Global Performances and Structural Health Monitoring Method of Single Unit or Multi Units Floating Offshore Wind Turbines

In the present study, the fully coupled wind-turbine/hull/mooring dynamics of the OC4 semi-submersible model and square-type semi-submersible multi-unit floating offshore wind turbine (MUFOWT) developed by Korea research institute of ships and ocean engineering (KRISO) are calculated by including viscous and second-order difference-frequency wave effects with finite element-based mooring dynamics module. All the simulation results were systematically compared against the corresponding experimental results. In addition, dynamic motions of KRISO MUFOWT in the fully blade broken case was analyzed. Lastly, as results of the sensitivity tests of the Blade Pitch Control Natural Frequency (BPCNF) and Blade Pitch Control Damping Ratio (BPCDR), the appropriate BPCNF and BPCDR are selected in order that the power regulation is effectively performed and the platform pitch caused by BPC is minimized. In addition, the structural health monitoring method for a wind turbine by using operational modal analysis (OMA) is introduced. The modal properties, such as natural frequencies, displacement mode shape, and curvature mode shape of the tower and blade are estimated by using OMA in the time domain numerical simulation. The modal properties are compared between the damaged condition and the intact condition, and the differences are used for the damage detection. Especially, the curvature mode shape is used as the important indicator for finding the damage location. The feasibility whether the difference in curvature mode shape can catch the location and intensity of local damage or not is checked, and their results are validated by using the FEM analysis. In addition, this damage detection method is extended to the floating offshore wind turbine as calculating the fully coupled wind-turbine/hull/mooring dynamics of the floating offshore wind turbine (FOWT) model in the time domain numerical simulation. To the best of our knowledge, there is no open literature regarding the health monitoring of FOWT except for reviews or roadmaps, which do not provide effective technical solutions