Dynamic Field Data from Offshore Monopile Wind Turbines - Assessment of Natural Frequencies and Damping

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DYNAMIC FIELD DATA FROM OFFSHORE MONOPILE WIND TURBINES – ASSESSMENT OF NATURAL FREQUENCIES AND

DAMPING

Karin Norén-Cosgriffand Amir M. Kaynia

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Abstract.The dynamic characteristics of offshore wind turbines are heavily affected by the ambient excitations (e.g. wave excitations and wind excitations) and nonlinear soil behaviour.

For monopile structures, the fatigue design for the wind and wave loading is one of the most important problems to consider. Since the fatigue damage is sensitive to the foundation stiff- ness and damping, increasing the accuracy of analysis tools used in the design and optimiza- tion process can improve the reliability of the design and reduce conservatism, thereby leading to a more cost-efficient design. In this context, analysis of field data is important for calibrating and verifying purposes.

This paper presents analysis of measured accelerations and strains in two offshore wind tur- bines in the North Sea with monopile foundations. One of the turbines is in a site that is dom- inated by sand and the other by clay. Field data during idling conditions, collected over long periods of operation, are analysed and the natural frequencies and estimates of damping are determined. Further, the effect of load (wave, wind and dynamic bending moment) on the nat- ural frequencies is investigated and clear correlation between load level and natural frequen- cy is demonstrated, which points to the importance of soil nonlinearity.

EURODYN 2020 XI International Conference on Structural Dynamics M. Papadrakakis, M. Fragiadakis, C. Papadimitriou (eds.) Athens, Greece, 23–26 November 2020

Peer-review under responsibility of the Organizing Committee of EURODYN 2020.

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1 INTRODUCTION

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2 SITE CONDITIONS, TURBINE STRUCTURE AND INSTRUMENTATION

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3 WIND AND WAVE INFORMATION

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REFERENCES

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