A Study On Fatigue Analysis Method Of Support Structures For Offshore Wind Turbines

Generally, the support structures for offshore wind turbines are made of welded tubular. Under the complicated marine environmental and wind turbines loads, the stress concentration will often occur on the structural joint, and it will easily cause the structural fatigue damage. Thus, the structural joints are the keystone of structure design for offshore wind turbines. In order to ensure the normal operations of wind turbines during its design fatigue life, it is essential to analyze the fatigue life for the support structures of the offshore wind turbines.In this study, we focus on the three-pile support structure for offshore wind turbines in one wind farm, the critical fatigue damage zone is selected, and then the fatigue life of this structure is detailed analyzed. The study work will provide the references and experiences for resolving the similar problems and the further research. The primary works in this study are as follows:Firstly, the basic theories and methods of fatigue analysis are introduced, including the liner fatigue cumulative damage rules, the bilinear fatigue cumulative damage rules, the nonlinear fatigue cumulative damage rules, the probabilistic cumulative damage rules and the fatigue cumulative damage dynamic model rules. More attentions and analysis are given to the S-N curves and Palmgren Miner liner fatigue cumulative damage rule which is simple and widely used at present.Secondly, based on the general Finite Element Method (FEM) software, ANSYS, the overall finite element model of "pile and support structures" of three-pile support structure for offshore wind turbines is presented. In additional, the local refined finite element model of the critical fatigue damage zone is provided. The boundary displacements of local finite element model can be obtained from the calculation of the overall finite element model, which can obtain the accurate results of hot spot stresses and solve the problem that the shell element model could not calculate the wave and current loads directly, and the pipe element model could not obtain the accurate results of hot spot stresses at the same time.Thirdly, the effects of finite element mesh size on the hot spot stresses are analyzed. The finite element mesh for brace should be parallel to the brace’s axis, and the finite element mesh for chords should be perpendicular to the weld. The method for selecting the fatigue calculation spot of three-pile support structure for offshore wind turbines is presented. Fourthly, the effects of wave and wind loads on the fatigue damage of structures are considered by using the fatigue life analysis methods of hot spot stresses based on the S-N curves from offshore standard ("Design of Offshore Wind Structures") and the Palmgren Miner liner fatigue damage cumulative rules. Also, the effects of fatigue load directions and mean stress on fatigue life of structure are discussed.Finally, the effects of different coupled-field analysis methods under wave loads and wind turbines loads on fatigue life are studied, such as the simple liner superposition method, the quadratic superposition method, the in phase of equivalent fatigue stress superposition method and the out phase of equivalent fatigue stress superposition method, and the recommended approach is provided.