Natural frequency and mode shape analyses in structural design models are important for estimating accurate fatigue loads used to predict the design lifetime of the structure. The natural frequency depends on several parameters such as the mass of the rotor-nacelle assembly, the stiffness of the tower and support structure, corrosion, scour protection and soil-structure response. The uncertainty of the soil-structure response can for monopile support structures be relatively high depending on the soil models employed and can thus lead to large deviations between measured and predicted natural frequencies. API p-y curves have been used for monopile design in the offshore industry for many years, but they typically underpredict the soil stiffness for sands, which leads to erroneous fatigue load estimates. Different corrections to the initial stiffness of API p-y curves in sand have been proposed to correct this deficiency and get more accurate predictions of natural frequencies. This paper presents predictions of natural frequencies and the accompanying mode shape using standard API p-y curves, modified API p-y curves and PISA soil reaction curves. These predictions are compared to results from field measurements of operating offshore wind turbines and some of the impacts that deviations between predicted and measured values can have on monopile designs are highlighted.
Kris W. Andersen, Peter Duroska, Cameron Brown, Miguel Pacheco Andrade and Michael Harte
Keywords: monopiles, API, PISA, offshore wind farms, geotechnics
Publisher: DFI