Upending Elongate Structures Offshore

Abstract

A crane (16) for upending an elongate structure such as a monopile (28) supports elongate rigging elements that are connected to the structure at respective connection axes spaced longitudinally along the structure. At least one of those elements, of fixed length (36), extends to a first connection axis and at least one other of those elements, of variable length (34), extends to a second connection axis. A frame (24) suspended from the crane (16) supports a winch (46) that pays out the variable-length element (34) of the rigging to lengthen that element. This lowers the second connection axis relative to the first connection axis, which serves as a pivot axis about which the structure is upended.

Claims

1. A lifting arrangement for upending an elongate structure, the arrangement comprising: rigging of elongate elements that are connected to the structure at respective connection axes spaced longitudinally along the structure, at least one of those elements extending to a first connection axis and at least one other of those elements extending to a second connection axis and being of variable length, wherein the structure has a centre of gravity that lies in a vertical plane disposed between the first and second connection axes; and a frame suspended from a crane, the frame supporting at least one winch that acts on the at least one variable-length element of the rigging; wherein the at least one variable-length element of the rigging extends laterally from the winch and over a sheave that defines a suspension point of the frame that is offset laterally from said vertical plane, and said vertical plane intersects the winch.

2. The arrangement of claim 1, wherein the frame is suspended via a hoisting system of the crane.

3. The arrangement of claim 2, wherein the frame is suspended via a hook of the crane.

4. The arrangement of any preceding claim, wherein the first connection axis defines a pivot axis about which the structure is upended.

5. The arrangement of any preceding claim, wherein the structure has a pair of connection points on the first connection axis on mutually-opposed sides of the structure, to which respective rigging elements extend.

6. The arrangement of claim 5, wherein the connection points are trunnions that protrude from the structure on the first connection axis.

7. The arrangement of any preceding claim, wherein the at least one element extending to the first connection axis is of substantially fixed length.

8. The arrangement of any preceding claim, wherein the at least one element extending to the first connection axis is attached at an opposite end to the frame.

9. The arrangement of any of claims 1 to 7, wherein the at least one element extending to the first connection axis is attached at an opposite end to the crane.

10. The arrangement of any preceding claim, wherein a longitudinal distance along the structure between the second connection axis and said plane is greater than a longitudinal distance along the structure between the first connection axis and said plane.

11. The arrangement of claim 10, wherein the longitudinal distance between the second connection axis and said plane is at least five times greater than the longitudinal distance between the first connection axis and said plane.

12. The arrangement of any preceding claim, wherein the suspension point is offset laterally from said plane by a distance of at least half of a diameter of the structure extending through the second connection axis.

13. The arrangement of any preceding claim, wherein the structure is suspended by the crane over water.

14. A method of upending an elongate structure to an upright orientation, the method comprising: suspending the structure from a crane via elongate rigging elements that are attached to the structure respectively at first and second connection axes spaced longitudinally along the structure, at least one of the rigging elements extending over a sheave that defines a suspension point offset laterally relative to a vertical plane containing the centre of gravity of the structure; and by paying out said at least one of the rigging elements from a winch that is also suspended from the crane and that is intersected by said vertical plane, lowering the second connection axis relative to the first connection axis, hence pivoting the structure around the first connection axis to the upright orientation.

15. The method of claim 14, comprising holding the first connection axis at a substantially fixed height relative to the crane.

16. The method of claim 15, comprising maintaining a rigging element attached to the first connection axis at a substantially fixed length.

17. The method of any of claims 14 to 16, comprising suspending the winch from a hoisting system of the crane.

18. The method of claim 17, comprising suspending the winch from a hook of the crane.

19. The method of claim 18, comprising keeping the centre of gravity of the structure on a vertical axis that intersects the hook.

20. The method of any of claims 14 to 19, comprising bearing a majority of the weight of the structure through the first connection axis.

21. The method of claim 20, comprising bearing less than 20% of the weight of the structure through the second connection axis.

22. The method of any of claims 14 to 21, comprising, while upending the structure, transferring the weight of the structure from one or more rigging elements attached at the second connection axis to one or more rigging elements attached at the first connection axis.

23. The method of claim 22, comprising, after completing said transfer of weight, detaching the or each rigging element from the structure at the second connection axis while suspending the weight of the structure from the or each rigging element attached at the first connection axis.

24. The method of any of claims 14 to 23, wherein the second connection axis is offset radially relative to a longitudinal axis of the structure.

25. The method of claim 24, wherein said lateral offset of the suspension point is at least as great as said radial offset of the second connection axis.

26. The method of any of claims 14 to 25, wherein the suspension point remains in fixed relation to the winch while upending the structure.

27. The method of any of claims 14 to 26, wherein, when the structure is upright, one or more rigging elements extend to the first connection axis substantially in alignment with the centre of gravity of the structure.

28. An upending device, comprising: a frame having at least one lifting point for suspending the frame from a hook of a crane such that a common vertical plane intersects the hook and the frame, wherein the frame comprises: a winch for paying out a rigging element, wherein said vertical plane intersects the winch; and a suspension point for supporting the rigging element at a position offset laterally from said vertical plane.

29. The upending device of claim 28, wherein the suspension point is defined by a sheave spaced laterally from the winch.

30. A crane supporting the upending device of claim 28 or claim 29.

Description

[0045] 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:

[0046] FIG. 1 is a side view of an installation vessel about to upend a monopile that is initially suspended horizontally from a crane of the vessel;

[0047] FIG. 2 is an enlarged schematic cut-away view corresponding to Detail II of FIG. 1;

[0048] FIG. 3 corresponds to FIG. 1 but shows the monopile partially upended; and

[0049] FIG. 4 corresponds to FIGS. 1 and 3 but shows the monopile fully upended.

[0050] FIGS. 1, 3 and 4 show an installation vessel 10 floating on the surface 12 of the sea at an installation site. The vessel 10 has a main deck 14 surmounted by a crane 16 whose jib or boom 18 can slew around a vertical slew axis 20. The slew axis 20 and the centreline of the boom 18 both lie on a common vertical plane.

[0051] As is conventional, the hoisting system of the crane 16 comprises a main hook 22 that is suspended from an array of sheaves 24 on the boom 18. The main hook 22 also lies on the common vertical plane that contains the slew axis 20 and the centreline of the boom 18.

[0052] In accordance with the invention, a horizontally-extending spreader frame 24 is suspended from the crane 16, in particular from the main hook 22 via downwardly-diverging spreader rigging 26 in this example. The spreader frame 24 is shown in detail in FIG. 2.

[0053] Via the spreader frame 24, the crane 16 supports an elongate load that is exemplified here by a monopile 28. In this example, the monopile 28 is a hollow tubular structure that is rotationally symmetrical about a central longitudinal axis 30.

[0054] The monopile 28 shown in the drawings comprises a relatively narrow top portion 28A and a relatively wide base portion 28B joined by a frusto-conical intermediate portion 28C. Thus, the centre of gravity 32 of the monopile 28 is offset longitudinally along the central longitudinal axis 30 toward the base end of the monopile 28; indeed, in this example, the centre of gravity 32 lies within the enlarged base portion 28B of the monopile 28.

[0055] The centre of gravity 32 of the monopile 28 lies directly beneath the main hook 22 of the crane 16, hence lying on the common vertical plane that contains the main hook 22, the slew axis 20 and the centreline of the boom 18.

[0056] The monopile 28 is suspended by load rigging that extends from the spreader frame 24 to longitudinally-spaced, horizontally-extending connection axes on the monopile 28. The load rigging comprises three rigging elements, one element 34 being of variable length and the other two elements 36 being of fixed length in this example. One of fixed-length elements 36 is hidden behind the other fixed-length element 34 in the side views of the drawings.

[0057] To define one of the connection axes, the variable-length element 34 of the load rigging is fixed by a hook at its lower end to a radially-protruding lifting shoe 38 at one end of the monopile 28, which is the lower or bottom end when the monopile 28 is upended as shown in FIG. 4. The lifting shoe 38 is on an upper side of the monopile 28 when the monopile 28 is in a horizontal orientation. Thus, the lifting shoe 38 lies on a vertical plane that contains the central longitudinal axis 30 of the monopile 28.

[0058] The two fixed-length elements 36 of the load rigging comprise slings that are fixed at their lower ends to respective trunnions 40 of the monopile 28. The trunnions 40 are diametrically opposed to each other about the monopile 28 on the other horizontal connection axis, which intersects the central longitudinal axis 30 of the monopile 28. Thus, the connection axis that joins the trunnions 40 is orthogonal to the vertical plane that contains both the central longitudinal axis 30 and the lifting shoe 38 of the monopile 28.

[0059] The variable-length element 34 of the load rigging emerges from the spreader frame 24 at a suspension point 42 that is offset laterally from the vertical plane containing the main hook 22, the slew axis 20 and the centreline of the boom 18. The lateral offset of the suspension point 42 from that vertical plane corresponds approximately to the radial offset of the lifting shoe 38 relative to the central longitudinal axis 30 of the monopile 28. Thus, that lateral offset is slightly greater than the outer radius of the base portion 28B of the monopile 28.

[0060] As FIG. 2 shows, the spreader frame 24 comprises a truss structure 44 defining a winch platform that supports a winch 46. The variable-length element 34 of the load rigging comprises a wire 48 extending from the winch 46 and a sling 50 that is joined end-to-end to the wire 48 by a sling coupling 52. The sling 50 extends from the sling coupling 52 to the lifting shoe 38 at the bottom end of the monopile 28.

[0061] In this example, the laterally-offset suspension point 42 of the spreader frame 24 is defined by a sheave 54 that is offset laterally from the centrally-located winch 46. The wire 48 extending between the winch 46 and the sling coupling 52 passes over the sheave 54. However, in principle, it would be possible instead to offset the winch 46 laterally and to dispense with the sheave 54.

[0062] The spreader rigging 26 is fixed to the truss structure 44, arranged symmetrically with respect to the vertical plane 56 that bisects the spreader frame 24 and that also contains the main hook 22, the slew axis 20 and the centreline of the boom 18. The fixed-length elements 36 of the load rigging are also fixed centrally to the truss structure 44.

[0063] The monopile 28 is shown in FIG. 1 in a horizontal orientation, suspended over the side of the vessel 10 above the surface 12, having been lifted in that orientation from a barge (not shown) on which the monopile 28 was transported to the installation site. The clearance between the monopile 28 and the surface 12 ensures that the monopile 28 is not directly subject to wave action at this initial stage.

[0064] As the centre of gravity 32 lies between the trunnions 40 and the lifting shoe 38 at the bottom end of the monopile 28, the weight of the monopile 28 is shared between the elements 34, 36 of the load rigging, which are therefore all in tension. However, as the centre of gravity 32 is much closer to the trunnions 40 than to the lifting shoe 38, the vast majority (about 90%) of the weight of the monopile 28 is borne by the fixed-length elements 36 of the load rigging and the remainder (hence only about 10%) is borne by the variable-length element 34 of the load rigging. The winch 46 of the spreader frame 24 that acts against the tension in the variable-length element 34 therefore needs only to have a correspondingly small capacity.

[0065] FIG. 3 shows the upending operation in progress. It will be apparent that by paying out the wire 48 from the winch 46 and hence lengthening the variable-length element 34 of the load rigging, the bottom end of the monopile 28 is lowered relative to the trunnions 40. Thus, the monopile 28 tilts toward being upright.

[0066] The trunnions 40 remain at a substantially unchanged height above the surface 12 by virtue of the fixed-length elements 36 of the load rigging and so define a nearly-fixed pivot axis as the monopile 28 tilts away from the horizontal. The trunnions 40, suspended on the fixed-length elements 36, merely swing slightly toward the vertical plane that contains the main hook 22, the slew axis 20 and the centreline of the boom 18. This minor movement of the trunnions 40 allows the centre of gravity 32 of the tilting monopile 28 to remain on that vertical plane.

[0067] FIG. 4 shows the upending operation now completed after paying out more of the wire 48 from the winch 46, further to lengthen the variable-length element 34 of the load rigging and hence further to pivot the monopile 28 about the trunnions 40. The monopile 28 is now upright, with its central longitudinal axis 30 substantially vertical and in the vertical plane that contains the main hook 22, the slew axis 20 and the centreline of the boom 18. The trunnions 40 and the fixed-length elements 36 of the load rigging also now lie on that vertical plane, directly above and aligned with the centre of gravity 32 of the monopile 28.

[0068] When the monopile 28 is vertical as shown in FIG. 4, the variable-length element 34 of the load rigging is also close to vertical but is offset laterally from the vertical plane that contains the main hook 22, the slew axis 20 and the centreline of the boom 18. The lateral offset of the variable-length element 34 is determined by the lateral offset of the suspension point 42 defined by the sheave 54 of the spreader frame 24, and also by the radial offset of the lifting shoe 38 relative to the central longitudinal axis 30 of the monopile 28. In practice, the variable-length element 34 need not be perfectly vertical at this stage and may, conveniently, converge downwardly toward the plane of the fixed-length elements 36 as shown in FIG. 4.

[0069] As the monopile 28 reaches verticality, all of its weight is supported by the fixed-length elements 36 of the load rigging via the trunnions 40. Thus, the minor portion of the weight load supported by the variable-length element 34 is transferred progressively to the fixed-length elements 36. When the variable-length element 34 is no longer under tension, it can be detached from the lifting shoe 38. This allows the crane 16 to transfer the upright monopile 28 into a conventional outrigger/gripper of the vessel 10 to be driven into the seabed as noted previously.

[0070] FIG. 4 shows that the spreader frame 24 can tilt relative to the boom 18 of the crane 16 as may be required to balance the system.

[0071] In the orientations shown in FIGS. 3 and 4, the bottom end of the monopile 28 is submerged beneath the surface 12. However, partial submergence of the monopile 28 in this manner is not essential: in principle, with a shorter monopile 28 and/or a higher crane hook 22 or shorter rigging, uprighting could be performed entirely in air.

[0072] Other variations are possible within the inventive concept. For example, the fixed-length elements of the load rigging could be attached directly to the main hook of the crane and not via the spreader frame.

[0073] The load rigging could comprise more than one variable-length element or any number of fixed-length elements.

[0074] It is not essential that a variable-length rigging element is attached to an end of the structure being upended. The variable-length rigging element could instead be attached to the structure at a connection point inboard from its end.

[0075] The structure could be lowered into or onto the water in a horizontal orientation and then upended by paying out a variable-length rigging element. In that case, a lower end of the structure may be flooded with ballasting water either through an open end or a bottom plug.

[0076] In a broad sense, all elements of the load rigging could be of variable length, with the spreader frame supporting two or more winches acting on respective elements. However, it is preferred that a majority of the weight of the structure being upended is suspended from fixed-length rigging, which defines a pivot axis for the structure to be upended under the control of variable-length rigging that supports a minority of the weight of the structure.