Numerical and experimental study of parametric roll with water on deck

A combined numerical and experimental investigation is performed on a FPSO ship at the rest and free to oscillate in heave, pitch and roll when interacting with regular head sea waves. Occurrence and features of parametric roll and water on deck are examined by varying the incident wavelength and wave steepness. The numerical strategy is based on a hybrid potential-flow method able to capture weakly nonlinear wavebody interactions, within the weak-scatterer hypothesis, and the green-water loads, within the shallow-water approximation for the shipped water. The solver is globally consistent with the experiments for both the roll-instability and water-on-deck events. The major differences are documented for cases where the occurrence of the phenomena is sensitive to the involved nonlinearities and so to the model approximations. In particular, the method tends to slightly overestimate the water-shipping occurrence. For the examined values of incident wavelength-to ship length ratio lambda/L causing experimentally parametric roll, numerically it is found that the green-water loads support the parametric roll excitation. As expected the instability develops after a transient phase where the roll motion is characterized by both the excitation (incident-wave) frequency and the rollnatural frequency that eventually dominates the signal. The roll natural period, say T4n, is affected by the nonlinear wave-body interactions which modify the restoring and added moment. For the examined cases, T4n tends to reduce as lambda/L decreases.