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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Christof Devriendt OWI-lab / VUB, Belgium
Co-authors:
Christof Devriendt (1) F P Wout Weijtjens (1) Gert De Sitter (1)
(1) OWI-lab / VUB, Brussels, Belgium

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Christof is the scientific coordinator of “ The Offshore Wind Infrastructure Lab” or OWI-lab (www.owi-lab.eu). The Offshore Wind Infrastructure project consists in the realisation of a number of investments allowing for the monitoring and modeling of offshore wind energy resources and of the behaviour of systems components in offshore wind farms. On the short term, the project aims at the setup of a complete wind-monitoring and testing infrastructure. It aims at improving the lifetime of offshore wind turbine components, at optimising the Operation and Maintenance strategies for offshore wind parks and maximising the energy output of offshore wind farms.

Abstract

The overall damping of an offshore wind turbine on a monopile foundation during different operating conditions

Introduction

Damping has an important impact on the fatigue damage of offshore wind turbines. This contribution aims to improve the understanding of damping in offshore wind turbines through long term monitoring. A better understanding on the damping of offshore wind turbines can improve design and thus reduce the initial cost of future substructures. Damping ratios are very difficult to predict by numerical tools therefore measurements on existing offshore wind turbines are crucial to verify the existing design assumptions. These measurements allow linking the damping to different operational and environmental conditions, which can then be returned as a design input.

Approach

The current contribution relies on measurements conducted at the Belwind offshore wind farm 46km outside the Belgian coast. One of the 55 Vestas V90-3.0MW turbines on monopile foundations was equipped with an array of accelerometers in 2011. The obtained measurements are continuously analysed using state-of-the-art Operational Modal Analysis techniques. These results are then put next to a subset of SCADA and meteorological data. This allows estimating a damping ratio for each environmental and operational condition of the wind turbine (EOC).


Main body of abstract

For this contribution three years of measurements were processed with state-of-the-are Operation modal analysis techniques for obtaining the damping. This resulted in a database of more than 100000 damping ratios (each 10 minutes a new damping ratio is obtained). The obtained turbine SCADA along with the meteorological measurements conducted at the site allow to link each damping ratio to an operational condition. Given the large database it then is possible to set up statistics for damping ratio a given operational condition, e.g. mean damping in parked conditions or at rated power. It is readily understood that damping values are highly dependent on these operating conditions and might vary significantly over different operational conditions of the turbine. For instance, during parked conditions there is a reduced damping due to low wind speeds. At rated power the increase of wind speeds results in additional aerodynamic damping thus a higher overall damping. A key result is the damping ratio for each operating conditions and for each ambient condition.

Conclusion

This research developed an approach to continuously estimate and classify the damping ratios on an operating offshore wind turbine. While these measurements can serve as an important input for design, they can also help to develop a better understanding on fatigue life consumption. This contribution showed how damping ratios are related to the operational and ambient conditions and defined a damping ratio for different operating conditions. Ultimately, this helps us to better understand some of the important damping contributions acting on an offshore wind turbine and distinguish them where possible such as aerodynamic damping and soil damping.


Learning objectives
The two main goals of this contribution are to demonstrate the feasibility of the continuous assessment of damping ratios during the operation of an offshore wind turbine and to illustrate and quantify the effect different EOC’s will have on these damping ratios. This research will allow evaluating the agreement between design assumptions and real-life conditions.