# Collaborative Unmanned Vehicles for Inspection, Maintenance, and Repairs of Offshore Wind Turbines
## 3 Implementing Collaborative UVs for the OWT IMR Operations
### Collaborative Strategies to Prolong the Operation Time  
Extending a UV operating time can be considered another important factor to be considered for offshore IMR activities. Currently, UAVs or UUVs depend heavily on a crewed vessel for energy supply, either by using tethered power cables or onboard chargers for the batteries. The most conventional method to achieve an uninterrupted power supply is by using a tethered system from a USV through a power umbilical cable. Although tethered systems are often associated with issues such as cable entanglement or jamming, significant strategies to optimize cable length and tension may however prevent these problems [53]. For OWT operations, using tethered UAV or UUV powered by a USV may solve the energy supply issue, especially when operating power-draining tools for maintenance or repair operations. While using a tethered system might solve the energy issue, however, there are many other weighing factors that might hinder its use, such as the manoeuvring flexibility around the OWT structure and the limitation of the cable's length in covering the whole WT structure. Thus, a wireless battery-powered UAV or UUV is often seen as a more practical solution to operating near a complex OWT structure. There are many strategies being proposed and developed for automatic UAV battery charging, beginning with a battery swapping system [125; 126] and during flight battery charging [127] to wireless charging [52; 128; 129]. In a battery swapping system, autonomous hot battery swapping has been used to ensure no data losses during the process [125]. In this process, a UAV needs to land precisely on a ground charging station consisting of a battery carousel platform. As soon as the landing takes place, direct electrical contacts between the charging platforms will supply enough energy to the UAV while a battery swapping takes place. A grasping mechanism will pull off the depleted battery and place it into an empty slot on the carousel. The carousel will then rotate to align a fresh battery to be inserted into the UAV battery slot. Another battery swapping method uses an inverted docking station as a battery swapping platform [126]. In this method, a quadrotor UAV will precisely attach itself to an inverted platform on a ceiling where further battery swapping using a gripper will take place. The purpose of introducing the inverted docking station is to allow a UAV to dock safely with a load attached below it. This method could be useful in the case where a load or a tool with different sizes would not enable a UAV to dock on a charging platform. However, for such a system to be implemented on a USV would require a special platform or space with a ceiling to be constructed. Another advanced method to ensure continuous UAV operation is by using a battery switching method during flight [127]. In this system, a quadcopter UAV (primary UAV) with a docking platform attached on top of it will be approached by a small UAV (secondary UAV) carrying an extra battery. The small UAV equipped with four docking legs will then mate with the primary UAV while still in the air. While docking, the mating legs function as connectors to power the primary UAV. At this stage, through a carefully designed battery switching circuit, the primary UAV will switch off its own battery and totally rely on the secondary battery. As soon as the second battery is fully depleted, the main UAV will switch back to its own battery and the small UAV will return to its base. This battery switching process is continuously repeated, with many small UAVs successively docking and being retrieved to provide extra energy supply to the main UAV. Eventually, the main battery will be fully depleted. To achieve a longer operating time, the interval time between successive changing of the secondary UAVs needs to be minimized as little as possible. While the battery swapping and battery switching system might be useful, wireless charging of a UAV has gained significant attention due to the flexibility it brings to the charging process.