Research On Control Method For Ship's Dynamic Positioning Assisted Mooring System

Due to the need for long operational period in marine environmental conditions,the thruster assisted mooring positioning(TAMP)system is widely adopted as an economical positioning method for station keeping in deep water.How to improve the economy and safety of operation are two important contents in the research of TAPM control method.Considering the safety and economy of positioning system,a new Dynamic Positioning Assisted Mooring(DPAM)system is designed in this paper.To improve the economy of DPAM system and maximize the utilization of the mooring system,the optimal ship position setpoint is selected according to different sea conditions,and the corresponding controller is designed.Moreover,in order to improve the safety of operations,a mooring monitoring system is designed,which can monitor the position of the target ship in real time,infer the corresponding mooring cable status,and realize the switching between different control strategies.The main research contents of this paper include the following aspects.Firstly,the working principle,operation flow and system structure of the DPAM are introduced.Secondly,the mathematical model of the DPAM is established,including the 3-DOF kinetic model under the mooring system,the marine environment disturbances models and the mooring system model.Thirdly,the ship motion characteristics under different sea conditions are analyzed and the suitable control strategy under different sea conditions are determined.Aiming at the high positioning accuracy and real-time demand of DPAM in moderate sea conditions,the system control method is designed,including the ship position setpoint optimization and controller design.By optimizing the position setpoint,the positioning system has better economy.Considering the advantages of finite time control technology such as fast convergence speed,good anti-disturbance ability and high accuracy,a finite time state observer is designed for the problem of unmeasured ship speed and unknown external interference,so that its estimation error is bounded within a finite time.According to the speed estimated by the observer and environmental interference,an output feedback positioning control law is designed.By using homogeneous and Lyapunov stability theory,the proposed position control system is proved to be finite-time convergence,so that the designed control system has a faster convergence speed,better anti-interference and anti-parameter perturbation ability and higher accuracy.Aiming at the operational safety requirement of DPAM in extreme sea conditions,the system control method is designed,including the ship position setpoint optimization and controller design.There are two strategies to optimize the position setpoint to ensure the safety of the positioning system and meet the economic demand.To avoid the ship position exceeding the critical equilibrium position and causing the mooring line fracture,a model predictive controller is designed by using a stair-like implicit generalized predictive control algorithm,which can predict the ship's position.Finally,a multi-objective thrust allocation optimization method is developed to reduce the energy consumption of propulsion system and provide the required control force and moment timely and effectively.In the process of thrust allocation,the optimization objective function takes into account factors such as the energy consumption,the wear and tear of the thruster,and the thrust error.Moreover,the constraints such as thrust prohibited area,dead-zone,saturation,and azimuth and thrust change rate are also be considered.The non-dominated sorting genetic algorithm-II(NSGA-?)is adopted to solve the proposed multi-objective thrust allocation optimization problem.To improve the population diversity,solution convergence and save computing time,the thrust allocation method is improved.For the designed DPAM system,the control method under moderate and extreme sea conditions and the thrust allocation method are studied in this paper.The proposed control methods are verified by numerical simulations.The results demonstrate that the proposed finitetime output feedback system can better meet the high-precision positioning target of the DPAM under moderate sea conditions.The designed model predictive control system not only guarantees the safety of the DPAM system,but also makes full use of the mooring system to saves costs and meet the safety requirements of the DPAM under extreme sea conditions.The thrust allocation method based on NSGA-? algorithm can effectively reduce the energy consumption of the propulsion system and make the DPAM system more economical.