On the intact stability of a ship in head and following sea: an analysis of the dynamic roll angle due to sudden heeling moments

The verification of ship stability has been historically carried out on the GZ curve and on the dynamic lever curve. They are both evaluated in still water. Recently, new methods have been developed, accounting for dynamic instabilities in waves. The present research study aims to investigate on the dynamic stability of a ship in waves. This is carried out by means of numerical simulations of the transient roll response of the ship, caused by sudden heeling action. The effective influence of longitudinal waves (that is head and following sea) on the dynamic angle of roll is investigated at different frequencies. The effects of the wave amplitude, of the damping and of the relative position of the wave crest on the ship, when the heeling action is applied, are analyzed. The study is carried out using a numerical model that takes into account the pertinent non-linearities on restoring and Froude-Krylov actions. Step functions are used to simulate heeling moments on the ship to check transient ship roll responses and detect possible unstable behaviors. The applications are carried out for the DTMB 5415 frigate hull.