Fatigue analysis of offshore wind turbines with soil-structure interaction and various pile types

Monopile offshore wind turbines (OWTs) are susceptible to fatigue damage, and time domain fatigue analysis is time-consuming and unsuitable for simulating various loading conditions. In this study, frequency domain fatigue analysis of OWTs with soil-structure interaction (SSI) and different monopile designs (the conventional monopile, monopiles with an extension, a wheel, and four spoiler ribs) is conducted. First, detailed 3D finite element models of OWTs are developed. The tower, monopile, and soil are modelled using solid elements, and SSI is accounted for using the Mohr-Coulomb failure criterion and surface-to-surface contact. Next, the power spectral densities of wind and wave loads on the tower and monopile are derived. Subsequently, the fatigue damages of the OWTs are evaluated under individual and combined wind and wave loads. The results show that damage increases with wave height, peaking at the rated wind speed and wave frequency close to the first frequency of the OWTs. Wind-induced damage is significantly greater than wave-induced damage, and the combined wind and wave loads produce more damage than the sum of the individual loads. Additionally, alternative monopile designs, particularly the monopile with a wheel, can reduce fatigue damage, though stress concentration effects on the wheel and ribs require careful consideration.