Fatigue Data for Reliability‐Based Offshore Platform Inspection and Repair

Abstract

Fatigue of welded fixed offshore platforms is a primary design and operational concern. Environmental, fatigue data, and method uncertainties, as well as inspection and repair activities, are major parameters that influence the welded joint reliability. This paper investigates and establishes the relation between welded tubular joint fatigue life computed from a deterministic design process and the probability of occurrence of: (1) First detectable crack; (2) first through‐thickness cracking; and (3) total joint failure stages. Several experimental fatigue data bases were used to derive the appropriate probabilistic distributions and relations between different joint failure stages. The effect of repair methods, such as repair welding and grinding, on the fatigue life is studied, and parameters that modify the probability of joint failure states are introduced. The data examined show that time to first discernible surface cracking for welded tubular joints is about 10‐65% of the time to through‐thickness cracking, with a mean value of 35%. The total joint failure will occur at about 150% of the time to through‐thickness crack. Simplified relations between crack size and remaining fatigue life are derived based on recent experimental results. Data presented in this paper provide the basic tools for performing a reliability‐based inspection and repair assessment.