Uncertainties in Spectral Fatigue Damages and an Efficient Simple-Reliability Calculation of Offshore Structures

Abstract

This paper presents uncertainties in spectral fatigue damages of offshore structures firstly. Then, attention is given to the formulation and procedure of a fast and efficient computation of fatigue reliability estimates. Most of uncertainties are embedded in response characteristics of the stress process and the damage-model used. Uncertainties in stress statistical characteristics are associated with the modeling of structures and random wave environment as well as wave loading and the analysis used. Uncertainties arising from degradation of member stiffness, wave-current and water-structure interactions can be considered in the modeling of structures, wave environment and loading. In the fatigue damage, there are additional uncertainties arising from the modeling of damage-mechanism. These uncertainties are due to experimental fatigue data and structural joint configurations. All these uncertainties can be classified into aleatory (naturally inherent) and epistemic (due to lack of knowledge) categories. The second part of the paper is devoted to a fast and efficient computation of fatigue reliability. This algorithm eliminates repetitive execution of spectral analysis procedure. It is performed only once for all reliability iterations. In this technique, a suitable spectral formulation of the stress process is used and a new uncertainty parameter is introduced to represent most of uncertainties in the stress spectrum. A detailed modeling of the fatigue-related uncertainties is presented. The failure function of the reliability analysis is expressed independently of the spectral analysis. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty parameters. The procedure is demonstrated by an example jacket structure and the results are compared with previously available ones.