Natural Frequencies Estimation of a Monopile Supported DTU-10MW Offshore Wind Turbine-Paper

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The study evaluates the natural frequencies of a monopile-supported DTU 10 MW offshore wind turbine, focusing on how soil–structure interaction affects its dynamic behavior. The authors developed a three-dimensional finite-element model in ABAQUS that represents the turbine superstructure, monopile, surrounding sand, and their interaction. They compared its predictions with several simplified foundation models and examined the effects of monopile diameter, embedment depth, and sand density.<br /><br />The first two vibration modes are tower bending in the side-to-side and fore-aft directions, with frequencies of 0.201 and 0.203 Hz in the reference model. Accounting for soil flexibility lowers the first natural frequency: a fixed-base model predicts it 12.4% higher. Of the simplified approaches tested, distributed springs based on the modulus of subgrade reaction provide the closest estimate, with an error of about 5%. Other models either overestimate or underestimate the frequency more substantially.<br /><br />Increasing monopile embedment raises the natural frequency until a critical depth, beyond which further embedment has little effect. This critical depth is lower for larger-diameter piles and denser sand. Increasing the pile diameter from 8.3 m to 10 m raises the frequency by about 9.1% in loose sand.<br /><br />The results highlight the importance of representing soil flexibility when assessing resonance risk and indicate that identifying the critical embedment depth can help avoid unnecessary monopile length.

Keywords

offshore wind turbine

monopile

soil-structure interaction

natural frequency

finite element model

ABAQUS

tower bending modes

sand density

embedment depth

resonance risk