Fatigue analysis of offshore platforms with uncertainty in foundation conditions

Foundation flexibility is an important consideration, in designing offshore structures against fatigue, since it has a significant effect on the system's dynamic response. However, a considerable amount of uncertainty is expected in the estimation of foundation flexibility due to the natural nonhomogeneity of the soil as well as laboratory and in-situ testing errors. The proposed method explicitly uses of the statistical nature of uncertainty in the soil stiffness to estimate the variability in the instanteneous and fatigue response using the First-Order Second Moment technique, with emphasis on uncertainties in the dynamic shear modulus along with random wave loading. Uncertainties in the response are estimated considering the randomness in the eigen values, eigen vectors, and the mechanical transfer function. Fatigue life is estimated at different levels of uncertainty for the dynamic shear modulus. It is observed that the uncertainty in the dynamic shear modulus of the soil has more significant effect on the fatigue life of the joints close to the base. The variability in the fatigue response increases with a reduction in the mean dynamic shear modulus. The response is more sensitive to changes in the dynamic shear modulus at its lower values.