Recent advances in the analysis of joint seakeeping processes are applied to the prediction of Von Mises stress. The methods of cross co-spectral moments are applied to the estimation of the probability distribution of Von Mises stress in each stationary joint environmental and operating condition. The important class of Von Mises stress problems subject to conditions with steady stress components (i.e., still water stresses) is considered, and the probability distribution is developed through application of the method of statistical principal axes for joint processes. An explicit method for estimating the lifetime probability distribution of Von Mises stress is described. From this lifetime distribution, the expected extreme value can be determined. This expected extreme value of Von Mises stress is suitable and appropriate for use as a dynamic loads parameter (DLP) in a global dynamic loads analysis (DLA) similar to that promoted by the American Bureau of Shipping. The method of cross co-spectral moments applied to the prediction of Von Mises stress is validated by comparison with Fourier-Stieltjes time-domain simulations. As an aid to understanding, the ellipsoidal distributions of Von Mises stress and probability are discussed, and the statistical principal axes are determined.
Authors: Bruce L. Hutchison, PE; Benjamin B. Ackers, PE