GB2498037A - Lifting system for an offshore wind turbine - Google Patents

Abstract

A lifting system, for a wind turbine on an offshore site, comprises an elongate primary boom 106 pivotally mounted at its proximal end to a wind turbine tower base. At least two elongate secondary booms are pivotally mounted at their proximal ends. First and second flexible members (cables) 302, 306 connect the distal ends of each secondary boom to the primary boom, and third and fourth flexible members 301, 304 connect the distal ends of each secondary boom to a mount. First and second adjustable counterweights 112, 114 are connected to the distal ends of each secondary boom. A method of erecting a wind turbine using the lifting system comprises the primary boom being elevated, from a substantially horizontal position, to a substantially vertical position by adjusting the counterweights. The wind turbine is connected to a hoist on a distal end of the primary boom, and is hoisted to a desired position by adjusting the counterweights. The relative lengths of the flexible members may be adjusted by at least one winch to elevate and hoist the wind turbine.

Description

Lifting System and Method for Erecting an Offshore Wind Turbine

Field of the Invention

The present invention relates to a lifting system for a wind turbine on an offshore site. The invention also relates to a method of erecting a wind turbine on an offshore site.

Background of the Invention

The erection of modern offshore wind turbines often requires highly specialised cranes that are able to lift wind turbine components to significant heights above sea-level.

io The weight of the wind turbine and its associated components, and the changing conditions of the surface of the sea also provide particular problems that increase the complexity of the operation.

Due to the nature of erecting offshore wind turbines, it is common to mount a crane on a vessel, and conduct the lifting operation from the vessel. In this case, the wind turbine may also be transported to the offshore site on the crane vessel, or on a separate vessel.

The known systems require a high level of skill of the crane operators in order to control the lifting of the heavy wind turbine from a vessel that is moving relative to the offshore wind turbine foundation. To reduce the complexity and increase the safety of the operation, the crane operators often require very calm sea conditions for the duration of the lift.

The present invention seeks to provide a lifting system, and associated method which simplifies the process of erecting an offshore wind turbine.

Summary of the Invention

According to one aspect of the present invention, there is provided a lifting system for a wind turbine on an offshore site. The lifting system comprises: an elongate primary boom, adapted to be pivotally mounted at its proximal end to a wind turbine tower base; at least two elongate secondary booms, wherein the proximal ends of the at least two secondary booms are pivotally mounted such that the axis of rotation of the primary boom and the axes of rotation of the secondary booms are substantially parallel; a first flexible member connecting the distal end of the first secondary boom to the primary boom; a second flexible member connecting the distal end of the second secondary boom to the primary boom; a mount, adapted to be mounted to a wind turbine base; a third flexible member connecting the distal end of the first secondary boom to the mount; a fourth flexible member connecting the distal end of the second secondary boom to the mount; a first adjustable counterweight connected to the distal end of the first secondary boom; and a second adjustable counterweight connected to the distal end of the second secondary boom, wherein, in use, one or more winches are used to adjust the position of the primary boom, and the first and second counterweights rotates at least the secondary boom about its proximal end.

Providing such a system allows wind turbines to be more easily erected in offshore sites. Furthermore, modifying the position of the lifted wind turbine may be accomplished more easily.

The lifting system is preferably adapted for use with a pre-installed wind turbine base structure. The base structure may comprise foundations in the sea-bed and a first portion of a tower of a length such that the top protrudes above the surface of the sea.

is Preferably, the secondary booms are positioned on opposite sides of the primary boom. By positioning the secondary booms on opposite sides of the tower, the counterweights are able to more effectively stabilise the primary boom.

Preferably, the at least two secondary booms are pivotally mounted such that the axis of rotation of the primary boom and the axes of rotation of the secondary booms are substantially coincident.

Preferably, the third and fourth flexible members are connected to the mount, spaced apart from the axes of rotation of the primary boom. By spacing apart the connection point from the axes of rotation a more stable lifting system may be provided.

Preferably, the first and second flexible members connect the secondary booms at a connection point adjacent to the distal end of the primary boom. More preferably, the connection point is between approximately 20% and approximately 50% of the length of the primary boom from the distal end of the primary boom. Yet more preferably, the connection point is between approximately 30% and approximately 40% of the length of the primary boom from the distal end of the primary boom. By connecting the first and second flexible members at adjacent the distal end of the primary boom greater control over the primary boom, and hence the wind turbine, may be provided.

The mount may comprise: first and second pairs of mounting members attached to a base for a wind turbine, each pair having a mounting member on opposite sides of the base for a wind turbine; and first and second pairs of corresponding engaging portions, each engaging portion being adapted to engage with a respective mounting member.

Preferably, the first and second pairs of mounting members are spaced apart along the longitudinal axis of the base. The term "longitudinal" as used herein connotes the direction along the wind turbine tower from its base to the nacelle.

Preferably, each engaging portion comprises a locking portion adapted to secure the engaging portion to the mount. Each engaging portion may comprise a female engaging portion adapted to engage with the mount. Preferably, the mounting members of each pair of mounting members are arranged on opposite sides of the wind turbine tower. The mounting members may each be substantially cylindrical. The engaging means may comprise at least two female engaging portions each adapted to engage with the mount and at least two corresponding locking portions adapted to secure the engaging portions to the mount. Preferably, the pivot axis of the primary boom is substantially coincident with the axes of the first pair of the substantially cylindrical members.

Preferably, the third and fourth flexible members are connected to the mount at the second pair of mounting members. By connecting the third and fourth flexible members to the mount at the second pair of members a more stable lifting system may be provided.

is The lifting system may further comprise a winch adapted to act on the first flexible member, and a winch adapted to act on the second flexible member. The lifting system may also further comprise a winch adapted to act on the third flexible member, and a winch adapted to act on the fourth flexible member. The winches are adapted to modify the position of the primary boom. By winching in, and winching out respective flexible members the relative positions of the primary and secondary booms can be modified. The winches are preferably adapted to maintain tension within the flexible members during operation of the lifting system in order to improve the stability of the lifting system. The winches are preferably attached to the mount. By attaching the winches to the mount the mass of the primary and secondary booms may be minimised. In addition, because the winches are not supported on the booms, the required structural rigidity of the booms may be reduced.

The first and third flexible members may be a first integral flexible member, and the second and fourth flexible members may be a second integral flexible member. The distal ends of the first and second secondary booms may comprise respective first and second pulleys, the first and second pulleys being adapted to receive the first and second integral flexible members respectively. The first and second pulleys preferably each comprise a controllable brake. By providing such integral flexible members, the complexity of the system may be reduced as only two winches are required, one each for the first and second integral flexible members.

The flexible member may be: a cable, such as a high tension steel cable; a chain; a rope, such as a steel rope; a wire; or any other suitable type of flexible member as will be readily apparent to the skilled person.

The primary boom preferably comprises two elongate spars, and at least one cross member connecting the elongate spars at the distal end. By providing such an arrangement the lifting system may be provided with the advantage of allowing the load being lifted by the system to be suspended directly below the cross member, and between the spars.

Where the primary boom comprises two spars, each secondary boom also comprises two spars. Each spar of each secondary boom may be independent. Each spar of the first secondary boom preferably comprises a first adjustable counterweight, and each spar of the second secondary boom preferably comprises a second adjustable counterweight.

Preferably, each of the said first adjustable counterweights are linked, such that the masses of each first adjustable counterweight remain substantially equal, and preferably each of the said second adjustable counterweights are linked, such that the masses of each second adjustable counterweight remain substantially equal. By linking the counterweights the lifting system may be more stable during operation.

The lifting system preferably further comprises at least one pump adapted to adjust a is quantity of water stored in the respective counterweights. The pump is preferably adapted to transfer water between the first adjustable counterweight and the second adjustable counterweight.

Preferably, the boom comprises a hoist at the distal end. The hoist is preferably adapted to be able to lift a complete wind turbine comprise a tower portion, a nacelle, and a rotor hub and associated blades. Alternatively, the hoist may be adapted to be able to lift a wind turbine in at least two portions. The portions may be the wind turbine tower, and the nacelle and rotor hub and associated blades.

According to a further aspect of the present invention, there is provided a method of erecting a wind turbine on an offshore site. The method comprises: pivotally mounting the proximal end of an elongate primary boom of a lifting system at the base of a wind turbine tower, the boom being in a substantially horizontal position; pivotally mounting the proximal end of at least two elongate secondary booms of a lifting system at the base of a wind turbine tower, each secondary boom having: first and second adjustable counterweight connected to a respective distal end; first and second flexible members connecting the respective distal ends of each secondary boom to the primary boom; and third and fourth flexible members connecting the respective distal ends of each secondary boom to a mount. The method further comprises elevating the primary boom to a substantially vertical position by adjusting the first and second counterweights; connecting a wind turbine located on a vessel to a hoist on a distal end of the primary boom; hoisting the wind turbine from the vessel, and modifying the position of the hoisted wind turbine to a desired erecting position by adjusting the first and second counterweights; and disconnecting the wind turbine from the hoist.

Preferably, the method further comprises adjusting the relative lengths of the flexible members to elevate the primary boom. At least one winch may be utilised to adjust the relative lengths of the flexible members.

Furthermore, the method preferably further comprises adjusting the relative lengths of the flexible members to modify the position of the hoisted wind turbine. Similarly to the method step for elevating the primary boom, to modify the position of the hoisted wind turbine, the relative lengths of the flexible members may be adjusted by winching.

Preferably, the first and second counterweights are adjusted by adjusting a quantity of water stored in the respective counterweights. The water is preferably transferred between the counterweights. Alternatively, sea-water may be utilised. In this case, the water being pumped to a counterweight may be taken directly from the sea, and the water being pumped from the counterweights may be pumped directly back into the sea. As such, the quantity of water may be adjusted by pumping sea-water to and from the counterweights.

is The invention extends to methods and apparatus substantially as herein described with reference to the accompanying drawings.

As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

Brief description of the Figures

These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which: Figure 1 shows a vessel with a lifting system according to the present invention approaching a wind turbine tower base; Figure 2 shows the vessel of Figure 1 engaging with the wind turbine tower base; Figure 2a shows the magnified portion of Figure 2 in more detail; Figures 3 and 4 show the primary boom of the lifting system being hoisted into an operational position from the deck of the vessel; Figure 5 shows the vessel retreating from the wind turbine tower base, and the boom secured in position on the base; Figure 6 shows a secondary vessel approaching the lifting system with a wind turbine; and Figure 7 shows the lifting system erecting the wind turbine.

Detailed Description of Preferred Embodiments

Figure 1 shows a vessel 100 with a lifting system approaching an offshore wind turbine tower base 102. As shown in the Figure, the tower base 102 is a column extending, for example, from the sea-bed to just above sea-level. The tower base 102 comprises foundations in the sea-bed, and a tower mounted on the foundations and extending to a position just above sea-level. The vessel 100 carries the lifting system 104 to the wind turbine tower base 102 for installation.

The lifting system 104 comprises a primary boom 106, secondary booms 108 and 110, counterweights 112, 114, 116 and 118, mount 120 and flexible members 300 connecting the booms to each other and to one or more winches (as will be described later). In this example, the tower 102 comprises two sets of mount members 122 adapted to receive the mount 120 of the lifting system in its installed position. In Figure 1, the lifting system 104 is shown in its stored position; i.e. the position utilised for transporting the lifting system to the offshore wind site. In this position, the counterweights are substantially void of ballast. Alternatively, the counterweights 112 and 116 may be provided with sufficient ballast to ensure the lifting system is stable during transportation.

The primary boom 106 comprises two spars 124 and 126, and a cross member having 128 a hoist 129 for lifting a load, such as a wind turbine that is to be mounted on the tower base 102. The joints between the cross member and the spars may be reinforced with bracing to improve the racking strength of the primary boom.

Correspondingly, the secondary booms 108 and 110, comprise respective spar pairs 130 and 132, and 134 and 136. On each of the spars 130, 132, 134 and 136 a respective counterweight 112, 114, 116 and 118 is mounted. In this example, each of the secondary booms 108 and 110 is mounted on the outside of the primary boom 106.

Figure 2 shows the vessel 100, and the lifting system 104, engaging with the wind turbine tower base 102. Figure 2a is a further enlarged version of the magnified portion of Figure 2 showing the engagement with the tower 102 in more detail. The base 102 comprises mounting members 122 for engaging with the mount 120 of the lifting system 104. The mounting members are formed by two sets of two substantially cylindrical members 122 extending from the wind turbine base 102 on opposite sides. One set of mounting members is located above the other. Each mounting member may be welded directly to the wind turbine base, or may be connected to a bracket that is attached to the wind turbine base 102 via bolts or welding for example.

The lifting system mount 120 comprises a collar 200 having two sets of engaging portions 202. Each engaging portion 202 comprises a female engaging portion or shoulder adapted to engage with one of the cylindrical members 122. In this example, the mount collar 200 comprises two vertically extending beams 204 and horizontal cross beams 206 to brace the vertically extending beams 204 and provide structural support. In use, the collar is coupled to the wind turbine tower base 102 with the vertically extending beams 204 located adjacent either side of the wind turbine tower 102. The engaging portions 202 are located on the vertically extending beams 204 such that when the collar 200 is in place, the two engaging portions 202 on one of the vertically extending members 204 will engage is respective cylindrical mounting members 122 on one side of the tower base 102.

In order to secure the mount 120 to the wind turbine base, each engaging portion is also provided with a locking portion or clamp 208. The locking portion 208 is pivoted about one end, and is adapted to hook around the cylindrical member 122. Each locking portion 208 is provided with a hydraulic ram that rotates the locking portion from an unlocked, open, position, to a locked, closed, position. Alternatively, a single hydraulic ram may be provided that acts simultaneously on each locking portion. By hooking the locking portion 208 around the cylindrical members 122, the cylindrical members are held securely between the female engaging portions 202 and the locking portions 208. In this way, the mount 120 is secured to the wind turbine baselo2, and the lifting system 104 can be free-standing while attached to the wind turbine base; i.e. it does not require any additional tethering or attachment to the vessel 100 or the wind turbine base 102. As shown in Figure 2a, in order to provide resistance to rotational, torque, forces on the mount the two sets of engaging portions 202 are located in vertically spaced apart positions on the vertically extending members of the mount 120. The vertical distance between the sets of engaging portions 202 and the corresponding mount members 122 is determined based on the size of the wind turbine to be erected utilising the system, and hence on the loads to be lifted by the lifting system 104.

The higher the load to be lifted, the greater the required distance between the engaging portions and the mount members.

Also connected to the vertically extending member 204 is a pivot 210 on which the spars of the primary 106 and secondary booms 108, 110 are mounted. As shown in Figure 2a, spar 126 of the primary boom is mounted closest to the vertically extending member 204, with spars 130, and 136 mounted externally and on the same pivot. In one example, the pivot is implemented as a single common axle, on which the respective spars are threaded.

Winches 212 are also mounted on the collar 200 by a stepper bar 214 attached to the vertically extending member 204. The winches 212 receive one or more of the flexible members (cables) mentioned above 300 that aid in positioning the primary and second booms when in use.

Figure 3 shows more clearly the arrangement of flexible members 300 and counter weights. Counter weights 112 and 114 are located at respective ends of the spars 130 and 132, the spars 130 and 132 forming the secondary boom 108 can rotate around the pivot under the influence of the torque from the counter weights 112 and 114. In this example, the ballast in counterweights 112 and 114 is a fluid, and a pipe 308 comprising a pump and a two way valve is used to connect the counterweight 112 with counter weight 114. Ballast can then be transferred between the counterweight 112 and the counterweight 114 using is the pipe 308 and pump to rotate the secondary boom 108 as desired around the pivot 210.

Secondary boom 110 has an identical structure with spars 134 and 136 as well as counter weights 116 and 118, on the opposite side of the primary boom 106 to secondary boom 108. Secondary boom 110 is not shown in Figure 3.

The flexible members 300 are in the form of a cable, such as a high-tension steel cable, connected to the winch 212. From there, the cables 300 pass over pulleys mounted on the end of secondary spars 132 and 136 to a terminating position on the primary spar 124, 126. In Figure 3, the cable 300 is represented as four separate cable lengths 301, 302, 304, and 306, to indicate that a single cable or a plurality of cables could be used to the same effect. Where a single cable is used, single winch 212 is sufficient to drive the cable system. Where separate cables are used, separate winches will need to be provided accordingly. Gable brakes may also be provided at the winch 212 and on the spars (where the cables pass through spar mounted pulleys) in order to lock the cables and the primary and secondary booms in place.

As will be understood from Figure 3, controlling the winch 212 to tighten the cable sections 301 and 302 on the right of the primary boom 106, and slacken the cables 304 and 306 on the left, allows the primary boom 106 to be pulled clockwise and upwards against the support structure of the secondary boom 108, 110. Similarly releasing the tension in cables 301 and 302, and increasing the tension in cables 304 and 306, allows the primary boom 124, 126 to be lowered anticlockwise in a controlled fashion. The secondary booms 108 and 110 therefore provide a support structure against which the winch 212 and cables 300 can act to position the primary boom in a variety of positions. In order to provide this stability, the counterweights 112 and 114 are filled with ballast, such as sea water. As illustrated in Figures 4, this arrangement also allows the primary boom 106 to pass the over centre position without any loss of stability.

The position of the secondary booms 130, 132 can be carefully controlled using the adjustable ballast of the counter weights 114 and 118. Figure 3, shows the secondary booms 130 and 132 in an initial position. In this position, one spar 130 is located over the vessel 100 close to the vessel deck with the counterweights 112 and 116 in a position on the deck. The corresponding spar 132 at the front of the boom 108 and the counterweight 114 is in a raised position. Water, advantageously sea water, can be pumped into the counter weight 112 while it is situated on the deck of the ship in order to provide the ballast.

The pump in pipe 308 can then be operated to transfer ballast to counterweight 114. The size of the counterweights is such that when sufficient ballast has been transferred to one of the counterweights, the secondary boom 108 will be rotated around the pivot 210. The operation of the other secondary boom 110 is identical.

is Also, as can be seen in Figure 3, if the cables 300 are locked in place with a winch brake, the counterweights will also act to provide a force acting on the primary boom. In Figure 3, for example, if cables 300 are unlocked then the winch 212 can be used to lift the primary boom 106 against the secondary boom. In this arrangement, the inertia of the secondary boom and the counterweights is assumed to be sufficient to provide a stable base. Also, if the a winch brake is used to lock the cable 300 in place, then adding ballast to counterweight 114 will provide an additional force acting on the primary boom 106 to pull it into an upright position. Using the counterweights allows fine positioning of the primary boom without requiring the use of the winch 212. This can be helpful for example if the primary boom is bearing a heavy load and the winch would have to work hard to perform the lift. As will be appreciated therefore, the action of the secondary boom provides an enhanced lifting effect for assisting the lifting of the primary boom, as well as a stable structure of the initial lift of the primary boom.

The operation of the lifting system 104 will now be described with reference to Figures 2, 3 and 4. Figure 2 shows the Hfting system in an initial position in which the primary boom 106 and secondary booms 108, 110 are in a collapsed or stowed position against the deck of the vessel 100. In this position, the counterweights 112 and 116 (on the deck of the vessel 100) are preferably loaded with ballast and the cables 300 are locked into place using a winch brake so that the spars 132 and 136 of the secondary booms 108, remain lifted in a high position and cannot rotate towards the sea surface.

The vessel approaches the wind turbine tower base 102 with the secondary booms 108 and 110 passing on either side until the mounting collar can be engaged. The lifting system 104 is then attached to the base 102 using the engagement portions 208 acting on mounts 122.

Figure 3 shows the primary boom 106 of the lifting system being hoisted into operational position from the deck of the vessel 100. The flexible members 301 and 302 are winched in using winch 212 and pass the pulley at the distal end of the spars 132 and 136.

Ballast is pumped into counterweights 114 and 118 on the end of spars 132 and 136 so that there is sufficient ballast weight against which the primary boom 106 can rotate. The primary boom 106 rotates (clockwise, as shown) in favour of the secondary booms 132 and 136 rotating (anti-clockwise, as shown). The cables 304 and 306 are correspondingly winched out to allow both the primary boom 106 and the secondary boom 108 to rotate (clockwise, as shown) about their respective proximal ends. The secondary booms 108 and rotate because of the redistribution of ballast.

Figure 4 shows the lifting system in an operational position, ready to begin lifting a load such as a wind turbine. The primary boom 106 is positioned over-centre and thus the is counterweight ballast has been adjusted to provide more ballast in counterweights 112 and 116 to compensate.

In this stable position, the vessel 100 may move away from the wind turbine base as shown in Figure 5 as it will not be required for the subsequent lifting process. Figure 6 next shows a transport vessel 500 carrying a wind turbine moving into a position where the wind turbine can be lifted by the lifting system 104 and positioned on the wind turbine base. The transport vessel 500 docks with the wind turbine tower base as before.

The lifting operation may be conducted as follows, and with reference to Figure 7.

The wind turbine is attached to the hoist 129 on the primary boom 106, and lifted from the deck of the transport vessel 500. 1iile lifting the wind turbine the ballast is adjusted by pumping water between the counterweights to maintain the stability of the lifting system 104, and minimise the tension in the cables 300. Where necessary, the cables 300, 302, 304 and 306 are winched to maintain the stability of the lifting system. Once lifted from the support vessel 500, the position of the primary boom 106 is modified to erect the wind turbine on the wind turbine base 102. As described above, the position is modified by adjusting the counterweights 112, 114, 116 and 118 and the relative lengths of the cables 301, 302, 304 and 306. In particular, the following process is utilised to move the lifted wind turbine from the position shown in Figure 6 via the lifted position shown Figure 7, to the position where the wind turbine is mounted on the wind turbine base: a) the wind turbine is lifted from the deck of the vessel 500 using the hoist 129 to a position where the bottom of the wind turbine tower is higher than the top of the wind turbine base 102; during lifting, ballast is added to counterweights 112 and 116 to counter-act the weight of the wind turbine; b) the wind turbine is then moved to a position over the wind turbine base by adjusting the counterweights 112, and 116 and 114, and 118, and the relative lengths of the cables 300, 302, 304 and 306. This moves the primary boom 106 backwards so that it is now in an over centre position. To do this the lengths of cables 304 and 306 are shortened utilising the winch, the lengths of cables 301 and 302 lengthening, while ballast is added to counterweights 112 and 116. Wtiile doing this, the primary boom is therefore caused to rotate about its proximal end, while the secondary booms remain substantially stationary c) the wind turbine is then lowered by the hoist 129 into position on the wind turbine base, and then secured in position; d) the hoist 129 is then disconnected from the wind turbine.

Once erected, and the hoist is disconnected from the wind turbine, the vessel 500 retreats from the wind turbine. The vessel 100 is then repositioned adjacent the wind is turbine to retrieve the lifting system 104, and the lifting system 104 is then lowered to the deck of the vessel 100 by reversing the process described above.

Modifying the position of the various booms can be controlled manually by operating the winch controls, and counterweight controls individually. However, it is preferred that a control system is provided, such that the user merely indicates that the position of the lifting system need be modified, and the control system provides the required inputs to the winches, counterweights and pulley brakes to modify the position accordingly. In this preferred embodiment, the control system is provided with safety limits to prevent the lifting system from becoming unstable.

As will be appreciated, each secondary boom 108, 110 may move as independent unit. However, it is preferred for stability of the two booms operate in tandem with one another, both therefore being controlled so that they adopt corresponding position on either side of the primary boom. Without this, the lifting system may become unstable as the forces could be unbalanced. Furthermore, it is possible that the spars of the respective secondary booms can move with respect to one another, essentially allowing the spars of the secondary boom to adopt any position desired. This can be achieved if the tension of the cables, and counterweights are carefully controlled.

The system preferably comprises cable brakes in order to improve the control over the lifting system. However, in an alternative example, each of the cables 300, 302, 304 and 306 are provided with a winch, and are winched independently to control the relative positions of the primary and secondary booms.

Claims (1)

1. <claim-text>CLAIMSA lifting system for a wind turbine on an offshore site, comprising: an elongate primary boom, adapted to be pivotally mounted at its proximal end to a wind turbine tower base; at least two elongate secondary booms, wherein the proximal ends of the at least two secondary booms are pivotally mounted such that the axis of rotation of the primary boom and the axes of rotation of the secondary booms are substantially parallel; a first flexible member connecting the distal end of the first secondary boom to the primary boom; a second flexible member connecting the distal end of the second secondary boom to the primary boom; a mount, adapted to be mounted to a wind turbine base; a third flexible member connecting the distal end of the first secondary boom to the mount; a fourth flexible member connecting the distal end of the second secondary boom to the mount; a first adjustable counterweight connected to the distal end of the first secondary boom; and a second adjustable counterweight connected to the distal end of the second secondary boom, wherein, in use, one or more winches are used to adjust the position of the primary boom, and the first and second counterweights rotates at least the secondary boom about its proximal end.</claim-text> <claim-text>2. A lifting system according to Claim 1, wherein said secondary booms are positioned on opposite sides of the primary boom.</claim-text> <claim-text>3. A lifting system according to Claim 1 or 2, wherein the at least two secondary booms are pivotally mounted such that the axis of rotation of the primary boom and the axes of rotation of the secondary booms are substantially coincident.</claim-text> <claim-text>4. A lifting system according to Claim 1, 2 013, wherein the third and fourth flexible members are connected to the mount spaced apart from the axes of rotation of the primary boom.</claim-text> <claim-text>5. A lifting system according to any of Claims 1 to 4, wherein the first and second flexible members connect the secondary booms at a connection point adjacent to the distal end of the primary boom.</claim-text> <claim-text>6. A lifting system according to Claim 5, wherein said connection point is between approximately 20% and approximately 50% of the length of the primary boom from the distal end of the primary boom.</claim-text> <claim-text>7. A lifting system according to Claim 5 or 6, wherein said connection point is between approximately 30% and approximately 40% of the length of the primary boom from the distal end of the primary boom.</claim-text> <claim-text>8. A lifting system according to any of the preceding claims, wherein the mount comprises: first and second pairs of mounting members attached to a base for a wind turbine, each pair having a mounting member on opposite sides of the base for a wind turbine; and first and second pairs of corresponding engaging portions, each engaging portion being adapted to engage with a respective mounting member.</claim-text> <claim-text>9. A lifting system according to Claim 8, wherein the first and second pairs of mounting members are spaced apart along the longitudinal axis of said base.</claim-text> <claim-text>10. A lifting system according to Claim 8 or 9, wherein each engaging portion comprises a locking portion adapted to secure the engaging portion to the mount.</claim-text> <claim-text>11. A lifting system according to Claim 8, 9 or 10, wherein the mounting members of each pair of mounting members are arranged on opposite sides of the wind turbine tower.</claim-text> <claim-text>12. A lifting system according to any of Claims 8 to 11, wherein each mounting member is substantially cylindrical.</claim-text> <claim-text>13. A lifting system according to Claim 12, wherein the pivot axis of said primary boom is substantially coincident with the axes of the first pair of said substantially cylindrical members.</claim-text> <claim-text>14. A lifting system according to any of Claims 8 to 13, wherein the third and fourth flexible members are connected to the mount at the second pair of mounting members.</claim-text> <claim-text>15. A lifting system according to any of the preceding claims, further comprising a winch adapted to act on the first flexible member, and a winch adapted to act on the second flexible member.</claim-text> <claim-text>16. A lifting system according to any of the preceding claims, further comprising a winch adapted to act on the third flexible member, and a winch adapted to act on the fourth flexible member.</claim-text> <claim-text>17. A lifting system according to Claim 15 or 16, wherein the winches are adapted to modify the position of the primary boom.</claim-text> <claim-text>18. A lifting system according to Claim 15, 16 or 17, wherein the winches are attached to the mount.</claim-text> <claim-text>19. A lifting system according to any of the preceding claims, wherein the first and third flexible members are a first integral flexible member, and the second arid fourth flexible members are a second integral flexible member.</claim-text> <claim-text>20. A lifting system according to Claim 19, wherein the distal ends of the first and second secondary booms comprise respective first and second pulleys, the first and second pulleys being adapted to receive the first and second integral flexible members respectively.</claim-text> <claim-text>21. A lifting system according to Claim 20, wherein the first and second pulleys each comprise a controllable brake.</claim-text> <claim-text>22. A lifting system according to any of the preceding claims, wherein the primary boom comprises two elongate spars, and at least one cross member connecting the elongate spars at the distal end.</claim-text> <claim-text>23. A lifting system according to Claim 22, wherein each secondary boom comprises two spars.</claim-text> <claim-text>24. A lifting system according to Claim 23, wherein each spar of the first secondary boom comprises a first adjustable counterweight, and each spar of the second secondary boom comprises a second adjustable counterweight.</claim-text> <claim-text>25. A lifting system according to Claim 24, wherein each of the said first adjustable counterweights are linked, such that the masses of each first adjustable counterweight remain substantially equal, and wherein each of the said second adjustable counterweights are linked, such that the masses of each second adjustable counterweight remain substantially equal.</claim-text> <claim-text>26. A lifting system according to any of the preceding claims, further comprising at least one pump adapted to adjust a quantity of water stored in the respective counterweights.</claim-text> <claim-text>27. A lifting system according to Claim 26, wherein the pump is adapted to transfer water between the first adjustable counterweight and the second adjustable counterweight.</claim-text> <claim-text>28. A lifting system according to any of the preceding claims, wherein the boom comprises a hoist at the distal end.</claim-text> <claim-text>29. A method of erecting a wind turbine on an offshore site, comprising: pivotally mounting the proximal end of an elongate primary boom of a lifting system at the base of a wind turbine tower, the boom being in a substantially horizontal position; pivotally mounting the proximal end of at least two elongate secondary booms of a lifting system at the base of a wind turbine tower, each secondary boom having: first and second adjustable counterweights connected to a respective distal endsof each secondary boom; first and second flexible members connecting the respective distal ends of each secondary boom to the primary boom; and third and fourth flexible members connecting the respective distal ends of each secondary boom to a mount; elevating the primary boom to a substantially vertical position by adjusting the first and second counterweights; connecting a wind turbine located on a vessel to a hoist on a distal end of the primary boom; hoisting the wind turbine from the vessel, and modifying the position of the hoisted wind turbine to a desired erecting position by adjusting the first and second counterweights; and disconnecting the wind turbine from the hoist.</claim-text> <claim-text>30. A method according to Claim 29, further comprising adjusting the relative lengths of the flexible members to elevate the primary boom.</claim-text> <claim-text>31. A method according to Claim 29 or 30, further comprising adjusting the relative lengths of the flexible members to modify the position of the hoisted wind turbine.</claim-text> <claim-text>32. A method according to Claim 30 or 31, wherein the relative lengths of the flexible members are adjusted by winching.</claim-text> <claim-text>33. A method according to any of Claims 29 to 32, wherein the first and second counterweights are adjusted by adjusting a quantity of water stored in the respective counterweights.</claim-text> <claim-text>34. A method according to Claim 33, wherein the water is transferred between the counterweights.</claim-text> <claim-text>35. A method according to Claim 33 or 34, wherein the quantity of water is adjusted by pumping sea-water to and from the counterweights.</claim-text>