Development of a Minimum Stability Criterion to Prevent Large Amplitude Roll Motions in Following Seas

The intact stability of ships, their ability to withstand external forces and loads without capsizing, to date is still evaluated on the basis of empirical and semi-empirical criteria. A number of accidents in the past years show that these existing criteria do not provide a suffcient safety level, especially with respect to certain ship types, like RoRo-vessels and large Container Ships. In particular, the dynamic behaviour in longitudinal waves is not addressed by the existing criteria. In the present work a new type of intact stability criterion is developed, which is based on the direct assessment of ship responses in irregular short crested seaways. The idea behind the new approach is to evaluate the ship's behaviour in a representative number of possible operating conditions. For each operating condition a limiting wave height is determined by the Blume-criterion. Above this wave height a ship is considered to be unsafe. All unsafe situations contribute to the Insuffcient Stability Event Index (ISEI) with their individual probability of occurrence. This index is a measure for the overall vulnerability of a ship in the dynamic wave environment. The threshold value for this index is obtained from the investigation and analysis of a number of capsizing accidents. Finally, a simplified criterion is derived from the simulation-based index in order to provide the findings obtained from the numerical simulations also in situations where direct calculations are not accessible. The simplified criterion is mainly based on lever arm alterations in waves.