An adaptive operation of a disturbance accommodating controller(DAC) depending on the seastate number for roll motion of a ship is suggested. DAC suggested in this paper assumes the shape of the disturbance, i. e. the wave slope excitation as a harmonic signal that consists of two sine waves of different frequencies and the dynamic model of this kind of disturbance is adopted in the controller to estimate the disturbance from the measurement of roll and to generate the control force to counteract exactly the estimated disturbance. Frequencies of the disturbance model used in the controller are selected so that the energy of the roll response in each seastate is to be minimized. Pole placement technique is combined to enhance the performance of DAC. A necessity of an adaptation scheme i. e. tuning the suggested DAC according to the varying sea environment is also shown by the results of the performance assessment of DAC's with different sets of design frequencies optimized for each seastate.
Major developments of ship motion theories have been said to be attributed to two important foundations : the development of strip theory and the treatment of ship motions in irregular waves as the ‘black box’ eletronic filter. "The signal received by the filter contains a number of different frequency components and these are amplified or attenuated to produce some modified output signal according to the characteristics of the filter. Extendng the latter idea further into modeling the wave elevation or the wave slope as a filter of the white noise lets us adopt the stochastic control theory in suppressing the unwanted ship motions (Sgobbo and Parsons, 1999) or enhancing the estimation of the state-variables of ship motions in irregular waves (Reid et al., 1984; Triantafyllou and Athans, 1981).