Direct Stability Assessment

Direct stability assessment is the highest tier of the Second Generation Intact Stability Criteria, in which the simplified Level 1 and Level 2 formulae are replaced by numerical simulation of the specific hull in irregular waves. It is the route available to a ship found vulnerable at Level 2 whose owner does not want to accept an operational limitation, and it is the only tier that models the actual vessel rather than a calibrated approximation of a fleet.

The difficulty is statistical rather than hydrodynamic. A stability failure is a rare event, so a direct simulation of realistic duration will usually contain no failures at all, and the assessment has to estimate a very small probability from data that does not contain it. Methods developed for this include the split-time approach, critical wave group methods and multi-level splitting, each of which decomposes the rare event into conditions that can be simulated in reasonable time.

The full article will cover the acceptable numerical methods, the software validation requirements and who performs them, the treatment of extrapolation to rare events, the acceptance criteria applied to the result, the cost and duration of a real assessment, and the way the result binds to the loading conditions actually modelled.