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Monopile foundations are known as the most common foundation solution for offshore wind turbines (OWTs). However, the state of practice for designing monopile foundations in high seismicity areas is still limited. In this paper, three-dimensional, fully coupled, nonlinear finite-element analyses were used to evaluate the seismic performance of hypothetical 5MW offshore wind turbines on monopile foundations in layered, liquefiable sites under motions with and without forward directivity. The numerical results showed that pile movements induced by wind loading led to a bias in settlement accumulation near the foundation. Velocity pulses increased the cyclic stress demand in soil and the potential for liquefaction triggering. The foundation rotations were influenced by ground motion characteristics such as polarity and velocity impulse, and the magnitude and direction of the applied wind load. This study highlights the importance of considering the effects of combined loads and pulse-like motions in the design and performance of OWT systems.
Hwang et al. (Thu,) studied this question.