Effects of foundation stiffness on the linear vibrations of wind turbine systems are of concerns for both planning and construction of wind turbine systems. Current study performed numerical modeling for such a problem using linear spectral finite elements. The effects of foundation stiffness were investigated for various combinations of shear wave velocity of soil, size of tower base plate, and pile length. Multiple piles are also included in the models such that the foundation stiffness can be analyzed more realistically. The results indicate that the shear wave velocity of soil and the size of tower base plate have notable effects on the dominant frequency of the turbine-tower system. The larger the lateral dimension, the stiffer the foundation. Large pile cap and multiple spaced piles result in higher stiffness than small pile cap and a mono-pile. The lateral stiffness of a mono-pile mainly depends on the shear wave velocity of soil with the exception for a very short pile that the end constraints may affect the lateral vibration of the superstructure. Effective pile length may be determined by comparing the simulation results of the frictional pile to those of the end-bearing pile.
Six wind turbines were blown to the ground by the wind gust during the attack of Typhoon Soudelor in August 2015. Survey using unmanned aerial vehicle, UAV, found the collapsed wind turbines had been broken at the lower section of the supporting towers. The dynamic behavior of wind turbine systems is thus in need of attention. The vibration of rotor blades and supporting towers of two wind turbine systems have been measured remotely using IBIS, a microwave interferometer. However the frequency of the rotor blade can be analyzed only if the microwave measurements are taken as the wind turbine is parked and secured. Time-frequency analyses such as continuous wavelet transform and reassigned spectrograms are applied to the displacement signals obtained. A frequency of 0.44Hz exists in both turbines B and C at various operating conditions. Possible links between dynamic characteristics and structural integrity of wind turbine –tower systems is discussed.
Transient vibrations of the tower supporting a horizontal-axis wind turbine were recorded using a microwave interferometer. Variations in dominant frequencies have been reported in the previous study. Signal analyses aiming to uncouple different frequency components were performed using reassigned spectrogram, a time-frequency representation based on time-corrected short time Fourier transform. Optimal resolutions in both time and frequency domains were first investigated using synthetic signals. The goal was to seek out the favorable combinations of window size and overlapping portions of adjacent windows for a data sequence at a given sampling rate. The dominant frequency found in reassigned spectrogram agrees with that obtained using Fourier spectrum of the same transient measurements of the wind turbine tower under investigation.
In a typical vibration test of tensioned cables, tension forces are mostly estimated from theory of a vibrating string with the first natural frequency. To obtain slightly better estimations, formulas based on an axially loaded beam can be employed. However, uncertainty on both flexural rigidity and effective length of the vibrating cable raise difficulty in reliably determining the possible range of the tension value. From the previous work of the authors, an alternative approach for the calculation of tension forces without the need of rigidity data had been proposed, in which frequencies of high modes are instead required in recovering accurate results. This paper extends the previous work to also consider the discrepancy between the design length and effective length so as to further improve the results. Feasibility of the proposed methodology with enhanced equations was verified by actual cable forces measured in an extradosed bridge. Current study aims to apply the proposed approach to the dynamic monitoring of the in-situ stay cables so as to improve the traditional assessment results without increasing the testing costs.
Wind turbine towers are in need of condition monitoring so as to lower the cost of unexpected maintenance. Wind loading from turbulence and gusts can cause damage in horizontal axis wind turbines even the supporting towers. Monitoring of wind turbines in service using embedded data sensor arrays usually is not targeted at the turbine-tower interaction from the perspective of structural dynamics. In this study the remote monitoring of the tower supporting a horizontal-axis wind turbine was attempted using a microwave interferometer. The dominant frequency of one tower was found to be decreased by more than 20% in 16 months. Numerical modeling using spectral finite elements is in progress and should provide further information regarding frequency shift due to stiffness variation and added mass. Expected outcome will contribute to remote monitoring procedures and nondestructive evaluation techniques for local wind turbine structures during operation.
The stiffness of a bridge span is evaluated by the dynamic displacement response corresponding to a three-axial vehicle
load moving with constant speed. The dynamic displacement influence line obtained from the dynamic displacement
time history was filtered by window smoothing and empirical modal decomposition (EMD) methods to acquire the
quasi-static displacement influence line. The beam stiffness was obtained by dividing the moment diagram
corresponding to a concentrated load applying on the measuring position with the curvature of the quasi-influence line.
The effects of three-axial moving load, moving speeds, and measuring positions on the stiffness estimation are explored.
The results show the window smoothing method is a better technique to obtain the quasi-static influence line. The only
discrepancies in curvature for single and three-axial load cases are near both ends of the beam. A larger range of correct
stiffness can be recovered for load moving with lower speed. Similar stiffness diagram can be obtained from the
influence lines at different measuring positions.