Offshore Platform Fatigue Cracking Probability

Abstract

A systems approach is presented for fatigue reliability analysis of offshore platforms. The probability of fatigue crack occurrence in individual structural joints is considered. A nomograph relating deterministic fatigue life to probability of fatigue cracking is developed based on the lognormal fatigue reliability format. Joints are grouped according to ranges of fatigue life. Fatigue cracking probabilities of joints in each group are determined as a function of service life. Bernoulli trials are used to evaluate the probabilities of 0, 1, 2, 3,… cracks occurring in each group and also in the entire structure. The effects of fatigue life correlation are included. It is shown that the number of joints, years of service life, and correlation effects are important factors in assessing system fatigue reliability. Analysis of a North Sea platform indicates a high level of reliability against fatigue damage.