Reliability analysis of fatigue life of the connectors—the US Mobile Offshore Base

Abstract

Reliability-based design analysis of the fatigue life of the connectors of the five sections of the 2-km long US Mobile Offshore Base (MOB) is demonstrated. A performance function is defined in terms of the nominal stress range, inherent defect or starter crack, and appropriate material properties, which are considered random variables. The reliability analysis is performed for a sea state 1–8 (SS1–8) random loading having a Gumbel distribution. Where possible, uncertainty data for random variables are obtained from published data relating to the fatigue of metal and metal alloys. Otherwise, judgmental coefficients of variation are prescribed for purposes of demonstration. The fatigue life is assumed to follow the Weibull distribution. The reliability function is defined in terms of the mean life and the total uncertainty in the fatigue life. Preliminary reliability calculations suggest that current design stress levels be reduced to meet the current fatigue life target reliability level for the MOB connectors. An illustrative design is demonstrated and the metal selected for a fatigue design of the connectors for 10 million cycles with a reliability of 0.99 at a nominal stress of 203 MPa is HY-130 steel.