Collision Avoidance for Unmanned Surface Vehicle in Extreme Multi-Ship Encounter Situations

Unmanned surface vehicles (USVs) can encounter traffic ships while performing trajectory tracking missions. USVs need to take suitable collision avoidance (CA) action in accordance with international regulations for preventing collisions at sea (COLREGS). A novel CA approach is proposed for the problem of simultaneous collision avoidance of multiple ships. Different from the already existing CA approaches, our proposed approach can cope with extreme multi-ship encounter situations where USV is surrounded by multiple ships from all sides. Collisions can be caused by incongruous CA actions between encountering ships. So, a CA strategy is proposed with symmetric rules according to COLREGS. In general, CA velocity designed for USV is obtained by adding an offset velocity to its desired velocity. To avoid all encountering ships simultaneously, the consistent offset velocity direction (COVD) method is proposed to design the direction of offset velocity. In addition, an emergency CA module is added to further ensure that a safe distance is maintained between USV and traffic ships. The performance of the proposed CA approach is verified by physical simulations with an existing simulator. The simulation results show that multiple USVs can implement simultaneous collision avoidance and return to their respective desired trajectories when they all use the proposed CA approach.