Refinement of Fatigue-Life Estimates by Interpolation on Metocean Conditions

The fatigue life of ships and offshore structures is commonly based on the Miner-Palmgren hypothesis and S-N curves. Fatigue damage rates can be calculated for each short-term sea state, and then combined with the probabilities of the short-term sea states to obtain the long-term damage rate and the estimated fatigue life. Each short-term stress response is usually computed for one point within the corresponding bin of the scatter diagram. In order to minimize uncertainty arising from the choice of a single representative point within a relatively large bin, a refinement procedure is proposed by interpolation on metocean conditions. The proposed procedure is applied to the most heavily utilized girth weld of a TLP tether and discussed.