Installation of Subsea Risers

Abstract

A method of installing a wave-configuration subsea riser of unbonded flexible pipe comprises lowering the riser progressively into the sea from an installation vessel while suspending an elongate clump weight in a catenary shape that comprises first and 5 second limbs extending upwardly from a conjoining bottom portion. An upper end of the first limb, at a distal end of the clump weight, is attached to the riser and an upper end of the second limb, at a proximal end of the clump weight, is suspended from a winch or crane of the vessel. While lowering the riser from the vessel, the weight load applied to the riser by the clump weight is controlled by adjusting the relative lengths of the first 0 and second limbs of the clump weight.

Claims

1. A method of installing a wave-configuration subsea riser, the method comprising: suspending the riser from an installation vessel; suspending an elongate clump weight in a catenary shape comprising first and second limbs extending upwardly from a conjoining bottom portion, with an upper end of the first limb, at a distal end of the clump weight, being attached to the riser and an upper end of the second limb, at a proximal end of the clump weight, being suspended from a winch or crane of the installation vessel; and while lowering the riser progressively into the sea from the installation vessel, controlling a weight load applied by the clump weight to the riser by adjusting relative lengths of the first and second limbs of the clump weight.

2. The method of claim 1, comprising using the winch or crane to adjust the relative lengths of the first and second limbs.

3. The method of claim 1 or claim 2, comprising adjusting the relative lengths of the first and second limbs by adjusting relative lowering speeds of the riser and of the upper end of the second limb.

4. The method of any preceding claim, comprising bending the clump weight along its length around a curve that defines the bottom portion.

5. The method of claim 4, comprising bending the clump weight by articulation between links of the clump weight.

6. The method of any preceding claim, comprising attaching the distal end of the clump weight to the riser.

7. The method of claim 6, comprising attaching the distal end of the clump weight to the riser via a collar of the riser.

8. The method of any preceding claim, further comprising attaching buoyancy modules to a hogbend portion of the riser aboard the installation vessel.

9. The method of claim 8, further comprising submerging the hogbend portion, carrying the buoyancy modules, under tension in the riser applied to the riser by the first limb of the clump weight.

10. The method of claim 9, comprising reducing tension in the riser by shortening the first limb of the clump weight, allowing buoyant upthrust of the submerged buoyancy modules along the hogbend portion to form a hogbend in the riser.

11. The method of any of claims 8 to 10, comprising attaching the distal end of the clump weight to the riser beneath the buoyancy modules that are attached to the hogbend portion of the riser.

12. The method of any preceding claim, comprising laying the riser on the seabed continuously while varying the weight load applied to the riser by the clump weight.

13. The method of any preceding claim, comprising adjusting the relative lengths of the first and second limbs by adjusting relative levels of the distal and proximal ends of the clump weight.

14. The method of any preceding claim, wherein the installation vessel supports the full weight load of the clump weight before, during and after attachment of the clump weight to the riser.

15. The method of claim 14, wherein the clump weight is lowered from and recovered to the installation vessel using the winch or crane of the installation vessel.

16. The method of any preceding claim, further comprising detaching the clump weight from the riser and recovering the clump weight to the installation vessel.

Description

[0051] To describe the prior art background, reference has already been made to FIGS. 1 to 4 of the accompanying drawings, in which:

[0052] FIG. 1 is a flow diagram of a prior art method for installing a riser;

[0053] FIG. 2 is a schematic perspective view of a pipelay vessel and a support vessel in the process of installing a riser for connection to an FPSO, using a method of installing or recovering a clump weight as set out in FIG. 1;

[0054] FIG. 3 is a side view that exemplifies movement of the riser in the water column through typical steps of clump weight installation as known in the prior art; and

[0055] FIG. 4 corresponds to FIG. 3 but exemplifies movement of the riser in the water column through typical steps of clump weight recovery as known in the prior art.

[0056] 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:

[0057] FIG. 5 is a flow diagram of a riser installation method in accordance with the invention; and

[0058] FIGS. 6 to 10 are a sequence of side views of a pipelay vessel installing a riser in accordance with the method set out in FIG. 5.

[0059] The flow diagram of FIG. 5 sets out a method of the invention. Steps 10, 12, 14 and 16 are the same as the correspondingly-numbered steps of the prior art method shown in FIG. 1. Thus, at 10, a riser of flexible pipe is progressively unspooled aboard an installation vessel and launched from that vessel into the sea. When a determination is made, at 12, that a sufficient length of the riser has been launched into the sea, initial buoyancy modules are mounted to the riser, at 14, spaced along a hogbend portion of the riser that will form the desired hogbend. The hogbend portion may, for example, be a few hundred metres long. Lowering of the riser continues until, at 16, a determination is made that the leading buoyancy modules are underwater.

[0060] The method shown in FIG. 5 differs from that shown in FIG. 1 in that, at 50, a first, proximal end of an elongate clump weight is connected to a winch aboard the installation vessel. Then, after paying out the winch wire at 52 to suspend the clump weight in water beneath the installation vessel, an opposed second, distal end of the clump weight is connected to the collar of the riser beneath the submerged leading buoyancy modules at 54.

[0061] The winch wire is paid out further as the pipe, with buoyancy modules attached along the hogbend portion, is lowered further in the water at 56.

[0062] On being attached to the riser, the clump weight applies a weight load to the riser. By virtue of the invention, that weight load is gradually increased to maintain tension in the riser and to overcome the increasing aggregate buoyant upthrust as more of the buoyancy modules are pulled underwater with the downwardly-advancing riser.

[0063] When the buoyancy modules along the hogbend portion of the riser have all been pulled underwater on the riser and have reached the appropriate depth, the weight load applied by the clump weight to the riser is gradually reduced. To do so, the weight load of the clump weight is transferred progressively from the riser to the winch wire, hence causing tension in the winch wire to increase.

[0064] Reducing the weight load applied to the riser in this way allows buoyant upthrust of the buoyancy modules along the hogbend portion to form the desired hogbend in the riser at 58.

[0065] When formation of the hogbend is complete, the distal end of the clump weight is detached from the riser at 60 so that the clump weight can be lifted back to the installation vessel.

[0066] The method set out in FIG. 5 will now be explained in more detail with reference to FIGS. 6 to 10 of the drawings, in which like numerals are used for like features.

[0067] FIGS. 6 to 10 show an installation vessel 30 on the surface 42 in the process of laying a riser 32 of flexible pipe. The riser 32 extends from the surface 42 to the seabed 44. These drawings are not to scale: in practice, the depth of the water between the surface 42 and the seabed 44 will typically be much greater than is shown here.

[0068] As is conventional, the installation vessel 30 comprises a pipe storage facility in the form of under-deck carousels 62 in this example, an upright lay tower 64 and an abandonment and recovery (A&R) winch 66.

[0069] The flexible pipe of the riser 32 is stored in one or both of the carousels 62 in a coiled arrangement before being lifted to an intermediate level of the lay tower 64. From there, the riser 32 is launched downwardly into the water on an upright launch axis, for example through a moonpool in the hull of the installation vessel 30.

[0070] As is also conventional, the lay tower 64 comprises a tensioner system that supports the suspended weight of the riser 32 while controlling downward movement of the riser 32 along the launch axis. If needs be, the lay tower 64 can be tilted from the vertical so that the launch axis has corresponding inclination. Buoyancy modules 34 are fixed to the riser 32 aboard the installation vessel 30 as the riser 32 is launched into the water.

[0071] FIGS. 6 to 10 also show an elongate, flexible clump weight 38 that is bendable along its length. In this example, the clump weight 38 comprises articulated links in the form of a heavy chain. A proximal end of the clump weight 38 is suspended from the A&R winch 66 on an A&R wire 68. Conversely, a distal end of the clump weight 38 is suspended from a collar 46 or other support structure on the riser 32.

[0072] The effect of suspending the clump weight 38 from its ends in this way is to hang the clump weight 38 in a catenary shape from both the lay tower 64 and the A&R winch 66 of the installation vessel 30. The installation vessel 30 supports the full weight load of the clump weight 38 throughout but that load is shared between the tensioners of the lay tower 64 and the A&R winch 66.

[0073] By sharing the weight load of the clump weight 38 between the lay tower 64 and the A&R winch 66 in varying ratios, the invention allows continuous fine control of the weight load that is applied by the clump weight 38 to the riser 32, without interruption, throughout the riser installation operation. Also, as the clump weight 38 is always supported by the installation vessel 30, there is no need for an additional support vessel to handle the clump weight 38 or, therefore, for interrupting the riser installation operation to allow for the approach of such a vessel.

[0074] The catenary shape of the clump weight 38 comprises first and second limbs 70, 72 that extend upwardly from a conjoining bottom portion 74. An upper end of the first limb 70, at the distal end of the clump weight 38, is attached to the riser 32 via the collar 46. Conversely, an upper end of the second limb 72, at a proximal end of the clump weight 38, is suspended from the A&R winch 66 via the A&R wire 68.

[0075] While lowering the riser 32 from the installation vessel 30, the weight load applied by the clump weight 38 to the riser 32 may be adjusted continuously by adjusting the relative lengths of the first and second limbs 70, 72 of the clump weight 38. Thus, there is a corresponding change in the relative levels of the distal and proximal ends of the clump weight 38. To allow this change, the clump weight 38 bends around a curve that defines the bottom portion 74 and is moved longitudinally in either direction around that curve. In this respect, reference is made to FIGS. 6 and 7.

[0076] Adjustment of the weight load applied by the clump weight 38 to the riser 32 may be achieved in various ways. For example, the A&R wire 68 may be paid out more quickly than the riser 32 is lowered by the tensioners on the lay tower 64, which increases the length of the first limb 70 of the clump weight 38 and so applies a greater weight load to the riser 32 as tension in the A&R wire 68 decreases. This is shown in FIG. 7. Conversely, the first limb 70 of the clump weight 38 may be shortened by lowering the riser 32 more quickly than the A&R wire 68 is paid out. This reduces the weight load applied by the clump weight 38 to the riser 32, and correspondingly increases the tension in the A&R wire 68 as the second limb 72 of the clump weight 38 lengthens. This is shown in FIG. 6.

[0077] It will be apparent that similar adjustments could be made by moving the riser 32 when the A&R wire 68 is stationary or vice versa, or indeed by reversing the direction of the riser 32 or the A&R wire 68 to lift them temporarily toward the installation vessel 30.

[0078] In practice, the weight load of the clump weight 38 is borne exclusively by the A&R winch 66 until the distal end of the clump weight 38 is attached to the collar 46 of the riser 32. Then, some of the weight load of the clump weight 38 is transferred gradually to the riser 32 as shown in FIG. 6. This maintains tension in the riser 32 by counterbalancing the buoyant upthrust of the submerged buoyancy modules 34. As the riser 32 is lowered further into the water and more of the buoyancy modules 34 are submerged as a result, more of the weight load of the clump weight 38 is transferred gradually to the riser 32, as shown in FIG. 7. This offsets the increasing aggregate buoyant upthrust of the buoyancy modules 34.

[0079] When all of the buoyancy modules 34 required to support the hogbend 48 have been pulled underwater on the riser 32 and have reached the appropriate depth, the weight load applied by the clump weight 38 to the riser 32 is gradually reduced. This is done by gradually transferring most of the weight load of the clump weight 38 back from the riser 32 to the A&R wire 68, for example by pulling up the A&R wire 68. This allows the buoyant upthrust of the buoyancy modules 34 to initiate the hogbend 48 in the riser 32 as shown in FIG. 8 and in enlarged form in FIG. 9.

[0080] When the A&R wire 68 has resumed carrying the full weight load of the clump weight 38, the distal end of the clump weight 38 can be detached from the collar 46 of the riser 32. The clump weight 38 can then be recovered to the installation vessel 30 for possible re-use on other risers.

[0081] Conveniently, the clump weight 38 can be detached from the riser 32 at any stage, without requiring the riser installation operation to be interrupted. For example, FIG. 10 shows the clump weight 38 still attached to the riser 32, with a minor portion of its weight load borne by the riser 32, as the installation vessel 30 advances to continue laying the riser 32 and to continue forming the final lazy-wave shape of the riser 32 including the hogbend 48.

[0082] Many variations are possible within the inventive concept. For example, the clump weight could be supported by a crane of the installation vessel rather than by a winch.