Design Wave Approach For Fatigue Strength Analysis Of Ship Hull Structures

Fatigue damage is one of the main failure modes of ship and marine structures. Currently, the fatigue assessment methods proposed by classification societies are simplified method and direct calculation method. Because the structural stress response is calculated by finite element analysis, the structural details can be simulated better in the direct calculation method than in the simplified method. Then the direction calculation method is widely used by the classification societies in the fatigue assessment. The direct calculation method includes spectral-based fatigue analysis and design wave approach. However, because of the large work load and low efficiency of the spectral-based fatigue analysis, the design wave approach is worth to be studied. Especially in the engineering field, the design wave approach will have a good prospect because of the convenient calculation and the relatively reasonable result.The aim of this paper is to make a research on the design wave approach of the fatigue assessment. The main contents are as follows,1) The fatigue strength of five bulk carriers is calculated using the spectral-based fatigue analysis method. First, the finite element model of entire ship is built and the mesh in the vicinity of the hotspot is refined. Then the spectral-based wave load is calculated. Further a Patran Command Language program is written, by which the wave pressures and inertial cargo pressures are applied, the calculation is executed and the results are extracted automatically.2) Four different design waves are determined based on four kinds of dominant load parameters (DLP). After that the fatigue damage and fatigue life of the selected ship is calculated by using these design waves.3) Taking the spectral-based fatigue results as the target values, the fitting of the fatigue stress and fatigue damage is carried out to find a good combination of the design waves for the fatigue assessment.