Calculation method to include water on deck effects

Abstract

Green water is an important issue regarding ships stability as it may dramatically change the loading of the ship compared to its dry deck condition. Until now, computational methods capturing this event are very time consuming as they often try to capture the complete dynamics of the flow over the vessel’s structure and deck using CFD. Such methods are not practical when dealing with numerous lengthy time domain simulations for long term stability assessments. MARIN has developed a fast method to be implemented in its 6 DOF time domain program FREDYN. This method has as objectives to be as fast as possible, even real time if achievable, but at the same time take into account correctly the mass of water flooding on the deck during green water events. The method is based on pre-computing the steady forward speed wave pattern and diffracted and radiated waves. The steady wave is computed for a series of sailing conditions using the in-house 3D linear panel code DAWSON. The diffracted and radiated waves are pre-computed using in-house 2D strip theory potential code SHIPMO for a series of frequencies and sailing conditions. A ship generated wave is then computed at each time step during the simulation using the current position and motions of the ship. This improves the computation of a realistic wave elevation consisting of the incident, steady, diffracted and radiated waves along the hull of the ship. This wave profile is then used to feed our flooding module which computes flows in tanks, compartments and through openings. This flooding model is based on a quasi-static Bernoulli formulation and empirical discharge coefficients. It is used to compute the flow over the bulwarks and through the freeing ports to the deck.