J-tube.

[0079] In FIG. 5, the API flange at the top of the upper pipe section 30 is shown entering the bellmouth 20 during upward pull-in movement of the riser top arrangement 22. At this stage, the lower pipe section 24 and the upper pipe section 30 are substantially in mutual alignment via the flexible joint 26 and the ball joint 28, all at an angle of about 4° to the vertical.

[0080] In FIG. 6, the riser top arrangement 22 has been lifted up to the extent that the sleeve 38 has now engaged with the bellmouth 20. In this respect, the upwardly-tapering profile around the skirt 40 of the sleeve 38 serves as a mating formation for the sleeve 38 that complements the downwardly-flared profile of the bellmouth 20. In this way, the sleeve 38 mechanically connects the ball joint 28 and hence the remainder of the riser top arrangement 22 to an external support that is exemplified here by the bellmouth 20.

[0081] As noted above, engagement of the axial load area 34 of the upper pipe section 30 with an upper balcony 16 of a supporting structure supports the tension load of the catenary. By pulling upwardly on the riser top arrangement 22, this tension also holds the sleeve 38 in engagement with the bellmouth 20. The sleeve 38 and the bellmouth 20 then support the flexible joint 26 to absorb angular loads arising from deflection of the riser in operation. The ball joint 28 plays no part in handling these angular loads from the operational riser.

[0082] It will be noted in FIG. 6 that the upper pipe section 30 is now substantially vertical whereas the remainder of the riser top arrangement 22 matches the typical inclination of the top of the riser and the bellmouth 20, namely between about 7° and 9° from the vertical in this example. The ball joint 28 pivots to accommodate this relative angular displacement of the upper pipe section 30 within the confines of the surrounding sleeve 38.

[0083] FIGS. 7 and 8 and FIGS. 9 and 10 show the respective states of the ball joint 28 when the riser top arrangement 22 is in the straight and angled states shown in FIGS. 5 and 6.

[0084] FIG. 8 includes a detail view that shows that the outer face of the sleeve 38 comprises a tapered C-ring 44. The C-ring 44 is mounted on a rubber ring 46 to take up any clearance between the sleeve 38 and the I-tube that incorporates the bellmouth 20.

[0085] In the sectional views of FIGS. 7 and 8, it will be apparent that, as is conventional, the flexible joint 26 comprises an elastomeric element 48 that is sandwiched between part-spherical pivot formations 50. The flexible joint 26 contains a central tube 52 that effects fluid communication between the lower pipe section 24 and the ball joint 28.

[0086] Also, the flexible joint 26 is coupled to the ball joint 28 by a flange adaptor 54 that connects parallel flanges of the flexible joint 26 and the ball joint 28 to hold those structures together in face-to-face sealing contact.

[0087] The features of the ball joint 28 are best appreciated with reference to FIGS. 9 to 11.

[0088] FIGS. 9 and 10 show that the ball joint 28 comprises a hollow pivot element 56 surrounded by a ring structure 58. The ring structure 58 has an array of bores 60 on its upper side to receive bolts for flanged connection to the skirt 40 of the sleeve 38. The ring structure 58 surmounts a bottom flange 62 whereby the ball joint 28 is coupled to the flexible joint 26 using the aforementioned flange adaptor 54.

[0089] In this embodiment, the ball joint 28 has an optional locking mechanism. For this purpose, the ring structure 58 supports an angularly-spaced array of radially-movable dogs 64, shown here retracted radially in an unlocked configuration. By activating respective hydraulic dog cylinders 66 individually, the dogs 64 can be advanced in a radially-inward direction to the varying extents that may be necessary for them to bear against the pivot element 56 at one of its several orientations.

[0090] FIG. 11 shows that the pivot element 56 has a part-spherical base 68 in fluid communication with the central tube 52 of the flexible joint. FIG. 11 also shows that the base 68 of the pivot element 56 is received and retained in a complementary cavity within the ring structure 58. The cavity is defined by a guide insert 70 with part-spherical concave curvature. A sealant line 72 communicates with the cavity to admit a sealing fluid that forms a seal around the pivot element 56.

[0091] It will be apparent from FIG. 11 that the dogs 64 lie in a plane above the base 68 to bear against a narrower neck 74 of the pivot element 56. Working together, therefore, the dogs 64 can lock the pivot element 56 at any orientation relative to the ring structure 58. In this way, when the large pivot angle of the ball joint 28 has facilitated installation of the riser and installation is complete, the pivot element 56 of the ball joint 28 can be locked in an orientation matching that of the upper pipe section 30.

[0092] Locking the pivot element 56 of the ball joint 28 in this manner couples the upper pipe section 30 to the ball joint 28 and hence to the flexible joint 26 as a rigid system, strengthening the structure and avoiding further movement of the ball joint 28 that could induce fatigue. From that point onward, the only compliance in the system is that provided by the flexible joint 26, which conventionally supports the riser for cyclical movement during its operational life in a fatigue-resistant manner.

[0093] The upper pipe section 30 can be fitted offshore after the riser and flexible joint 26 has been installed on the surface facility. If the upper pipe section 30 is installed after the riser, the pivot element 56 and the guide insert 70 attached to the upper pipe section 30 can be inserted into the ring structure 58 and then locked by operating the dog cylinders 66. However, an advantage of the pivotable rigid upper section of the invention is to avoid the need for further connection because the rigid upper section can be passed through the tubes.

[0094] Finally, FIGS. 12 and 13 show that the riser top arrangement 22 of the invention can be applied not only to a balcony designed for a flexible riser but also to a hang-off structure 76 designed for a rigid riser. In this case, the flexible joint 26 is seated into the hang-off structure 76 as best appreciated in FIG. 13. This engagement with the hang-off structure 76 is facilitated by the downward taper of the housing of the flexible joint 26, which is evident from preceding drawings.

[0095] The top flange 32 of the upper pipe section seen in FIGS. 12 and 13 can be adapted for any type of connection to an upper balcony. The sleeve 38 is not required in this embodiment and so has been omitted.

[0096] Other variations are possible within the inventive concept. For example, a tapered stress joint could be used beneath the ball joint instead of a flexible joint.