Reliability-based fixed-time nonsingular terminal sliding mode control for dynamic positioning of turret-moored vessels with uncertainties and unknown disturbances

This paper presents an area keeping and heading control method for a turret-moored vessel subject to uncertainties and unknown disturbances. Specifically, the nonlinear motion model of the vessel is firstly transformed into the form of reliability-heading error dynamics by introducing a reliability-based matrix, which builds the relationship between the mooring line tension and vessel motion. Then, a fixed-time fast nonsingular terminal sliding surface is constructed based on the states of reliability-heading error dynamics. Next, a fixed-time extended state observer (ESO) based on position-heading measurements is designed to estimate the uncertainties and unknown disturbances. Based on the sliding surface and ESO, a fixed-time nonsingular terminal sliding mode controller (NTSMC) is proposed to ensure that the reliability and heading errors converge to the sliding surface in fixed time. By adjusting the reliability to the desired value, the capacity of mooring system can be fully utilized to keep the vessel within the allowable area. Meanwhile, the safety of mooring lines can be guaranteed. Finally, simulations and comparisons demonstrate the effectiveness of the proposed fixed-time ESO-based NTSMC for dynamic positioning of a turret-moored vessel.