Reliability-based fatigue analysis of mooring lines

Design against fatigue has not been a prime consideration for mooring systems, but has received increased attention during the past years for permanent floating units. Uncertainties both in the environmentally induced dynamic load effects and in the fatigue strength of mooring line components make a rational design against fatigue a complex task, and a number of the observed mooring line failures are certainly due to fatigue. Current fatigue design practice is based on the Miner Palmgren hypothesis for the accumulation of fatigue damage, considering standard T-N curves, together with various algorithms for fatigue damage calculation. The methods of structural reliability, taking both the uncertainties related to fatigue loading and fatigue strength into account, have been applied in a probabilistic analysis procedure of the fatigue limit state (FLS) for mooring lines. This paper outlines the philosophy and methodology for this procedure. The procedure is also applied in numerical examples for a semisubmersible and a turret moored ship. Both base case failure probabilities and results from a number of sensitivity analyses are presented. It is demonstrated that the reliability-based design procedure can be effectively utilized to quantify the safety against fatigue failure of moorings. The results of the case studies indicate thatmore » the largest uncertainty is associated with the T-N curve parameters. However, the sensitivity of the fatigue reliability to the number of chain links, especially those lying on bottom, and to the damage calculation algorithm is also appreciable. In the examples considered, the wave-frequency part of the loading tends to contribute more to the fatigue damage than the low-frequency part of the loading.« less