WO2022135640A1

WO2022135640A1 - Repairing a wind turbine blade

Info

Publication number: WO2022135640A1
Application number: PCT/DK2021/050352
Authority: WO
Inventors: Ivar J.B.K. JENSEN; Aksel PETERSEN; Christian SKOV; Claus Engholm Nielsen; Allan Korsgaard POULSEN; Rasmus Sole NØRGAARD
Original assignee: Vestas Wind Systems A/S
Priority date: 2020-12-23
Filing date: 2021-12-06
Publication date: 2022-06-30

Abstract

A method of repairing a blade (12) of a wind turbine generator (2) is provided. The method comprises lifting a blade repair assembly (104, 204, 304) to a position adjacent to a region of the blade (12) to be repaired; and orienting a repair device (112) of the blade repair assembly (104, 204, 304) towards that region (12). The method further comprises operating the blade repair assembly (104, 204, 304) to generate a lateral force to press the repair device (122) into engagement with the blade (12); and operating the repair device (112) to repair the blade (12) while continuing to press the repair device (112) into engagement with the blade (12).

Description

REPAIRING A WIND TURBINE BLADE

TECHNICAL FIELD

The present disclosure generally relates to schemes or apparatus for repairing a wind turbine blade. In particular, this disclosure relates to approaches for repairing the leading edges of wind turbine blades to mitigate erosion of the leading edges that occurs in operation.

BACKGROUND

When the rotor of a horizontal axis wind turbine (HAWT) rotates, the leading edge of each blade of the rotor cuts through the air, often at high speed. Consequently, over time the leading edges are eroded and damaged as they encounter and collide with raindrops, debris and other particles in the air. Such damage to the leading edge progressively degrades the aerodynamic performance of the blade and, in turn, the overall efficiency of the wind turbine. Accordingly, regular maintenance of the rotor blades is necessary to repair leading edge damage.

Repairing blades with leading edge damage typically involves stopping and locking the rotor while the repair is made in situ by personnel supported near to the damage site, for example by a temporary platform or rappel equipment. Alternatively, the blade may be disassembled from the wind turbine and lowered to the ground for the repair to be completed. These known approaches entail significant outage time and so impact the annual energy production (AEP) of the turbine, and can also incur risks to personnel.

It is against this background that the invention has been devised.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method of repairing a blade of a wind turbine generator is provided. The method comprises lifting a blade repair assembly to a position adjacent to a region of the of the blade to be repaired, and orienting a repair device of the blade repair assembly towards that region. The method further comprises operating the blade repair assembly to generate a lateral force to press the repair device into engagement with the blade, and operating the repair device to repair the blade while continuing to press the repair device into engagement with the blade. The method may further comprise operating a thruster of the blade repair assembly to generate the lateral force. According to some embodiments, the method may comprise controlling the direction in which the lateral force acts on the repair device by controlling the position and / or orientation of the blade repair assembly.

The method may comprise pitching the blade towards a feathered position or 90^e pitch angle, so that the leading edge of the blade faces into the prevailing wind. Correspondingly, the method may comprise pitching the blade towards a 0^e pitch angle, that is, with the chord of the blade substantially parallel to a plane of a rotor to which the blade belongs, and perpendicular to a rotor axis. More generally, the blade may be pitched at an angle that is closer to 90 “than to 0°, the latter corresponding to an orientation in which the chord of the blade is substantially parallel to the rotor plane. Pitching the blade towards a pitch angle of 90^e reduces the angle of attack and minimises any lift generated by wind flowing around the blade during the repair operation. Pitching the blade at or close to a pitch angle of 90° minimises any tendency of the lateral force produced by the blade repair assembly to rotate the rotor as the force is acting close to parallel with the rotor axis. Accordingly, pitching the blade in this manner potentially avoids a need to apply a rotor lock, as a brake of the wind turbine generator will typically be capable of resisting any small loads arising on the rotor to hold the rotor in a fixed position during the repair, for example in a vertical position. In some embodiments, the blade may be allowed to move as the repair is completed. For example, the blade may move between a substantially vertical orientation and a substantially horizontal orientation as the rotor rotates.

According to some embodiments, the method may comprise moving the repair device on a surface of the blade while repairing the blade. Optionally, moving the repair device comprises operating a drive system of the repair device. The step of moving the repair device may comprise adjusting a vertical position of the blade repair assembly. The method may also comprise moving the repair device along an edge of the blade whilst repairing damage to the edge, in which case the edge may be a leading edge of the blade.

In some embodiments, the step of lifting the blade repair assembly may be achieved using an aerial vehicle. In such embodiments, the method may further comprise flying the aerial vehicle to the wind turbine generator with the blade repair assembly coupled to the aerial vehicle. While the aerial vehicle flies to the wind turbine generator, the thruster of the blade repair assembly may be inactive. The blade repair assembly may be directly mounted to the aerial vehicle, and may be pivotably mounted to the aerial vehicle. In other embodiments, the step of lifting the blade repair assembly may be achieved using a crane.

In embodiments where the blade repair assembly is not directly mounted to an aerial vehicle, the method may comprise suspending the blade repair assembly on a line, for example a single line or a pair of parallel lines. In such embodiments, the method may comprise operating a winch to draw in or pay out the, or each, line to adjust the vertical position of the blade repair assembly.

According to some embodiments, the method may comprise lifting the blade repair assembly by a support body of the assembly to which the repair device is coupled. In such embodiments, the method may further comprise rotating, or allowing rotation of, the repair device relative to the support body while repairing the blade. For example, the repair device may couple to the support body by a flexible joint that allows relative rotation of the repair device and support body during the repair operation.

In some embodiments, the repair device may comprise a robotic device, for example, the repair device may be robotic and/or may comprise a robotic element such as a robotic arm. Operating the repair device to repair the blade may comprise abrasive and / or coating operations.

A second aspect of the invention provides a blade repair assembly for repairing a blade of a wind turbine generator, the blade repair assembly comprising a repair device and a propulsion system. The repair device is configured to engage and to repair a surface of the blade, while the propulsion system is arranged to generate a lateral force to press the repair device into engagement with the blade. The assembly also comprises a coupling by which it can be lifted so as to position the repair device in the vicinity of the blade.

The propulsion system may comprise a thruster which optionally comprises a rotor.

The repair device may comprise a drive system which is configured to move the device on a surface of the blade. The assembly may further comprise a support body that comprises the coupling and supports the repair device and the propulsion system.

The invention also extends to a blade repair system for repairing a blade of a wind turbine generator, the system comprising the blade repair assembly of the above aspect and an aerial vehicle configured to lift the blade repair assembly by the coupling, to engage the repair device with the blade.

It will be appreciated that preferred and/or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which like features are assigned like reference numerals, and in which:

Figure 1 is a front view of a horizontal axis wind turbine to which embodiments of the invention may be applied;

Figure 2 is a front view of a wind turbine blade of the wind turbine of Figure 1 , showing damage along a leading edge of the blade;

Figure 3 is a perspective view of a blade repair system according to an embodiment of the invention;

Figure 4 is a front schematic view of a blade repair system according to another embodiment of the invention;

Figure 5 is a front schematic view of a blade repair system according to another embodiment of the invention;

Figures 6a to 6c are front schematic views of robotic maintenance devices for use in embodiments of the invention; and Figure 7 is a side view schematic of an example wheel arrangement of a robotic maintenance device for use in embodiments of the invention.

DETAILED DESCRIPTION

In general terms, embodiments of the invention provide apparatus and corresponding methods for repairing damage on the leading edge of a wind turbine blade, although in principle the methods and apparatus of embodiments of the invention could be used to repair any part of a wind turbine blade.

The approach involves lifting a leading edge repair assembly to the site of damage on a wind turbine blade. The assembly may be lifted using an unmanned aerial vehicle (UAV) or a crane, for example, which then supports the repair assembly while the repair is carried out. As such, repair and maintenance operations can be completed in a timely manner without the need for manual intervention, or wind turbine disassembly.

In particular, use of a UAV in supporting the repair assembly improves the ease with which repairs can be completed on wind turbine blades that are in difficult to access locations, such as on offshore wind turbines.

Moreover, since the repair assembly is supported by a UAV or other means, embodiments of the invention allow for repair of a blade without having the blade itself support the weight of the repair assembly. This allows for the blade to be repaired when oriented generally vertically, or in any other position, and without requiring the blade to be horizontal and pitched at 0° with the leading edge facing up (i.e. with the blade chord generally vertical and parallel to the rotor plane). Instead, the blade can be pitched to reduce the angle of attack, for example by pitching the blade towards a pitch of 90°, namely a position where the leading edge faces forwards (i.e. into the prevailing wind) so that the blade chord is generally horizontal and perpendicular to the rotor plane. At such a pitch, the blade generates negligible lift and so the rotor remains generally stationary, thus avoiding a need for the rotor to be locked, using a rotor lock pin for example, while the repair is completed. Furthermore, a blade which is pitched so that blade chord is generally perpendicular to the rotor plane has a smaller area of resistance in a vertical plane than a blade which is pitched so that the blade chord is generally parallel to the rotor plane. The blade may also be ‘feathered’ according to known principles to minimise lift generated during the repair operation. As such, embodiments of the invention allow for the horizontal load on the rotor imparted by wind to be minimised when a repair of a blade is carried out.

It is further contemplated that use of a UAV in supporting the repair assembly would allow for a blade to be repaired even in circumstances where the rotor slowly rotates or moves, as the UAV could follow the blade and compensate for any movement.

Since embodiments of the invention allow for a blade to be repaired in any orientation, it is envisaged that multiple blades on a rotor may be repaired at the same time, thus improving the flexibility with which repair procedures can be carried out.

In embodiments of the invention, the leading edge repair assembly is configured to generate a lateral force to press a repair device of the assembly into engagement with the blade. This allows for the position of the repair device on the blade to be controlled precisely as the repair is completed, thereby promoting a high-quality repair. In general terms, the lateral force emulates the effect of the repair device being held in firm contact with the blade under self-weight if the device were positioned on top of a blade oriented horizontally. A ‘lateral force’ is a force having a non-zero horizontal component, and typically a force that is predominantly or even exclusive directed horizontally. It is noted that, where a blade is pitched at or close to 90° with the leading edge facing forwards, the lateral force applied by the leading edge repair assembly is applied in a direction that is generally perpendicular to the rotor plane and so does not prompt the rotor to rotate. This in turn further minimises any need to resist rotation of the rotor, any in particular avoids a need to use a rotor lock.

To provide context for the invention, Figure 1 shows a horizontal axis wind turbine (HAWT) 2 that includes a tower 4, a nacelle 6 mounted at the apex of the tower 4, and a rotor 8 supported on the nacelle 6. The rotor 8 includes a rotor hub 10 and three coplanar blades 12 supported by the rotor hub 10, each blade 12 extending radially from a root 14 at the rotor hub 10 to a tip 16. Each blade 12 has a generally aerofoil profile in transverse cross section which interacts with the wind to cause rotation of the rotor 8 which, in turn, drives a generator (not shown) to produce electrical energy.

The aerofoil profile of each blade 12 defines a leading edge 18 and a trailing edge 20 of the blade 12 with respect to a sense of rotation of the blade 12, in use. Referring to Figure 2, each blade 12 is formed of a shell of fibre composite, aluminium, or similar material which encloses the blade’s internal components. The outer surface 23 of the shell has an outer skin 24 which is defined by several different layers of material (polymeric, elastomers, paint, etc), including an outermost top coat, a second layer underneath the outmost top coat, and a third layer underneath the second layer.

When the rotor 8 rotates, the outer skin 24 of each blade 12 may be eroded along its leading edge 18 by particulates, debris or moisture in the air. Over time, this results in damage which is characterized as a loss of material from the blade 12 and may include pitting, gouges, or crack-like features. As Figure 2 shows, this damage first presents in an area 30 close to the blade tip 16, where the relative circumferential speed of the blade is highest in operation, although over time the damaged area 30 will creep towards the blade root 14. In the damaged area 30, the top coat has been eroded to expose parts of the second and third layers of the outer skin 24.

Such damage adversely affects the aerodynamic qualities of the blade 12, thus resulting in reduced operational efficiency of the wind turbine 2. Should the damage be allowed to continue to worsen, the structural integrity of the blade 12 may eventually be compromised.

The leading edge 18 damage may define differing levels of severity based on how deep the damage extends inwardly into the material layers defining the outer skin 24 of the blade 12. The topcoat, second layer, and third layer of the outer skin 24 may be formed from different colours of material so that the severity of the damage can be easily determined from a visual assessment.

Figure 3 illustrates a blade repair system 100 according to an embodiment of the invention for repairing damage on the leading edge 18 of a blade 12.

The system 100 includes an unmanned aerial vehicle (UAV) 102 which supports a leading edge repair assembly 104 via a cable arrangement 106. In other embodiments, the system 100 may include an alternative means of lifting the leading edge repair assembly 104, such as a crane.

The leading edge repair assembly 104 comprises a generally straight, rigid beam 108; a propulsion system 1 10 that acts as a thruster; and a robotic maintenance device 112 for carrying out maintenance on a blade 12. The propulsion system 110 is perpendicularly mounted at a first end 114 of the beam 108 and is configured to generate a lateral force that pushes the leading edge repair assembly 104 in the direction indicated by the arrow in Figure 3 towards the wind turbine blade 12. The robotic maintenance device 1 12 is hingedly mounted at an opposite, second end 116 of the beam 108.

The UAV 102 is depicted as a multi-rotor drone which can freely move in the air near to the blade 12, although it should be appreciated that any suitable aerial vehicle may be used in embodiments of the invention. The drone 102 includes a navigational system that is configured to use Lidar (light detection and ranging) technology to position the system 100 in front of the leading edge 18, the navigational system being fully or partially autonomous and/or controlled by a remote operator, for example.

The cable arrangement 106 connects the drone 102 with the beam 108 so that the beam 108 hangs freely beneath the drone 102. The cable arrangement 106 comprises a main cable 120 which extends from the drone to a junction, and two inclined cables 122 of equal length which diverge from the junction 124 to connect to opposite ends of the beam so that the cable arrangement 106 forms an inverted Y-shape that supports the beam 108 in a substantially horizontal orientation. The main cable 120 incorporates an in-line winch 126, suspended from the drone 102, which is operable for adjusting the length of the main cable 120 and thereby provide a means for adjusting the vertical position of the beam 108.

The robotic maintenance device 112 is configured to interface with the leading edge 18 of a blade 12 that is oriented substantially vertically. Advantageously, the drone 102 can position the robotic maintenance device 112 to interface with the leading edge 18 for most blade pitch angles, so that the robotic maintenance device 112 can engage a blade 12 that is pitched for normal operation, for example. In turn, the propulsion system 1 10 holds the robotic maintenance device 1 12 in firm engagement with the blade 12 during the repair process, allowing the position of the device 1 12 on the blade 12 to be controlled precisely.

The robotic maintenance device 112 comprises an elongate support body 128 that connects to the beam 108 approximately at its midpoint and orients such that its main longitudinal axis extends generally vertically and parallel to the leading edge 18 under repair, in use. The support body 128 supports a repair tool 130 for repairing leading edge 18 damage, and a drive system 132 which is operable to move the support body 128 along the length of the leading edge 18 as the repair is completed.

By virtue of drawing in or letting out the main cable 120, the winch 126 can compensate for movement of the device 112 along the leading edge 18 to keep the beam 108 horizontal. The horizontal angle of the beam 108 can also be maintained by moving the drone 102 itself upwardly or downwardly as necessary.

In this embodiment, the propulsion system 1 10 comprises a propeller 134 which is driven by a motor 136 to rotate on an axis substantially aligned with a longitudinal axis of the beam 108 to provide a lateral force in the form of horizontal thrust, which acts in a direction extending generally from the propeller 134 to the robotic maintenance device 112. In this way, the propeller 134 pushes the robotic maintenance device 112, via the beam 108, into engagement with the leading edge 18 of the blade 12 to achieve a secure interface between the edge 18 and the device 1 12. This secure interface ensures that the position of the device 1 12 on the leading edge 18 can be controlled effectively and thereby improves the precision with which the repair tool 130 repairs the damage, thus resulting in a high quality repair. Furthermore, a secure interface ensures that the traction required for the drive system 132 to be effective is achieved.

Indeed, in this embodiment the lateral force created by the propulsion system 1 10 contributes to holding the robotic maintenance device 1 12 in position on the blade 12 by counteracting the tendency of the device 1 12 to fall due to its weight under gravity. In this respect, the lateral force generates frictional engagement between the robotic maintenance device 112 and the blade surface 23. Meanwhile, the drone 102 and winch 126 are operated to position and orient the support beam 108 of the leading edge repair assembly 104 to control the direction in which the force generated by the propulsion system 110 acts.

Similarly, the propulsion system 110 must typically be kept within a certain vertical range of the level of the robotic maintenance device 112 to avoid the force generated by the propulsion system 1 10 from having a significant vertical component, such that it will tend to move the robotic maintenance device 1 12 on the blade 12 and thereby interfere with control of the device 1 12 position. Figure 4 shows a blade repair system 200 according to another embodiment of the invention, in which a leading edge repair assembly 204 is suspended from a drone 102 via two parallel, horizontally spaced support cables 206. The support cables 206 could be identical.

As in the embodiment of Figure 3, the leading edge repair assembly 204 of Figure 4 comprises a robotic maintenance device 112 and a propulsion system 110 comprising a propeller 134 which are disposed at opposite ends of a beam 208. In this embodiment, the assembly further comprises a counterweight 210. The counterweight may be mounted to an upwardly- inclined end portion of the beam 208, in which case the propellor 134 could be mounted on the counterweight 210. The propeller 134 is preferably close to the vertical level of the centre of gravity to maintain the leading edge repair assembly 204 in balance. The counterweight 210 balances the weight of the robotic maintenance device 112 at the opposite end of the beam 208 about a centroid 212 located approximately at the midpoint of the beam 208. The two parallel cables 206 are spaced apart so as to connect with the beam 208 on either side of the centroid 212. In this way, when the drone 102 is flying parallel to the ground, the beam 208 is held substantially horizontally.

Each support cable 206 includes an in-line winch 226, the winch being operable for adjusting the length of its respective support cable 206 so that the beam 208 can be kept horizontal, for example if the drone 102 flies at an angle to the ground.

In contrast to the embodiment of Figure 3, the beam 208 comprises a flexible portion 214 located between an end of the beam connected to the robotic maintenance device 112 and the support cable 206 closest to the robotic maintenance device 112. The flexible portion 214 accommodates relative rotation of the beam 208 and the robotic maintenance device 112 in all directions, and so allows the drone 102 to make small positional movements in the air without disrupting the position of the robotic maintenance device 112 on the blade 12, thereby isolating the robotic maintenance device 1 12 from the drone 102 to some extent and avoiding the need to synchronise movement of the drone 102 and device 1 12 perfectly. Conversely, isolating the robotic maintenance device 112 from the drone 102 in this way also allows for movement of the device 112 relative to the drone 102 due to movement of the blade 12, meaning that the drone 102 need not follow movement of the blade 12 precisely. Figure 5 shows a blade repair system 300 according to another embodiment of the invention, which comprises a leading edge repair assembly 304 pivotably coupled directly to the underside of a drone 102 via a universal joint 306.

As in the previously described embodiments, the leading edge repair assembly 304 comprises a robotic maintenance device 1 12 and a propeller 134 disposed at opposite ends of a beam 308. In this embodiment, the universal joint 306 connects to a symmetrically-angled, V-shaped portion of the beam 308 so that the leading edge repair assembly 304 can pivot freely beneath the drone 102. Similarly to the Figure 4 embodiment, in Figure 5 a counterweight 210 supports the propellor 134 and balances the weight of the robotic maintenance device 1 12 about the universal joint 306, so that the beam 308 is substantially horizontal when the repair assembly 304 is in equilibrium.

The universal joint 306 comprises a stabilisation mechanism, for example based on a three-axis gimbal, which is operable for stabilising movement of the universal joint 306 and keeping the beam 308 substantially horizontal when the drone 102 moves.

Similarly to the embodiment of Figure 4, the beam 308 comprises a flexible portion 214, located between an end of the beam 308 connected to the robotic maintenance device 112 and the V-shaped portion attached to the drone 102. The flexible portion accommodates relative rotation of the beam 308 and the device 1 12 in all directions, which prevents fluctuations in the drone’s position from affecting the robotic maintenance device 112. This allows the drone 102 to move to compensate for changes in wind, for example, without impacting the robotic maintenance device 112.

Examples of robotic maintenance devices 1 12 for use in embodiments of the invention are shown in Figures 6a to 6c, any of which may be used in any of the arrangements shown in Figures 3 to 5. As previously mentioned, in each example the robotic maintenance device 112 comprises a support body 128, a repair tool 130 for repairing damage, and a drive system 132 which is operable to move the support body 128 along a leading edge 18 as a repair is completed.

The repair tool 130 is coupled to a lower end of the support body 128 with respect to an operational position, the tool 130 being pivotably coupled to the support body 128 so that it may pivot about a transverse axis that is substantially horizontal in use, to follow the profile of a leading edge 18 over a blade tip 16 and thereby repair a portion of a leading edge 18 that extends around onto a blade tip 16.

Typically, a repair process involves at least abrading, cleaning and coating application actions. First, the outer skin 24 is abraded in the region of the damage in order to create a smooth surface free from imperfections. Any dust or debris created during the abrading action is then cleaned from the surface. Finally, one or more layers of coating material (such as protective paint) are applied to the abraded surface to restore the original profile of the blade 12.

The repair tool 130 is removable and carries a tool head 140 which is operable for carrying out at least one action during the repair process. The repair tool 130 may be interchanged with other repair tools as different actions are completed during the repair process. It will be understood that the particular design of each repair tool 130, as well as the total number of repair tools used during the repair process, may vary, as each repair tool 130 is designed to provide a certain functionality and such functionality needs may vary in different contexts and applications. The repair tool 130 may comprise a robotic mechanism configured to control the movements of the repair tool head 140.

Figure 6a shows an example of a robotic maintenance device 112 carrying a repair tool 130 equipped with an abrading tool head 140, the tool head 140 comprising a rotating deburring disc 144 which is operable for abrading the outer skin 24 of the leading edge 18 to the depth of the damage. Any dust or debris created during the abrading action may be blown away, or cleaned, from the blade surface 23 by the draft caused by the propeller 134. Additionally or alternatively, separate means of cleaning the blade surface 23 may be provided on the repair tool 130.

Figure 6b shows an example of a robotic maintenance device 1 12 carrying repair tool 230 equipped with a coating application tool head 240. This may be a different device 112 to that shown in Figure 6a, although in this example it is the same device 112 but fitted with a different repair tool 230.

The coating application tool head 240 comprises means of applying a coating material (such as paint) to the outer skin 24 of the leading edge 18. Such means may comprise one or more paint rollers, a spatula or the like. It is envisaged that the abrading tool head 140 of Figure 6a and the coating application tool head 240 of Figure 6b may be used in succession as the repair process proceeds. As such, once the abrading action is completed, the leading edge repair assembly 104, 204, 304 is flown back to an operator and the repair tool 130 carrying the abrading tool head 140 is manually removed from the repair device 1 12 and replaced with the repair tool 230 including the coating application tool head 240. Alternatively, the robotic maintenance device 112 may be capable of swapping the tool heads 140, 240 in situ, thereby avoiding the need to return the device 1 12 to an operator.

Figure 6c shows an example of a rotatable repair tool 330, which comprises an abrading tool head 140 facing in a first direction and a coating application tool head 240 facing in a second direction, the first and second directions being mutually opposed. The repair tool 330 can be selectively oriented according to the action to be carried out. In this way, the repair tool 330 does not need to be removed and replaced as the repair process proceeds.

For each of the examples shown in Figures 6a to 6c, the drive system 132 of the robotic maintenance device 1 12 comprises a wheel arrangement 146 which includes two pairs of wheels 148 driven by at least one motor (not shown) at a controllable speed along the leading edge 18. As shown schematically and in greatly simplified form in Figure 7, the wheel arrangement 146 is configured for a clamping movement to press each wheel 148 into engagement with the blade 12 whilst being able to compensate for changes in the blade profile as the robotic maintenance device 112 drives along the leading edge 18 of the blade 12.

More specifically, a first wheel 148a of a pair is coupled to a first end of a first curved axle 150a that is coupled to a first cog 152a at a second end. The first cog 152a interconnects with second cog 152b that is coupled to a second wheel 148b of the pair via a second curved axle 150b. The first and second cogs 152a, 152b therefore define a geared interface between the first and second curved axles 150a, 150b, such that the first and second curved axles 150a, 150b can pivot in the plane of the cogs 152a, 152b in order to move the wheels 148a, 148b together or apart as required in a scissor-like action. This movement can be utilised to provide a clamping force that helps to hold the position of the maintenance device 112 on the blade 12, and therefore complements the traction created by the lateral force generated by the propulsion system 110. The drive system 132 is configured to move the robotic maintenance device 112 along the length of the leading edge 18 as the repair is completed, making use of the traction created by the pressing force provided by the propulsion system 132 together with the clamping action of the geared interface of the device wheels 148. In this way, the repair tool 130, 230, 330 is moved along the leading edge 18 of the blade and passes over the area to be repaired as required, so that the tool head 140, 240 can complete the necessary action.

The skilled person will appreciate that modifications may be made to the specific embodiments described above without departing from the inventive concept as defined by the claims.

Claims

1. A method of repairing a blade (12) of a wind turbine generator (2), the method comprising: lifting a blade repair assembly (104, 204, 304) to a position adjacent to a region of the blade (12) to be repaired; orienting a repair device (112) of the blade repair assembly (104, 204, 304) towards the region of the blade (12) to be repaired; operating the blade repair assembly (104, 204, 304) to generate a lateral force to press the repair device (122) into engagement with the blade (12); and while continuing to press the repair device (1 12) into engagement with the blade (12), operating the repair device (112) to repair the blade (12).

2. The method of claim 1 , comprising operating a thruster (1 10) of the blade repair assembly (104, 204, 304) to generate the lateral force.

3. The method of claim 1 or claim 2, comprising pitching the blade (12) towards a feathered position.

4. The method of any claims 1 and 2, comprising pitching the blade (12) towards a 0^e pitch angle with the chord of the blade substantially perpendicular to a rotor axis of a rotor to which the blade belongs.

5. The method of any preceding claim, comprising moving the repair device (112) on a surface (23) of the blade (12) whilst repairing the blade (12).

6. The method of claim 5, wherein moving the repair device (1 12) comprises operating a drive system (132) of the repair device (1 12).

7. The method of claim 5 or claim 6, wherein moving the repair device (1 12) comprises adjusting a vertical position of the blade repair assembly (104, 204, 304).

8. The method of any of claims 5 to 7, comprising moving the repair device (112) along an edge of the blade (12) whilst repairing damage to the edge.

9. The method of claim 8, wherein the edge is a leading edge (18) of the blade.

10. The method of any preceding claim, comprising lifting the blade repair assembly (104, 204, 304) using an aerial vehicle (102).

11 . The method of claim 10, comprising flying the aerial vehicle (102) to the wind turbine generator (2) with the blade repair assembly (104, 204, 304) coupled to the aerial vehicle (102).

12. The method of claim 11 , wherein the thruster (110) of the blade repair assembly (104, 204, 304) is inactive while the aerial vehicle (102) flies to the wind turbine generator (2).

13. The method of any of claims 10 to 12, wherein the blade repair assembly (104, 204, 304) is mounted directly to the aerial vehicle (102).

14. The method of claim 13, wherein the blade repair assembly (104, 204, 304) is pivotably mounted to the aerial vehicle (102).

15. The method of any of claims 1 to 9, comprising lifting the blade repair assembly (104, 204, 304) using a crane.

16. The method of any of claims 1 to 12 or claim 15, comprising suspending the blade repair assembly (104, 204, 304) on a line (106, 206).

17. The method of claim 16, comprising suspending the blade repair assembly (104, 204, 304) on a pair of parallel lines (206).

18. The method of claim 16 or claim 17, comprising operating a winch (126, 226) to draw in or pay out the line (106, 206) to adjust the vertical position of the blade repair assembly (104, 204, 304). 17

19. The method of any preceding claim, comprising lifting the blade repair assembly (104, 204, 304) by a support body (128) of the assembly to which the repair device (1 12) is coupled.

20. The method of claim 19, comprising rotating the repair device (1 12) relative to the support body (128) while repairing the blade (12).

21. The method of any preceding claim, wherein the repair device (112) comprises a robotic device.

22. The method of any preceding claim, wherein operating the repair device (112) to repair the blade (12) comprises abrasive and/or coating operations.

23. The method of any preceding claim, comprising controlling a position and/or orientation of the blade repair assembly (104, 204, 304) to control a direction in which the lateral force generated by the blade repair assembly (104, 204, 304) acts on the repair device (112).

24. The method of claim 1 or claim 2, wherein the blade (12) is oriented substantially vertically.

25. A blade repair assembly (104, 204, 304) for repairing a blade (12) of a wind turbine generator (2), the assembly comprising: a repair device (1 12) configured to engage and to repair a surface (23) of the blade (12); a coupling by which the blade repair assembly (104, 204, 304) may be lifted to position the repair device (112) in the vicinity of the blade (12); and a propulsion system (110) arranged to generate a lateral force to press the repair device (112) into engagement with the blade (12).

26. The assembly (104, 204, 304) of claim 25, wherein the propulsion system (1 10) comprises a thruster. 18 The assembly (104, 204, 304) of claim 26, wherein the thrustor comprises a rotor. The assembly (104, 204, 304) of any of claims 25 to 27, wherein the repair device comprises a drive system (132) configured to move the device on a surface (23) of the blade (12). The assembly (104, 204, 304) of any of claims 25 to 28, comprising a support body (128) that supports the repair device (1 12) and the propulsion system (1 10), the support body (112) comprising the coupling. A blade repair system (100, 200, 300) for repairing a blade (12) of a wind turbine generator (2), the system comprising the blade repair assembly (104, 204, 304) of any of claims 23 to 27 and an aerial vehicle (102) configured to lift the blade repair assembly (104, 204, 304) by the coupling, to engage the repair device (1 12) with the blade (12).