Fatigue assessment by spectral approach for the ISSC comparative study of the hatch cover bearing pad

Fatigue analyses were carried out using a spectral approach in the ISSC comparative study of the hatch cover-bearing pad (Marine Struct 15 (2001) 1; 14th International Ship & Offshore Structures Congress, Nagasaki, Japan, 2000). The predicted fatigue life was 5.3 years for the structural detail. For the same detail, the fatigue life predictions of the classification societies were between 2 and 21 years (Marine Struct 15 (2001) 1; 14th International Ship & Offshore Structures Congress, Nagasaki, Japan, 2000). In addition, several parameter variations were applied in the present study using various sea areas, speeds and three different models for the fatigue strength. The predicted fatigue lives varied from 3 to 6 years. In the results of the spectral analyses, clear differences in fatigue lives were observed between moderate and severe sea areas. This also indicates that defining the environmental conditions of the ship, i.e., sea areas and occurrence probabilities of different sea states, is important in addition to the operational profile of the ship. The predicted fatigue lives are short for the analysed detail. If the true fatigue life is over 20 years, the study indicates that the fatigue strength or the fatigue loading models underestimate the fatigue life. The methodologies in fatigue analysis include rather significant uncertainties, especially in defining the fatigue loading.