Prediction of extreme motions and vertical bending moments on a cruise ship and comparison with experimental data

The wave induced motions and vertical bending moments induced by extreme seas on a Cruise Ship is investigated numerically and experimentally. Six sea states and extreme wave sequences, which could trigger modulation instability and induce high nonlinear ship responses, were chosen. The ship responses are simulated in the time domain with a body nonlinear seakeeping code based on strip theory. Two levels of complexity are used and compared: (a) the first assumes that the body nonlinear effects in the vertical responses are dominated by the hydrostatic and Froude-Krylov components, therefore the related forces are nonlinear, while radiation and diffraction is kept linear (partially body nonlinear method); (b) the second considers all forces as body nonlinear and they are updated at each time instant during the simulation depending on the hull wetted surface (fully body nonlinear method). The paper presents comparisons of direct time domain simulations with experimental records, as well as probability distribution of maxima. The fully body nonlinear code compares very well with the experiments in extreme wave conditions.