13. A Consideration on the Phase Difference between the Wave Induced Stresses of Longitudinal and Transverse Strength

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Dynamic stress response of double bottom structures of a bulk carrier to wave loading is investigated by the stochastic method. Wave-induced loads on the double bottom, as well as the hull girder vertical wave bending moment, are calculated by a strip method for the ship motion analysis. The phase differences and the correlation coefficients between the dynamic water pressure and the cargo inertia force are evaluated, and it is found that these loads act in the opposite direction. The stress response function of double bottom structures is obtained by the quasi-static structural analysis of a 3-dimensional model for the entire hold part. The short-term and the long-term predictions of stresses in irregular seaways are performed on the base of the random response theory for a linear system. The study is concentrated on the bottom shell plating, for which the superposition of hull girder bending stress and double bottom bending stress is necessary. The correlation coefficient between these stresses is investigated in long-crested irregular seas. An approximate formula is presented for the superposition of these in terms of the long-term predicted values. And design criteria for the double bottom strength are proposed.
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