Investigation into control strategies for manoeuvring in adverse weather conditions

Manoeuvring in adverse weather conditions has significant challenges for the shipmaster especially when the ship is equipped with the smaller propulsion power. The automatic control methods for safe manoeuvring in severe weather conditions have not been studied extensively since manual control is applied in these conditions. Moreover, in most numerical simulations, PID control is used due to its simplicity. In this paper, the authors discussed the performance of a conventional PID autopilot under large external forces using the numerical simulation of a Handymax bulk carrier in various weather conditions. Then, a path-following control strategy was applied to eliminate the drifting problem found in the conventional PID autopilot alone. Moreover, Optimal Control (OC) based general-purpose trajectory optimization path-planner was proposed for obtaining obstacle-free optimal paths. Finally, the performance of the resulting path-planner-follower is analyzed under different severe weather conditions. A series of early small and precise continuous rudder commands are found to be used to execute the successful collision avoidance maneuver whereas a human operator may use discrete rudder angles with large steps. The head sea course is found to be the safest one in adverse weather conditions compared to the other oblique courses.