Marine Riser Fatigue Analysis Using Wave-Variance Spectra

The regular wave method is currently used in the fatigue analysis of marine riser systems. A wave height of the largest 1,000 waves is usually assumed, along with the ship response amplitude operator, RAO, at the significant period, to calculate the harmonic motion at the riser/ship interface. This approach is quite conservative, leading to life predictions much shorter than if the entire spectural content of the wave action were considered. This paper presents an alternative method that accounts for the entire wave spectrum. The Bretschneider wave-variance spectra, aIong with the ship RAO and the results of a marine riser analysis, provide the basis of the calculation. The distribution of wave peak amplitudes, and therefore the alternating stress amplitudes, is assumed to be Rayleigh distributed. The Palmgren-Miner linear damage law is written in an integral form that produces a most probable reduced stress. The formulation is based on a standard type of Stress-Number of Cycles to failure (S-N) curve. Since riser and coupling stress calculations are based on finite-difference and finite element techniques, curves X and X\´ of API standard RP 2A apply. The more conservative of the two, X\´,is used for the example calculation. Two sea states are considered, along with a typical drill ship RAO. Fatigue life calculations using the method presented give more than two orders of magnitude longer life compared to the regular wave method. The "flatter" the S-N curve, the larger the difference will be between the two methods.