CALIBRATION OF A FATIGUE LIMIT STATE FOR MOORING LINES

A structural reliability method is applied to calibrate the safety factor of a fatigue design method for the mooring lines of floating offshore platforms. Both methods are based on the Miner Palmgren hypothesis for the accumulation of fatigue damage. The reliability analysis takes explicit account of the various uncertainties in the fatigue capacity, the damage accumulation and the calculation of mooring line tensions. These uncertainties are all intended to be covered by the safety factor in the design analysis. The uncertainty in the fatigue capacity is developed for chain and steel wire rope. A test set of six floating platforms in various water depths is considered. The effect of varying numbers of mooring line components is taken into account. A target probability of failure is established. The safety factor of the design method is calibrated to yield designs close to the target probability. This calibration is effected by minimising an objective function based on the difference between target probability and calculated probability of failure for the mooring lines in the test set. Under-design is penalised more heavily than over-design.