COLREGs-aware Trajectory Planning and Collision Avoidance for Autonomous Surface Vessels

Abstract

This thesis considers trajectory planning and collision avoidance for autonomous surface vessels (ASVs) operating in complex domains in the presence of other vessels. In particular, the task of maneuvering in compliance with the International Regulations for Preventing Collisions at Sea (COLREGs), which are the rules of the road on water, is considered. The contributions are directed towards COLREGs-aware trajectory planning and collision avoidance, where COLREGs rules 8 and 13-17 are addressed. These rules consider the conduct of vessels in encounters where risk of collision is present. The rules address how the maneuvering obligations are assigned to the involved vessels as a function of the encounter geometry and relative velocity. Rules 13-15 are encounter-type specific and consider overtaking encounters, head-on encounters, and crossing encounters, respectively. Rules 8, 16, and 17 address in more general terms how vessels that have either give-way or stand-on obligations are to maneuver to reduce the risk of collision. The main motivation behind the work is to enable electric autonomous passenger ferries as an efficient and environmentally friendly means of transporting pedestrians in urban environments. Still, the concepts and methods are applicable to most surface vessel operations. The first step in maneuvering in compliance with the COLREGs is to determine which rules that apply to the ASV. In this work, a COLREGs classification algorithm has been developed, to determine the encounter type and hence the maneuvering obligations of the ASV in a vessel-to-vessel encounter between the ASV and each so called target ship, which is another vessel that the ASV must avoid collision with. Determining the obligations of the ASV is, however, the easy part, whereas maneuvering in compliance with the obligations is a more challenging one. The COLREGs are written by humans and for humans, and its formulation is in some parts qualitative, to allow for humans to assess the situation based on experience and skills. This poses a challenge when it comes to evaluating and acting on these rules through machine code, where quantitative statements are preferred. This thesis presents a novel mechanism for enforcing maneuvering in compliance with the COLREGs. It comprises a target ship domain with broad consideration to the regulations, where the encounter type, encounter geometry, relative velocity and available space to maneuver are considered. The domain is designed such that if the ASV maneuvers as to not violate the domain, the ASV is consequently maneuvering in compliance with the encounter-type specific COLREGs rules 13-15 and 17. By enforcing the target ship domains as strict constraints in the trajectory planning and collision avoidance algorithms, the proposed domain robustly enforces COLREGs compliance independently of other objectives such as trajectory tracking, energy efficiency and passenger comfort. Several reactive collision avoidance methods are also proposed for ensuring safe operation of ASVs in dynamic and unstructured areas with other vessels and restricted space to maneuver. The methods include capacity for COLREGs-aware maneuvering when avoiding collision with target ships, and also collision avoidance with static obstacles with complex geometries. The methods have a varying degree of coupling with the ASV's guidance, navigation, and control (GNC) system, which makes the proposed mechanisms for COLREGs-aware and collision-free maneuvering easy to integrate in an arbitrary GNC architecture. A trajectory planner for path following and collision avoidance with static and dynamic obstacles is also proposed. The trajectory planner is formulated as an optimal control problem, minimizing the tracking error to the path and the induced accelerations. In addition to the COLREGs rules considered by enforcing the novel target ship domain, the trajectory planner includes consideration to rules 8 and 16, regarding making maneuvers that are readily apparent and performed in ample time to stay well clear of target ships which the ASV has give-way obligations to. This is achieved by assigning windows of reduced cost for the tracking error and the induced accelerations in the control horizon. These windows facilitate any maneuver to avoid collision to be performed within them. The windows are parameterized by a small set of intuitive parameters, and enable, if circumstances of the case admit, maneuvers to avoid collision to be conducted in ample time, in accordance with Rule 8 and Rule 16. The work in this thesis has both a theoretical and practical focus, to develop and also test new methods. The proposed navigation algorithms have been tested through an extensive set of simulations in relevant operational domains, where it is demonstrated that the proposed target ship domain robustly enforces compliance with COLREGs rules 13-15 and 17, and that the windows of reduced cost increase compliance with rules 8 and 16. Furthermore, some algorithms have been tested in full-scale experiments with an electric prototype autonomous passenger ferry. In the experiments, a radar- and lidar-based target tracking system has been applied to close the autonomy loop, demonstrating that the proposed methods are suitable for real-time operation, and are robust to a realistic level of noise and uncertainties in the tracking data.