Research On Simulating Of Thrust Allocation For Dynamic Positioning System Of The Semi-Submersible Vessel

The ocean is rich in natural resources, with the gradual depletion of the land oil and other energy, human’s dependence on marine resource is increasing rapidly. In order to exploit marine resources, especially the oil and gas in the deep sea, the ship or offshore platform with dynamic positioning system, which is not limited by water depth, has become a research focus. Dynamic positioning system is a kind of closed loop control system which can resist external interferences as wind, wave and ocean current relying on their own power. It also can keep ship steadily in given position and yaw angle. Thrust allocation is an important part of the Dynamic Positioning system. Its main role is to allocate the expected force and moment into speed and direction of the thrusters. Thrust allocation problem can be expressed as a constrained nonlinear optimization problem. Thrust allocation optimization plays an active role in reducing the error, reducing fuel consumption and equipment wear.This paper has mainly studied the thrust allocation of the ship dynamic positioning system, established mathematical model for the thrust allocation of the dynamic positioning system and carried out the simulation analysis of the thrust allocation. At the same time, in order to validate the effects of proposed thrust allocation algorithm, based on the SQP method, this paper has verified the ship motion model, designed the fuzzy motion controller, and made a simulation analysis for the thrust allocation algorithm in dynamic positioning of the ship.The main work of this paper is as follows:First, it describes the modeling idea and solution method of the MMG ship motion model, analyzes the environment disturbance model and gives the approximate calculation method. On this basis, it verifies the validity of the ship motion model at the help of the ship turning cycle test and zig-zag test.Second, this paper gives a simple overview of the thrust allocation problem, and according to the objective functions and constraints of the thrust allocation problem, sums up the mathematical model of multi-target thrust allocation problem. Meanwhile, it details the sequential quadratic programming, which can solve the thrust allocation problem. At last, using MATLAB, it makes a thrust allocation simulation on the model ship.Third, it introduces the basic principles of fuzzy control and the design procedures of the fuzzy controller, and using the fuzzy inference system of MATLAB, it designs the fuzzy controller of ship dynamic positioning system.Fourth, according to the ship motion mathematical model and wind, wave, current interference model and the fuzzy controller of ship dynamic positioning system, this paper makes a tracking simulation under a certain sea conditions. It gets the three-degree-of-freedom ship position and heading variation curves, and the thrust and angles variation curves of every thruster, which validates the efficiency of thrust allocation algorithm.