PREDICTION OF CRITICAL WAVE CONDITIONS FOR EXTREME VESSEL RESPONSE IN RANDOM SEAS

The paper describes a method for predicting the occurrence of conditions which can excite ship motions. Earlier experimental studies have suggested that large roll motions and capsizing are often associated with the ship encountering a number of steep and almost regular waves. Most capsizes have been observed in following and quartering seas. In this work, the wave conditions encountered by a ship are characterised in terms of wave groups that consist of a high run and a low run of waves. In order to study the waves encountered by a ship, the spectral information measured at a fixed point was transferred to a moving co-ordinate system. The wave groups and the high runs were then predicted in the moving co-ordinate system using envelope theory. These and other properties of the encountered waves were found to depend strongly on the ship speed. Using these results, the speeds and the headings at which the ship is likely to encounter long high runs were predicted. Critical conditions were determined by comparing the period and the length of the dominant encountered waves with the characteristics of the ship. The probability of the ship encountering a critical high run was then determined for a given speed and seastate.