Estimation of Parametric Roll in a Stochastic Seaway

The present paper describes a model for prediction of parametric roll which is sufficiently fast to give predictions in real time for the probability of different levels of parametric roll angles in given long crested sea states. The procedure is based on an extension of a simplified two-degree of freedom (roll and heave) time domain model [10]. In the present paper the effect of the increased added resistance when the bow heaves and pitches down in a wave crest is introduced. Due to the resulting forward speed variation the roll resonance condition will be changed. The influence of ship speed on roll angle has in the past been noted by e.g. [6], where large roll angles in combination with the ship speed reduction can be seen from time traces from model tests.

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