Research On Control Methods For Dynamic Positioning Ships Based On Discrete System Theory

Dynamic Positioning(DP)ships at sea act as the sum of low-frequency motion and high-frequency motion due to the action of the control inputs and environmental disturbances such as wind,wave and currents.The ship motion control is mainly designed for low-frequency motion.When the ship conducts in long operation,if the heading is chosen inappropriately,there exists an angle between the disturbance force and heading.In order to maintain the position and heading,the ship needs to provide larger propulsion to compensate the environmental forces,so that the fuel consumption will increase.When the ship conducts in offshore operation for a long time,it will often encounter a variety of sea conditions,and need to undertake kinds of operation modes and switch between different operation modes.Currently,the ship control systems are almost computer control systems,which deal with the discrete-time signal processing.Based on the theory of discrete systems,and aiming at the positioning control and switch control for multi-operation DP ships,many research was carried out as follows:First,the DP ships sailing in the horizontal plane is a three degrees of freedom(DOF)motion.A 3-DOF discrete nonlinear ship motion model was designed,meanwhile,the wind,wave and current models were estiblished.The simulation tests were carried out and the results proved the legitimacy and correctness of the model.Secondly,for the low and high frequency superposition characteristics of the DP ships,discrete nonlinear observers were designed for different environmental disturbance to estimate the motion state of the ship,and filter out the wave frequency data when there exists high frequency disturbance.In order to improve the estimation accuracy,increase the robustness of the observer and solve the difficulties brought by the Coriolis-centripetal matrix,the acceleration feedback technology is introduced to the observer design to eliminate the impact of Coriolis-centripetal matrix,and reduce the sensitivity to the disturbances and uncertainties of the ship.Thirdly,aiming at the fuel consumption of the DP ships engaging long-time offshore operations,environment adaptive heading controller and positioning controller were designed according to the principles of pendulum.The controllers make the ship adaptive to the environment and reduce the acting force due to the disturbance,so that the fuel consumption(?)(?).The neading of the ships under "green DP" control is not the optimal.Aiming this problem,the environmental adaptive heading control strategy is adopted and an environmental adaptive region positioning controller is designed.The environmental adaptive heading controller provides the optimal heading,and the discrete nonlinear model predictive controller is introduced to predict the position of the ship and maintain the ship in the set-region.The controller designed can further reduce the fuel consumption and extend the operating time at sea.Finally,an integrated controller is designed for multi-operation DP vessels.The ship can conduct in different operation modes with the integrated controller.A separate controller for each operation mode is designed,and then the weighting function,which is designed according to the velocities,the distance between the ship and the target,and the time,is introduced to combine the controllers.The weighting function can make the transform between different controllers smoothly.The simulation results prove the validity and affectivity of the controller designed.