Natural Frequency Analysis of a Wind Farm Turbine-Pile-Foundation System

The accurate estimation of the natural frequency of a turbine-pile-soil system is one of the key design requirements for an offshore wind farm. The natural frequency of the soil-structure interaction system must fall within a narrow window. A FLAC^3D model is used to analyze an offshore wind turbine with a 6.5m diameter pile. Soils were modeled with solid zones using a Mohr-Coulomb model with strain-dependent moduli, the pile with linear elastic liner elements, and upper structure with beam elements. The interface between the pile and soil was also specified. The soil and interface parameters were carefully calibrated with advanced laboratory data including the small strain soil stiffness, and in-situ data from a pile test. Therefore, the numerical model could reproduce the test pile behaviors when subjected to both vertical and lateral static loads. The analysis suggests that both the soil strain-dependent modulus and the lumped rotational masses of the upper structure play significant roles in the turbine natural frequency analysis and should be particularly addressed during the design.