Prescribed-Performance Green Dynamic Positioning for Fully Actuated Vessels Under Input Magnitude and Rate Saturations

For the green dynamic positioning (DP) of the fully actuated vessel under input magnitude and rate saturations, unknown disturbances, and safe operational region limitations, this paper develops a prescribed-performance Lyapunov-based nonlinear model predictive control (PL-NMPC) scheme with nonlinear thrust allocation (TA). The input magnitude and rate saturations are considered as constraints in the receding horizon optimization (RHO) model. A disturbance observer is designed to provide estimates of the unknown disturbances. A prescribed performance function and an associated error transformation are introduced. Leveraging the disturbance observer and the error transformation, a robust prescribed-performance auxiliary controller is designed to establish a contractive constraint for the RHO model. With the contractive constraint, the developed PL-NMPC scheme inherit the stability of the designed auxiliary controller. Recursive feasibility and the closed-loop stability under the PL-NMPC scheme are provided. Through the theoretical analyses, the fully actuated vessel maintains its position and heading within the safe operational region with minimized energy consumption. Simulation results and comparisons demonstrate the effectiveness and the energy-saving capability of the developed PL-NMPC scheme. Note to Practitioners—In engineering practice, thrusters of DP vessels are subject to physical limitations, and constraints on vessel positions, headings, and velocities arise due to specific DP missions, safety and energy-saving concerns. Unlike the existing NMPC-based DP controllers with TA, the developed PL-NMPC scheme is applicable to vessels with azimuth thrusters, able to avoid singular thruster configurations, and is supported by comprehensive analyses of recursive feasibility and closed-loop stability. Comparative results on a fully actuated model vessel show that the PL-NMPC scheme can achieve the energy-saving rate higher than 10%. Our scheme offers significant potential for improving the energy-saving capability and ensuring safe DP operations in offshore engineering and ocean transportation. In the future, our scheme can be applied to a wide range of multi-input and multi-output systems with factors of disturbance uncertainties, energy-saving and safety concerns.