Gust Response Factor Methodology for Wind Turbine Tower Assemblies

This paper proposes a methodology based on the gust response factor (GRF) for evaluating the along-wind response of wind turbine towers. The approach presented differs from conventional GRF methods as the GRF contains contributions from two resonant modes, mainly due to blade/tower interaction effects. The wind turbine tower model considered contains two interconnected flexible subsystems, representing the tower and the rotor system. Each component is initially modeled as a separate degree-of-freedom (DOF), and these are coupled together to form a two DOF reduced order model of the coupled tower/rotor system. Thus, the resonant component of the response contains energy output from the two modes of the coupled system. The GRF is obtained for both tower tip displacement and base bending moment through numerical integration, with a closed form expression included for the former. A series of numerical examples are included to investigate the magnitude of GRFs obtained for the two DOF assembly which allows for blade/tower interaction, and these are compared with GRF values obtained from an equivalent singe DOF model which ignores blade/tower interaction by lumping the mass of the blades in with that of the nacelle.