An opto-mechanical system has been developed to measure the dynamic behaviour of multi-megawatt wind turbines. This portable system is easier and less expensive to use than previously used methods. Thus it is feasible to use the system to develop a large database of the modal damping characteristics of operational full-scale wind turbines for the development of the improved fatigue life prediction tools that are needed in the rapidly growing global wind industry. The opto-mechanical system and a 3D scanning pulsed Doppler LIDAR system are used to make simultaneous measurements of the dynamic response and wind field in three different utility-scale wind farms. The wind farms are located in different types of terrain, ranging from the flat terrain through to highly complex terrain. The measurements are made on five different multi-megawatt wind turbines (1.8MW Vestas V90; 2.0MW Vestas V80; 2.3MW Enercon E70; 3MW Vestas V90; and 3.6MW Siemens SWT). A single-degree-of-freedom dynamic model is used to determine the modal damping parameters from the measured spectra of the tower deflections. It is shown that the aeromechanical damping ratios range from 0.4% to 0.8%. Measurements in the operating and idling phases of a turbine are used to show that the aerodynamic damping, which arises from the interaction between the rotor and wind, is the dominant damping mechanism for an operating wind turbine, and accounts for two-thirds of the overall damping; the material damping accounts for one-third of the overall damping. The 3.6MW Siemens SWT wind turbine has the smallest overall damping, whereas the 3MW Vestas V90 has the largest damping as well as the largest dynamic deflections. However, an assessment of the Goodman diagram shows that in its location of flat terrain, the 3MW Vestas V90 wind turbine may likely meet its 20-year design life. Nevertheless, for other locations, such as in complex terrain, in-situ measurements should be made to verify the suitability of the wind turbine for wind farms in such locations. This work demonstrates the feasibility of using the opto-mechanical system to develop a large database of the modal damping characteristics of operational full-scale wind turbines.

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