Importance of higher modes for dynamic soil structure interaction of monopile-supported offshore wind turbines

Offshore wind turbines (OWTs) have emerged as one of the most sustainable and renewable sources of energy. The size of OWTs has been increasing, which creates more challenges in the design of foundations due to the potential higher-mode effects involved in the dynamic soil-structure interaction (DSSI) response. Several foundation modeling techniques are available for calculating the OWT fundamental frequency; however, their capability to predict the higher modes by considering real geometric configurations is unclear. The main aim of this study is to perform a rigorous modal analysis of the NREL 5MW reference OWT to investigate the higher mode effects using the 3D finite element method. A detailed parametric analysis is also performed to study the effects of soil inhomogeneity, initial soil modulus, and the monopile dimensions (diameter, thickness, and embedded pile depth) on higher modes' natural frequencies and effective mass participation ratios. The study shows that dynamic soil-structure interaction has a significant role in modal response and the simplified foundation models are not accurate enough. Given the significant contribution from higher modes, they should not be simply ignored in the OWT design, particularly in earthquake-prone zones.