Practical Review on Fatigue Damage Estimation Under Combinations of Global and Local Loadings

Abstract

In this paper, reviews on the fatigue damage estimation are presented in case that both the global and the local loadings are applied and two practical approaches are proposed if the statistical data of the local loading are not available. So far the fatigue damage has been estimated through Rules from Classification of Societies if the statistical data such as the standard deviation and the up-crossing rate of the stress processes are available. However, in many cases, it’s hard to prepare enough data for the fatigue damage estimation, which results in very severe design. Hence two practical approaches are presented which can be applied if the statistical data of the local loading are not available. In practice, only the maximum stress range and the total cycles of the local loading are available very often. In the practical approach 1, it is assumed that the maximum stress range always occurs during the total cycles. Then, the local fatigue damage is very conservatively estimated and the total fatigue damage is obtained by the way of summation of the global fatigue damage and the local fatigue damage. In the practical approach 2, the local fatigue damage is estimated based on the assumption of the Weibull fitted local loading. And the use of the cube root summation is proposed between the global fatigue damage and the local fatigue damage. The presented two practical approaches are applied to the side appurtenances of a FPSO and the resultant fatigue damages are compared and discussed. The results of this study can be expected as practical and useful data for the fatigue analysis of the marine and offshore structures.