On the Motion and Relative Water Surface of a Moored Semisubmersible Platform under Combined Conditions of Irregular Waves and Winds

The critical motions and relative water surface of a moored semisubmersible are normally very complicated under combined external disturbances namely steady wind, dynamic variable winds and irregular waves. This paper deals with the steady and variable components using a frequency-domain approach. Analyses are made for wave frequency and very low frequency, with a good agreement being obtained between theoretical predictions and experimental results. The effect of the damping coefficient on the response spectrum estimation of very low frequency is discussed. The cause of the scattered statistical values estimated from the response spectrum is investigated and a practical method for the total prediction of the motion and relative motion, including the very slowly-varying phenomena and steady drift components, is introduced.