  Parametric rolling is a parametric excitation phenomenon caused by GM
variation in waves. There are a lot of studies of the estimation the
conditions, the occurrence, and the amplitude of parametric rolling. On the
other hand, there are relatively few cases in which theoretical methods for
estimating parametric roll amplitudes in irregular waves have been validated in
tank tests. The primary objective of this study is to validate theoretical
estimation methods for the parametric roll amplitude in irregular waves and
improve their accuracy. First, the probability density functions (PDF) of the
parametric roll amplitude obtained from the model ship motion experiment in
irregular waves are compared with that obtained from theoretical estimation
methods. Second, the method to improve the accuracy of estimation of the roll
restoring variation in irregular waves is suggested. Third, the method to
estimate the distribution of the maximum amplitude of parametric rolling in
irregular waves. As a result, the PDFs of the roll amplitude obtained from the
experiments differ from the results of theoretical estimation. After that, by
correcting GM variation, the results of theoretical estimation are closer to
the experimental results. Moreover, by the theoretical estimation method using
the moment equation, the qualitative estimation for the PDF of the maximum roll
amplitude is succeeded.
