Fatigue Reliability Analysis of a Mono-Tower Platform

In this paper, a fatigue reliability analysis of a Mono-tower platform is presented. The failure mode, fatigue failure in the butt welds, is investigated with two different models. The one with the fatigue strength expressed through SN relations, the other with the fatigue strength expressed through linear-elastic fracture mechanics (LEFM). In determining the cumulative fatigue damage, Palmgren-Miner's rule is applied. Element reliability, as well as systems reliability, is estimated using first-order reliability methods (FORM). The sensitivity of the systems reliability to various parameters is investigated. The systems reliability index, estimated by using the fatigue elements with the fatigue strength expressed through SN relations, is found to be smaller than the systems reliability index estimated by using LEFM. It is shown that the systems reliability index is very sensitive to variations of the natural period, damping ratio, current, stress spectrum and parameters describing the fatigue strength. Further, soil damping is shown to be significant for the Mono-tower.