Abstract
Offshore wind turbines are dynamic structures that require an accurate estimate of the main eigenfrequencies to avoid resonance and, as a result, increased fatigue damage. This contribution focuses on the first and second mode eigenfrequencies measured over an entire offshore wind farm in the Belgian North Sea. The comparison between the field measurements during parked conditions and design estimations yields the same conclusions as previous studies reported in the literature; eigenfrequencies are systematically underestimated. A comprehensive step-by-step fleet-wide numerical benchmark is thus presented to improve the model predictions and understand the causes of the mismatch. Finite-element analyses, based on the current state-of-the-art soil-monopile interaction models, were considered since the foundation stiffness is identified as a key source of the discrepancy between in situ observations and design models. The importance of environmental parameters in the second mode is studied. The consideration of the contribution of scour protection for a realistic representation of foundation stiffness is discussed as well. The final results show that simulations agree considerably better with the measurements, although a significant bias is still observed for the second mode.
