PREDICTING FATIGUE DAMAGE MATRICES FOR FLOATING STRUCTURES ACROSS MULTIPLE SEASTATES: THE DAMMAT ROUTINE

This document illustrates the use of the routine DamMat. It seeks to facilitate fatigue analysis of floating structure components, such as the tethers of a tension-leg platform. Its output takes the form of damage matrices, which give the mean damage a specific floating platform experiences across a range of steady-state ocean conditions. Separate matrices are produced for a range of assumed fatigue properties; specifically, of the exponent b which relates the applied stress S to fatigue life N. Thus, these matrices conveniently summarize all relevant structural information, including (1) the hull geometry, which governs forces, (2) the tether stiffness, which governs resonant response, and (3) the fatigue behavior, as characterized by the exponent b. These matrices do not depend, however, on the long-term relative frequencies of seastate parameters such as significant wave height Hs, peak spectral period Tp, and heading. As a result, after a single DamMat analysis yields an appropriate damage matrix, long-term damage rates can be estimated for multiple sites by applying appropriate (site-dependent) seastate weighting probabilities.