Setpoint Chasing for Thruster-Assisted Position Mooring

Abstract

In the present industrial thruster-assisted position mooring (PM) systems, thrusters are used to damp the vessel's dynamical motions and to provide compensation of any line breakage. To increase robustness and extend the weather window for PM systems, this paper proposes a new concept of setpoint (SP) chasing for moderate and extreme conditions. The SP in moderate conditions is the vessel's equilibrium position where the environmental loads are balanced by the mooring forces to avoid the conflict between the control action of thruster assistance and the mooring system. To chase this SP, the thruster assistance provides additional damping and restoring forces to compensate the dynamic environmental loads while the mooring system compensates the mean environmental loads. The simulations and experiments suggested that the proposed SP chasing for moderate conditions improved the vessel performance while the utilization of the mooring system was maximized. Due to risk for mooring line breakage, the SP in extreme conditions is the position closer to the field zero point than the equilibrium position. To chase this SP, the thruster assistance provides damping, restoring force and mean force controls to compensate both the dynamic and a part of the mean environmental loads while the mooring system compensates the rest of the mean environmental loads. The simulations indicated that the proposed SP chasing for extreme conditions reduced the vessel's drift and tension in the most loaded line.