An offshore wind turbine generator with tensioned guide wires

Abstract

An offshore wind turbine generator (1) comprising a hoisting arrangement for hoisting and / or lowering a main component (6) to / from a nacelle (4) is disclosed The hoisting arrangement comprises an up-tower crane (5) and at least two tensioned guide wires (9), each guide wire (9) being connected at a first end to an up-tower position (10) of the offshore wind turbine generator (1), and at a second, opposite, end to a down-tower position (2) of the offshore wind turbine generator (1). At least one guiding element (12) interconnects the connecting interface (8) and / or the main component (6) to each of the guide wires (9). A tension system (11) introduces a tension in each guide wire (9), and also dampens oscillating movements of the suspended main component (6) along a direction being substantially transverse to a direction defined by a movement path defined by the guide wires (9).

Description

[0001] AN OFFSHORE WIND TURBINE GENERATOR WITH TENSIONED GUIDE WIRES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an offshore wind turbine generator with a hoisting arrangement for hoisting and / or lowering a main component to / from an up-tower position, such as to / from a nacelle. The main component being hoisted or lowered is guided by means of tensioned guide wires. The offshore wind turbine generator according to the invention allows oscillating movements of the main component being hoisted or lowered to be appropriately handled without introducing excessive tension in the tensioned guide wires.

[0004] BACKGROUND OF THE INVENTION

[0005] Wind turbine generators are normally provided with a number of heavy main components, such as gearbox, generator, transformer, main bearing, etc. Such main components are often arranged in or near a nacelle located at the top of a tower. Thus, in the case that it is necessary to replace one of the main components, the component to be replaced needs to be lowered from the nacelle, and a replacing component needs to be hoisted to the nacelle and installed there. This requires suitable hoisting gear, e.g. with sufficient hoisting capacity to handle the weight of the main components. Furthermore, the main components need to be guided during the lowering or hoisting, in order to ensure that they are lowered or hoisted in a controlled manner, e.g. minimising the risk of the components colliding with the tower and / or the nacelle structure.

[0006] For onshore wind turbine generators, a ground based crane and a ground based guiding system, e.g. including tag lines, may be applied for lowering and hoisting main components from and to the nacelle. However, for offshore wind turbine generators, this is not an option. Therefore, for offshore wind turbine generators, it is desirable to apply an up-tower crane, e.g. arranged in or on the nacelle, or on the tower at a position near the top of the tower. However, this introduces a risk of collisions between the main component being lowered or hoisted and the tower of the wind turbine generator. Moreover, this risk is increased due to the fact that offshore wind turbine generators are expected to be more exposed to impact from wind and waves than onshore wind turbine generators.

[0007] DESCRIPTION OF THE INVENTION

[0008] It is an object of embodiments of the invention to provide an offshore wind turbine generator with a hoisting arrangement, where the risk of collisions between a main component being lowered from or hoisted to the nacelle is minimised, without introducing excessive loads on the wind turbine generator.

[0009] It is a further object of embodiments of the invention to provide a method for replacing a main component in a nacelle of an offshore wind turbine generator, in which the risk of collisions between the main component being lowered from or hoisted to the nacelle is minimised, without introducing excessive loads on the wind turbine generator.

[0010] According to a first aspect the invention provides an offshore wind turbine generator comprising a foundation, a tower supported by the foundation, a nacelle mounted on the tower, and a hoisting arrangement for hoisting and / or lowering a main component to / from the nacelle, wherein the hoisting arrangement comprises:

[0011] - an up-tower crane arranged in or on the nacelle, the up-tower crane comprising a connecting interface for connecting a suspended main component to the up-tower crane,

[0012] - at least two tensioned guide wires, each guide wire being connected at a first end to an up-tower position of the offshore wind turbine generator, and at a second, opposite, end to a down-tower position of the offshore wind turbine generator, a tension system configured to introduce a tension in each guide wire, and - at least one guiding element interconnecting the connecting interface and / or the main component to each of the guide wires, so as to enable a suspended main component being hoisted or lowered by means of the up- tower crane to be guided by means of the guide wires, along a movement path, wherein the tension system in use is also configured to dampen oscillating movements of the suspended main component along a direction being substantially transverse to a direction defined by the movement path.

[0013] Thus, according to the first aspect the invention provides an offshore wind turbine generator. In the present context the term 'offshore wind turbine generator' should be interpreted to mean a wind turbine generator that is positioned at an offshore location, such as at sea or on a lake.

[0014] The offshore wind turbine generator comprises a foundation, a tower supported by the foundation and a nacelle mounted on the tower. A number of main components, e.g. a gearbox, a generator, a transformer, a main bearing, etc., are housed in the nacelle.

[0015] The offshore wind turbine generator further comprises a hoisting arrangement for hoisting and / or lowering a main component to / from the nacelle, e.g. as part of a process for replacing one of the main components being housed in the nacelle.

[0016] The hoisting arrangement comprises an up-tower crane arranged in or on the nacelle, at least two tensioned guide wires, and a tension system. The up-tower crane comprises a connecting interface for connecting a main component to the up-tower crane, thus allowing the main component to be suspended from the up-tower crane while being lowered or hoisted. The connecting interface could, e.g., be in the form of a hook arranged at an end of a hoisting wire, or it could be any other suitable kind of connecting interface which allows a main component to be detachably connected to the up-tower crane, so as to enable that the main component is lowered or hoisted by means of the up-tower crane. Each of the at least two guide wires is connected at a first end to an up-tower position of the offshore wind turbine generator, and at a second, opposite, end to a down-tower position of the offshore wind turbine generator. In the present context the term 'up-tower position' should be interpreted to mean a position which is at or near an upper part of the tower. Thus, the up-tower position could be a position on the tower, near the top of the tower, e.g. immediately below the nacelle. Alternatively, it could be a position on the nacelle. Similarly, in the present context the term 'down-tower position' should be interpreted to mean a position which is near a lower part of the tower. Thus, the down-tower position could be a position on the tower, near the base of the tower. Alternatively, it could be a position on the foundation, a transition piece or a landing platform near the base of the tower. Accordingly, the tensioned guide wires each interconnects an up-tower position of the offshore wind turbine generator and a down-tower position of the offshore wind turbine generator.

[0017] When a main component is being lowered from or hoisted to the nacelle by means of the up-tower crane, the connecting interface of the up-tower crane and / or the main component is / are connected to each of the guide wires by means of at least one guiding element. Accordingly, the at least one guiding element is configured to interconnect a main component being supposed to be lowered or hoisted and / or the connecting interface, on the one hand, and the guide wires, on the other hand. The main component is thus guided by means of the guide wires from the down-tower position to the up-tower position, or from the up-tower position to the down-tower position, along a movement path being defined by the guide wires. The movement path may advantageously be arranged with a suitable distance to the tower, so as to minimise the risk of collisions between the main component and the tower.

[0018] Since the connecting interface and / or the main component is / are connected to at least two tensioned guide wires, rotations of the main component during hoisting or lowering is essentially avoided.

[0019] The tension system is configured to introduce a tension in each of the guide wires. The introduced tension ensures that the movement path followed by the main component is well defined, and that the main component is not allowed to deviate significantly from the movement path defined by the guide wires. During use of the hoisting arrangement, the tension system is further configured to dampen oscillating movements of the suspended main component along a direction being substantially transverse to a direction defined by the movement path. The tension system may comprise separate tension mechanisms for each guide wire. In this case the tension introduced in one guide wire may differ from the tension introduced in another guide wire. As an alternative, all guide wire may be connected to the same tension mechanism.

[0020] When the main component being lowered or hoisted is exposed to impact from wind and waves, it will be pushed away from the movement path defined by the tensioned guide wires, in a direction which is substantially transverse to the movement path. This will result in a pull in the guide wires. In the present context the term 'substantially transverse' should be interpreted to mean a direction which is perpendicular to the direction defined by the movement path, or at least has a significant component which is perpendicular to the direction defined by the movement path. Thus, the substantially transverse direction is not parallel or substantially parallel to the direction defined by the movement path. The substantially transverse direction may, e.g., be substantially horizontal.

[0021] The impact from wind and waves on the main component described above may cause the main component to perform oscillating movements along the substantially transverse direction, and this may result in a periodic pull in the tensioned guide wires. If the tensioned guide wires were simply connected to two fixed end points, such an oscillating movement of a suspended main component being connected to the guide wires, and the resulting periodic pull on the guide wires, would cause elongation of the guide wires, resulting in a significant increase in the tension in the guide wires. This would result in significant loads on various parts of the wind turbine generator, e.g. on the guide wires and on parts of the wind turbine generator where the guide wires are attached. Thus, such parts of the wind turbine generator would need to be dimensioned so as to be able to handle these loads. This adds to the weight as well as to the costs of the wind turbine. However, in the wind turbine generator according to the first aspect of the invention, the tension system is configured to dampen such oscillating movements of the main component. Accordingly, the oscillating movements are handled without introducing undue increase of the tension in the guide wires, thus allowing affected parts of the wind turbine generator to be dimensioned to handle significantly lower loads. Furthermore, the tension introduced in the guide wires when the main component is not performing oscillating movements, or when it is in a position defining a rest position of the oscillating movement, is sufficient to ensure suitable guidance of the main component being lowered or hoisted. Accordingly, the pull in the guide wires caused by the oscillating movement of the main component is handled in such a manner that the tension in the guide wires is maintained substantially constant, and with minimal elongation of the guide wires.

[0022] For instance, the tension system may be designed in such a manner that deviations from the movement path of the main component being lowered or hoisted causes a transfer of energy to the tension system, rather than causing elongation of the guide wires, and thus an increase in the tension in the guide wires. This may be regarded as the tension system 'absorbing' the additional tension forces which would otherwise have been introduced in the guide wires, as a result of the oscillating movement of the suspended main component. Thus, built up of excessive tension in the guide wires is avoided. Furthermore, slack in the guide wires is also avoided, when the deviation from the movement path is decreased, thus decreasing the pull in the guide wires. In other words, the tension system ensures that a substantially constant tension is maintained in the guide wires.

[0023] The tension system may comprise a member connected to each guide wire, which member moves when the suspended main component performs oscillating movements along the substantially transverse direction. According to this embodiment, the oscillating movements of the main component causes a periodic pull in the guide wires, and this periodic pull causes the members connected to the respective guide wires to move, rather than resulting in elongation of the guide wires, and thus in an increase in the tension in the guide wires. The member may, e.g., be in the form of a balancing weight, a movable piston or any other suitable kind of member being able to move in response to an oscillating movement of a main component being connected to the guide wires.

[0024] For instance, the tension system may be configured so that a movement of the main component away from a rest position of the oscillating movements along the substantially transverse direction causes energy transfer from the oscillating movement to movement of the member.

[0025] The oscillating movement of the main component defines a rest position which substantially coincides with a position along the movement path defined by the guide wires, and two extreme positions defining the largest deviations from the movement path, on either side of the rest position. When the suspended main component moves away from the rest position towards one of the extreme positions, this causes an increase in the deviation from the movement path, and thus an increase in a pull on the guide wires. According to this embodiment, this increase in the pull on the guide wires causes an energy transfer from the guide wires to movement of the member connected to the respective guide wires.

[0026] Thus, energy is transferred from the oscillating movement of the main component to movement of the respective members, via the guide wires.

[0027] When the main component performs an opposite movement, i.e. when it moves from one of the extreme positions towards the rest position, the pull on the guide wires is similarly reduced, and as a consequence, energy may be transferred from the moving members to the respective guide wires, so as to avoid slack in the guide wires.

[0028] The energy transfer may cause an increase in potential energy of the moving member. According to this embodiment, the moving member is lifted, thus increasing the potential energy of the member, when the pull in the guide wire is increased as a result of the oscillating movement of the main component. Similarly, the moving member may be lowered, thus decreasing the potential energy of the member, when the main component moves towards the rest position of the oscillating movement. The energy transfer described above may cause friction between the moving member and an adjacent structure, e.g. in the form of a housing accommodating the moving member. According to this embodiment, the energy originating from the oscillating movement of the suspended main component is at least partly absorbed by the moving member and the adjacent structure, due to the friction there between. Alternatively or additionally, the energy transfer may cause friction in other parts of the tension system, such as in pulley wheels.

[0029] As an alternative, the energy transfer may cause a change of pressure in a fluid. For instance, the moving member may be a movable piston, and the movement thereof may cause a change of a pressure in a cylinder accommodating the movable piston. The fluid could, e.g., be a hydraulic fluid or air.

[0030] The tension system may comprise at least one constant tension winch. According to this embodiment, the tension in the guide wires is provided by means of a constant tension winch, i.e. a winch being capable of providing a substantially constant tension, for instance by winding and unwinding guide wire in response to variations in pulling forces applied to the guide wires. This would contribute to avoiding excessive tension in the guide wires as a consequence of an increase in pulling force, as well as to avoiding slack in the guide wires as a consequence of a decrease in pulling force. The constant tension winch may, e.g., be actively controlled. Appropriate control of the constant tension winch may contribute to damping of oscillating movements of the suspended main component. Damping may further be provided by means of friction, e.g. in one or more pulley wheels arranged in front of the constant tension winch.

[0031] The at least one guiding element may be a rigid element. According to this embodiment, the connection between the connecting interface of the up-tower crane and / or the main component and each of the guide wires is a rigid connection. This ensures that a substantially constant and well defined distance is maintained between the suspended main component and the respective guide wires, thus ensuring that the suspended main component is guided in a safe, reliable and well defined manner. The foundation may be a floating foundation. According to this embodiment, the foundation of the wind turbine generator is not fixedly connected to the seabed. Instead, the foundation is connected to one or more anchoring points on or in the seabed via wires, chains or similar connectors. Thus, the exact position of the wind turbine generator is allowed to shift within a certain area defined by the position of the one or more anchoring points and the length of the connectors, subject to impact from wind and waves. Furthermore, offshore wind turbine generators with floating foundations follow vertical movements of passing waves, and they may tilt due to the impact of waves. Thus, offshore wind turbine generators with floating foundations are more exposed to impact from wind and waves than offshore wind turbines with fixed foundations. Accordingly, the risk of oscillating movements of suspended main components may be considered particularly high in such wind turbine generators, and therefore the present invention is particularly relevant in this case.

[0032] As an alternative, the foundation may be a fixed foundation, such as a jacket, a monopile or other suitable kinds of bottom fixed structures.

[0033] The tension system may be mounted at a down-tower position on the foundation. For instance, the position of the tension system may define the down-tower position where the second ends of the respective guide wires are connected to the offshore wind turbine generator. This ensures a certain minimum distance between the movement path defined by the guide wires and the tower. As an alternative, the tension system may be mounted at a downtower position directly on the tower. As another alternative, the tension system may be mounted at an up-tower position, e.g. on or immediately below the nacelle, or on an arm or beam extending from the nacelle, and for instance at the up-tower position where the first ends of the respective guide wires are connected to the offshore wind turbine generator. It should be noted that it is not ruled out that the offshore wind turbine generator comprises a tension system mounted at a down-tower position as well as a tension system mounted at an up-tower position.

[0034] The suspended main component may be a gearbox, part of a gearbox, a generator, a sub-component for a generator, a main bearing, a blade bearing or a transformer. All of these components are relatively heavy, and they are often arranged in or near the nacelle. Thus, it is relevant to lower or hoist such components by means of a hosting arrangement as described above.

[0035] According to a second aspect the invention provides a method for replacing a main component in a nacelle of an offshore wind turbine generator, the offshore wind turbine generator comprising a foundation, a tower supported by the foundation, a nacelle mounted on the tower, and a hoisting arrangement, the method comprising the steps of:

[0036] - dismantling the main component from an installed position in the nacelle,

[0037] - connecting the main component to a connecting interface of an up-tower crane of the hoisting arrangement,

[0038] - connecting the connecting interface and / or the main component to at least two tensioned guide wires of the hoisting arrangement, by means of at least one guiding element, each guide wire being connected at a first end to an up-tower position of the offshore wind turbine generator, and at a second, opposite, end to a down-tower position of the wind turbine generator, and

[0039] - lowering the main component from the nacelle towards a down-tower position of the offshore wind turbine generator, by means of the up-tower crane, while guiding the main component by means of the at least two tensioned guide wires, along a movement path, wherein the method further comprises the step of introducing a tension in the guide wires by means of a tension system, and wherein, during lowering of the main component, the tension system dampens oscillating movements of the main component being lowered, along a direction being substantially transverse to a direction defined by the movement path.

[0040] The method according to the second aspect of the invention may advantageously be performed at an offshore wind turbine generator according to the first aspect of the invention. The remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.

[0041] In the method according to the second aspect of the invention, a main component to be replaced is initially dismantled from an installed position in the nacelle, and possibly moved within the nacelle to a position where it is possible for the dismantled main component to exit the nacelle. Next, the dismantled main component is connected to a connecting interface of an up-tower crane of the hoisting arrangement of the offshore wind turbine generator.

[0042] Furthermore, the connecting interface and / or the main component connected thereto, is / are connected to at least two tensioned guide wires of the hoisting arrangement, by means of at least one guiding element, e.g. at least one rigid guiding element. Each of the guide wires is connected at a first end to an up- tower position of the offshore wind turbine generator, and at a second, opposite, end to a down-tower position of the wind turbine generator. This has already been described above with reference to the first aspect of the invention.

[0043] The main component is then lowered from the nacelle towards a down-tower position of the offshore wind turbine generator. This is done by appropriately operating the up-tower crane, and while guiding the main component by means of the at least two tensioned guide wires. Thus, the main component essentially follows a movement path defined by the guide wires, in the manner described above with reference to the first aspect of the invention.

[0044] Furthermore, a tension is introduced in the guide wires by means of a tension system, and during lowering of the main component, the tension system dampens oscillating movements of the main component being lowered, along a direction being substantially transverse to a direction defined by the movement path. This has already been described in detail above with reference to the first aspect of the invention.

[0045] The tension system may comprise a member connected to each guide wire, and the method may further comprise the step of moving the member when the main component being lowered performs oscillating movements along the substantially transverse direction. This has already been described in detail above with reference to the first aspect of the invention.

[0046] When the lowering of the main component has been completed, a replacement main component may be connected to the connecting interface of the up-tower crane, and the replacement main component may be hoisted towards the nacelle by means of the up-tower crane, while being guided by means of the guide wires, essentially in the manner described above. Once the replacement main component has reached the nacelle, it may be installed therein, thus replacing the previously lowered main component.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention will now be described in further detail with reference to the accompanying drawings in which

[0049] Fig. 1 is a perspective view of an offshore wind turbine generator according to an embodiment of the invention,

[0050] Figs. 2 and 3 show a detail of the wind turbine generator of Fig. 1,

[0051] Figs. 4 and 5 illustrate connection of a main component to guide wires in accordance with an embodiment of the invention,

[0052] Figs. 6-8 illustrate a tension system for a hoisting arrangement of a wind turbine generator according to an embodiment of the invention, and

[0053] Figs. 9 and 10 illustrate lowering of a main component as part of a method according to an embodiment of the invention.

[0054] DETAILED DESCRIPTION OF THE DRAWINGS

[0055] Fig. 1 is a perspective view of an offshore wind turbine generator 1 according to an embodiment of the invention. The wind turbine generator 1 comprises a foundation (not shown) with a landing platform 2. The foundation could, e.g., be a floating foundation. A tower 3 is supported by the foundation, and a nacelle 4 is mounted on top of the tower 3. The nacelle 4 houses various main components, e.g. in the form of a gearbox, a generator, a transformer and / or a main bearing.

[0056] An up-tower crane 5 is mounted in or on the nacelle 4 and protrudes from the top of the nacelle 4. A main component 6 to be lowered from the nacelle 4 is connected to the up-tower crane 5 via a wire 7 and a connecting interface 8, e.g. in the form of a hook or similar device allowing the main component 6 to be connected or attached to the wire 7, and thus to the hoisting mechanism of the up-tower crane 5. Thus, the main component 6 is suspended from the up-tower crane 5.

[0057] The wind turbine generator 1 is further provided with two tensioned guide wires 9. The guide wires 9 are, at a first end, connected to a beam 10 mounted on the nacelle 4, and, thus at an up-tower position, and, at a second end, to a tension system 11 mounted on the landing platform 2, and thus at a down-tower position. Accordingly, the guide wires 9 define a movement path between the up-tower position at the beam 10 and the down-tower position at the landing platform 2. The beam 10 ensures that the guide wires 9, and thus the movement path defined thereby, are arranged with a minimum distance to the tower 3.

[0058] The tension system 11 introduces a tension in each of the guide wires 9. This ensures that the movement path defined by the guide wires 9 remains reliable and well defined. Furthermore, the tension system 11 is configured to dampen oscillating movements of a main component 6 being connected to the guide wires 9. This will be described in further detail below.

[0059] Figs. 2 and 3 show a detail of the wind turbine generator 1 of Fig. 1, more particularly of the upper part of the wind turbine generator 1, from two different angles. Thus, the up-tower crane 5, the suspended main component 6 and the connection of the guide wires 9 to the beam 10 can be seen. Similarly to Figs. 2 and 3, Fig. 4 also shows the upper part of the wind turbine generator 1 of Fig. 1. However, in Fig. 4 the suspended main component 6 is connected to each of the guide wires 9 by means of guiding elements 12. Thereby, when the suspended main component 6 is lowered (or hoisted) by means of the up-tower crane 5, it is guided by the guide wires 9, and therefore essentially follows the movement path defined by the guide wires 9.

[0060] Fig. 5 is a detailed view of the connection between the suspended main component 6 and the guide wires 9. It can be seen that the guiding elements 12 interconnecting the main component 6 and the guide wires 9 are rigid elements. This ensures a substantially constant or fixed distance between the main component 6 and the guide wires 9.

[0061] Fig. 6 shows another detail of the wind turbine generator 1 of Fig. 1, in the form of the lower part of the wind turbine generator 1. Thus, in Fig. 6 the landing platform 2 and the tension system 11 can be seen. The tension system 11 will be described in further detail below with reference to Figs. 7 and 8.

[0062] Figs. 7 and 8 show a tension system 11 for a hoisting arrangement of a wind turbine generator 1 according to an embodiment of the invention, e.g. the wind turbine generator 1 of Fig. 1. Fig. 7 is a side view of the tension system 11 and Fig. 8 is a perspective view of the tension system 11.

[0063] The tension system 11 comprises two movable members 13, in the form of balancing weights, each connected to an end of one of the guide wires 9 via a set of pulleys 14. Thus, the weight of the members 13 introduces the required tension in the respective guide wires 9. Furthermore, the pulleys 14 allow the members 13 to move along a substantially vertical direction, i.e. up and down, in response to changes in a pulling force applied to the guide wires 9. This will be described in further detail below with reference to Figs. 9 and 10.

[0064] Figs. 9 and 10 illustrate lowering of a main component 6 from the nacelle 4 of the offshore wind turbine generator 1 of Fig. 1, as part of a method according to an embodiment of the invention. Fig. 9 is a front view of the wind turbine generator 1 and Fig. 10 is a perspective view of the wind turbine generator 1. The main component 6 is connected to the up-tower crane 5 via the connecting interface 8, and it is further connected to each of the guide wires 9 by means of guiding elements (not shown), in the manner described above with reference to Figs. 4 and 5. Thus, as the main component 6 is being lowered from the position near the beam 10 towards the landing platform 2, by appropriately operating the up-tower crane 5, the main component 6 is guided by the tensioned guide wires 9, thus essentially following the movement path defined by the guide wires 9. Thereby collisions between the main component 6 and the tower 3 are avoided.

[0065] As the main component 6 is lowered, the wind turbine generator 1 as well as the main component 6 is exposed to wind and waves. This may cause the main component 6 to deviate from the movement path defined by the guide wires 9, along a direction being substantially transverse to the direction of the movement path, such as in a substantially horizontal direction. This may further cause the main component 6 to perform oscillating movements along this transverse direction.

[0066] Such an oscillating movement defines a rest position and two extreme positions at opposing sides of the rest position. When the main component 6 moves away from the rest position and towards one of the extreme positions, it pulls the guide wires 9 along, due to the connection established by the guiding elements (12 in Figs. 4 and 5), as can be seen in Figs. 9 and 10. This results in a pulling force on the guide wires 9. If the guide wires 9 were fixedly connected to the wind turbine generator 1 at the up-tower position as well as at the down-tower position, this pulling force would result in elongation of the guide wires 9, causing a significant increase in the tension in the guide wires 9. This would, e.g., require various parts of the wind turbine generator 1, notably the beam 10 and the guide wires 9, to be designed and dimensioned to handle such increased tension.

[0067] However, as described above with reference to Figs. 7 and 8, the guide wires 9 are connected to the movable members 13 of the tension system 11. Thus, when a pulling force is applied to the guide wires 9, due to an oscillating movement of the suspended main component 6, the pulling force is transferred to the tension system 11, in the sense that the members 13 are moved in an upwards direction, thus increasing their potential energy. Accordingly, the tension in the guide wires 9 is not increased, or is only increased slightly. This may be regarded as the tension system 11 'absorbing' the potential increased tension in the guide wires 9 caused by the oscillating movement of the main component 6.

[0068] When the main component 6, as part of the oscillating movements, moves from an extreme position towards the rest position, the pulling force applied to the guide wires 9 is gradually decreased. This will be balanced by the movable members 13 in the sense that gravity acting on the members 13 will partly overcome the reduced pulling force, thus causing the members 13 to move in a downwards direction. This ensures that the tension in the guide wires 9 is maintained at an essentially constant level, also during this part of the oscillating movements, thus preventing slack in the guide wires 9.

[0069] Thus, the risk of collisions between the main component 6 and the tower 3 is minimised, without introducing excessive loads on the wind turbine generator 1, notably on the guide wires 9 and the beam 10, and thereby without the need for designing and dimensioning the wind turbine generator 1 to handle such excessive loads.

Claims

CLAIMS1. An offshore wind turbine generator (1) comprising a foundation, a tower (3) supported by the foundation, a nacelle (4) mounted on the tower (3), and a hoisting arrangement for hoisting and / or lowering a main component (6) to / from the nacelle (4), wherein the hoisting arrangement comprises:- an up-tower crane (5) arranged in or on the nacelle (4), the up-tower crane (5) comprising a connecting interface (8) for connecting a suspended main component (6) to the up-tower crane (5),- at least two tensioned guide wires (9), each guide wire (9) being connected at a first end to an up-tower position (10) of the offshore wind turbine generator (1), and at a second, opposite, end to a down-tower position (2) of the offshore wind turbine generator (1),- a tension system (11) configured to introduce a tension in each guide wire (9), and- at least one guiding element (12) interconnecting the connecting interface (8) and / or the main component (6) to each of the guide wires (9), so as to enable a suspended main component (6) being hoisted or lowered by means of the up-tower crane (5) to be guided by means of the guide wires (9), along a movement path, wherein the tension system (11) in use is also configured to dampen oscillating movements of the suspended main component (6) along a direction being substantially transverse to a direction defined by the movement path.

2. The offshore wind turbine generator (1) according to claim 1, wherein the tension system (11) comprises a member (13) connected to each guide wire (9), which member (13) moves when the suspended main component (6) performs oscillating movements along the substantially transverse direction.

3. The offshore wind turbine generator (1) according to claim 2, wherein the tension system (11) is configured so that a movement of the main component (6) away from a rest position of the oscillating movements along the substantially transverse direction causes energy transfer from the oscillating movement to movement of the member (13).

4. The offshore wind turbine generator (1) according to claim 3, wherein the energy transfer causes an increase in potential energy of the moving member (13).

5. The offshore wind turbine generator (1) according to claim 3 or 4, wherein the energy transfer causes friction between the moving member (13) and an adjacent structure.

6. The offshore wind turbine generator (1) according to claim 3, wherein the energy transfer causes a change of pressure in a fluid.

7. The offshore wind turbine generator (1) according to any of the preceding claims, wherein the tension system (11) comprises at least one constant tension winch.

8. The offshore wind turbine generator (1) according to any of the preceding claims, wherein the at least one guiding element (12) is a rigid element.

9. The offshore wind turbine generator (1) according to any of the preceding claims, wherein the foundation is a floating foundation.

10. The offshore wind turbine generator (1) according to claim 8, wherein the tension system (11) is mounted at a down-tower position on the foundation.

11. The offshore wind turbine generator (1) according to any of the preceding claims, wherein the suspended main component (6) is a gearbox, part of a gearbox, a generator, a main bearing, a blade bearing or a transformer.

12. A method for replacing a main component (6) in a nacelle (4) of an offshore wind turbine generator (1), the offshore wind turbine generator (1) comprising a foundation, a tower (3) supported by the foundation, a nacelle (4) mounted on the tower (3), and a hoisting arrangement, the method comprising the steps of:- dismantling the main component (6) from an installed position in the nacelle (4),- connecting the main component (6) to a connecting interface (8) of an up-tower crane (5) of the hoisting arrangement,- connecting the connecting interface (8) and / or the main component (6) to at least two tensioned guide wires (9) of the hoisting arrangement, by means of at least one guiding element (12), each guide wire (9) being connected at a first end to an up-tower position (10) of the offshore wind turbine generator (1), and at a second, opposite, end to a down-tower position (2) of the wind turbine generator (1), and- lowering the main component (6) from the nacelle (4) towards a downtower position of the offshore wind turbine generator (1), by means of the up-tower crane (5), while guiding the main component (6) by means of the at least two tensioned guide wires (9), along a movement path, wherein the method further comprises the step of introducing a tension in the guide wires (9) by means of a tension system (11), and wherein, during lowering of the main component (6), the tension system (11) dampens oscillating movements of the main component (6) being lowered, along a direction being substantially transverse to a direction defined by the movement path.

13. The method according to claim 12, wherein the tension system (11) comprises a member (13) connected to each guide wire (9), and wherein the method further comprises the step of moving the member (13) when the main component (6) being lowered performs oscillating movements along the substantially transverse direction.

Images