Maintenance system for offshore wind turbines

Abstract

A maintenance system for offshore wind turbines (20) comprises a base location (101) that stores a drone (102) and a maintenance robot (103). The maintenance robot (103) is for performing maintenance tasks on the offshore wind turbine( 20). It includes a movement system (106) configured for movement around the wind turbine using access designed for human maintenance personnel, an imaging system (107) for providing images of the wind turbine, and a tool (108) for carrying out maintenance tasks. The drone (102) is for transporting the maintenance robot (103) from the base location (101) to the offshore wind turbine( 20). The drone (102) is an unmanned vehicle comprising: a propulsion system (109) for transporting the drone and its payload, and a payload handling system (112) for holding the maintenance robot (103) and for loading / unloading the maintenance robot (103) at a landing area (15) of the offshore wind turbine (20). A navigation and positioning system (114, 115) is used for guidance of the drone (102) to the offshore wind turbine (20) and for positioning the drone (102) at the landing area (15).

Description

[0001] MAINTENANCE SYSTEM FOR OFFSHORE WIND TURBINES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a maintenance system and a method of providing maintenance for offshore wind turbines. The invention also relates to a robot apparatus for use in the system. The offshore wind turbines may for example be floating offshore wind turbines.

[0004] BACKGROUND OF THE INVENTION

[0005] Offshore wind farms typically include an array of wind turbines that are spaced apart at an offshore site. The offshore site should have connectivity for transfer of power to the power consumer, which may involve power cables connecting to the land or in some cases to an offshore consumer and / or energy storage facility, such as an offshore hydrogen production facility. It also needs to be possible to install suitable structures in and below the water using fixed foundations or via wind turbines with floating foundations. This can be in the sea or in another body of water, with the location being selected for its weather conditions and / or for convenience of access / installation. Offshore wind farms can have less impact on people and the landscape by virtue of their offshore location, which can be in a remote area. It is also generally possible to find offshore sites with more favourable wind conditions than on land, such as with higher wind speeds and / or more consistent wind conditions, so offshore farms generate more electricity per amount of capacity installed.

[0006] Global offshore wind energy production capacity has risen significantly in recent times, and this is expected to continue to increase rapidly. Studies have found that for some years offshore wind power in Europe has been competitive on price with conventional power sources such as fossil fuel power sources. There are numerous motivations for replacement of non-renewable power sources with wind power. There will be considerable benefits from enhancing the productivity of offshore wind energy, both in terms of maximising green energy production from existing wind farms as well as boosting the uptake of wind energy as an alternative to non-renewable energy. A key factor in the productivity of offshore wind energy is maintenance. There is a significant loss in productivity if a wind turbine has to be shut down or run at less than 100% capacity due to a need for maintenance. An offshore location can be difficult to access as well as requiring potentially expensive marine or air transport to bring out skilled maintenance staff and materials to service or repair the wind turbines and associated offshore infrastructure. Locations that are attractive for wind energy production are often subject to wind / weather conditions that restrict access for maintenance staff / materials, especially during seasons with a high occurrence of adverse weather. This has the result of increasing the downtime for offshore wind turbines that require maintenance, which adversely impacts the overall productivity for an offshore wind farm in a way that does not occur onshore, where year-round access is generally possible. Moreover, maintenance of turbines in offshore locations has a significant cost impact in addition to lost productivity due to the required transport of personnel / materials by sea or air.

[0007] SUMMARY OF THE INVENTION

[0008] Viewed from a first aspect, the present invention relates to a maintenance system for offshore wind turbines, the system comprising: a maintenance robot for performing maintenance tasks on an offshore wind turbine, the maintenance robot comprising: a movement system configured for movement around the wind turbine using access designed for human maintenance personnel, an imaging system for providing images of the wind turbine, and a tool for carrying out maintenance tasks; a base location for storing the maintenance robot; a drone for transporting the maintenance robot from the base location to the offshore wind turbine, the drone being an unmanned vehicle comprising: a propulsion system for transporting the drone and its payload, and a payload handling system for holding the maintenance robot during transport and loading / unloading the maintenance robot at a landing area of the offshore wind turbine; a navigation and positioning system for guidance of the drone from the base location to the offshore wind turbine and for positioning the drone at the landing area on the offshore wind turbine in order to unload the maintenance robot; and a control system for control of the drone and the maintenance robot in response to maintenance requirements of the offshore wind turbine.

[0009] Viewed from a second aspect, the invention provides a method of providing maintenance for offshore wind turbines, the method comprising using a system as defined above.

[0010] The invention further extends to a combination of a drone and maintenance robot that are configured for use in the maintenance system. This combination may thus comprise: a maintenance robot for performing maintenance tasks on an offshore wind turbine, the maintenance robot comprising: a movement system configured for movement around the wind turbine using access designed for human maintenance personnel, an imaging system for providing images of the wind turbine, and a tool for carrying out maintenance tasks; a drone for transporting the maintenance robot from a base location to the offshore wind turbine, the drone being an unmanned vehicle comprising: a propulsion system for transporting the drone and its payload, and a payload handling system for holding the maintenance robot during transport and loading / unloading the maintenance robot at a landing area of the offshore wind turbine; a navigation and positioning system for guidance of the drone from the base location to the offshore wind turbine and for positioning the drone at the landing area on the offshore wind turbine in order to unload the maintenance robot; and a control system for control of the drone and the maintenance robot in response to maintenance requirements of the offshore wind turbine.

[0011] With this approach the need for human intervention for maintenance tasks can be reduced or removed and the tasks can be completed with the offshore wind turbine remaining fully unmanned. The drone may for example be an aerial drone in the form of an unmanned aerial vehicle, e.g. with a flight controller and a landing gear for enabling landing on the base location and / or the landing area of the offshore wind turbine. Alternatively the drone may be an unmanned marine vehicle, e.g. with a marine navigation system and an automated gangway device for coupling the drone to the landing area of the offshore wind turbine for loading and unloading the maintenance robot, e.g. by transferring it from a cargo area of the drone onto the landing area of the offshore wind turbine. The automated gangway device may be a part of the payload handling system. The marine vehicle may be designed to float and to transport the maintenance robot over the surface of the water, or alternatively it may be a submersible or semi-submersible vehicle allowing for transport beneath the surface of the water. The landing area is advantageously an existing landing area of the offshore wind turbine, e.g. a landing area provided for access by humans who may arrive by boat and / or by air transport. Thus, the landing area may be a platform located near to sea-level that can be accessed via a floating vessel and / or from the air. Such a landing area may for example be located on a buoyancy member of a floating offshore wind turbine. Alternatively, or additionally, the offshore wind turbine may include a high level landing area designed only for access from the air.

[0012] In relation to the above aspects “maintenance” should be understood in a broad sense and could for example include repair, servicing and / or inspection. Thus, possible maintenance tasks may comprise physical repair or replacements of parts, fluids or paint, adjustment, replacement or repair of existing parts and equipment, and / or pure inspection tasks without any need for physical repair actions. The maintenance tasks may for instance be the monitoring and inspection of new or existing fatigue cracks in welds such as steel weldments.

[0013] The access designed for human maintenance personnel may for example be one or more access space(s) intended for use by human maintenance personnel. The access may thus be spaces designed to be entered / occupied by a human during maintenance tasks. The maintenance robot is advantageously configured to enter and / or occupy such access, e.g. moving around the interior and / or exterior of the wind turbine within spaces that have been included for use by humans.

[0014] By means of the invention any human involvement would involve people at a remote location with no human presence at the offshore wind turbine. This means that maintenance tasks can be done without the need to wait for transport of human maintenance personnel to the offshore wind turbine and without the need to fit around weather conditions permitting such transport. A drone is capable of transporting the maintenance robot to the offshore wind turbine in conditions where human access to the offshore wind turbine may not be permitted, e.g. due to adverse wave conditions or other weather preventing use of boats and / or use of aircraft capable of carrying people. Even in instances where human access could be permitted it may with the present invention involve reduced costs to carry out maintenance tasks without the need for physical transport of humans to the offshore wind turbine. The drone and robot can also be deployed from a base location at any time without working in shifts or requiring meal breaks. There is no problem in having the drone and robot wait for long periods before immediate activation in response to a call for maintenance. It will also not be a problem to mobilize the drone and robot to the wind turbine, carry out the maintenance work, and then simply leave the drone and robot behind on the offshore wind turbine for days, or even weeks or months, in case of bad weather or any technical problems met during the return of the drone and robot. This means that the maintenance operation could be commenced without having to take a “weather window” for the safe return into consideration. On the other hand, when humans are entering an offshore wind turbine, one would always have to wait until a sufficient long weather window is available for the transit to the wind turbine, carrying out the maintenance work, and the safe return of the personnel to the ship or helicopter. This is in particular an issue during the winter season with few “weather windows” available of any length. The base location can advantageously provide for storage of multiple drones and robots. The base location may itself be unmanned and in a remote location, where it is not possible to “store” human maintenance personnel but where there is greater convenience for access to the offshore wind turbines.

[0015] As well as this there are fewer constraints on the operating status of the wind turbine itself. For human access there is often a requirement to shut down the wind turbine or sub-systems thereof, with the shutdown being needed at all points when people are on board. With the proposed maintenance system some maintenance tasks can be completed without any loss of energy production, and others will involve shorter shut down periods with the wind turbine still running at other times, e.g. during unloading / loading of the maintenance robot and movement of the maintenance robot to a maintenance location on the wind turbine.

[0016] There are thus significant technical improvements for this approach compared to using human maintenance personnel where there are constraints from the location and availability of people. This provides for higher productivity of offshore wind farms since the downtime for maintenance is significantly reduced and since the periods / weather conditions when maintenance access is possible is greatly increased.

[0017] The system may comprise just one maintenance robot or it may comprise multiple maintenance robots that can optionally be deployed together in order to perform multiple maintenance tasks in parallel and / or to perform more complex maintenance tasks. Alternatively, or additionally, the maintenance robot may itself comprise multiple robots, e.g. several robots working together to perform the maintenance task. Thus, the maintenance robot may consist of one robot or may comprise several separable robots capable of working together or separately. For example, there may be a transport robot for transporting one or more other robot(s) and optionally materials and tools, wherein the other robot(s) include one or more of an inspection robot, a tool operating robot, or any other form of robot. In the discussion below the references to the maintenance robot should be taken to include the option of several separable robots as discussed above.

[0018] The maintenance robot may be at least partially remote controlled. It may include some autonomous systems such as for movement guidance or basic maintenance tasks. However, in some example embodiments the maintenance robot is configured to be remoted controlled in order to allow for a remote operator to use the tool to perform a maintenance task. The maintenance robot may for example be provided with remote control systems similar to those known for remote surgery. In this way the system can provide the advantages discussed above in relation to unmanned access for maintenance, whilst also keeping the ability to perform maintenance tasks that require skilled maintenance personnel. Advantageously, and unlike traditional human maintenance, the human operator can change for different tasks or different phases of maintenance allowing for specialists to be brought in as needed as well as permitting efficient use of more highly skilled maintenance personnel with their time being focussed on jobs requiring their specialism, whilst another operator takes over control of the maintenance robot at other times, e.g. during manoeuvring around the wind turbine from the landing area to the maintenance location. This is a marked contrast to maintenance routines using a human presence on the offshore wind turbine, where a majority of the time of a skilled maintenance engineer is lost to transport time and the time needed to move about the wind turbine. The maintenance robot may comprise a robot communication system for transmitting and / or receiving data and / or control signals. The robot communication system may for example: receive control signals providing instruction to the movement system, transmit image data from the imaging system and / or receive control signals providing instruction to the tool. The robot communication system may be configured to communicate with the control system in order to facilitate control of the maintenance robot, e.g. remote control for maintenance operations guided by a human operator. The robot communication system may be configured for communications with the offshore wind turbine, e.g. for wireless transmission or receipt of data. The robot communication system may be configured to communicate with the drone and / or with the navigation and positioning system.

[0019] The drone may include a drone communications system for transmitting and / or receiving data and / or control signals. The drone communication system may for example: receive control signals providing instruction to a movement controller of the drone such as the flight controller of an unmanned aerial vehicle or the marine navigation system of an unmanned marine vehicle, transmit data from an imaging system or other sensor system of the drone, and / or receive control signals providing instruction to the payload handling system. The drone communication system may be configured to communicate with the control system in order to facilitate control of the drone, e.g. to receive instructions about a designation location or for remote control for operations guided by a human operator such as landing and take-off / cast-off. The drone communication system may be configured for communications with the offshore wind turbine, e.g. for wireless transmission or receipt of data. The drone communication system may be configured to communicate with the robot and / or with the navigation and positioning system.

[0020] The drone communication system and / or the robot communication system may include a short range communication device for local communications and, for example, not capable of communicating with the base location when it is a larger distance away (so that the short range communication is not effective). In this case the communication system may be adapted to interface with a long range communication system of the offshore wind turbine or of the other of the drone or robot. For example, the offshore wind turbine may be able to communicate using data connections included in power-to-shore electrical cables, such as fibre optic cables incorporated in the power-to-shore electrical cables. In this case the short range communication device may be used for communication with the offshore wind turbine to allow for longer range communication (e.g. with the base location) to be “piggy -backed” via the communication system of the offshore wind turbine. It will be appreciated that in some embodiments it is not required for both of the drone and the robot to be capable of long range communications, but instead they may share a long range communications capability of just one of the drone or robot, or of the offshore wind turbine.

[0021] The short range communication device may for example be a short-range device (SRD) as described by ECC Recommendation 70-03. Suitable short range communications technologies include Bluetooth such as Bluetooth Low Energy (BLE), Wi-Fi, near-field communication (NFC), LPWAN, ultra-wideband (UWB) and IEEE 802.15 based specifications such as Zigbee. The short range communication device may have a range of several hundred metres. The maintenance system may be configured to use short range communications technologies for communication over distances of 500m or less, optionally 250m or less, or in some cases 100m or less. Technologies allowing for mesh networking may be used to create a larger area over which short range communications devices can be used, e.g. a mesh network in which each offshore wind turbine in an offshore field can be a part of the mesh network along with the maintenance robot and / or the drone.

[0022] Alternatively, or additionally, the drone communication system and / or the robot communication system may comprise a long range communication device for long range communication, such as for communication with the base location. The long range communication device may also allow for short range communication such as between the robot and the drone. The long range communication device may for example be a wireless device operating with signals in the radio frequency range. Suitable long range communications technologies include DigiMesh, Long Range Radio (LoRa), and cellular networks including 4G or 5G systems. The long range communication device may have a range in excess of 500m. The maintenance system may be configured to use long range communications technologies for communication over distances of 500m or more, optionally 1 km, 2 km, 5 km, 10 km, 100km or more, or in some cases the range may be effectively global, such as via use of cellular networks linked into the global cellular network system.

[0023] The communication devices, either short range or long range, preferably make use of a low latency streaming technology advantageously having a latency of less than 100ms. Possible streaming technologies include open source video transport protocols such as Secure Reliable Transport (SRT), application-level network protocols such as Real Time Streaming Protocol (RTSP), and Web Real- Time Communication (WebRTC). In an example embodiment a suitable low latency streaming technology is used to transmit image data from the imaging system of the maintenance robot to the control system and / or to a remote operator. This may for example be done in the form of video feed as an underlying layer. Other data, e.g. for the navigation and positioning system may also be sent via the low latency streaming technology. Reductions to latency may allow for a higher resolution of data (e.g. higher resolution image data, higher frame rate of video feed, higher frequency of position updates) to be sent so that there is more accurate control of remote operations, such as movement of the drone or the maintenance robot, or maintenance tasks being carried out by the maintenance robot. Using low latency systems may also allow for the processing of data at a remote location (e.g. in the cloud) using higher power processing than that which is available at the drone or at the maintenance robot, as the case may be. This minimises the processing power / software requirements for the mobile parts of the system, whilst still allowing for full use of the data that is obtained at the offshore wind turbine. The processing of data may for example include machine image processing for image recognition and / or to enhance operation of the navigation and positioning system.

[0024] The maintenance robot comprises a movement system for movement of the robot around the wind turbine using access designed for human maintenance personnel. As noted above the movement system may be provided by a transport robot that forms a part of the maintenance robot, e.g. together with other robots such as an inspection robot. The required movement may include movement inside the wind turbine tower, rotor blade, rotor hub, nacelle and / or foundation to a maintenance location. The movement may alternatively or additionally include movement on the outside of the wind turbine tower, rotor blade, rotor hub, nacelle or foundation. This is done without having people onboard said offshore wind turbine. The movement system can use access designed for humans and advantageously makes use of one or more spaces, structures or features of the offshore wind turbine that are designed for human use. The maintenance robot may thus be configured to use one or more of: ladders, stairways, lifts, doorways, walkways and / or any other pre-existing human infrastructure of the wind turbine.

[0025] The maintenance robot may be configured to pass through an opening of less than Im wide by 2.5m high, preferably less than 0.5m wide by 1.5m high. The robot may be configured to be able to mount stairs with width of 40 cm or less, depth of 20 cm or less and height of 15 cm or more. The robot may be configured to fit into a space (e.g. as in a lift) with floor area of less than Im x Im, or less than 0.5m x 0.5m. The robot may have a weight of less than 100kg, preferably less than 75kg. The robot may be configured to have a vertical reach of at least 1.5 m, preferably at least 2 m.

[0026] The maintenance robot and / or the drone may be configured to operate a human accessible entry system of the wind turbine. This may for example involve opening of a doorway or hatch, movement of the maintenance robot through the resulting access way, then closing of the doorway or hatch. This may include unlocking and / or unsealing a weatherproof door. Alternatively, or additionally, the maintenance system may be designed to operate with a modified design of wind turbine including specific adaptations for access of the maintenance robot, such as an actuation system for the (human access) door that can open it in response to a remote command, or a special access point for the maintenance robot.

[0027] The maintenance system will use the drone to deliver the maintenance robot to the offshore wind turbine and may then use the same drone to return the maintenance robot to the base location. The drone may stay at the offshore wind turbine whilst the maintenance task is completed. Alternatively, if for instance an electrically powered drone with batteries is used, a second drone could be stationed and pre-charged on the wind turbine and used for the return journey. The maintenance robot may be configured to return to the drone, either automatically or under remote control once the maintenance task is completed. During return of the maintenance robot to the drone then a human accessible entry system of the wind turbine may be operated as above in order to allow the robot to exit the wind turbine. The movement system of the maintenance robot may include one or more of wheels, legs, tracks, propellers, magnetic grippers, or other mechanisms. It may for example be include a quadruped robot movement system of the type used for so- called “robot dog” designs such as Boston Dynamics’ Spot, MIT’s cheetahs and Unitree’s Gol. A quadruped design can be sized to use human infrastructure as set out above, including being able to reach sufficiently high either by means of a suitable arm or by standing on two legs. The legs may include grippers (e.g. hooks and / or clamps) for climbing on ladders.

[0028] The robot may comprise a robot positioning system for guiding movement and / or for providing data to a remote user who controls movement of the robot. This may for example be a part of the robot’s movement system. The robot positioning system may include sensors or the like for providing data to be used alongside data from the imaging system. For example, the robot positioning system may include one or more of GPS / GNSS receivers, inertial navigation systems (INS), LiDAR scanners, ultrasonic sensors and cameras. The robot positioning system may make use of input from the navigation and positioning system of the maintenance system and / or the robot positioning system may provide input to the navigation and positioning system of the maintenance system and hence may form a part thereof. Increased accuracy for GPS and / or GNSS may be provided by so called GNSS enhancement, such as via real-time kinetic positioning (RTK) or other means to use external information, e.g. relating to known positions of markers or beacons at the offshore wind turbine. The robot positioning system may be provided with a map or model of the wind turbine in order to aid movement of the robot, e.g. around internal structures thereof.

[0029] The imaging system of the maintenance robot may allow for machine image recognition and / or for capture of image data for transmission to a remote human operator. This imaging system may be used to provided information to the maintenance robot’s movement system for aiding manoeuvring around the wind turbine and the foundation structure. By means of the imaging system the maintenance robot (and optionally the remote operator) is aware of its surroundings, which is important for control of movement as well as for performing maintenance tasks. The imaging system may include a sensor (such as a digital camera sensor) for providing images using visible light, in which case the imaging system may also comprise a light source. Alternatively, or additionally, the imaging system may use other principles to obtain images of the surrounding environment and / or the maintenance location, such as thermal imaging systems, non-visible light, acoustic imaging systems, laser imaging systems and / or other machine imaging / inspection devices. Alternatively, or additionally, the maintenance robot movement system and / or the maintenance robot imaging system may make use of beacons or transponders placed around the wind turbine and / or the foundation structure at fixed predefined points to assist in the navigation on board the offshore wind turbine. In this way the maintenance robot may be configured to operate in conditions where human maintenance personnel may struggle, e.g. dark, smoke, excessively bright light, as well as obtaining image information that is not possible with the human eye, such as for assessment of the condition of internal parts without disassembly as well as for mapping of temperature or noise that may indicate possible failures giving rise to undesirable wear / friction, vibration or leaking of fluids.

[0030] The imaging system may be provided via an imaging and signal system that provides images as well as other data about the wind turbine and / or has a built in data transmission capability.

[0031] The maintenance robot includes at least one tool for carrying out the maintenance tasks, which may include a tool to replicate a human maintenance task and / or a tool for performing work that is not possible for a human, or both in the same tool. The maintenance robot may be configured to carry out inspection as well as maintenance and / or as a part of its maintenance tasks. In this regard maintenance may include adjustment, replacement, or upgrade of parts including physical parts as well as software. Maintenance may also include servicing tasks of any type, such as replacement of consumables (e.g. fluids) and so on. The maintenance robot may comprise multiple tools, which could be several tools available to use at the same time and / or may include a modular arrangement where a single tool holder can be used for several tool types.

[0032] The tool(s) may include a manipulator for physical maintenance tasks and / or a computer tool for software maintenance tasks. There may be inspection tools to augment the data provided by the imaging system, such as motion or noise sensors for vibration analysis, gas sensors, laser measurement systems, microphones, electrical sensors, weld crack detection sensors and systems and the like. Where electrical sensors are present these may provide the capabilities of a multi-meter, e.g. being able to measure voltage, resistance, and current, as well as optionally capacitance.

[0033] The maintenance robot may comprise a storage system for carrying spare parts and / or tools. The maintenance robot may thus comprise one or more compartment and / or holder for providing storage capacity.

[0034] The maintenance system may be configured to provide the maintenance robot with one or more tool(s) whilst the maintenance robot is at the base location and optionally before it is loaded onto the drone. Thus, the base location may comprise a tool store and / or a tool production facility. In this way the maintenance robot can be provided with the particular tool(s) required for the maintenance task(s) to be performed for a given trip of the drone. The maintenance system may also be capable of manufacturing or assembling a specific tool for a particular maintenance task, and thus the base location may include a facility for manufacture of tools as well as optionally other parts. The maintenance system may be arranged to carry out an on-site inspection in order to identify maintenance requirements (e.g. with a first trip of a drone with a maintenance robot or a separate inspection robot), to prepare one or more tool(s) and / or part(s) for a required maintenance task, then to send a drone with a maintenance robot (e.g. on a second trip) with the pre-prepared tool(s) and / or part(s) so that the robot may perform the maintenance task.

[0035] The base location may comprise a parts store and / or a parts production facility. The parts production facility may also provide the facility for manufacture of tools, where present. The parts store may hold commonly required spare parts for the offshore wind turbine. The parts production facility may for example include an additive manufacturing device for manufacture (e.g. “3D printing”) of replacement parts, which may be done based on data obtained via an imaging system of the maintenance robot. For example, a bespoke part may be made to match the profile of a broken part at the offshore wind turbine, wherein this profile may be obtained via an imaging system of the maintenance robot.

[0036] The maintenance robot is stored at the base location when it is not in use. The base location may provide storage for multiple maintenance robots. In that case the multiple maintenance robots may be of the same design, or they may be of differing design, e.g. for performing different maintenance tasks. The drone may also be stored at the base location when not in use, and the base location may provide storage for multiple drones to allow for multiple simultaneous trips for maintenance of different offshore wind turbines.

[0037] The base location may include a base communication system, which may comprise a short range and / or a long range communications device for communications with one or more of the maintenance robot, the drone, the offshore wind turbine, the control system and / or a remote user interface. The base communication system may comprise a device of the type discussed above. It may also include wired communications to other facilities either offshore or on land.

[0038] The base location may be on land or offshore. It may be located within the area of a wind energy farm, i.e. within a field in which an array of wind turbines is located. In that way the base location may provide a maintenance function that is shared between many closely located wind turbines yet requires only a single base location that may have a few as one robot and one drone. The base location may be offshore but located remote from the offshore wind turbines, e.g. serving several fields. Another option is for the base location to be provided onboard a ship, which may also provide a location for a human operator.

[0039] The base location may include a power system and / or power storage. In example embodiments the base location may receive power from wind energy, such as from a wind farm that is served by the base location. In some possible implementations, the base location may be installed at an offshore wind turbine, e.g. at a conveniently located wind turbine in a wind farm that is served by the base location.

[0040] The drone may be an unmanned aerial vehicle and should be capable of landing at the base location as well as at the offshore wind turbine. It is for transporting the maintenance robot and therefore should be able to comfortably fly holding a payload of appropriate weight and size, i.e. holding the maintenance robot and optionally a docking station for the maintenance robot. Thus, the drone of example embodiments may be able to lift at least 75kg, preferably at least 100kg. The drone may be able to fly whilst carrying a payload fitting in a cuboid of up to 1.5m x Im x Im (length x width x height), or preferably up to 2m x 1.5m x 1.5m.

[0041] The drone may be an unmanned marine vehicle and should be capable of landing (e.g. docking) at the base location as well as at the offshore wind turbine. It is for transporting the maintenance robot and therefore should be able to carry a payload of appropriate weight and size, i.e. holding the maintenance robot and optionally a docking station for the maintenance robot. Thus, the marine drone of example embodiments may be able to transport at least 75kg, preferably at least 100kg. The drone may be able to carry a payload fitting in a cuboid of up to 1.5m x Im x Im (length x width x height), or preferably up to 2m x 1.5m x 1.5m.

[0042] The drone is used to transport the maintenance robot from the base location to the wind turbine that requires maintenance. It should therefore have a range that allows for this length of flight whilst carrying a payload including the weight of the maintenance robot. The range may also allow for a return j ourney in the case that it is not possible to replenish the power supply, e.g. recharged batteries, whilst the drone is at the offshore wind turbine. The drone may for example have a minimum range of 5 km, more preferably at least 10 km, and in some examples at least 15 km. This range should be calculated including the possibility of adverse weather, e.g. head winds potentially in both directions of travel.

[0043] It is an advantage for the drone to be able to land at the offshore wind turbine even when the wind turbine blades are turning. There are particular constraints when landing an aerial vehicle at an offshore wind turbine as there is generally limited space and because the aerial drone must fly accurately and stably to avoid adverse impact from the moving turbine blades and the resulting turbulence. The wind turbine location will be an area subject to strong winds and to resist these winds as well as cope with the added turbulence from the turbine blades the aerial drone of preferred embodiments may be able to fly in wind speeds of up to 30 knots (or up to 55 kph), preferably up to 50 knots (or up to 93 kph). knots. Similarly, a sea-borne drone may be able to maintain a required speed and hearing in the same types of wind speeds, as well as coping with the associated sea conditions.

[0044] In example embodiments the drone may be capable of unloading / loading the maintenance robot in a target area of 5m x 5m or less and / or of safely landing at a landing area of 6m x 6m or less. In this regard the landing area may be an area provided at the offshore wind turbine for use by visiting human maintenance personnel. The landing area may be a platform for landing an aerial transport vehicle such as a helicopter and / or a platform for landing a marine transport vehicle such as a boat. In some examples the landing area is a platform provide at a lower part of the offshore wind turbine that is accessible by sea and by air. The target area may be a target area of a landing area, i.e. a section of the landing area where it is desired for the maintenance robot to be unloaded and / or where it is desired for an aerial drone to land.

[0045] The aerial drone may be capable of landing in a target area of 5m x 5m or less and / or of safely landing in a landing area of 6m x 6m or less. The maximum external dimensions of a lower part of the aerial drone in plan view may be less than 5m x 5m, optionally less than 4m x 4m, and in some embodiments less than 3m x 3m. A small size for the lower parts of the drone, e.g. including the landing gear and payload handling system, allows for ease of access to small landing areas. Upper parts of the aerial drone, which may comprise parts of the propulsion system, may be wider than the lower part. For example, they may be permitted to overhang from the landing area and thus may be larger than the landing area. It will be appreciated that the aerial drone must be able to land with an accuracy that is smaller than the total dimensions of the available landing area in order to allow room for sideways movement during take-off / landing (e.g. from wind gusts or turbulence) as well as room for unloading of the maintenance robot and / or opening of the access (e.g. a door) into the wind turbine.

[0046] In the case of an unmanned marine vehicle (i.e. a sea-borne drone) the drone should be able to transport the maintenance robot to / from the offshore wind turbine and unload / load it at the landing area, for example by holding station adjacent to the landing area and conveying the maintenance robot from the drone to the landing area by using the payload handling system. The payload handling system may comprise a crane system and / or a gangway system. The unmanned marine vehicle may be configured to dock with the offshore wind turbine in order to hold station adjacent to the landing area.

[0047] The drone may comprise an automatic or remotely controlled loading and unloading system for the maintenance robot. This may be a part of the payload handling system. The loading and unloading system may include a mechanism for attaching and detaching the maintenance robot, e.g. via actuatable fixings such as clamps, hooks, electromagnets or other gripper systems. The drone may be configured to transport the maintenance robot and / or parts to be replaced on the wind turbine by carrying it in a docking station, i.e. a housing or fixing structure that holds the maintenance robot and / or parts to be replaced, with the loading and unloading system, which may include a door mechanism of the docking station. In the case of an aerial drone then the drone may lift the docking station, e.g. underslung beneath other parts of the drone. The use of a docking station can allow for a standardised design of payload handling system to be used with multiple different robot types or payloads, since similar docking stations may be used for different forms of maintenance robot. The drone may be permanently attached to the docking station, or there may be a releasable connection so that the drone can leave the docking station behind and, if needed, pick up a different docking station. This may be beneficial when selecting different robot types at the base location, or when replacing a used robot with a fully charged robot.

[0048] The docking station may include connections for power and / or data in order that the maintenance robot can connect to the docking station when it is loaded into it. This may include connectivity for one or more of: recharging batteries of the maintenance robot, communication of data to or from the maintenance robot, control of the maintenance robot, software updates and / or shared use of data between the drone and the maintenance robot (e.g. for purposes of navigation and / or positioning).

[0049] In example embodiments the drone is an unmanned aerial vehicle with a vertical take off capability and may for example be a rotary wing vehicle. The drone may for instance be a quadcopter or other multi-blade rotary wing drone. This type of drone provides suitable manoeuvring and lifting capabilities within a form factor that can be landed in the available landing area for typical offshore wind turbine designs.

[0050] The drone and / or the maintenance robot may be battery powered, such as by means of rechargeable batteries. Lithium ion technologies may be used. Each of the drone and the maintenance robot may include a battery power system. In an alternative configuration there may be a single shared battery power system that is used for the drone propulsion system when the maintenance robot is loaded on the drone, and used to power the functions of the maintenance robot when it is unloaded at the offshore wind turbine. In that case the connection of the maintenance robot to the drone via the payload handling system may include an electrical connection for transfer of power as well as a mechanical connection for carrying the weight of the maintenance robot.

[0051] The base location may include a recharging system for recharging and / or maintaining charge levels of the battery power system(s). The offshore wind turbine may be provided with a recharging system for recharging and / or maintain charge levels of the battery power system(s), e.g. using electrical energy generated at the wind turbine or from the grid.

[0052] Alternatively, the drone and / or the maintenance robot may have a nonelectrical power system and or a non-battery power source, e.g. using any form of engine, motor, fuel cell or generator, such as devices powered by hydrocarbon fuels or by hydrogen. In that case the base location and / or the offshore wind turbine may be adapted to refuel the drone, e.g. by means of fuel such as hydrogen that is stored at the base location and / or the offshore wind turbine.

[0053] By allowing for recharging / refuelling at the offshore wind turbine the maintenance system can have drones with longer ranges and / or maintenance robots with smaller batteries / smaller fuel tanks. There is no need for the drone to retain enough range for a return trip if it can recharge or refuel whilst the maintenance tasks are being completed. If the maintenance robot can recharge / refuel whilst aboard the offshore wind turbine then this removes limitations on the operating time of the robot.

[0054] The maintenance system includes a navigation and positioning system. This may be a system comprising elements on the drone and / or on the maintenance robot, as well as optionally external elements installed at the base location or at an offshore wind energy farm. Thus, the navigation and positioning system may include components distributed over different parts of the maintenance system, which may work together and / or which may work separately. The navigation and positioning system is for guidance of the drone from the base location to the offshore wind turbine, which may be implemented by means of a satellite navigation system, e.g. using GPS / GNSS receivers that may be provided on the drone. The navigation and positioning system is also for positioning the drone at the landing area on the offshore wind turbine in order to unload the maintenance robot. This may be achieved using one or more of GPS / GNSS receivers, inertial navigation systems (INS), LiDAR scanners, ultrasonic sensors and camera. These may once again be provided on the drone, although in some cases there may be s ens ors / sy stems on the maintenance robot that provided data to the navigation and positioning system.

[0055] It is important to have a high degree of accuracy during landing / take off of an aerial vehicle and / or lan ding / cast-off for a marine vehicle, especially where the landing area at the offshore wind turbine is small. The drone may use a combination of systems to achieve increased accuracy, e.g. both of GPS and inertial navigation and / or enhanced accuracy by use of local reference points that may be fixed to the offshore wind turbine.

[0056] The drone may be required to land on and depart from a floating structure, which may hence include a moving landing area. The base location or the offshore wind turbine may have a floating foundation or may otherwise be floating, e.g. in the case of a base location on board a ship. To enhance positioning of the drone in relation to the moving landing area then the navigation and positioning system may comprise a local reference system using reference points at the floating structure, e.g. one or more of visual markers, wireless beacons, base stations and so on.

[0057] Increased accuracy for GPS and / or GNSS may be provided by so called GNSS enhancement, such as via real-time kinetic positioning (RTK) or other means to use external information, e.g. relating to known positions of markers or beacons at the offshore wind turbine.

[0058] A camera or other imaging system may be used to guide the drone to a landing area, e.g. via use of painted markings and / or lights to provide guidance features that are visible during landing via the camera or other imaging system of the drone or of the maintenance robot. This may involve the use of machine vision for detecting and identifying guidance features and / or transmission of images, e.g. via a low latency system as discussed above, to enable real-time viewing by a remote operator who can remotely control the drone. Machine vision may be used in combination with automatic control of the movement of the drone. Alternatively or additionally machine vision may be used to introduce augmented reality images into the images viewed by a remote operator, e.g. to highlight markings at the landing area, or to introduce an augmented reality representation of the landing area based on positioning information, e.g. from detection of markers or use of a GPS RTK system. In that regard the navigation and positioning system may also include motion and / or orientation sensors on the drone, e.g. one or more accelerometers and / or gyroscopes, such as a six degree of freedom system, in order to determine the orientation of the drone and aid in controlling it during movement.

[0059] The maintenance system includes a control system for control of the drone and the maintenance robot in response to maintenance requirements of the offshore wind turbine. This control system may comprise computer processors for local control of the drone and / or of the maintenance robot. It may comprise remote controller devices providing a user interface for a remote operator to interact with and control activities of the drone and / or of the maintenance robot. There may be a manned command centre at the base location or remote from the base location, where the command centre may be manned with maintenance personnel and / or remote controller devices of the control system. The control system may be configured to receive indications of maintenance tasks and to plan trips for the drones / robots to complete those tasks. Thus, the control system may comprise a prioritisation system and / or a route planning system that is adapted to assign maintenance tasks to particular maintenance robots.

[0060] The control system may additionally be configured to control the offshore wind turbine, or to interact with control systems of the offshore wind turbine. This may be done to control the operational statues of the offshore wind turbine, e.g. by triggering a shut down, and / or to facilitate remote access to the offshore wind turbine, e.g. by actuating door opening systems.

[0061] As noted above the offshore wind turbine may be a floating wind turbine, which may hence be a floating structure comprising: an offshore floating foundation having at least one buoyancy member; and a wind turbine installed on the offshore floating foundation. The wind turbine may be located on a buoyancy member. Optionally the offshore floating foundation comprises a plurality of buoyancy members. There may be a framework for mechanically connecting the buoyancy members to each other. In one example configuration the floating foundation comprises three buoyancy members in a triangular configuration. The floating foundation may comprise four buoyancy members, the fourth buoyancy member placed between three buoyancy members which are positioned in a triangular configuration. As used herein, the term “buoyancy member” is used to denote a hull or other type of housing, which contains a material having a density lower than the density of water. The material having a density lower than the density of water will typically contain a gas, such as air.

[0062] The landing area on the wind turbine may be arranged on top of one of the buoyancy members.

[0063] The offshore wind turbine may be configured to provide power to one or more power consumer(s). The power consumer(s) may comprise an offshore hydrocarbon or hydrogen production and / or storage installation. The power consumer(s) may comprise a power consumer on land or on an island electrical grid connected to the offshore wind turbine via a subsea cable.

[0064] In some examples the floating foundation comprises an energy storage located on or inside the offshore foundation. The energy storage may be a reservoir for fuel such as hydrogen or hydrocarbon based fuel. The energy storage may be a rechargeable battery or a plurality of rechargeable batteries. A connection may be provided to allow the energy storage to be accessed by the drone and / or the maintenance robot, e.g. for refuelling or for recharging of battery systems.

[0065] The maintenance system may be provided as a part of an offshore power farm. Thus, in a further aspect the invention provides an offshore power farm comprising a plurality of offshore wind turbines and a maintenance system as discussed above.

[0066] LIST OF FIGURES

[0067] Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0068] Figure 1 is a perspective view of a floating wind turbine;

[0069] Figure 2 is a schematic perspective view of the floating wind turbine;

[0070] Figure 3 shows a maintenance system for offshore wind turbines;

[0071] Figure 4 is a schematic diagram of a landing area with a combination of a drone and a maintenance robot in the maintenance system of Figure 3;

[0072] Figure 5 shows an alternative maintenance system for offshore wind turbines; and Figure 6 is a schematic diagram of a landing area with a combination of a drone and a maintenance robot in the maintenance system of Figure 5.

[0073] DETAILED DESCRIPTION

[0074] Figures 1 and 2 show an example of a floating offshore structure 1 including a wind turbine 20. The floating offshore structure 1 is connected to a power consumer 14 via a power cable connected to a cable terminal 13. The power consumer 14 may be any type of power consumer on land or on an island. The power consumer 14 may be an offshore hydrocarbon producing installation, such as a drilling platform type of installation (floating types, seabed fixed types), an oil and / or gas production platform such as a FPSO (a floating production, storage and offloading) type of installation, or any other type of offshore installation involved with production of hydrocarbons.

[0075] The floating offshore structure 1 comprises a floating offshore foundation 10 with three buoyancy members I la, 11b, 11c in a triangular configuration. The floating offshore foundation 10 also comprises a framework 12 for mechanically connecting the buoyancy members I la, 11b, 11c to each other. The floating offshore structure 1 comprises a wind turbine 20 installed on the offshore floating foundation 10, and a power converter 25 located inside the first buoyancy member I la. The power converter 25 receives power from the wind turbine 20 and optionally receives power from at least a second power source 16 electrically connected via a power cable to a second cable terminal 15. The power converter 25 supplies power to a power consumer 14 electrically connected via a power cable to the cable terminal 13. In this embodiment, the wind turbine 20 is a wind turbine located on the third buoyancy member 11c. The weight of the power converter 25 is used as part of the ballast of the floating foundation 10.

[0076] As shown in Figure 1 the floating offshore structure 1 is separated from the power consumer by a distance D. This distance D may be from 150m and up to several hundred kilometres. It will be appreciated that the offshore site may be remote from other installations and may be difficult to access, e.g. for maintenance.

[0077] The term floating offshore structure 1 refers in this case to the combination of the floating foundation 10 and the wind turbine 20. The wind turbine 20 in this sense is the entire system needed to produce electric energy from wind, e.g. a tower with a nacelle containing a shaft connected to rotor blades and a generator. The wind turbine 20 may further comprise mechanical and / or electrical equipment such as gearboxes, brakes, frequency converter, transformer, and controllers for controlling the electrical and / or mechanical equipment etc. It also includes an access way and other infrastructure for permitting access by human maintenance personnel, such as a doorway, a lift, internal or external stairways or ladders, internal walkways and so on. The floating foundation 10 comprises a landing area 15, e.g. on one of the buoyancy members I la, 11b, 11c, wherein the landing area 15 is for access by a person from a boat and / or a helicopter. This, the landing area 15 may be arranged for connection of a gangway for landing from a boat, and / or it may be arranged as a helipad for a helicopter.

[0078] Figure 3 shows an offshore power farm 100 along with a maintenance system

[0079] 101, 102, 103 for the offshore wind turbines 20 of the offshore power farm. The offshore power farm comprises an array of offshore wind turbines 20a to 20N that together provide power to a power consumer 14. The offshore wind turbines 20 can be electrically connected to a common power converter 25 or may each have individual power converters 25. Thus, the floating offshore wind turbines 20 may be connected in series, optionally with a few parallel circuits.

[0080] The maintenance system comprises a base location 101, a drone 102 and a maintenance robot 103. In this example the drone 102 is an unmanned aerial vehicle. The base location 101 may have features as discussed above and, in this example, it houses multiple maintenance robots 103 as well as at least one drone

[0081] 102. The base location 101 includes infrastructure for communications with the drone 102, the robots 103, the offshore wind turbines 20 and a control system 104 which can be located remotely, e.g. in a remote server such as in the cloud, and / or can include parts distributed through different parts of the maintenance system, e.g. at the base location 101, in the drone 102 or robot 103, and / or at some user interface where a remote operator is located, e.g. in a command centre. The drone 102 transports the maintenance robot 103 to a wind turbine 20 that requires maintenance. The maintenance robot 103 is held in a docking station 105 during transport. The docking station 105 can also be used to store the robot 103 at the base location 101. The drone 102 is guided by a navigation and positioning system 114, in this case represented as a satellite system, which also aids in correct positioning the drone 102 at the landing area 15 on the floating offshore structure 1 in order to unload the maintenance robot 103.

[0082] Figure 4 is a schematic diagram that shows the combination of the drone 102 and the maintenance robot 103 of Figure 3 at a landing area 15 of an offshore foundation 10, which also supports a wind turbine 20. The maintenance robot 103 comprises: a movement system 106 configured for movement around the wind turbine using access designed for human maintenance personnel, an imaging system 107 for providing images of the wind turbine, and a tool 108 for carrying out maintenance tasks. It will be appreciated that the present Figures use a schematic illustration since the robot 103 can be embodied in various forms, e.g. as a dog robot or a bipedal robot, and that the imaging system 107 and the tool 108 can also take numerous forms.

[0083] The drone 102 is an unmanned aerial vehicle and, in this case, it takes the form of a quadcopter. The drone 102 comprises: a propulsion system 109 for lifting and transporting the drone 102 and its robot payload, a flight controller 110, landing gear 111 for enabling landing on the base location 101 as well as at the offshore wind turbine 1, and a payload handling system 112 for carrying the maintenance robot 103 in flight and for loading / unloading the maintenance robot at a landing area 15 of the floating offshore structure 1.

[0084] To land accurately at the landing area 15 the drone 102 uses the navigation and positioning system, which here includes satellite navigation 114 as well as a localised reference point provided by beacon 115. This may for example be a reference point used in GPS RTK. Alternatively, it may use a different system for relative location and optionally also triangulation via other reference points, e.g. on adjacent wind turbines 20 or on other parts of the floating structure, such as at the buoyancy members 11. The wind turbine 20 has an access way 113, which is operated either manually by the maintenance robot 103, or automatically via a remote signal that may be from any part of the system. It will be appreciated that the various parts are shown entirely schematically and not necessarily to scale. When the robot 103 is unloaded it can enter the wind turbine 20 and move to a maintenance location, e.g. using a lift or other human infrastructure. The imaging system 107 is used to help aid movement, e.g. with machine vision systems and / or via remote control. At the maintenance location a maintenance task can be carried out using the tool 108. After maintenance is completed then the maintenance robot 108 will return to the docking station 105 at the drone 102, and then the drone 102 can return to the base location 101.

[0085] Figures 5 and 6 show an alternative implementation in which the drone 102 is an unmanned marine vehicle. As with Figures 3 and 4 the maintenance system comprises a base location 101, a drone 102 and a maintenance robot 103. The base location 101 and navigation and positioning system 114 are similar to that of Figures 3 and 4.

[0086] In place of transport by flight the marine drone 102 of Figures 5 and 6 transports the maintenance robot 103 by sea to a wind turbine 20 that requires maintenance. The maintenance robot 103 is held in a docking station 105 during transport. The navigation and positioning system 114 aids in correct positioning the drone 102 at the landing area 15 on the floating offshore structure 1 in order to dock with the floating offshore structure 1 and unload the maintenance robot 103, as depicted in Figure 6. The maintenance robot 103 is as described in relation to the preceding embodiment, and once on the offshore wind turbine it can perform maintenance tasks in the same way. The drone 102 comprises a hull for buoyancy, where the hull is provided with a propulsion system 109, e.g. a marine engine, for moving the hull to transport the drone 102 and its robot payload. A payload handling system 112 is included and in this case takes the form of a gangway system for allowing loading / unloading of the maintenance robot 103 at the landing area 15 of the floating offshore structure 1.

Claims

CLAIMS:

1. A maintenance system for offshore wind turbines, the system comprising: a maintenance robot for performing maintenance tasks on an offshore wind turbine, the maintenance robot comprising: a movement system configured for movement around the wind turbine using access designed for human maintenance personnel, an imaging system for providing images of the wind turbine, and a tool for carrying out maintenance tasks; a base location for storing the maintenance robot; a drone for transporting the maintenance robot from the base location to the offshore wind turbine, the drone being an unmanned vehicle comprising: a propulsion system for transporting the drone and its payload, and a payload handling system for holding the maintenance robot during transport and loading / unloading the maintenance robot at a landing area of the offshore wind turbine; a navigation and positioning system for guidance of the drone from the base location to the offshore wind turbine and for positioning the drone at the landing area on the offshore wind turbine in order to unload the maintenance robot; and a control system for control of the drone and the maintenance robot in response to maintenance requirements of the offshore wind turbine.

2. A maintenance system for offshore wind turbines as claimed in claim 1, wherein the maintenance robot comprises a robot communication system for transmitting and / or receiving data and / or control signals; and the drone includes a drone communications system for transmitting and / or receiving data and / or control signals; wherein the robot communication system is configured to: receive control signals providing instruction to the movement system, transmit image data from the imaging system, receive control signals providing instruction to the tool and / or communicate with the control system in order to facilitate control of the maintenance robot; andwherein the drone communication system is configured to: receive control signals providing instruction to a movement controller of the drone, transmit data from an imaging system or other sensor system of the drone, receive control signals providing instruction to the payload handling system, and / or communicate with the control system in order to facilitate control of the drone.

3. A maintenance system for offshore wind turbines as claimed in claim 2, wherein the drone communication system and / or the robot communication system includes a short range communication device for local communications that is not capable of communicating with the base location, wherein said communication system is adapted to interface with a long range communication system of the offshore wind turbine or of the other of the drone or robot.

4. A maintenance system for offshore wind turbines as claimed in claim 1, 2 or 3, wherein only one of the drone and the robot is capable of long range communications, and wherein communications systems of the drone and the robot are configured to share the long range communications capability of said only one of the drone and the robot.

5. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot comprises a movement system configured to use one or more of: ladders, stairways, lifts, doorways, walkways and / or another pre-existing human infrastructure of the wind turbine.

6. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot comprises a quadruped robot movement system.

7. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot is configured to pass through an opening of less than Im wide by 2.5m high, preferably less than 0.5m wide by 1.5m high,wherein the robot is configured to be able to mount stairs with width of 40 cm or less, depth of 20cm or less, and height of 15 cm or more, and wherein the robot is configured to fit into a space with floor area of less than Im x Im, or less than 0.5m x 0.5m.

8. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot and / or the drone is configured to operate a human accessible entry system of the wind turbine.

9. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot comprises a robot positioning system for guiding movement and / or for providing data to a remote user who controls movement of the robot, wherein the robot positioning system makes use of data from the imaging system along with data from one or more of GPS / GNSS receivers, inertial navigation systems (INS), LiDAR scanners, ultrasonic sensors and cameras.

10. A maintenance system for offshore wind turbines as claimed in claim 9, wherein the robot positioning system makes use of input from the navigation and positioning system of the maintenance system and / or the robot positioning system provides input to the navigation and positioning system of the maintenance system.

11. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the imaging system of the maintenance robot includes a sensor for providing images using visible light along with an imaging device that uses other principles to obtain images not visible to the human eye, such as one or more of thermal imaging systems, non-visible light, acoustic imaging systems, laser imaging systems and / or other machine imaging / inspection devices.

12. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot comprises multiple tools with a modular arrangement where a single tool holder can be used for several tool types.

13. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the tool, or tools, for carrying out maintenance tasks includes a manipulator for physical maintenance tasks and / or a computer tool for software maintenance tasks.

14. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the tool, or tools, for carrying out maintenance tasks include one or more inspection tool(s) to augment the data provided by the imaging system, such as motion or noise sensors for vibration analysis, gas sensors, laser measurement systems, microphones, electrical sensors and the like.

15. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the base location comprises a tool store and / or a tool production facility, and wherein the maintenance system is configured to provide the maintenance robot with one or more tool(s) whilst the maintenance robot is at the base location and optionally before it is loaded onto the drone.

16. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the base location comprises a parts store and / or a parts production facility.

17. A maintenance system for offshore wind turbines as claimed in claim 16, wherein the parts production facility includes an additive manufacturing device manufacture of replacement parts based on data obtained via an imaging system of the maintenance robot.

18. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone has a minimum range of 5 km, more preferably at least 10 km, and in some examples at least 15 km.

19. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone is able to transport the maintenance robot in wind speeds of up to 55 kph, preferably up to 93 kph.

20. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone is capable of unloading / loading the maintenance robot in a target area of 5m x 5m or less and / or of safely landing at a landing area of 6m x 6m or less.

21. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone comprises an automatic or remotely controlled loading and unloading system for the maintenance robot.

22. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone is configured to transport the maintenance robot by carrying it in a docking station that includes connections for power and / or data in order that the maintenance robot can connect to the docking station when it is loaded into it.

23. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the maintenance robot is connectable to the drone via the payload handling system through both an electrical connection for transfer of power as well as a mechanical connection for carrying the weight of the maintenance robot.

24. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the navigation and positioning system comprises a satellite navigation system for guidance of the drone from the base location to the offshore wind turbine along with inertial navigation and GNSS enhancement for positioning the drone at the landing area on the offshore wind turbine.

25. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the navigation and positioning system comprises a local reference system using reference points at the floating structure to enhance positioning of the drone in relation to a landing area on a floating structure that is moving.

26. A maintenance system for offshore wind turbines as claimed in any preceding claim, wherein the drone is an unmanned aerial vehicle, and wherein the drone comprises a flight controller and a landing gear for enabling landing on the base location and / or the landing area of the offshore wind turbine.

27. A maintenance system for offshore wind turbines as claimed in any of claims 1 to 25, wherein the drone is an unmanned marine vehicle, and wherein the drone comprises a marine navigation system and an automated gangway device for coupling the drone to the landing area of the offshore wind turbine for loading and unloading the maintenance robot.

28. A method of providing maintenance for offshore wind turbines using a maintenance system as claimed in any preceding claim, the method comprising: storing the maintenance robot and the drone at the base location; in response to maintenance requirements of the offshore wind turbine, transporting the maintenance robot from the base location to the offshore wind turbine; using the navigation and positioning system for guidance of the drone from the base location to the offshore wind turbine and for positioning the drone at the landing area on the offshore wind turbine; unloading the maintenance robot and using it to perform a maintenance task by moving the robot to a maintenance location using access designed for human maintenance personnel and then using the tool for the maintenance task.

29. A method as claimed in claim 28, wherein the use of the maintenance robot is controlled remotely by a human operator, and wherein the human operator is changed for different maintenance tasks or for different phases of performing the maintenance task.

30. A combination of a drone and maintenance robot for a maintenance system as claimed in any of claims 1 to 27, the combination comprising: a maintenance robot for performing maintenance tasks on an offshore wind turbine, the maintenance robot comprising: a movement system configured formovement around the wind turbine using access designed for human maintenance personnel, an imaging system for providing images of the wind turbine, and a tool for carrying out maintenance tasks; a drone for transporting the maintenance robot from a base location to the offshore wind turbine, the drone being an unmanned vehicle comprising: a propulsion system for transporting the drone and its payload, and a payload handling system for holding the maintenance robot during transport and loading / unloading the maintenance robot at a landing area of the offshore wind turbine; a navigation and positioning system for guidance of the drone from the base location to the offshore wind turbine and for positioning the drone at the landing area on the offshore wind turbine in order to unload the maintenance robot; and a control system for control of the drone and the maintenance robot in response to maintenance requirements of the offshore wind turbine.

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