Measurements and numerical studies on dynamic characteristics of prefabricated assembled steel-concrete hybrid towers for wind turbine systems

In recent years, the concept of the prefabricated assembled steel-concrete hybrid tower (SCHT) has been shown to be a very economical option for multi-megawatt wind turbines. However, with the common application of SCHT for wind turbines, concrete cracking problems caused by resonance effects have been observed in SCHTs after the operation of wind turbines. First, field measurements of a SCHT for a 6.0 MW wind turbine located in a mountainous area of Yunnan Province, China, revealed a significant deviation between the measured frequency and the designed frequency. Subsequently, a nonlinear dynamic model based on the mixed form of geometrically exact beam theory was developed, in which the blades, nacelle, and tower were explicitly taken into account. A simplified spring-dashpot-mass model was used to account for the soil-structure interaction (SSI) effect. The accuracy and efficiency of the proposed numerical model were validated by comparing it with the modal identification results based on the measured data. Finally, the effects of the modulus of elasticity of concrete, the top mass of the tower and the shear modulus of the foundation soil on the natural frequencies of the SCHT were investigated.