Mounting of Offshore Structures

The present invention relates to methods and apparatus for mounting offshore structures, and in particular for mounting offshore structures comprising principally of a relatively tall single column, mast, shaft or tower such as wind turbines. The invention also relates to methods and apparatus for transporting wind turbines and similar offshore structures to their location of use offshore.

BACKGROUND

Mounting large structures such as wind turbines in offshore locations involves a number of complexities. Because of the size of the structures they are inherently difficult to transport and there are significant handling issues in achieving a correct alignment of the structure with its offshore support base. These difficulties are aggravated when the sea conditions are less than ideal. In relatively shallow waters it is possible to use so-called jack-up barges which, when the jacks are extended, provide a stable platform from which the offshore structure can be mounted on its offshore support base. However, it is increasingly required to mount structures such as wind turbines in deeper water where jack-up barges cannot operate. The present invention seeks to provide a method and apparatus which is particularly suitable for the mounting of offshore structures in deeper waters.

BRIEF SUMMARY OF THE DISCLOSURE

According to a first aspect of the present invention there is provided an apparatus for use in mounting an offshore structure on an offshore support base, the apparatus comprising: a first supporting component operatively attachable to a lower part of the offshore structure; a cushioning arrangement operatively disposed between the first supporting component and the offshore support base and operable to hold the offshore structure in spaced apart relation from the offshore support base and to lower the first supporting component carrying the offshore structure until the offshore structure is mounted on the offshore support base.

Preferably the cushioning arrangement comprises

a second supporting component operatively attachable to the offshore support base; a linkage by which the first and second components are connected and operable to permit the first and second supporting components to move with respect to each other; and control means operable to adjust the stiffness of the linkage between a first condition in which the first and second supporting components can move substantially freely and second condition in which relative movement of the supporting components is resisted.

Preferably the linkage comprises: at least one pair of linkage arms, the first of which is pivotally attached at a first end to the first supporting component, the second of which is pivotally attached at a first end to the second supporting component, the respective linkage arms being pivotally attached to one another at a location spaced apart from their respective first ends; and a control device attached to the respective linkage arms and operatively controllable to vary its resistance to relative movement of the first and second linkage arms between a first condition in which the arms can move substantially freely and second condition in which relative movement of the linkage arms is resisted.

Preferably the linkage comprises a plurality of pairs of linkage arms and a respective control device for each pair of linkage arms.

Preferably the control device is a hydraulic piston and cylinder arrangement.

In an alternative embodiment of the first aspect of the invention, the cushioning arrangement comprises a plurality of hydraulic piston and cylinder arrangements and control means operative to control the extension and retraction of the cylinders.

Preferably the first supporting component is shaped to encompass the outer surface of the said lower part of the offshore structure. It is particularly preferred that the first supporting component is substantially annular.

Preferably the second supporting component comprises engagement formations operative to releasably engage, or to be releasably engaged by, corresponding engagement formations of the offshore support base thereby to releasably attach the second supporting component to the offshore support base.

In some preferred embodiments of the invention, the second supporting component is attached to the offshore support base (prior to the mounting of the offshore structure).

In preferred embodiments of the invention the apparatus further comprises a plurality of reaction arms extending upwardly from the first supporting component and each including reaction surface arranged operatively to contact the offshore structure and react against tilting moments thereof.

Preferably the reaction arms are pivotally attached to the first supporting component, the apparatus further comprising respective biasing means operable to urge the respective reaction surfaces towards the offshore structure.

According to a second aspect of the invention there is provided a system for mounting an offshore structure on an offshore support base, the system comprising: a crane arrangement for lifting and lowering the offshore structure; the crane arrangement comprising at least two lifting cables; a spreader beam, a heave compensation assembly disposed between the lifting cables and the spreader beam, and carrying cables extending from the spreader beam; and an apparatus of the first aspect of the invention, the apparatus being attached to the carrying cables.

Preferably, the crane arrangement is mounted on a transport vessel for transporting the offshore structure.

Preferably in the first and second aspects of the invention, the offshore structure is a wind turbine.

Preferably the offshore structure is carried in a substantially upright orientation.

According to a third aspect of the invention there is provided a method of installing an offshore structure on an offshore support base, the method comprising; (i) providing an apparatus comprising: a first supporting component operatively attachable to a lower part the offshore structure; and a cushioning arrangement disposed between the first supporting component and the offshore support base and operably moveable

between an extended configuration in which the offshore structure is in spaced apart relation from the offshore support base and a retracted configuration;

(ii) attaching the offshore structure to the first supporting component; (iii) supporting the offshore structure and the first supporting component with the cushioning means in said extended configuration; and (iv) adjusting the cushioning arrangement to lower the first supporting component such that the offshore rests on the offshore support base.

Preferably in this third aspect of the invention, the cushioning arrangement comprises a second supporting component operatively attachable to the offshore support base; a linkage by which the first and second components are connected and operable to permit the first and second supporting components to move with respect to each other; and control means operable to adjust the stiffness of the linkage between a first condition in which the first and second supporting components can move substantially freely and second condition in which relative movement of the supporting components is resisted, the method comprising, (iii.i) prior to step (iii) above, setting the control means to said second condition, and at step (iv) adjusting the control means towards said first condition.

Preferably the method includes the step of, prior to, or contemporaneously with, step (iii.i), adjusting the control means so that the cushioning means is in an extended configuration with the first supporting component spaced apart from the second supporting component.

Preferably the linkage comprises: at least one pair of linkage arms, the first of which is pivotally attached at a first end to the first supporting structure, the second of which is pivotally attached at a first end to the second supporting structure, the respective linkage arms being pivotally attached to one another at a location spaced apart from their respective first ends; and the control means comprises a control device attached to the respective linkage arms and operatively controllable to vary its resistance to relative movement of the first and second linkage arms between a first condition in which the arms can move substantially freely and second condition in which relative movement of the linkage arms is resisted.

Preferably the method comprises, in step (ii) the steps of:

(ii.i) attaching a lower part of the offshore structure to the first supporting component and, subsequently,

(ii.ii) attaching the cushioning arrangement to the offshore support base.

Preferably in the step (ii.ii), the second supporting component is attached to the offshore support base.

Preferably the second supporting component is releasably attached to the offshore support base.

Preferably, in one embodiment of this aspect of the invention, the cushioning arrangement of said apparatus is previously attached to said offshore support base, the method comprises at step (ii) receiving the lower part of said offshore structure in the first supporting structure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made a way of example only to the following drawings in which:

Figure 1 A is a front view of an apparatus according to the invention;

Figure 1 B is a side view of the apparatus in Figure 1 A;

Figure 2 is an enlarged view of the apparatus of Figure 1A;

Figure 3 is a side view of the apparatus of Figure 2;

Figure 4 shows an apparatus according to the invention attached to an offshore structure and approaching an offshore support base;

Figure 5 shows apparatus according to the invention disposed on the offshore support base in one configuration;

Figure 6 shows the apparatus of Figure 5 in another configuration;

Figure 7 shows an alternative form of the apparatus according to the invention;

Figure 8 shows apparatus according to Figure 1 when mounted on an offshore support base;

Figure 9 shows another variation of the apparatus according to the invention;

Figure 10 illustrates one embodiment of the invention including latching means for attaching the apparatus to the offshore support base; and

Figure 11 shows the apparatus of Figure 10 in the latched condition.

DETAILED DESCRIPTION

Referring now to the drawings, and the apparatus 10 of the invention is used for transporting an offshore structure more especially an offshore structures comprising principally a relatively tall single column, mast, shaft or tower, such as wind turbines, to its location of use and for mounting the structure on its offshore support base. The apparatus 10 comprises a first supporting component 12 which, in use, is attached to the offshore structure 14 at a lower part thereof, and preferably proximate its base. The first supporting component 12 preferably encompasses the outer peripheral surface of the offshore component 14. Typically, the offshore structure is a wind turbine the external wall of which is generally circular in cross-section. Preferably therefore the first supporting component 12 is substantially annular.

The apparatus 10 also comprises a cushioning arrangement which in the embodiment illustrated in Figures 1 to 6 comprises a second supporting component 16 and linkage means 18 which unites the first and second supporting components 12, 16.

The linkage means 18 is operable to permit the first and second supporting components 12, 16 to have freedom of movement with respect to one another in both the vertical and horizontal directions. The linkage means 18 also comprises control devices 20 which are operable to control the relative movement on the first and second supporting components 12, 16. In the illustrated embodiment, the control devices 20 comprise

hydraulically operated piston and cylinder arrangements. The linkage means comprises at least one pair of arms 22, 24. Preferably, the linkage means 18 comprises a plurality of pairs of arms 22, 24 which may be equidistantly spaced around the offshore structure 14. First arms 22 are connected that their first end to the first supporting structure 12 and may pivot with respect to the first supporting structure 12. Second arms 24 are connected at their first end to the second supporting structure 16 and may pivot with respect to the second supporting structure 16. The respective first and second arms are pivotally joined together at joint 26.

Thus, by increasing the hydraulic pressure in the cylinders of the piston and cylinder arrangements 20 the pistons can be extended so that the first and second supporting components are, or become, spaced apart. Also, the hydraulic pressure in the piston and cylinder arrangement can be used to limit, restrict or prevent relative movement between the first and second supporting components 12, 16. For example, when the offshore structure 14 is being moved to its location of use it may be subject to turning moments and other motions from external causes such as movement of the transporting vessel in response to wave action. By choosing an appropriate stiffness of the pistons and cylinder arrangement 20 movement of the offshore structure 14 can be resisted.

The apparatus 10 preferably further comprises a plurality of reaction arms 40 extending upwardly. Preferably the reaction arms 40 depend from the first supporting component 12. Each reaction arm 40 includes a reaction surface 42 which may be brought to bear against the outer peripheral surface of the offshore structure 14. The reaction arms 40 may be fixedly attached to the first supporting component 12 or, more preferably, may be pivotally attached thereto. In the latter case, a hydraulic control device such as hydraulic piston and cylinder arrangement 44 (figure 7) may be disposed between the reaction arm 40 and the first supporting component 12. Thus, the reaction arms 40 provide a means of controlling and limiting the extent of any rotation or tilting of the offshore structure 14 with respect to the first supporting component 12, such as may be caused by the rolling or pitching motion of the transport vessel 33 at sea. The movement of the reaction arms 40 may be independently controlled by hydraulic control devices 44, and/or the reaction arms 40 and the control devices 44 may be interlinked. As illustrated in Figure 9, the respective reaction arms 40 may be linked by a linking ring 46 at the upper ends 4OA. The linking ring 46 may be segmented with individual segments allowed to move with respect to each other under suitable control means, such as hydraulic control means. Alternatively, the linking ring 46 may be rigid so that

the respective reaction arms 40 are maintained in fixed relation to one another. The reaction arms 40 are particularly useful in controlling that motion of the offshore structure 40 attributable to the movement of the transport vessel 33 as the offshore component 14 is mounted on to the offshore base 28.

The piston and cylinder arrangements 20 can operatively be adjusted to a condition which is sufficiently stiff so that the apparatus 10 can be made to support the load of the offshore structure 14.

In use of the apparatus 10 of the invention, the apparatus 10 may be initially attached to the offshore structure 14. In an alternative arrangement, the apparatus 10 may initially be mounted on the offshore support base 28.

In the embodiment illustrated in figures 1 to 6, the apparatus 10 is initially attached to the lower portion of the offshore structure 14. Suitably, the apparatus 10 may be bolted to the lower portion of the offshore structure 14. The apparatus 10 is employed in conjunction with a system 30 for carrying and moving the offshore structure 14 and the apparatus 10. The system 30 includes a crane arrangement 32 on which the offshore structure 14 is carried and which is operable to raise and lower the offshore structure 14. The crane arrangement 32 is mounted on a transport vessel 33. The crane arrangement 32 comprises a plurality of lifting cables 34 and means (not shown) for moving the lifting cables 34 to raise and lower the offshore structure 14. The crane gantry, cables and lifting means may be conventional. A spreader beam 36 is disposed below the lower ends of the lifting cables 34 and a heave compensation assembly is interposed between the lower ends of the lifting cables 34 and the spreader beam 36. The heave compensation assembly may be active or passive. Preferably, the main body of the heave compensation assembly, principally comprising accumulators, is disposed on the spreader beam 36 which has the advantage of reducing the lengths of requisite hydraulic hoses and hence reducing hydraulic losses. Carrying cables 38 extend from the spreader beam 36 to the apparatus 10.

The procedure for mounting the offshore structure 14 on the offshore support base 28 is as follows. The offshore structure 14 and apparatus 10 (which is attached by the first support component 12 to the lower portion of the offshore structure 14) are carried for transport by the crane arrangement 32 on the vessel 33. It is advantageous in this respect that the offshore structure 14 is maintained in its upright condition. The vessel

33 approaches the offshore support base 28 is manoeuvred until the offshore structure 14 and the apparatus are above the offshore support base 28. At this stage, the linkage 18 and its control mechanism 20 are in a condition such that the first and second supporting components are free to move with respect to one another. In other words, the linkage 18 and its control mechanism 20 do not offer significant resistance to relative movement of the first and second support components 12,16. The first and second support components 12, 16 are thus spaced as far apart as the linkage 18 permits. The heave compensation assembly is activated. The crane arrangement 32 then lowers the offshore structure 14 until the cushioning arrangement (in the illustrated embodiments of Figures 1 to 5, the lower supporting component 16) rests on the offshore support base 28. The lower supporting component 16 is attached to the offshore support base 28. This may be done manually or, more preferably, automatically such as by latching means. An example of a latching arrangement is shown in Figures 10 and 11. In Figure 10 latches 48 are shown in an open condition ready to receive the lower, second, supporting component 16. When the second supporting component 16 is received on the offshore support base 28, the latches 48 move to the position shown in Figure 11 , to thereby retain the second supporting component 16 on the offshore support base 28. Actuators 50 may be provided to move the latches 48 to and/or from the retaining position. The second supporting component 16 is preferably annular and the offshore support base 28 may include a correspondingly shaped boss 52 about which the second supporting component sits.

After the second supporting component 16 has been attached to the offshore support base 28, the crane arrangement 32 is used to raise the offshore structure 14 until the cushioning arrangement is in its extended condition, that is, until the first and second supporting components 12, 16 are spaced apart to their maximum extent. The control arrangement is then operated to increase the hydraulic pressure in the piston and cylinder arrangements 20, typically to a maximum, so that the linkage 18 achieves a stiffness sufficient to support the load of the offshore structure 14. The offshore structure 14 is then lowered as necessary by the crane arrangement 32 so that the load is transferred from the crane arrangement 32 to the apparatus 10. At this time, the reaction arms 40 are used to control or restrain movement of the offshore 14 arising from, for example, movement of the vessel 33 in the prevailing sea conditions. The piston and cylinder arrangements 20 are then controlled to allow the first supporting component 12 to move downwardly, thereby lowering the offshore structure 14 into its position of use on the offshore support base 28. The offshore structure 14 is then

secured to the offshore support base 28. The above process is reversible and may be used to remove the offshore structure 14 from the offshore support base 28, such as for maintenance, repair or replacement.

In a variation of the invention, the apparatus 10 is not initially attached to the lower portion of the offshore structure 14. Rather, the apparatus 10 is mounted, permanently, semi-permanently or removeably on the offshore support base 28. Thus, the procedure for mounting the offshore structure 14 on the offshore support base 28 differs in some respects. The offshore structure 14 is carried by the crane arrangement 32 on vessel 33 to the offshore support base 28 and the vessel 33 and crane arrangement 32 are manoeuvred to position the offshore structure 14 above the apparatus 10 on the support base 28 (Figure 8). The heave compensation assembly is activated. At this stage, the linkage 18 and its control mechanism 20 may be in a condition such that the first and second supporting components are fee to move with respect to one another. In other words, the linkage 18 and its control mechanism 20 do not offer significant resistance to relative movement of the first and second support components 12,16. As shown in Figure 8, the linkage 18 may be provided with sufficient stiffness to raise the first supporting component 12 above the second supporting component 16. The offshore structure 14 is then lowered until it engages the apparatus 10, preferably entering the first supporting component 12. As the offshore structure 14 engages the apparatus 10, the linkage 18 through its control mechanism 20 is stiffened so that load is transferred from the crane arrangement 32 to the apparatus 10, until the offshore structure 14 is supported by the apparatus 10. At this time, the reaction arms 40 are used to control or restrain movement of the offshore 14 arising from, for example, movement of the vessel 33 in the prevailing sea conditions. The piston and cylinder arrangements 20 are then controlled to allow the first supporting component 12 to move downwardly, thereby lowering the offshore structure 14 into its position of use on the offshore support base 28. The offshore structure 14 is then secured to the offshore support base 28. The above process is reversible and may be used to remove the offshore structure 14 from the offshore support base 28, such as for maintenance, repair or replacement.

Figure 7 shows a further alternative embodiment of the invention in which the second supporting component is absent. The first supporting component 120 is attached to the offshore support structure 28 through a hydraulic cushioning arrangement 60. As the offshore structure 14 is lowered by the crane arrangement 32, it engages the first supporting component 120. The reaction arms 40, whose movement is controlled

through hydraulic piston and cylinder arrangements 44 control tilting and other movements of the offshore structure 14 as it approaches the first supporting structure 120. When the offshore structure 14 is received by the first supporting component 120, the hydraulic arrangement 60 cushions the movement of the offshore structure 14 as it is brought to rest. The offshore structure 14 may then be attached to the offshore support base 28.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.