Study On Modeling And Stochastic Control Of Ship's Dynamic Positioning System

As the rapid development of exploitation and production in deep sea, positioning technology with anchor is of limitation, More Dynamic Positioning (DP) systems are used in offshore ships and platforms. Dynamic positioning system has broad development in the market. The key technology of DP system is control technology which represents the development level of DP system. Dynamic change of ships'structures and parameters is of random characteristics. That is the motion model with uncertainty. In addition, the disturbance caused by wind, wave and current is of random characteristics too. Even if the disturbance caused by wind, wave and current is modeled partly, the part that is not modeled is of uncertainty. Therefore, one of key problems in dynamic position control is to meet improving performance of ship's dynamic positioning system with uncertainty mentioned above.The work of this dissertation is about the key problem mentioned above, the main contents are as follows:(1) Considering dynamic uncertainty caused by random wind, wave and current, Ship's dynamic positioning model is built using stochastic differential equations. Estimation method of stochastic coefficient is brought out, simulation results show that stochastic differential dynamic system describes is better than deterministic differential dynamic system on describing ship's motion response to dynamic uncertain sea conditions. Response results described by stochastic differential dynamic system are more coincide with actual system than deterministic differential dynamic system. For states of ship's dynamic positioning system need to be estimated and theirs parameters need to be identified simultaneously, extended Kalman filter is used to estimate states and identify parameters of ship's dynamic positioning system with four fixed blade propellers. And system's identifiability is analyzed. Identification precisions with different inputs, initial states, noise standard deviation of identification algorithm are analyzed. Results are used to guide engineering practice.(2) Stochastic nonlinear backstepping controller is designed based on the dynamic plant described by stochastic differential equations. In order to get the states that could not be measured to realize state feedback control, stochastic nonlinear observer is built based on ship dynamic positioning equation described by stochastic nonlinear differential equation. Then stochastic nonlinear backstepping controller with stochastic nonlinear observer is designed based on ship dynamic positioning equation described by stochastic nonlinear differential equation. Stability of controllers and observer mentioned above are analyzed and their simulations are implemented. That shows effectiveness of controllers and observer mentioned above.(3) Considering thrusters' constrains, predictive controller on nonlinear dynamic positioning system under stochastic wave is designed based on rolling horizon optimization. In order to improve the efficiency of searching for optimum controller sequence, controller structure based on the output of Proportional-Integral-Differential (PID) is brought out. The method above is of feed-forward characteristics using predictive controller and feedback characteristics using PID controller, the controlled system is of fast convergence and performance indexes of least fuel and overshoot and rolling horizon optimization is implemented. Simulation shows that efficiency of controller.(4) To implement thrusters'allocation of supply ship with two tunnel thrusters and two azimuth thrusters, direct allocating algorithm is built. Dynamic positioning capability plot of DP 43092 is drawed in Matlab based on direct allocating algorithm, results are compared with KONGSBERG's StatCapV2.6.1, that shows effectiveness of direct allocating algorithm. In order to minimize fuel consumption, decrease thruster force, balance load and thruster force needed by controller, direct allocating+genetic algorithm are built to solve the nonlinear programming problem. The algorithm could implement optimal thrusters' allocation rapidly and it is of practical value. Optimal thrusters'allocation problem is built as an uncertain programming problem. And relaxation factor is built to aid to solve uncertain nonlinear programming problem. Searching space is expanded by relaxation factor, probability of obtain solutions is improved without lose quality of solutions.(5) Stochastic controller design based on Lyapunov function has been induced in controlling ship avoiding roll capsize. The controller is designed based on method of construction. Considering deviation between reference model and actual system, robust bounds of reference models is analyzed. Capsize probability simulation algorithm is designed based on semi-discrete numerical algorithm considering that existing capsize probability simulation algorithm based on Fokker-Plarrck equation is complex and computation cost is large. Results show the efficiency of the algorithm and controller designed, and robustness have been validated.(6) Hardware-in-the-loop simulation platform of ship dynamic positioning control has been developed and applied in analyzing and designing an actual dynamic positioning control system. In the system, computer models simulator on ship motion and sea environment, full-scale controller and full-scale operation terminal are implemented and ship motion in three dimensions is shown by three dimensions simulator based on DirectX. Full-scale controller has the same network interface, signals input/output interface with the practical controller used in ship. Validated simulation could be taken using the hardware-in-the-loop simulation platform to decrease the cost of test and risk and increase test efficiency of tank experiment and practical test in sea.