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Fatigue Life Evaluation of Tripod Offshore Structure Using 3D Fatigue FE Analysis
Muzaffer Shazia,장경호,신왕섭 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.6
Off shore wind power became an important source of renewable energy that is rapidly growing over the past several years. Large off shore wind turbines require strong and large support structures. The support substructures are exposed to diverse dynamic and static loads such as waves, snow, wind ice, earthquakes, and tides. These unstable, destructive repeated loads lead to material degradation or damage accumulation which in turn leads to the failure of off shore structures. Fatigue is considered one of the signifi cant modes of failure in off shore structures. The constant exposure of welded multiplanar tubular joints of tripod structure under cyclic loading or the presence of fl aws due to fabrication and construction induces high-stress concentration at these joints. The material degradation or damage accumulation at welded joints gives rise to the formation or propagation of cracks, and ultimately leads to a collapse of giant steel structures. This research aimed at investigating the origin of crack positions and fatigue life in tripod off shore structures. The 3-D fatigue FE analysis was carried out in two steps, Firstly, the welding deformation and welding residual stress were calculated in thermal and mechanical analysis. Secondly, 3D fatigue crack analysis based on cyclic hysteresis constitutive equation and fatigue damage theory was used to calculate the fatigue life and crack initiation in tripod off shores structure. Results obtained from the analysis were compared with the results from hot-spot stress method and Eurocode 3. From the results, it was considered that 3D-fatigue FEM is a useful method to estimate fatigue life and crack initiation.
Shazia Muzaffer,장경호,Mikihito Hirohata 대한토목학회 2024 KSCE Journal of Civil Engineering Vol.28 No.2
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.
Fatigue analysis of overlapped tubular joints
Shazia Muzaffer,Kyong-Ho Chang,ZhenMing Wang,Wang-Sub Shin 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
The increasing demand of on the use of wind energy during recent years, have led to installation of more durable, safe and stable offshore structures in deep waters. The circular hollow section tubular joints are generally preferred in offshore structures. The overlapped joints are considered better than simple gap K-joints because of their higher axial strength, high stiffness and high durability. These tubular joints being subjected to combination of wind and waves load during their design period, the constantly acting loads deteriorates the structure, which often results in total collapse of offshore structures. In order to allow for cost effective and reliable design, the substantial requirement is to do accurate and reliable simulation. In this study in order to understand the behavior of crack initiation and the fatigue life of overlapped joints, the 3D fatigue FEM analysis of overlapped joint was carried out. The 3D fatigue FEM study based on continuum damage mechanics and constitutive model, was carried out in two steps namely 3D thermo elastic-plastic analysis and 3D non-steady heat conduction analysis. The fatigue life and crack initiation positions were determined. Finally, S-N curves were plotted and the results from FEM analysis were compared with Hot Spot Stress and with S-N curves recommended by Eurocode 3.
Fatigue Analysis of Tripod wind turbine support structure
Shazia Muzaffer,Kyong-Ho Chang(장경호),ZhenMing Wang(왕진명),Wang sub Shin 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
The offshore wind turbines in deep waters requires building large foundations, which transfers loads from wind turbine to ground. The tripod being considered the most robust and secure supporting structure due to its good strength and stability. The welded joints in the structure being exposed to different environmental loads such as wave load, ice load, wind load and earthquake for longer periods increases the stress concentration at hotspot positions which in turn leads to crack initiation and ultimately the failure of whole structure. In this work the main objective is to investigate the fatigue life and crack initiation positions of welded joint in Tripod structure by using 3D fatigue FEM analysis. Firstly, in 3-D Fem analysis, welding residual stresses and welding deformation is calculated by employing sequentially coupled 3-D thermo-mechanical analysis. Secondly, a nonlinear damage model based on continuum damage mechanics for multiaxial high cycle fatigue is used to predict the fatigue life and fatigue crack initiation. The results obtained from analysis were quite satisfying, so 3D fatigue FEM can predict the fatigue life of structures with great accuracy.