In this paper, the problem of dangerous conditions induced by variations of restoring in waves is
addressed, with particular attention to parametric roll. An analytical model for the study of roll
motion in irregular longitudinal long crested waves is presented. Stochastic stability boundaries in
the first parametric resonance region are provided analytically. Nonlinear/non gaussian roll
response above threshold is determined numerically and compared with experiments. A semi-
empirical methodology is proposed to extend the classical deterministic static stability assessment
to a rational probabilistic framework taking into account hydrostatic restoring variations in waves
and different environmental conditions. Examples of application are provided.
