Research On Control Method For Dynamic Positioning Of Ships In High Sea States

With the exploitation of sea resource and the need of the other ocean engineering, the working range of the ocean engineering ships are being extended to the deep and ultra deep water, and the requirement for the dynamic positioning systems that installed on the engineering ships are increasingly too high. When working in the deep water areas, the sea environment that the engineering ships subject to is often harder than the offshore areas, that is to say, the engineering ships are often working in high sea states. The control method for the dynamic positioning system that suitable for the moderate sea states can not insure the same performance in high sea states, so it is necessary to expand research on control method for dynamic positioning of ships in high sea states,and design the feasible control strategy for the dynamically positioned ships in high sea states.Up to now, there is no precise definition of high sea states. The high sea states in this article is understood as the extreme seas that is still in the controllable range of the dynamically positioned ships, that is to say, by improving the control method can still improve the control performance to some extent,and guarantee the normal working of ships.The sea states are the result of wind, wave, current and other common effects of the marine environment. Complicated ocean environment in high sea states have a more significant uncertain, coupling, time-varying characteristics, so the motions of the dynamically positioned ships in high sea states show a more pronounced nonlinear characteristics.For question raised above, this paper mainly does the following research work:Firstly, in order to describe the ship motion state more accurately in high sea states, after expounding the characteristic of scakceping theory and maneuvering theory, based on the ideas of unified theory, a nonlinear unified mathematical model which can be used to describe the ship motion state in different sea states is established. In order to verify the performance of the designed controller and observer, appropriate mathematical models of the ocean environment are established. The wave loads are modeled by means of wave force response amplitude operator, which describes the wave loads directly as the first-order wave force and the second-order wave force acting on the ship, more in line with the actual physical effects of wave loads.Secondly, since in high sea states the period of the waves get longer , resulting in increasing wave frequencies, it is difficult to separate the wave frequency component from the ship motion, a nonlinear passive observer specifically applicable to high sea states is designed based on the unified mathematical model, which is used to estimate the unmeasured velocity and the bias. The designed observer has the same analysis process in passivity and stability,which reduce the design difficulty to a certain extent.Thirdly, Given that a more accurate nonlinear unified mathematical model has already been established, at the same time treating the control problem of dynamically positioned ships in high sea states as a class of nonlinear control problems, so the backstepping control method can be used to design the controller for it, for which has a strong advantage in dealing with nonlinear systems. By selecting the proper state variable, and constructing the corresponding Lyapunov function, a backstepping controller is obtained based on the unified model. Although the more accurate unified mathematical model is used to describe the motion of ships in high sea states, the wave force in practice can not be measured directly, and there still exists unknown and uncertain environmental disturbance for the complicated ocean environment,a sliding mode term is incorporated into the backstepping controller to improve the robust of the controller. For the chattering problem of control forces and moments introduced by the sliding mode term, quasi-sliding mode boundary layer method is designed to deal with it.Finally, based on Matlab / Simulink, and MSS (Marine Systems Simulator) marine system simulation toolkit which is developed by Norwegian University of Science and Technology, corresponding to the design process of the above controller and observer,simulation platform for backstepping controller, backstepping and sliding mode controller,backstepping and sliding mode controller with no chattering, and nonlinear passivity observer are constructed respectively. Simulation results show that the designed controller and observer can meet the control requirements of dynamically positioned ships in high sea states.