Comparison of fatigue life and crack initiation of tubular joints due to the difference in the brace position

Jacket structures are widely used in oil or gas production facilities and wind turbines. The bracing members of jacket structures must be designed to withstand intense and repeated stresses, such as those caused by wind and wave forces. The continual exposure to fluctuating stresses and the harsh marine environment can lead to material deterioration, resulting in stress concentrations at tubular joints that may induce fatigue cracks. This study aims to investigate the fatigue life due to differences in the brace position and crack initiation during cyclic loading. The study employs 3D fatigue finite element (FE) analysis based on cyclic hysteresis elastoplastic constitutive theory and continuum damage mechanics. Initially, to reproduce the welding members, a 3D non-steady heat conduction analysis was conducted to calculate the temperature history. Subsequently, residual stresses and welding deformation were calculated using thermal elastoplastic analysis. Finally, a 3D fatigue FE analysis was conducted to calculate fatigue life and predict the crack initiation position due to differences in brace positions. The S-N curves obtained from the 3D fatigue FEM were compared with the S-N curves recommended by Eurocode 3 and IIW. The FEM data was compared with the hotspot stress method (HSS).