Wave load compensation in DP control systems

This master thesis presents alternative methods for wave load compensation in dynamic positioning (DP) operations, where stable and robust conventional feedback DP controllers typically are used. Such DP feedback control laws, compensates slowly-varying second order wave loads through the integral action. The problem is that these mechanisms are subject to changes caused by low-frequency (and possible mid-frequency) contributions, which are forcing an offset in DP stationkeeping accuracy. In addition, feedback mechanisms requires such offsets to be induced before the control law can mobilize any counteractions, making the control system less reactive to disturbances. Therefore, this thesis aims to find better strategies for eliminating offsets due to slowly varying second order wave loads. A background study and literature review have been performed, in order to gain fundamental knowledge on relevant topics; that is, hydrodynamical theory in terms of wave models and loads, sea state estimation, relevant instruments for wave load estimation, DP control systems in general, and information on C/S Arctic Drillship in the Marine Cybernetics Laboratory. In order to test different control strategies, a high-fidelity simulator in six degrees of freedom have been developed and implemented, such that relevant sensor measurements and wave loads are provided. The simulator is based on parameters from the physical model vessel C/S Arctic Drillship, such that the implemented control systems can be tested directly on the physical model vessel in MC-Lab. However, the latter have not been performed due to external factors. Five control strategies have been presented, including a DP-observer estimate as feedforward compensation, direct integral action in DP PID-control, acceleration feedforward compensation, adaptive control using the internal model principle, and a spectrum-based method. The two first were considered as conventional DP feedback controllers, and were used as baseline to compare the effectiveness against the other developed methods. A performance analysis was submitted, based on results from the control systems applied to the high-fidelity simulator. In order to compare the results as fairly as possible, much time was put into tuning the control laws. Consequently, only surge direction was tuned properly enough to be part of the analysis. The performance was evaluated and compared based on key performance indicators, in terms of positioning performance and thrust effort. Results showed that the adaptive control method, using the internal model principle, was able to counteract the second order wave loads well in a long term perspective, while on a short term, the adaption was unable to match the conventional controllers, and the overall thrust effort was significantly higher than the conventional feedback controllers. The acceleration feedforward held, in general, the best performance in both a long term and short term period, by providing an aggressive response to all accelerations. However, mean-drift loads were not successfully compensated, since these lead to zero acceleration. Moreover, the latter method showed tendencies to be sensitive towards uncertain measurements. Finally, a spectrum based method, utilizing a wave spectrum estimate to compensate mean-drift loads, was implemented. However, the algorithm was too time-consuming for the control system, but alternatives to overcome the issue were discussed. Taken into consideration that only surge direction was analyzed, there are reasons to believe that there exists better strategies for wave load compensation, than the conventional feedback mechanisms.