Fatigue damage summation with bias correction

This paper presents a framework for establishing bias factors for combining fatigue damage from two independent stochastic processes considering linear SN-curves. The proposed design approach for calculating fatigue damage consists of identifying appropriate bias factor from bias factor contours pre-generated by simulations, and correcting the linear sum fatigue estimate to account for stress amplification. Depending on input parameters, the approach can account for Gaussian, non-Gaussian, wide-banded and narrow-banded processes in a practical way, which is not achieved by existing methods. For long-term fatigue assessments, simplified approaches are possible by selecting a conservative representative bias factor at global minimum, while more accurate estimates can be found by more careful selection of bias factor per short-term condition. Case-specific design contours tailored to the application can be generated using the methodology described. In this paper examples of design contours for combination of wave induced- and vortex induced vibrations (VIV) fatigue relevant for marine risers are presented.