FATIGUE RELIABILITY OF TUBULAR JOINTS IN OFFSHORE STRUCTURES - RELIABILITY ANALYSIS

A reliability analysis methodology for the limit state of fatigue failure and the limit state of fatigue-fracture is presented. A probabilistic fracture mechanics fatigue crack growth model is used to quantify fatigue damage in a given period of time. Ligament fracture is assessed using the CEGB-R6 elasto-plastic fracture mechanics procedure. The important sources of uncertainty considered include environmental conditions, hydrodynamic loading, global structural analysis, local stress calculation at the tubular joints and fatigue crack growth using fracture mechanics. Initial defect size, the fatigue threshold, crack propagation parameters, fracture toughness, the yield strength of the material and welding induced residual stresses are treated as basic random variables. Suitable probabilistic models for the above variables are presented and a broad outline of the level 2 reliability analysis procedure based on the Hasofer and Lind reliability index is given. Two examples of reliability analysis showing the variation of reliability with time are presented and the important factors which govern the failure of the joint either by fatigue, fracture or plastic collapse are discussed.