On parametric roll predictions

Experiments have been carried out with a model of the KCS container vessel. The model tests focussed on three out of five stability failure modes of the Second Generation Intact Stability Criteria that are currently being developed by the IMO. This paper focusses on two aspects of the prediction of the risk on parametric roll in regular waves. The first aspect is a check on the assumption of the IMO that simulation programs exist that properly can predict the risk on parametric roll; the second aspect is the effect of the roll damping model on the predicted parametric roll amplitudes. The first aspect has been investigated by asking members of the CRS community1 to do simulations using proprietary programs. Five members responded to this request. The paper shows that a prediction of the roll damping based on exclusively geometrical information results in quite different answers. If the coefficients of a quadratic damping model are fixed in the input, the predictions of parametric roll angles in regular waves as a function of the wave amplitude are quite close for the different simulation programs. However, there is a significant discrepancy between simulations and experimental results with respect to the threshold wave amplitude at which the parametric roll phenomenon starts. An investigation in the modelling of the damping shows that this has some effect, but it does not explain the large difference. A final conclusion is, that the studied simulation programs will benefit from further improvements to predict all aspects of parametric roll events accurately. A good understanding of these aspects is considered important for a reliable Direct Stability Assessment.