Research On Optimization Algorithm Of Thrust Allocation For Dynamic Positioning System

With the development of dynamic positioning technology, the range of applications of dynamic positioning vessel is more extensive. Thrust allocation unit is a vital part of a dynamic positioning system. Its main function is to receive the instructions of three degrees freedom including the longitudinal force, lateral force and the rotary torque of the bow which are issued by DP controller. The thrust and angle after optimizing by the optimization algorithms are allocated to each thruster. This paper mainly researches on the "Offshore Oil201" with7steerable thrusters through the simulation on Matlab platform aiming at the thrust allocation problem for dynamic positioning system.Firstly, a mathematical model of dynamic positioning system for the pipe-laying crane ship is established which contains coordinate system model of the movement of the ship, the environmental interference force model(wind, wave, current) and the propulsion system model. Moreover, three degrees freedom movement of the ship in the horizontal plane is modeled that is the foundation of the subsequent studies.Secondly, a nonlinear objective function of the thrust allocation problem with the purpose of optimal energy consumption is built according to the target of the thrust allocation. Meanwhile the maximum thrust of the thruster of the pipe-laying crane ship and the change rate of the thrust and angle are also taken into account.Thirdly, based on the sequential quadratic programming algorithm the penalty function and filter are chosen to be the value function. But it may have Marotos effect which leads to the convergence variation no matter what kind of value function is selected. Subsequently, the SQP filter algorithm is improved that conducts the second-order correction for those who do not accept the tentative step to overcome the Marotos effect. This provides the theoretical basis for the subsequent simulation of the thrust allocation.Finally, a PID controller is designed in this paper. The longitudinal force, lateral force and the rotary torque of the bow are calculated by controller through the position and heading deviation of the ship, as well as the instructions of three freedom degrees is transmitted to the thrust distribution unit which is used to conduct the thrust allocation. Then the thrust allocation simulation using the optimization algorithm in the target pipe-laying ship is completed in the same controller and simulation environment on Matlab platform. According to the simulation results, the running rate, the positioning accuracy and the energy loss of the three optimization algorithms are compared and analyzed.