Investigation of bottom slamming on ships in irregular waves

The bottom slamming of a chemical tanker and a LNGcarrier advancing in irregular waves is investigated numericallyand compared with experiments. The probability of slammingoccurrence on longitudinal locations and the slamming inducedpressures on the bottom of two ships are discussed. It isconsidered that slamming occurrence at a point is dependentupon two conditions: the relative vertical motion at the samelongitudinal position of the ship being larger than the verticaldistance from the still water to the concerned position, and theentry velocity exceeding some threshold velocity.Ship motions in irregular waves predicted by a time domainseakeeping code and measured from the model tests are used tocalculate the slamming occurrence statistically based on the twoconditions mentioned above. Only heave and pitch motions areconsidered in the calculations. The seakeeping code combinesbody linear radiation and diffraction forces with body nonlinearFroude-Krylov forces, hydrostatic forces and shipping of greenwater on the bow. The effects of body nonlinearity areconsidered by a simplified method: the memory functions,infinite frequency added masses and the radiation restoringcoefficients are assessed at each time instant as function of theinstantaneous wetted surface. A similar procedure is used tocalculate the diffraction forces.The experimental data of the wave-induced loads on thesetwo vessels in different sea states are analyzed statistically.Probability of exceedance of entry velocities and pressure peaksfor the sections at the bow and stern are computed for variousirregular sea states. The results of the slamming occurrence onlongitudinal locations and wave induced loads on these twotypes of ships are discussed.