Fatigue Assessment and Extreme Response Prediction of Ship Structures

Doctoral thesis, 2010

In this thesis, a simplified narrow-band approximation model is proposed to estimate fatigue damage of ship structures, and an efficient method for extreme response predictions is also developed using upcrossing spectrums of ship responses. The proposed fatigue model includes two main parameters, significant stress range hs and zero upcrossing frequency fz. The first parameter is assumed to be proportional to significant wave height Hs through a factor C, derived from a linear hydrodynamic theory. The value of C depends on the mission conditions. The zero upcrossing frequency is approximated by the encountered wave frequency, where the wave period Tz is deduced to be an explicit function of Hs. The fatigue model is validated by the “accurate” rainflow method with less than 10% of discrepancy. The uncertainties of fatigue life predictions are studied by the safety index, employing the proposed fatigue model. It is shown that the safety index computed using the fatigue model agrees well with that computed from the measurements. With respect to the fact that ship responses are non-Gaussian, the Laplace Moving Average (LMA) method and a transformed Gaussian approach are studied to model the non-Gaussian responses. The transformed Gaussian approach is adopted for the computation of the upcrossing spectrums. The extreme ship responses are then estimated from the upcrossing spectrums. The standard deviation, zero upcrossing frequency, skewness and kurtosis of responses are needed to compute the upcrossing spectrums. It is shown that the extreme responses computed by the proposed method agree well with those computed by the standard engineering method using the measured responses.