Dynamic analysis of fixed offshore structures subjected to random ocean waves is presented. The nonlinear drag force is included in the formulation. The analysis is performed in the time domain as well as in the frequency domain with the aid of equivalent linearization technique.
In fatigue failure analysis the hybrid time-frequency domain method is used. This method defines pseudo transfer function and calculates power spectral densities of stress for representative seastates and predicts power spectral densities of stress for other seastates by the interpolation of pseudo transfer function.
After stress time series and power spectral densities of stress are calculated for every seastate, accumulated damage is calculated by two method. The one utilizes narrow band assumption of power spectral density of stress and the other uses the rainflow counting of stress time series.
In this paper three different structures are studied for several seastates and the results are compared.