This paper presents formulations and procedure of a fast and efficient computation of fatigue reliability estimates of offshore structures, which eliminates repetitive execution of spectral analysis procedure so that it is performed only once for all reliability iterations. This is archived by a suitable uncertainty modelling and spectral formulation of the stress process. For this purpose, a new uncertainty variable is defined to represent all uncertainties in the stress spectrum, except those in the damping and inertia force coefficients, thicknesses of marine growths and structural members, which are represented by their own uncertainty variables. Apart from uncertainties in the stress spectrum, a detailed modelling of the fatigue-related uncertainties is presented. Uncertainties in SCF, damage model (S-N line), analytical modelling of the probability distribution of non-narrow banded stress process, long-term probability distribution of sea states and in the reference damage at which failure occurs, are all considered in the group of fatigue-related uncertainties. Formulation of the stress spectrum and stress spectral moments is presented explicitly in the idealized uncertainty space. Then, the failure function of the reliability analysis is expressed in terms of uncertainty variables as being independent of the spectral analysis. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty origins. The procedure presented in the paper is demonstrated by an example jacket type structure and the results are compared with previously calculated results using more sophisticated uncertainty modelling of the stress spectrum.
Probabilistic Assessment of Degree of Bending in Tubular X-Joints of Offshore Structures Subjected to Bending Loads
