On the Wave Loads for Accurate Structural Design of Ships

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As for the recent structural design of a ship, direct strength calculations are approved in classification society's rules, and wave loads for the calculation are given. However, it is questionable to apply such wave loads to the investigation of fatigue strength of a ship. In fatigue analysis of a ship, it is most important to estimate accurate pressure fluctuations in time and space. The stress fluctuation can be calculated with the use of the pressure fluctuations.In this research, the fluctuations of wave pressure in time and space were calculated firstly. Surveying the pressure distributions along ships' hull, 4 typical patterns of distribution of pressure fluctuation were introduced for fatigue analysis of ships. Using this pressure, structural analyzes for fatigue strength of a ship can be conducted with the use of a sub-structured model of the ship.In the structural analysis using the sub-structured model, however, it is not easy to apply the adequate unbalanced force and to set up the proper boundary conditions. Several empirical techniques were adopted to overcome such the difficulties so far. In this research, several methods to treat the unbalanced force and the boundary conditions in the sub-structured model of the ship were investigated secondly, and the effects of each treatment on the stress were clarified.With the use of the introduced pressure distribution patterns, structural analyzes were performed for a sub-structured model with proper unbalanced forces and boundary conditions surveyed herein. The stress distributions on the inner bottom plate and the bilge hopper tank plate were investigated, and the design wave loads for fatigue analysis of a ship are classified.