Rapid Ship Motion Simulations For Investigating Rare Stability Failures In Irregular Seas

The use of time-domain numerical simulations for the investigation of stability failures and other rare events in random, irregular seas requires a challenging combination of speed and accuracy. Simulations must be fast enough to observe a statistically significant number of failure or near failure events in order to build a reliable stochastic model of the event or conditions leading up to the event, while also being accurate and complete enough to capture the physical behavior that drives the event. Of particular importance are the body-nonlinear hydrostatic and Froude-Krylov forces, which are critical for large-amplitude roll motion and may also play a significant role in the surge and sway forces involved in surf-riding and broaching.This paper presents a volume-based evaluation of the body-linear Froude-Krylov and hydrostatic pressure forces, which retains the inseparability of hydrostatic and Froude-Krylov forces and the effects of large-amplitude relative motion along the length of a ship. Implementation of the method requires a minimum number of evaluations of the incident wave, so it can run at a fraction of the computational cost for traditional surface pressure integration schemes. The calculation has been implemented in a hybrid numerical method that incorporates ordinary different equation (ODE) like models for wave-body perturbation forces. The hybrid method has been used to produce a very large number of realizations of irregular sea responses, including a statistically significant number of stability failures, for validating schemes for the extrapolation of extreme motion responses.