A better understanding of ship stability is necessary to prevent losses of ships due to unstable motions, particularly in high seas. We analyze the stability of ship motions using a hydrodynamic model accounting for all the rigid body motions of a ship, as well as memory effects in the fluid. A comparison with model tests shows that the computer model provids reasonable results. The influence of the steady wave pattern is assessed for a container ship. Simulations show that ship's dynamics depend strongly on the nonlinearities of the ship-fluid-system. When wave heights increase we notice a bifurcation before the ship capsizes. A path following method is employed to determine in a systematic manner the stability limits. The application for a simplified system shows the importance of the nonlinearities of the mathematical model.
Each year, almost one hundred ships of tonnage greater than 500 gt are lost in the world's oceans. Economical and environmental risks are important of course; but even more important is the danger for human lives. Thus, there is still a lot of research necessary aimed at improving the tools for analysis and prediction of ship motions in severe seas. The largest portion (one third) of the total losses results from severe weather conditions. That is why we concentrate our research on capsizings due to severe weather conditions. Current stability criteria are empirical and they are based on the properties of the righting-lever. The hydrostatic roll-restoring moments, called righting moments in calm water, are calculated at various heeling angles. By dividing the righting moment by the weight of the ship the righting lever curves are obtained. National and international rules on intact stability make demands on minimum values and characteristics of the righting-lever curves (IMO, 1995). These rules are accompanied by rules on damage stability.