Spectral fatigue damage calculation in the side shells of ships, with due account taken of the effect of alternating wet and dry areas

Abstract

A method for calculating the wave load induced fatigue damage in ship side shells in connections between longitudinals and transverse web frames is presented. The method is based on the Miner–Palmgren fatigue damage formulation and a spectral approach is used to determine the necessary variances of the stress processes. The traditional linear frequency domain analysis is expanded in order to incorporate the non-linear nature of the external water pressure. The pressure process is determined by use of pressure RAOs, and not as previously by relative motion RAOs. Effects of load combinations are included in the method, both combinations of the global vertical and horizontal hull girder bending and combinations of the combined global bending with the local pressure induced load process. The proposed method is used to analyze a double hull tanker, and the results show reasonable agreement with results published by other authors. Effects of load combinations are shown to be significant, and especially it is confirmed that the external pressure is very important to include in the analysis as it accounts for the majority of the calculated damage.