# Collaborative Unmanned Vehicles for Inspection, Maintenance, and Repairs of Offshore Wind Turbines
## 3 Implementing Collaborative UVs for the OWT IMR Operations
### Strategies to Perform Complex Tasks  
UAVs on their own can be utilized to inspect high-rise structures, including wind turbines. Currently, they are being used for non-contact visual inspection and NDT ultrasonic inspection [33]. However, a great amount of effort still needs to be carried out for a single drone to perform inspections with high task complexity, such as having to carry heavy payloads (sensors and actuators) or performing inspections in inaccessible areas (narrow spaces or difficult angles). For maintenance or repair operations, the task complexity is even higher as it might involve performing a series of operations with different tools. Hence, a single UAV performing repair or maintenance work on a high-rise wind turbine structure is seen as far-fetched due to the current capabilities of a single UAV. The level of complexity to perform maintenance tasks ranges from very low complexity up to very high complexity. Performing visual inspection can be considered a very low complexity task while, in contrast, gearbox replacement activities are at a very high complexity level. Unmanned technology for undertaking very high complexity tasks such as replacing wind turbine blades or gearbox does not exist yet and is currently deemed unsuitable. Apart from that, there are still many opportunities for unmanned operations to be deployed for OWT, which mostly depend on the level of complexity. In most cases, a single UV is insufficient to perform unmanned maintenance tasks, thus requiring collaborative strategies to be deployed. One of the sources of complexity arises when a task requires using multiple tools for completion. For instance, repairing surface damages on a wind turbine blade usually involves the processes of cleaning, sanding, filling and putting, coating, and painting [64]. The strategies that can be implemented to perform this type of task are fulfilled by using multiple homogenous UAVs equipped with a different set of tools [65] or by using a single UAV with the capability of carrying and changing tools for different tasks [66], as illustrated in Figure 1a,b. Both strategies will depend on a USV as a docking and power charging station; however, a tool-changing UAV will rely more on a USV as a tool changing station. For a tool-changing UAV, it may also be a UAV that is equipped with a manipulator [66; 67], an arm [43; 68; 69], or multiple arms [37; 70] to perform more dexterous manipulation during maintenance or repair works. Table 2 summarizes the different features and potential collaboration or adaptation methods that can be achieved with these types of tool-changing UAVs. \begin{table}
\begin{tabular}{l l l} \hline \hline
**End Effector** & **Ref.** & **Special Features** & **Potential Collaboration/Adaptation Methods** \\ \hline Manipulator & [66] & An elastic gripper with a gripp and release mechanism. & An automated tools magazine can be placed on a USV platform where a UAV will fly over it and select a desirable tool to exchange by grasping or releasing. \\ \cline{2-3}  & [67] & A 2-Degree-of-Freedom & With a rotating gripper mechanism, a UAV will have greater flexibility to pick up tools on a USV which is placed without fixed orientation. \\ \cline{2-3}  & & servomotors-based gripper with 360 degrees of rotation. & With a rotating gripper mechanism, a UAV will have greater flexibility to pick up tools on a USV which is placed without fixed orientation. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Potential collaboration/adaptation methods between a USV and a tool-changing UAV. Figure 1: Different types of UAVs to perform complex tasks: (**a**) multiple homogenous UAVs with different fixed tool, (**b**) single UAV with multiple tools changing, (**c**) multiple UAVs with multiple tools changing, (**d**) single UAV with a magazine of tools, and (**e**) single UAV with multiple robotic arms holding different tools.