Anchoring Subsea Flexible Risers

Abstract

A subsea flexible riser installation has a seabed foundation and a steep-configuration flexible riser anchored to the seabed by the foundation. An attachment formation is fixed relative to the riser and a locating formation is fixed relative to the foundation, both at positions elevated above the seabed. The locating formation is engaged with the attachment formation on the riser. This holds the attachment formation and the riser against movement when engaged, protecting the base of the riser from over-bending and fatigue.

Claims

1. A subsea flexible riser installation, comprising: a seabed foundation; a steep-configuration flexible riser anchored to the seabed by the foundation; an attachment formation fixed relative to the riser at a position elevated above the seabed; and a locating formation fixed relative to the foundation at a position elevated above the seabed, the locating formation being engageable with the attachment formation on the riser to hold the attachment formation against axial, lateral and rotational movement when so engaged.

2. The installation of claim 1 further comprising an upright rigid locating structure that is attached rigidly to or integral with the foundation and that supports the locating formation rigidly.

3. The installation of claim 1, wherein the attachment formation is fixed directly to the riser.

4. The installation of claim 1 further comprising a pulling system acting directly or indirectly between the riser and the foundation for pulling the attachment formation toward the locating formation.

5. The installation of claim 1 further comprising a locking mechanism for locking the attachment formation to the locating formation.

6. The installation of claim 5, wherein the locking mechanism holds a convex surface of the attachment formation in engagement with a complementary concave locating surface of the locating formation.

7. The installation of claim 1, wherein the riser extends from a generally horizontal bottom section to a generally upright ascending section via a curved sag bend section extending upwardly from the seabed, and the attachment formation is positioned above the sag bend section.

8. The installation of claim 7, wherein the attachment formation is positioned level with an ascending section of the riser adjacent to the sag bend section.

9. The installation of claim 7 further comprising a lower bend controller positioned between the attachment formation and the seabed to act on the sag bend section of the riser.

10. The installation of claim 7 further comprising an upper bend controller positioned between the attachment formation and an upper end of the riser to act on the ascending section of the riser.

11. The installation of claim 9, wherein said bend controller is supported by the attachment formation.

12. The installation of claim 1, wherein the locating formation is oriented to engage an attachment formation that is within 15° of vertical.

13. The installation of claim 1, wherein the attachment formation is movable along and lockable relative to the riser.

14. A method of anchoring a steep-configuration flexible subsea riser to the seabed, the method comprising engaging an attachment formation with a locating formation, wherein the attachment formation is fixed relative to the riser and the locating formation is fixed relative to a seabed foundation, both at positions elevated above the seabed.

15. The method of claim 14 further comprising pulling the attachment formation into engagement with the locating formation and then locking the engaged attachment formation to the locating formation.

16. The method of claim 14 further comprising restricting or stiffening bends in an ascending section and/or a sag bend section of the riser while feeding corresponding reaction loads to the foundation via the attachment formation.

17. The method of claim 14 further comprising positioning the attachment formation at a position elevated above the seabed by moving the attachment formation along the riser before locking the attachment formation relative to the riser.

Description

[0041] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0042] FIG. 1 is a side view of an anchor arrangement for a flexible riser in accordance with the invention, with the riser being pulled toward an upstanding rigid locating structure atop a subsea anchor; and

[0043] FIG. 2 corresponds to FIG. 1 but shows the riser engaged with a locating formation of the locating structure.

[0044] FIGS. 1 and 2 show an anchor arrangement 10 in accordance with the invention for anchoring a flexible riser 12 in a steep-wave or steep-S configuration in shallow water.

[0045] ‘Shallow’ means that the water is shallow enough for wave or tide action typical of that location to impart movement along the length of the riser 12 during operation. Such a depth may, for example, be 100 m to 500 m, with about 150 m being typical.

[0046] The riser 12 extends in a continuous length through the anchor arrangement 10, hence obviating a subsea connection such as a flange connection that is commonly used at the base of a riser in steep-wave configurations. The riser 12 comprises an upright ascending section 12A extending toward the surface (not shown) and a bottom section 12B extending from the ascending section 12A generally horizontally in parallel to the seabed 14. A sharply-curved sag bend section 12C is disposed between the ascending section 12A and the bottom section 12B. The sag bend section 12C extends upwardly along the riser 12 from a touchdown point 16, at which the riser 12 starts to bend away from contact with the seabed 14.

[0047] In this example, the ascending section 12A is steeply inclined at an angle of up to 15° to the vertical adjacent to the anchor arrangement 10, although this inclination will vary in accordance with any curvature of the ascending section 12A along its length.

[0048] The anchor arrangement 10 comprises a tubular sleeve or clamp 18 that encircles the riser 12 and is fixed to the riser 12 at a position elevated above the seabed 14. In this example, the clamp 18 is positioned around the bottom of the ascending section 12A, just above the sag bend section 12C. Here, the riser 12 and hence the clamp 18 experiences zero bending moment in a nominal configuration.

[0049] The clamp 18 may be attached to the riser 12 by friction or by welding; the attachment method will depend upon the material from which the riser 12 is made. The clamp 18 may be attached to the riser 12 at an onshore fabrication site or offshore on board an installation vessel.

[0050] A subsea anchor 20 or foundation such as a pile or a deadweight block is embedded in the seabed 14. The anchor 20 is surmounted by an upstanding rigid locating structure 22, whose lower end is fixed to the anchor 20. An upper end of the locating structure 22 comprises a locating formation 24 that is shaped as a receptacle to interface with and to hold the clamp 18 that encircles the riser 12. Specifically, in this example, the locating formation 24 presents a complementary concave part-cylindrical seating surface 26 to the clamp 18 at the same elevation as that of the clamp 18 above the seabed 14. The seating surface 26 is oriented to match the inclination of the riser 12 and hence of the clamp 18 at that elevation.

[0051] The locating structure 22 supports a locking mechanism 28 acting in opposition to the seating surface 26 of the locating formation 24. In this example, the locking mechanism 28 comprises restraining bands 30 although other locking arrangements are possible.

[0052] A wire 32 is attached to the clamp 18 to pull the clamp 18 into engagement with the seating surface 26 of the locating formation 24 during installation of the riser 12, as shown in FIG. 1. The anchor 20 supports a return sheave 34 around which the wire 28 passes so that upward tension on the wire 32 pulls the clamp 18 and the riser 12 downwardly. The clamp 18 terminates in flanges 36 that engage with the locating formation 24 above and below the seating surface 26 to ensure axial location of the riser 12.

[0053] Once the wire 32 and the sheave 34 have been used to pull the clamp 16 into engagement with the seating surface 26 of the locating formation 24 as shown in FIG. 2, the locking mechanism 28 is operable by a diver or ROV to embrace the clamp 18 on the riser 12. The locking mechanism 28 pulls the clamp 18 into closer engagement with the locating formation 24 or at least prevents the clamp 18 being pulled away from and hence disengaging from the locating formation 24.

[0054] When the clamp 18 is engaged rigidly with the locating formation 24 of the locating structure 22, the clamp 18 and the riser 12 are restrained against axial and lateral movement and also against rotation. Thus, when so engaged, the clamp 18 serves as an attachment formation for holding the base of the riser 12 in a fixed position relative to the seabed 14.

[0055] The anchor arrangement 10 further comprises upper and lower bend controllers positioned around the riser 12 respectively above and below the clamp 18. In this example, an upper bend controller comprises a bellmouth 38, flare or ‘tulip’ that mitigates overbending and fatigue of the riser 12 by managing the bending moment between the fixed clamp 18 and the ascending section 12A of the riser 12.

[0056] The bellmouth 38 is an upwardly-flared generally conical bend restrictor that is supported by the clamp 18, for example by being fixed to an upper end of the clamp 18 by bolts. The bellmouth 38 has a horn- or trumpet-like shape that is rotationally symmetrical about a central longitudinal axis, which axis is aligned with the central longitudinal axis of the clamp 18. The bellmouth 38 may be in two parts to enable it to be assembled around the riser 12 offshore.

[0057] It will be apparent that the rigid engagement of the clamp 18 to the locating formation 24 of the locating structure 22 holds the bellmouth 38 rigidly relative to the anchor 20, hence increasing the effectiveness of the bellmouth 38 to protect the riser 12.

[0058] In this example, the lower bend controller is a vertebrae bend restrictor 40 comprising interacting elements of steel or polymer. An upper end of the bend restrictor 40 is attached to the clamp 18 such that the bend restrictor 40 hangs from the clamp 18 around the riser 18. The bend restrictor 40 extends from the clamp 18 along the sag bend section 12C and past the touchdown point 16 to the bottom section 12B of the riser 12. The bend restrictor 40 particularly protects the sag bend section 12C of the riser 12 from overbending during installation as shown in FIG. 1, but may also provide protection to the riser 12 during operation.

[0059] In a possible variant of the invention, the clamp need not be installed at an angle that gives zero bending moment at a nominal configuration. Instead, for example, the clamp could be held generally horizontal to create an overbend section of the riser between the sag bend section and the ascending section of the riser within the clamp. In that case, the bellmouth of FIGS. 1 and 2 could be replaced with an underbender guide serving as an upper bend controller.

[0060] In other variants of the invention, the upper bend controller need not be a bellmouth or an underbender guide but could instead be a bend stiffener, which again is suitably fixed to the clamp. A bend stiffener is distinguished from a bend restrictor in that it resists bending with progressively increasing resistance, particularly adjacent to the interface between the flexible riser and the fixed clamp. Similarly, the lower bend controller need not be a bend restrictor but could instead be a bend stiffener or a downwardly-flared bellmouth, either of which is also suitably fixed to the clamp.

[0061] Again, where a fixed clamp supports the upper and/or lower bend controllers, this increases their effectiveness to protect the riser. Also, reaction loads arising from controlling bends in the riser may conveniently be fed to the anchor and the seabed via the clamp and the locating structure.

[0062] Many other variations are possible within the inventive concept. For example, the sheave may be a snatch block whose side plate can be opened to insert the wire without having to thread the wire through the block. Alternatively, a winch may replace the sheave. To overcome high alignment loads during the final part of pull-in, a hydraulic pulling system could be used in addition to a wire, sheave or winch, thus potentially reducing the size or weight of the anchor.

[0063] The clamp and/or the upper and/or lower bend controllers may be arranged to slide along and then lock to the riser for precise positioning relative to the sag bend section. Such operations may be performed above or preferably below the water surface.