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PO016: The Impact of Scour on Natural Frequency
Kester Gunn, Modelling and Analytics Expert, RWE Renewables
This paper presents novel methods that allow the natural frequency of the structure to be reliably estimated from measurement of the nacelle acceleration using standard, pre-existing sensors. Offshore wind turbines installed in sites with currents are often impacted by scour; a process in which the accelerated flow around the turbine's foundation erodes the sea-bed. Alternatively, the natural movement of sand-waves within a site can both lower and raise the sea-bed level globally. These processes can significantly change the effective embedment of the wind turbine foundation, and hence significantly change the natural frequency of the turbine structure. The majority of offshore wind turbines are designed in a “soft-stiff” configuration, in which the structure's natural frequency is tightly bounded by the rotor's 1P and 3P excitation frequency ranges. Hence, excursion of the natural frequency close to the turbine's 1P or 3P frequency ranges can negatively impact the wind turbine: drastically increase the fatigue damage of the foundation; require curtailment of the turbine to modify the 1P and 3P frequency range; or, in extreme conditions, require the turbine to cease operation. Such outcomes can reduce the value of a project by tens of millions of euros. By robustly estimating the natural frequency from readily available measurements, it is possible to show the evolution of the natural frequency of the turbine structure over time. The evolution of the structural frequency is then correlated with bathymetry surveys. This shows how the natural frequency of the structure decreases with scour, and also how it is restored when scour-protection is installed. It is further demonstrated that this approach makes it is possible to predict the date at which a specific frequency threshold will be reached, allowing interventions to be planned. These methods will also be invaluable if life-extension is considered, where they can be used in estimating the fatigue utilisation.