Modeling strategies for wind turbines considering soil-structure interaction

Reduced-order models (ROMs) are developed for obtaining the natural frequencies of offshore wind turbines (OWTs) considering the soil-structure interaction (SSI) by distributed springs along the monopile and employing three different modeling strategies. The ROM is derived considering a non-prismatic beam with mass at the tip. Once the mathematical model is obtained, the natural frequencies of the NREL 5 MW reference wind turbine are analytically obtained and the results are compared with those obtained by higher-order hierarchical models based on the Finite Element Method (FEM). The novelty here is the presentation of a mathematical model for this SSI representation that allows obtaining different natural frequencies and the vibration modes representation in an analytical scheme. The analyses developed raise a discussion about the choice of the shape function in the use of Galerkin's method. The results show that, depending on the modeling strategy, the fundamental frequency of the OWT studied is obtained with great accuracy when compared to the FEM analysis (considered as a reference value), with difference below 3.5%. The ROMs obtained allow evaluating the natural frequencies and mode shapes with very low computational cost, a feature that can be useful especially in the early stages of design.