Non-Gaussian response of offshore platforms: Fatigue

A fatigue analysis procedure is proposed for offshore platforms where the response is non-Gaussian due to nonlinear wave kinematics and free-surface fluctuations. The non-Gaussian distribution of the stress process is considered to be a mixture of Gaussian and shifted exponential distributions. The level crossings of this process are estimated considering it a translation process, and the probability density function of the peaks is estimated numerically. Using Palmgren-Miner's rule for fatigue damage accumulation, the total cumulative damage over the long-term wave climate is estimated. It is shown that at higher sea states, where the effect of intermittent loading is significant, the probability density function of the peaks of the stress deviates significantly from the normally used Rayleigh density. The assumption of Rayleigh density for peaks at higher sea states is shown to be unconservative, and the unconservatism increases with an increase in significant wave height. Since field observations confirm that the dynamic response of offshore platforms is non-Gaussian, the method proposed could be used to check the fatigue design of offshore platforms to avoid overestimating the fatigue life.