Suresh Rajendran

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.

Vertical bending moment (VBM) is an importantparameter for the structural safety of any sea going ship.Generally, it is expected that ships encounter largest VBM inhead seas. However, when it comes to flexible ships, it is notnecessary that the largest VBM always occurs in head sea. Itcan also occur in oblique waves and high frequency response inwaves with shorter period can be as large as the wavefrequency response or can be even larger. Studies conducted inthe recent past by other researchers has shed some light intothis peculiar characteristics of VBM of flexible hulls.Therefore, it will be worthwhile to further investigate to checkwhether the nonlinear design load calculation of the flexiblehull in head seas will lead to conservative results or should betested for a range of headings with different combinations ofwave period in order to estimate the largest VBM acting on thehull. In this paper, the numerical and the measured VBM of anultra large container ship (ULCS) in low to severe sea states areanalyzed and compared. The effect of speed, significant waveheight, springing and bow flare slamming on the response isstudied. The measured VBM in head and oblique waves arecompared and interesting findings are observed.