On the assessment of parametric roll in random sea

The aim of the present paper is to advocate for a very effective stochastic procedure, based on the First Order Reli-ability Method (FORM) well-known within structural reliability. Due to the efficient optimisation procedures imple-mented in standard FORM codes and the short duration of the time domain simulations needed (typically 60-100s to cover the structural memory effects in the response) the calculation of the upcrossing rate of a given level of the re-sponse is very fast. Thus complicated non-linear effects can be included. An application for roll motions of ships will be discussed. A simple non-linear dynamic model for roll including the effect of heave is formulated in the time domain. Closed form expressions are used for the heave motion and for the wave-induced roll moment. The instantaneous GZ curve is derived numerically, but only for one regular wave. Inclu-sion of parametric roll in the example has not been possible due to shortcomings in the present model of the GZ curve and the lack of a coupling to pitch. However, provided more accurate hydrostatic and hydrodynamic models are used, parametric roll could be accounted for, too, in the proposed stochastic procedure.