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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Whole-life foundation and structure integrity' taking place on Wednesday, 12 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Rasoul Shirzadeh Vrije Universiteit Brussel (VUB), Belgium
Co-authors:
Rasoul Shirzadeh (2) F P Wout Weijtjens (2) Patrick Guillaume (1) Christof Devriendt (2)
(1) Vrije Universiteit Brussel (VUB), Brussels, Belgium (2) Offshore Wind Infrastructure Lab, , Belgium

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Rasoul Shirzadeh has joined the department of Mechanical Engineering of the Vrije Universiteit Brussel (VUB) in January 2010 where he is a PhD-student in the Acoustic & Vibration Research Group (AVRG). He is mainly interested in the field of Fluid Structure Interaction (FSI), Aeroelasticity of wind turbines, Offshore Wind Turbines and Operational Modal Analysis. He is now involved in the Offshore Wind Infrastructure Project (www.owi-lab.be) which aims to increase the reliability and efficiency of offshore wind farms by investing in testing and monitoring of an offshore wind turbine in the Belgian north sea.

Abstract

An experimental and computational investigation on the dynamics of an offshore wind turbine in parked conditions

Introduction

This paper present a comparison between measurements and simulations to study the dynamics of an offshore wind turbine in parked conditions. The model is created using the HAWC2 according to current standards based on the design basis of the experimentally investigated Vestas V90 offshore wind turbine on a monopile foundation installed in the Belgian North-Sea. The damping value of the first for-aft mode has been tuned based on measurements obtained from a long-term monitoring. The paper will present some of the effects of the different design parameters and the different ambient conditions on the dynamics of an offshore wind turbine.

Approach

Damping ratios are crucial for lifetime predictions as the amplitude of vibrations at resonance are inversely proportional to these ratios. The overall damping of the first bending mode of an offshore wind turbine consists of a combination of aerodynamic damping and damping due to constructive devices, such as a tuned mass damper (TMD), and additional offshore damping, e.g. structural damping. A limited number of offshore measurements deal with the dynamic behaviour of the wind turbines.

This paper does not focus on the theoretical background of the utilized algorithms but on the application of these methods on measured and simulated data for an offshore wind turbine. The paper is structured as follows. First, the utilized measurement system is summarized. The main features of the aeroelastic simulations of the offshore wind turbine, implemented in HAWC2, are then described.

We will recapitulate some of the results in order to compare them with the simulations. The measurements are obtained during a measurement campaign on an offshore wind turbine in the Belgian North-Sea. These results have been presented at two EWEA conferences and are discussed in detail in [1] and [2]. Using the simulation model of the offshore wind turbine the paper will end with illustrating and explaining some of the effects of the different ambient conditions, e.g. changing tidal level, wave height, wave periods and wind speeds on the dynamics of an offshore wind turbine in parked conditions. The obtained natural frequencies, damping values and acceleration levels will be compared with those obtained from the measurement.

The paper will discuss the accuracy of the results and the limitations of the used simulations in modeling an offshore wind turbine according to best practice and current standards and using state-of-the art modeling software.

Main body of abstract

Within the OWI-project (www.owi-lab.be) measurement campaigns have been performed at the Belwind wind farm, which consists of 55 Vestas V90 3MW wind turbines. The wind farm is located in the North Sea on the Bligh Bank, 46 km off the Belgian coast.

The numerical simulations have been carried out using HAWC2 aeroelastic code developed at Risø DTU. HAWC2 uses a multi-body formulation, allowing the user to model each component of the turbine as a separate body. The implementation of each body is carried out adopting a finite element theory. The code is capable to simulate the structural response of a pitch controlled horizontal axis wind turbine (HAWT) subject to aerodynamic, hydrodynamic and soil loads. The detailed specifications of the blade aerodynamic properties; monopile foundation, tower, nacelle and drivetrain structural properties are provided as an input file for the HAWC2 code. The hydrodynamic and elastic properties of the offshore support are also collected.

The natural frequencies of the structure were identified using automated operational modal analysis applied to the measurement data. The natural frequencies and mode shapes of the structure are then compared with the results from an eigenvalue analysis and time domain simulations in HAWC2.

Table compares the obtained frequencies from measurements and simulations for the first 10 lowest identified modes. We can conclude that the eigen-frequencies and mode shapes identified during the monitoring period compare well with the time domain simulations. The highest difference on both frequency and mode shape can be found for the blade mode (B1AEP). This can be explained by the fact that we did not use the real blade data but a downscaled version of the NREL 5 MW reference wind turbine as mentioned earlier. In the real measurements we also found a slightly larger difference between the second bending modes in the FA and SS directions then in the simulations. This could be due to the soil stiffness which in reality might not be symmetrically distributed . Another reason might be the boat-landing which is not considered in HAWC2. The boat-landing might considerably affect the stiffness or frequencies of the second bending modes, considering the relative large displacements of these modes around the transition piece. In the measurements the frequency of the second bending mode in the FA direction is significantly larger, this makes sense considering that the yaw angle is most of the time around 210 degrees, perfectly aligned with the boat-landing.

Figure shows a comparison between some of the fundamental tower-support structure modes obtained during the measurement period while the wind turbine was in parked conditions and the equivalent modes shapes found in the simulations.

Damping ratios are crucial for lifetime predictions as the amplitude of vibrations at resonance are inversely proportional to these ratios. The overall damping of the first bending mode of an offshore wind turbine consists of a combination of aerodynamic damping and damping due to constructive devices, such as a tuned mass damper (TMD), and additional offshore damping, e.g. structural damping [3] [4]. A limited number of offshore measurements deal with the dynamic behaviour of the wind turbines.

The damping contribution that is mostly affected by ambient conditions is the aerodynamic damping. Therefore, we will analyze the effect of wind speed on the damping values of the fundamental tower modes in FA and SS directions. The damping values for FA and SS modes are compared with measurements in Figure .

In both measurements and simulations we find a slightly higher damping for the first SS mode (S1SS) in comparison with the first fore-aft mode (S1FA) as mentioned already. This can be explained due to the presence of some extra aerodynamic damping effects in the SS direction considering the high pitch angle in parked conditions. According to [5] the aerodynamic forces are present even at standstill due to the larger blade surface that interacts with surrounding air when the tower vibrates in the SS direction.

Conclusion

This paper studies the dynamics of an offshore V90 3MW wind turbine installed at Belwind wind farm. The structural properties of wind turbine and its support structure as well as the wave kinematics, wind conditions and soil parameters are implemented in HAWC2. The natural frequencies of the structure identified from eigenvalue analysis and time domain simulations in HAWC2 are then compared with measurements. Both simulations and measurements results are processed using the state-of-the-art operational modal analysis technique PolyMAX. The resonance frequencies obtained from the simulations show a satisfactory agreement with the measurement results.

The overall damping of the first FA mode has been estimated from both overspeed stop test and ambient excitations measurements. The measured FA damping values are 1% and 1.6% for the cases without and with the tuned mass damper respectively. The overall damping of the model was tuned to be in agreement with the measurements. The tuning procedure has been discussed in this paper. Using the updated model, the influence of different wave and wind parameters on the dynamic response of the wind turbine has been studied. The damping ratios of the first FA and SS modes have been estimated at different wind speeds and the results compared with the measurements. They showed good agreement especially for the SS mode.

Furthermore, we have analysed the simulated acceleration levels for different parameters e.g. wave period, wave height and wind speed. The overall trend of the simulations and measurements was in good agreement, however the simulations seemed to overestimate the vibration levels particularly in those cases where the aerodynamic damping was negligible. So it was concluded that although the overall damping of the simulations was in good agreements with the measurements, the simulations did not correctly predict the acceleration levels. It is obvious that this can have important consequences on the correct calculation of the fatigue life of an offshore wind turbine. It has been shown that the distribution of the additional offshore damping has an influence on the predicted acceleration levels.



Learning objectives
This paper will present a detailed comparison between an experimental and numerical investigation to study the dynamics of an offshore wind turbine in parked conditions.


References
[1] Devriendt C, Jan Jordaens P, Van Ingelgem Y, De Sitter G, Guillaume P. Monitoring of resonant frequencies and damping values of an offshore wind turbine on a monopole foundation. EWEA 2013 conference, Vienna, Austria, 2012.
[2] Devriendt C, Jordaens PJ, De Sitter G, Guillaume P. Damping estimation of an offshore wind turbine on a monopile foundation. EWEA 2012, Copenhagen, Denmark, 2012.
[3] Petersen B, Pollack M, Connell B, Greeley D, Davis D, Slavik C, Goldman B, Medley L. Evaluate the effect of turbine period of vibration requirements on structural design parameters: Technical report of findings 2010.
[4] Tarp-Johansen NJ, Energy D. Comparing sources of damping of cross-wind motion. European Offshore Wind Conference (EOW). DONG Energy, 2009.
[5] Hansen M, Thomsen K, Fuglsang P, Knudsen T. Two methods for estimating aeroelastic damping of operational wind turbine modes from experiments. Wind Energy 2006; 9(1-2):179–191.