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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
Yukiko Tsugawa The University of Tokyo, Japan
Co-authors:
Yukiko Tsugawa (1) F P Noboru Sakamoto (2) Shozo Kawasaki (2) Chuichi Arakawa (1) Makoto Iida (1)
(1) The university of Tokyo, Tokyo, Japan (2) Electric Power Development Co.,Ltd., Tokyo, Japan

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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

Ms. Tsugawa is a graduate student of the Department of Mechanical Engineering at the University of Tokyo, Japan. Her research interests are in load analysis and design of wind turbine, with focus in offshore wind conditions.

Abstract

Analysis of wind turbine fatigue loads for proper estimation of offshore turbulence intensity

Introduction

It is said that the turbulence model of IEC 61400-3 does not properly express characteristics of offshore wind. It is because this IEC offshore model following the onshore model of IEC 61400-1. In fact, the turbulence intensity in some offshore site is more variable than that in the IEC standard. On the basis of this characteristic of offshore turbulence intensity, H. Wang et al. (2014) proposed that offshore turbulence intensity should be estimated at the 99% quantile rather than the 90% quantile.

Approach

A field data in an offshore site shows that turbulence intensity of offshore wind is variable with seasons and wind direction. Based on the feild data, the quantitative analyses of wind turbine loads were performed in two approaches. In one approach, the fatigue loads calculated under the turbulence condition following the IEC standard and under that considering the variability of turbulence intensity were compared. Both turbulence conditions were estimated at the 90% quantile of turbulence intensity. In the other approach, the effect of the difference between the estimations of turbulence intensity at the 90% and 99% quantile on fatigue loads was analyzed.

Main body of abstract

Turbulence intensity of the field data varied with seasons, i.e., summer or winter, and with wind direction, i.e., whether wind was from the sea direction or the land direction. The wind from the sea direction exhibited particularly high turbulence intensity during winter. The variability was considered by analyzing under the four conditions distinguished by seasons and sea directions. The numerical aeroelastic analysis was performed using NREL FAST. Loads and moments on a blade root and a tower base were calculated. The load amplitudes and the number of cycles were counted on the basis of the rain flow method. The number of cycles was weighted following the proper wind speed distribution, i.e., Rayleigh distribution, for each condition of analysis. Finally, the comparison was performed as damage equivalent fatigue load for each load or bending moment. The fatigue lifetime was estimated following Miner’s rule.
Without the consideration of variability of turbulence intensity, the damage equivalent fatigue load and bending moment of the inflow direction on a blade root was about 2% smaller than these with the consideration. This difference means that the fatigue lifetime is estimated about 20% longer when the variability of turbulence intensity is ignored.

Conclusion

According to the analyses in this research, the variability of turbulence intensity had a negative effect on fatigue loads of wind turbine. The disregard for the variability made the damage equivalent fatigue loads smaller and the fatigue lifetime longer in the inflow direction on a blade root. Hence, turbulence intensity should be estimated considering its variability in order to improve the reliability of offshore wind turbine’s design.
For the future works, it is needed to analyze fatigue loads and lifetime on other parts of wind turbine.



Learning objectives
Turbulence intensity affects the fatigue life of wind turbines. Therefore, the proper estimation of turbulence intensity improves the reliability of their design. The objective of this research is the quantitative analysis of not only wind characteristics but also wind turbine loads. The effect of the offshore variability of turbulence intensity on fatigue loads of wind turbine was analyzed. It is supposed that these analyses suggest the proper way how the turbulence intensity should be estimated.