Coordinated weathervaning control of two surface vessels in a tandem configuration

During a tandem offloading operation between a floating production storage and offloading (FPSO) unit and a connected shuttle tanker (ST), the control of the relative position and heading between two vessels is crucially important, since large relative motion may cause serious accidents such as collision or hose disconnection. Dynamic positioning (DP) systems can be used to control the motion of the vessels during the operation. Among various DP control strategies, weathervaning control enables each vessel to be oriented toward the direction of the mean environmental force induced by wind, wave and ocean current so as to improve the operational safety and reduce the overall thruster power consumption. This study proposes a controller design for DP in the task space of the vehicle dynamics that allows for weathervaning control without explicit measurements or accurate estimation of environmental disturbances. A coordinated control scheme assuming a reliable communication channel between two vessels is also introduced to ensure the safety of the vessels and continuous oil transfer by suppressing excessive relative heading changes under significant environmental disturbances. Numerical simulation results for several DP control scenarios under different control settings and environmental conditions are presented.