Distributed coordination for collision avoidance of multiple ships considering ship maneuverability

Ship collisions and related accidents still happen every now and then despite the development and installation of advanced assistant systems on ships for collision prevention. With the development of larger ships, more advanced collision avoidance methods are highly desirable. This requires improvement in intelligence and autonomy for safe and quick decisions in avoiding potential collisions especially with the growing interest in smart and autonomous ships. Up to date, several methods such as radar and look-out systems have been adopted to describe the potential collision scenarios between ships and offer guidelines for safe maneuvering. However, most of these methods are focused on one-to-one and one-to-many ships encountering situations with little work on many-to-many situations.

Recently, Dr. Shijie Li and Dr. Jialun Liu from the Wuhan University of Technology in collaboration with Professor Rudy Negenborn from the Delft University of Technology designed a distributed coordination strategy to optimize the collision avoidance operation in multiple ships. They took into consideration the ship dynamics and the internal characteristics of the anti-collision decision making among multiple ships. The work is currently published in the research journal, Ocean Engineering.

The coordination strategy comprised of two phases. Firstly, the ship trajectories were predicted based on the ship dynamics, given different candidate rudder angles as inputs. Next, the collision risk parameters were calculated to evaluate the potential collision risk associated with each rudder angle selection. Secondly, the authors formulated a distributed optimization strategy to determine the most efficient ship collision avoidance maneuver, considering the candidate rudder angle taken by each ship and the corresponding steering time that takes to avoid collisions. Eventually, simulations experiments were carried out to evaluate the effectiveness and efficiency of the proposed method in terms of time, computation costs and communication.

Based on the assumption that the ships could exchange information regarding their position, speed and course with other ships, the proposed distributed coordination method could minimize the total sum of time taken by all involved ships to avoid collision with other ships. Also, the simulation experiments allowed the determination of the type, number, and size of messages that are ideal in estimating the communication costs required for implementing such coordination strategies. This could, therefore, enhance the safety and reliability of ship navigation, operation, and reduction in collisions.

The proposed method showed compatibility with the 1972 International Regulations for Preventing Collision at Sea (COLREGs) approach. Therefore, their integration will enhance its applicability in intelligence collision avoidance and especially in the future generation of unmanned ships.

The authors focused on the collision avoidance problem in multiple ships based on a distributed decision-making perspective with no consideration of the tracking control regarding ship courses and trajectories. This is however considered a key consideration in improving the anti-collision operations. Therefore, the authors proposed further integration of the coordination strategy with advanced heading control algorithms to further enhance efficient and accurate ruder steering operations. From the experimental results, the proposed method can be applied to solve similar collision avoidance problems in restricted waters.