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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 Tim Verbelen (1) Gert De Sitter (1) Tim Verbelen (1) Gert De Sitter (1) Gert De Sitter (1) Gert De Sitter (1)
(1) OWI-lab / VUB, Brussels, Belgium (2) 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 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

Design verification of offshore wind turbines on monopile foundations.

Introduction

Monopile foundations are currently the most common type of offshore substructures for wind turbines. However there are indications that the current design guidelines under-predict the soil stiffness for large-diameter monopoles. As a result the resonance frequencies of the first modes of the offshore wind turbine can differ significantly from the designed resonance frequencies. This can lead towards higher vibration levels, due to an increased interaction with the blade passing frequencies. Underestimating the dynamic loads can result in reduced lifetime of the offshore structures and increased maintenance costs.

Approach

OWI-Lab initiated an extensive design validation campaign at the Belgian Belwind and Northwind windfarms. This contribution presents the results of this design verification campaign and discusses the results in great detail. During 2014 short-term measurement campaigns have been performed at 10 turbines. Therefore biaxial accelerometers have been mounted on the transition piece of each turbine. Measurements have been performed during 20 minutes. The vibration data has been processed using state-of-the-art operational modal analysis techniques. This approach allowed to identify the resonance frequencies of the first 2 bending modes in both the for-aft and side-side direction with great accuracy.

Main body of abstract

A lot of design parameters are very difficult to predict by numerical tools and therefore performing dedicated tests on existing offshore structures is vital to verify the existing design assumptions. To increase power generation and to limit weight, offshore wind turbines are becoming structurally more flexible, thus an accurate prediction of their dynamic behavior is mandatory. Underestimating the stiffness and the damping of offshore structures in the design phase for example, inevitable results in the use of more steel and thus higher constructions and installation costs. Often the resonance frequencies and damping ratios of the first modes of an offshore wind turbines differ significantly from the designed resonance frequencies and damping ratios. This can result in higher vibration levels, due to an increased interaction with the blade passing frequencies.

Therefore OWI-Lab initiated an extensive design validation campaign at the Belgian Belwind and Northwind windfarms. When performing a design verification campaign one needs to take into account that the resonance frequencies can shift significantly due to changing operational and environmental conditions. The frequencies of the higher modes are strongly affected by the tidal level and even depend on secondary structures, e.g. an overall stiffening of the turbine in the FA-direction can be found when the nacelle is aligned with the J-Tube. The results will be corrected for these influences. Finally a detailed analysis of the obtained resonance frequencies is performed versus e.g. waterdepth, monopile length and monopile fixity.


Conclusion

Results indicate a general underestimation of the soil stiffness. The first resonance frequency is between 5% and 10% higher then designed. The second resonance frequency is between 20% and 40% higher then designed. It was found that the relative difference with as designed values increased with water depth and was independent of the monopile length or fixity. Moreover it was found that the second bending mode frequencies coincided with the 6P blade passing frequencies. This can result in higher loads and therefore reduced life-time or increased O&M costs.


Learning objectives
The main goal of this contribution is to share the results of an extensive design verification camping. The result will allow improving current standards. Ultimately, reducing the cost of offshore wind energy.