# 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
To increase the flexibility in using different tools for limited assets, the multiple tool-changing UAVs (Figure 1c) can also be implemented as substitutes for the multiple fixed-tool UAVs. There is also a possibility of introducing a UAV which carries a magazine with multiple tools (Figure 1d) or a UAV with multiple arms carrying different tools (Figure 1e), providing that the payload capacity is manageable. The illustrations in Figure 1 show different types of UAVs that can be deployed in this strategy, while Table 2 summarizes the potential collaborations or adaptation methods between a USV and a tool-changing UAV. Using multiple UAVs which hover close to each other during a maintenance operation, a task can be accomplished relatively fast by quickly taking turns to perform different roles such as drilling [71], cleaning [72], coating [73], or painting [74; 75]. A careful sequence of operations can be planned for multiple UAVs or for multi-robot [76] performing such operations by modelling roles and task allocation within a cooperative heterogeneous system. Based on many-sorted first-order logic, assigning roles and tasks for heterogeneous multi-robot operations can be achieved using temporal relations. The temporal relations enable expressing a wide class of collaborative tasks among the robots. To reason on the multiple choices faced during role and task assignment, tensor decomposition has been proposed. However, due to hovering instability caused by high wind, these UAVs need to attach themselves tightly to the blade during operation. A UAV with the capability of attaching itself firmly to a WTB or any structure either horizontally or vertically is of great advantage. To land and stick firmly on an irregular surface as on a WTB, a mechanism that uses two swivel arms with four vacuum suction cups for a UAV can be implemented [77]. The two swivel arms can be adjusted accordingly, using a gear system that also acts as a counterbalance to ensure greater stability during operations. Alternatively, a crawler robot (CR) that can attach firmly to a WTB can also be deployed by a UAV [24], using a gripping mechanism for deployment and retrieval. To stick to the WTB, this crawler robot also uses vacuum suction cups as a gripping mechanism attached to each of its six legs. For underwater maintenance and repair operations, similar approaches to UAV-USV collaboration can be devised for implementing a collaborative USV-UUV. A UUV can be equipped with a manipulator, a robotic arm, as well as multiple robotic arms [78]. Regarding a tool-changing UUV [79], it already exists and has been commercialized for underwater IMR activities. To facilitate the tool-changing operation, this UUV depends on an underwater facility that stores a set of tools. This underwater facility is purposely built to enable continuous operations without the presence of a tools-carrying vessel, making it greener. However, problems might arise in certain situations such as operating in a vast underwater area and difficult human access to tools for maintenance or repair. In a vast underwater area such as under OWT farms, the distance a UUV needs to travel back and forth for tool-changing will not be practical. Using a collaborative USV-UUV approach, a green USV can be carefully designed to be equipped with a submersible tool-changing facility. If any of the tools need to be replaced, repaired, or maintained through human intervention, the USV can simply return to the onshore base. There are many collaborative strategies between UAVs that can be planned and implemented for unmanned OWT IMR operations. In most cases, the involvement of a USV in any collaboration will assist a UAV or UUV in terms of functioning as a docking and power charging station [50; 80], providing extra computing power and data storage capacity [60; 61], as a communication hub between all other UAVs and GCS [25; 51], as well as providing a reliable localization and navigation system [60], as will be further discussed in this section.