Dynamic Positioning of Ships
A nonlinear control design study
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Abstract
Dynamic positioning (DP) is relatively a new technique used to maintain the position and heading of ships in various offshore operations. Due to the features like better safety and operating efficiency, DP systems are becoming more and more popular. This thesis mainly focusses on the control system design part of the DP system. The theoretical study is based on a naive mathematical model of a ship and the forces and disturbances acting on it. The design objective is the regulation of a vessel to a desired operating point. Two main concepts for the DP control system design presented in this thesis are: the control system design based on the state dependent Riccati equation and the control system design using the port-Hamiltonian framework. In this paragraph, I highlight some important results presented in this thesis. Before really addressing the DP control system design problem, we studied the stability properties of a special type of autonomous nonlinear systems. The main idea was to find a set of sufficient conditions which ensure the global asymptotic stability of these systems. We concluded this study with two counterexamples which prove that some existing results on this subject are in fact incomplete and the problem is still open for further research. These counterexamples have been published as two separate articles in IMA Journal of Mathematical Control and Information. We also proposed the Fourier series interpolation method to solve a special type of matrix Riccati equations. This method requires lesser computations. Therefore, it is a better method for online computation of the solution of the matrix Riccati equations. Another important result in this thesis is a family of controllers for regulation of a DP ship by using the port-Hamiltonian framework. This result is to appear soon in IET Control Theory and Applications.