Structural reliability analysis of offshore jackets for system-level fatigue design

Lattice systems such as offshore jackets and towers are weight-efficient support structures for wind energy and oil and gas units. Nevertheless, their light-weight is achieved at the expense of increasing their proneness to fatigue failure, due to their many welded connections. Consequently, special attention is to be dedicated to the design of the fatigue hot spots. System-level fatigue design methods aim at calibrating the reliability of the fatigue components to achieve a desired target system reliability. These methods rely on the accurate assessment of the system probability of failure, which is computationally demanding due to the statistical dependence among fatigue limit states and the large number of possible deterioration states that need to be taken into account. In the present paper, we develop a novel approach, called the truncation algorithm, to estimate lower and upper bounds of the system reliability due to extreme environmental and fatigue limit states within feasible computational time. The proposed approach is applied to assessing existing system-level fatigue design methods and to study system effects, such as the effects of redundancy and the correlation among fatigue limit states on the system reliability.