Investigation of Bottom Slamming on Ships in Irregular Waves

The bottom slamming of a chemical tanker and a LNG carrier advancing in irregular waves is investigated numerically and compared with experiments. The probability of slamming occurrence on longitudinal locations and the slamming induced pressures on the bottom of two ships are discussed. It is considered that slamming occurrence at a point is dependent upon two conditions: the relative vertical motion at the same longitudinal position of the ship being larger than the vertical distance from the still water to the concerned position, and the entry velocity exceeding some threshold velocity. Ship motions in irregular waves predicted by a time domain seakeeping code and measured from the model tests are used to calculate the slamming occurrence statistically based on the two conditions mentioned above. Only heave and pitch motions are considered in the calculations. The seakeeping code combines body linear radiation and diffraction forces with body nonlinear Froude-Krylov forces, hydrostatic forces and shipping of green water on the bow. The effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces. The experimental data of the wave-induced loads on these two vessels in different sea states are analyzed statistically. Probability of exceedance of entry velocities and pressure peaks for the sections at the bow and stern are computed for various irregular sea states. The results of the slamming occurrence on longitudinal locations and wave induced loads on these two types of ships are discussed.