Fatigue reliability analysis of offshore structures

Abstract

In this dissertation, two subjects related to the fatigue reliablity analysis of offshore structures are studied. One is the fatigue analysis of the fixed offshore structrues subjected to random wave loadings. The other is the reliability analysis for the propressive fatigue failure. For the fatigue analysis, an effeicent method for the dynamic stress analysis is presented. In this method, the power spectral densities of the stresses due to random waves are obtained in the frequency domain by using the equivalent lienearization technique. Sample time histories for random stresses are simulated, thereafter. Two stress cycle counting methods are utilized for the fatigue damage estimation, i.e., the narrow band and the wide band methods. Example analysis is carried out to investigate the effects of the bandwidth of the random stresses on the fatigue life estimation. Numerical results indicate that the wide band approach, which is more appropriate to the realities, predicts smaller values of fatigue damages compared with those by the narrow band approach for all seastate conditions. For the system reliability analysis of offshore structures against progressive fatigue failure, the second moment reliability methods are developed. Two methods are presented. One is the improved first order reliability method (IFORM), and the other is the modified probabilistic network evaluation technique(MPNET). Using idealized parallel member models, reliability analyses for progressive fategue failures are carried out for various cases with multiple members composed of multiple connecting joints per member. Numerical results indicate that the effectiveness of the proposed methods over the conventional ones (i.e., the FORM and the PNET) increases very significantly as the numver of failure modes of the system increases.