STOCHASTIC FATIGUE DAMAGE ACCUMULATION DUE TO NONLINEAR SHIP LOADS

Efficient methods are described here to predict the stochastic accumulation of fatigue damage due to nonlinear ship loads that are produced in random seas. The stochastic analysis method, which may be applied both to overload and fatigue limit states, is based on a relatively new concept: the “nonlinear transfer function ” (NTF) method. The basic goal of this method is to require the use of a generally expensive, nonlinear time-domain ship load analysis for only a limited set of idealized, regular waves. This establishes the so-called nonlinear transfer function; i.e., the generally nonlinear transformation from wave amplitude and period to the load amplitude measure of interest (e.g., total load range for rainflow-counting, tensile portion for crack propagation, etc.). Stochastic process theory is used (1) to identify a minimal set of regular waves (i.e., heights and periods) to be applied, (2) to assign an appropriate set of “side–waves ” to be spatially distributed along the ship, and (3) to determine how these results should be weighted in predicting statistics of the loads produced in random seas. The result is compared here with full nonlinear analysis of a specific ship, over long simulations of an irregular sea. A ship with relatively flared cross-section is chosen, which 1 shows marked nonlinearity and hence asymmetry in its positive and negative (sag and hog) mid-ship bending moment. The NTF method is shown to accurately predict the results of the long nonlinear simulations. This suggests the potential for considerable reduction in analysis costs: time-domain analysis over many cycles of an irregular sea is replaced by a limited number of regular wave analyses