Qualitative and quantitative characteristics of parametric ship rolling in random waves in the light of physical model experiments

Abstract

An extensive series of experiments on the parametrically excited rolling motion of a partially restrained post-panamax containership in longitudinal irregular long crested waves has been performed at Osaka University. The paper addresses some basic aspects of the problem such as the influence of the initial conditions, the problem of the length of the initial stochastic transient, the dispersion in the estimated statistical characteristic of measured processes, i.e. roll, pitch and wave elevation. It is shown that the uncertainty in the statistical estimations from time histories for roll can be significantly larger than the uncertainty in the estimations for pitch and waves, especially when the environmental conditions are close to the parametric roll stability boundaries. In that respect, the problem of “practical ergodicity” is also addressed. From the comparison of roll time series obtained from repeated experiments with different initial conditions we have clearly proved the (expected) coexistence of symmetric solutions. In addition, from the same comparisons, the conjecture on the possible presence of multiple non-trivial attractors appears to be supportable.