A new frequency-domain method for fatigue prediction of offshore structures under wideband random loadings

Abstract

The frequency-domain methods used to predict fatigue damage in offshore structure are widely employed in practice due to their simple algorithms and fast computation speed. While reliable accuracy can be achieved with current prediction methods for narrowband stress processes, actual offshore fields often exhibit wideband stress responses, resulting in more complex relations between the rainflow stress distribution, power spectral density, and fatigue damage. This study proposes a novel fatigue prediction method for wideband stress processes that utilizes Monte Carlo simulation for designed standard spectra. An empirical formula is derived to calculate the expected damage rate, considering material properties, spectral moments, and rainflow range distribution moments. To verify the accuracy of the model, 3-month onsite measured stress data from a semi-submersible platform are used, and the proposed method is compared with several typical prediction methods with explicit expressions. The results indicate that the expected damage rate is determined by the spectral moments of power spectral density, assuming fixed material properties. The proposed method shows good prediction accuracy for the field data and has a significant improvement of fatigue damage prediction for various bandwidth processes.