Fatigue Reliability for Offshore Structures

The results of a four-year project, sponsored by the American Petroleum Institute, to investigate the fatigue design process in the welded joints of steel offshore structures are summarized. Fatigue damage expressions were formulated. A simple analytical expression for damage based on the rainflow method of cycle counting in a wide band process was constructed. Because fatigue design factors have significant uncertainty, a reliability approach, using the lognormal format, was developed. The performance of Miner's rule was characterized statistically, as was modeling error associated with the process of computing fatigue stresses in a joint from oceanographic data. Procedures of S-N data analysis were developed, and characteristic S-N data sets were presented. The reliability format was employed to construct a design rule for low period structures. It is argued that the new design rule is more discriminating in that it can account for service life, wave spectra, water depth, platform dynamics, as well as the location of the joint in the structure.