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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
Jun Tanemoto The University of Tokyo, Japan
Co-authors:
Jun Tanemoto (1) F P Takeshi Ishihara (1)
(1) The University of Tokyo, Bunkyo-ku, 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

Mr. Tanemoto is a Ph.D student of the university of Tokyo. He studied about global wind and wave prediction for ship's weather routing in Kobe university, and got master degree of maritime science. He moved to the university of Tokyo 3 years ago. After that he has studied about numerical prediction method of extreme winds and waves, which are needed to design wind turbines, in his Ph.D process. In this presentation, he focuses on general wind and wave relations, and proposes a joint probability distribution model of these relations, for which estimates fatigue loads.

Abstract

A numerical study of wind wave and swell by using wave prediction models and combined wind fields

Introduction

Wave simulations are commonly used for the predictions of extreme wave height and associated wave period, which are necessary for the design of offshore wind turbines. Waves around Japan are very complex due to the coexistence of wind wave and swell, which induced by propagating waves over the Pacific Ocean.

In this paper, two numerical studies, the validation of two third generation wave models and the evaluation of the predicted waves by changing the wind field used as the sea surface condition of a wave model, are carried out for the prediction of both wind wave and swell.


Approach

A case study of the prediction of two peak spectra is carried out for the validation of third generation wave models, in which SWAN and WAVEWATCH III are used. Predicted waves are compared with observed significant wave height, period and wave spectra at Choshi offshore wind power test site.

Then, 20 years tropical cyclone induced waves are predicted by using wave simulation with wind fields obtained from mesoscale model, typhoon model and combined wind field of those models at Nakagusuku bay, which is frequently attacked by the strong tropical cyclones in Japan.


Main body of abstract

Third generation wave model SWAN and WAVE WATCH III are validated by using observation waves during an extratropical cyclone at Choshi offshore wind energy test site. Although predicted wave heights by SWAN show good agreement with observations, predicted wave periods are underestimated significantly due to underestimations of wave spectra in the low frequency range. WAVE WATCH III predicts wave heights, periods and spectra correctly.

Then, 20 years tropical cyclone induced waves are simulated at Nakagusuku bay and wind fields used as sea surface boundary conditions for wave simulations are validated. From a case study, predicted wave heights and periods by using wind field obtained from mesoscale model are underestimate observations. Although predicted waves by typhoon model underestimate observed wave height, those overestimate observed wave periods. The reason is underestimations of wind-waves before the typhoon attacked. As a result, waves are generated by the only the swell by the typhoon far from the site. These underestimations/overestimations are improved by using combined wind field.

It is also found that predicted extreme wave height and period by using the wind field obtained from the mesoscale model underestimate tropical cyclone induced extreme wave height and period and these underestimations are improved by using the wind fields obtained from the typhoon model and the combined wind field.


Conclusion

In this study, two numerical studies are carried out. Concluding remarks as follows: 1) Although predicted wave heights by SWAN show good agreement with observations, predicted wave periods are underestimated significantly due to underestimations of wave spectra in the low frequency range. WAVE WATCH III predicts wave heights, periods and spectra correctly. 2) predicted extreme wave height and period by using the wind field obtained from the mesoscale model underestimate tropical cyclone induced extreme wave height and period and these underestimations are improved by using the wind fields obtained from the typhoon model and the combined wind field.


Learning objectives
Our research focuses on the prediction of both wind wave and swell. Two important findings for the prediction of extreme wave heights and periods are obtained: 1) Selecting a wave model is one of the important point 2) The accuracy of wind field is important not only at the center of tropical cyclone but also outside.