Cooperative control of autonomous tugs for ship towing

Autonomous surface vehicles (ASVs) are seeing a significant development over the last decade. In recent years, their commercial applications are attracting the attention of many companies. One of the promising subjects is to develop autonomous tugs for ship berthing. This paper focuses on the cooperative control of autonomous tugs for ship towing in a berthing scenario. We propose a multi-layer optimal control strategy for a two-tug towing system to guarantee a ship reaching a desired position with a desired heading and velocity. In the higher layer supervisory control, an optimal control method is used to allocate towing forces and determine towing angles. With the help of these results and geometry relationships, the reference trajectories of the two autonomous tugs can be calculated online. Based on the reference trajectories, the trajectory tracking, which is in the lower layer, is addressed. Simulation results indicate that two autonomous tugs can cooperatively tow the unpowered ship to a desired position with a desired heading and velocity.