||S. Kruger, F. Kluwe
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In previous researches an intact stability criterion was developed which aims at the determination of sufficient intact stability of a ship in heavy weather. The aim of this dynamic intact stability criterion was to establish a minimum level of stability which should ensure that the remaining capsizing risk in heavy wether shall be as low as reasonably possible. The proposed criterion results in the computation of an index value. To make the criterion practically applicable, threshold values need to be defined for this index to distinguish between safe and unsafe ships or conditions on the one hand and to establish a required minimum level of safety in heavy weather on the other. To establish such a threshold value for the ISEI, we have investigated several full scale capsizing events with appropriate numerical methods and procedures. These methods or procedures are based on the direct computation of the non linear roll motions of the ship in time domain in a natural seaway. Such kind of methods or procedures have been developed during several decades with the special intention to analyze full scale capsizing events for German authorities. If these methods or procedures we have used in our analysis were found to give a reliable prediction of the ship's behaviour in the accident condition, it was then assumed that these methods or procedures will also deliver reliable results in other conditions (which must still be covered by the mathematical formulation of the problem and the related assumptions). This results in the possibility to analyze many situations that might be potentially as dangerous for the ship as the accident condition. Such kind of computations can be used to find the required threshold values for our dynamic intact stability criterion. An accident is an event which definitively proved that the stability condition was dangerous. Therefore, given that our numerical methods deliver the correct answer to the accident situation, then we can assume that the same type of computations for other conditions will give also a reliable answer. If we now use these methods to determine the ISEI- value for a loading condition which has been identified to be unsafe, then we can establish a relationship of the ISEI-value and stability conditions which are not acceptable. Further, we must not neglect the fact that over decades in different countries, a lot of useful knowledge has been generated which resulted in many types of stability criteria. Based on the individual kowledge of the time when such criteria were developed, and also based on the ship types that did exist in those times, these stability criteria represent a sufficient and practical level of safety for those ship types.