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Nonlinear Ship Loads and Ship Fatique Reliability
Stochastic me hods are described here to predict the effects of nonlinear ship loads on fatigue accumulation in random seas. These are found to be capable of predicting the net fatigue damage over many ship response cycles, at a fraction of the cost of direct time-domain analysis. It is also shown how these nonlinearship load models can be used within a full reliability analysis of ship components. This uses first-order reliability methods to estimate not only the net effect of all uncertainty sources, but also the relative contribution of each [e.g., load vs. material property variability]. In our ship load application, nonlinear effects are found to increase the relative impact of load (and wave) modelling and its uncertainty.
The ship load analysis for fatigue is based on a new concept, the "narrow-band transfer function" (NTF) method. The basic idea is to apply the nonlinear time domain ship load analysis to a limited set of regular, sinusoidal waves. This establishes the narrow-band transfer function; i.e., the ship load amplitude from nonlinear analysis as a function of the wave amplitude and period . Stochasti c process theory is used both to select which regular waves [i.e., heights and periods) to use, and to decide how these results should be weighted in predicting load statistics in an irregular sea.
The result is compared 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 shows marked nonlinearity and hence asymmetry in its positive and negative (sag and hog) mid-ship bend ing moment. The NTF method is shown to accurately predict the results of the long nonlinear simulations. This suggests great reduction in analysis costs: time-domain analysis over many cycles of an irregular sea is replaced.