Modeling And Simulation Of Control System For Ship Dynamic Positioning

Dynamic positioning system(DPS)is an important control system for floating structures such as engineering ships and ocean platforms,and is an effective guarantee for exploration into deep sea and development of marine economy.With the increasing strong development of ship's DPS and the involved submodules,the study on it has become a focus of attention in the field of ship and ocean engineering.Ship's DPS is a kind of device to keep the ship in a special position and heading by means of the thrust produced by the propulsion system to overcome the external disturbances of wind,current and waves.Since the ocean environment is complex and changeful,establish of the reasonable and valid mathematical models and design of the control algorithms are important issues of the study on control system for ship dynamic positioning.Taking the 1:20 model of a 75 m platform supply vessel with study object,this thesis focuses on design and simulation study of three important modules in the control system.Firstly,the mathematical models including kinematic model and dynamic model of three degrees of freedom motion under the dynamic positioning mode are established for a surface ship.The simulation models of marine environmental disturbance forces from wind,current and waves are discussed.The structure of the propulsion system and the mathematical model of thruster are put forward.Based on the ship motion model,Kalman filter is introduced for state estimation of the ship motion.The principle and application of the filter in the control system are explained.A discrete Kalman filter is adopted to filter out the high-frequency component and noise and figure out the low-frequency signal to be controlled for ship dynamic positioning.In addition,an adaptive factor is introduced to improve the algorithm of Kalman filter which performs as an adaptive Kalman filter.The proposed algorithm can reduce the effects of the historical estimates on state estimation,and increase the weight of new measurements.The simulation experiments indicate that the performance of the approach is achieved as expected.Since the control system is time-varying with strong nonlinearity and dynamic nature,in order to optimize the performance,a Backstepping controller with integral terms is designed,and the eventual control law is constructed with the help of Lyapunov functions.Simulations are implemented with the proposed control strategy and the traditional PID strategy,and the control mechanism and effects of the strategies are compared to testify the validity of the proposed control strategy.According to the various factors in the thrust allocation unit,including energy consumption,the type and layout of thrusters,limitation in the physical properties of thrusters and avoidance of singular structure,the constraints and objective function of optimization problem are constructed.A solution logic is derived according to the Sequential Quadratic Programming algorithm.Finally,the design of optimization algorithm for thrust allocation is verified through simulation experiment on the given model of a 75 m platform supply vessel.