Simplified Fatigue Assessment of Offshore Wind Turbine Full Height Lattice Structures in the Frequency Domain

New concepts for support structures of offshore wind turbines have gained interest in the industry and are examined for intermediate and deep water depths in order to unlock new offshore potential markets. One of the main proposals that has already been commercialized is the three or four leg full height lattice structure. The fatigue assessment of such structures is of primary significance, since it is one of the main design drivers. A Frequency Domain (FD) framework is developed in this study with the ability of analysing several topologies of lattice structures for dynamic and fatigue assessment. The concept of the model relies on the natural frequencies and modeshapes estimation with the employment of the Finite Element Analysis (FEA) and the fatigue damage prediction due to wind and wave loading with the utilization of a Transfer Function (TRF) that relates the input spectrum to output stress spectrum for a member of the structure. Furthermore, the method of mode superposition is adopted for the calculation of the response of the structure. The benchmarking of the model for the dynamic analysis with ANSYS for a reference structure and turbine yields sufficient results with errors around 5% for the two first natural frequencies and even smaller for the higher modes. A case study of the structure developed by the Dutch company 2-B Energy is performed and the fatigue damage values as well as the stress spectra as computed by the DM are compared with the equivalent results calculated by the Time Domain (TD) software package GH Bladed for three different members. The DM produces satisfactory results for all the members for cases with low or medium environmental loading and less accurate results, which are pointing in the right direction, for the cases with high environmental loading. With the utilization of the DM for a preliminary analysis of a structure a significant amount of time (several hours) can be saved.Aerospace EngineeringAerodynamics, Wind Energy & Propulsion / Wind EnergySustainable Energy Technolog