Fatigue Life Comparison of Tubular Joints in Tripod and Jacket Offshore Support Substructures Using 3D Fatigue FE Analysis

Abstract

In a wind energy system, the safety and stability of substructures plays an important role during the in service of offshore structure. Offshore structures are continuously subjected to high cyclic fatigue loads and may experience fatigue cracks due to the continuous accumulation of plastic strain and stress concentrations at welded joints. The fatigue life of welded tubular joints is one of the most important factors determining the life of offshore structures. In this study, the fatigue analysis of tubular joints of the tripod and jacket support structure was performed using 3D fatigue FEM to estimate the fatigue life and predict the positions of crack initiation. The 3D fatigue FE is based on constitutive equations and continuum damage mechanics. The welding state of tubular joints were reproduced to calculate the welding residual stresses and welding deformation. The residual stresses and weld deformation were used as input together with cyclic loading in the 3D fatigue FE to calculate the fatigue life and predict the crack initiation positions. The S-N curve calculated by the 3D fatigue FE analysis were compared with the SN curves of Eurocode 3. The results show that 3D fatigue FE analysis is an effective tool to analyze large and complex structures before installation to ensure the safety and stability of the structure. © 2023, Korean Society of Civil Engineers.