Comparing Controllers for Dynamic Positioning of Ships in Extreme Seas

Abstract

The primary purpose of this study is to develop two different dynamic position- ing (DP) controllers for a model-scale supply vessel Cybership III, and determine which control strategy provides a safer, smarter and greener control when the ves- sel is exposed to extreme seas. DP has since the 1960s contributed to the safety and efficiency of oil-related and other operations for marine vessels. With this computer-controlled system, vessels such as research vessels, supply vessels and cruise ships can automatically maintain position and heading by using propellers when the vessel is exposed to wind, waves and current.

A nonlinear passive observer (NPO), proportional-integral-derivative with acceler- ation feedback (PID-AFB) controller, sliding-mode control (SMC) and a reference model for Cybership III were derived. Test cases were first performed with the Marine Cybernetics Simulator (MCSim) in calm, harsh and extreme seas. This simulator consists of a high fidelity process plant model of Cybership III in Mat- lab/Simulink together with models for generation of environmental forces. Sec- ondly, tests in calm and harsh seas were performed on the model vessel in the Marine Cybernetics Laboratory (MCLab) at MARINTEK. The performance for tests in MCSim was evaluated by integral squared error (ISE), integral absolute error (IAE) and integral time-weighted absolute error (ITAE). The measures of performance for tests in the MCLab were performed by using ISE, IAE and ITAE in combination with the energy consumption of the thrusters on Cybership III in order to generate a cost function. This cost function is a new method used to eval- uate the performance of the controllers with the aim of achieving a safer operation meaning good accuracy, a greener operation by minimizing energy consumption and a smarter operation by achieving the two latter simultaneously. Other measures of performance used to evaluate controllers on Cybership III and similar vessels in MCLab have as far as the knowledge of the author, only been performed with respect to the error of the positions and heading. This thesis uses a new measure of performance not only evaluating the error, but also the energy consumption during tests, thus evaluating the controller with more realistic performance measures of performance.

The PID-AFB-controller provided the best performance when the vessel was tested in extreme seas in MCSim and was evaluated only by ISE, IAE and ITAE, while the SMC achieved the best performance when tested in calm and harsh seas both in the MCLab and MCSim. The tests performed in MCSim were not comparable with the tests performed in the MCLab and the reasons for this may be inaccurate model parameters, the lack of a proper thruster model, 3D effects of the waves in the basin and reflection on the tank wall. In retrospect of the experiments, Cybership III was found to weight 14% more than modeled in MCSim and hence the test cases in MCLab was considered to be more credible.