9. Motion and Longitudinal Strength of a Ship in Head Sea and the Effects of Non-Linearities

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In this paper, the authors investigate theoretically the motion and longitudinal strength of ships among waves taking account of the effects of nonlinearities such as the hull shape nonlinearity, bottom emergence, and hydrodynamic impact. The longitudinal bending moment in the main hull girder is calculated in case of bottom and/or bow-flare slamming, resulting whipping vibrations. Example calculations are performed for a flat-bottomed tanker-type ship with shallow fore draft and a fast container ship with large bow-flare. In the former case, special interests are concentrated to behaviors of a tanker in ballast condition according to the Regulation for the Segregated Ballast Tank proposed by IMCO. In the latter case, vibratory responses caused by bow-flare slamming are investigated in fully loaded condition, and the interaction of bottom slamming and bow-flare slamming is clarified. A series of model tests as carried out in a model basin for a tanker-type ship, measuring the heave and pitch, the acceleration at the bow, and the bottom pressure along the base line; the experimental results were compared with calculations, which show good agreement. Results obtained are as follows: (1) The heaving and pitching amplitudes themselves are affected only slightly by slamming, while the acceleration at the bow increases due to the effect of nonlinearity. (2) The time histories of the calculated pressure and the measuree are similar in shape. (3) As for the whipping vibrations in a tanker-type ship, the maximum bending moment can be estimated by assuming the ship's hull as a rigid body, and becomes large in the sagging side. (4) In the case of a container ship, whipping is largely influenced by the interaction of bottom and bow-flare slamming, and the maximum whipping moment can be reduced by adjusting the ship's speed. If the time difference of these two kinds of slamming is of order of the ship's natural period of vibration, serious whipping g may occur. (5) As far as the ship's rolling motion can be ignored, the maximum whipping moment I determined only by the height and period of encountering waves. (6) The minimum fore draft according to the Regulation for the Segregated Ballast Tank proposed by IMCO is insufficient to avoid structural damages in rough seas.
社団法人日本船舶海洋工学会の論文

9. Motion and Longitudinal Strength of a Ship in Head Sea and the Effects of Non-Linearities

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