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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
Yong Jun Cho University of Seoul, Korea, Republic of
Co-authors:
Yong Jun Cho (1) F P Kee Sok Yang (2) Byeong Kyu Kim (2)
(1) University of Seoul, Seoul, Korea, Republic of (2) Korea Port Engineering Cooperation, Seoul, Korea, Republic of

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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

Dr. Cho has been working in the coastal engineering for almost 25 years. Now he works for University of Seoul, Seoul, South Korea. He earned His PhD degree from the North Carolina State University, Raleigh, U.S.A in 190.

Abstract

Nonlinear difference interaction of wind load with wave load acting the offshore wind energy converter

Introduction

Even when random waves simultaneously attack the offshore wind energy converter with the wind, random waves and gusty wind don’t always enforce the offshore wind energy converter be deformed.

Aforementioned nonlinear difference interaction has their roots on varios mechanisms like the thickened boundary layer near the sea surface, the coherent largy eddies over bumpy sea surface due to random waves,
, and the aerodynamic damping.

Hence, the nonlinear difference interaction of wave with wind force deserve much more attention, otherwise we can end up with very conservative design, which eventually hampers the development of offshore wind energy industry.


Approach

we carry out the numerical simulation using the structural dynamic model developed in this study to estimate the nonlinear difference interaction of random waves with gusty wind.

For the aeroelastic and hydroelastic analysis to explain the interaction of offshore wind energy converter with waves and wind, we used the beam element method.

The time series of wind velocity with gust effect indispensable for the buffeting analysis were numerically simulated using Monte Carlo technique based on the Kamail spectrum, and the time series of random waves, velocity and acceleration are also simulated using Random Phase method using JONSWAP spectrum.

Main body of abstract

Nonlinear difference interaction of random wave force with gusty wind on the substructure of offshore wind energy converter consists of various physical mechanisms

Random waves could intermittently exert a resistant force against the deformation of offshore wind energy converter via the dissipation of wind energy at the sea surface, and the coherent largy eddies triggered by bumpy sea surface due to random waves.

Aforementioned resistant force can alos be found at the reversal stage of acting direction of wave force when the free surface turns into its trough, which eventually stands against the wind direction.

The aerodynamic damping, triggered whenever there are changes in the relative velocity of wind with respect to offshore wind energy converter, could be the last contributor.

Therefore, if the nonlinear difference interaction of random waves with wind force is not taken into the full consideration, the design of offshore wind energy converter could be too conservative, that only hamper the growth of the offshore wind industry due to the excessive initial capital investment problem.

In this rationale, we carried out the numerical simulation for the offshore wind energy converter of 5MW to quantitatively evaluate the nonlinear difference interaction of random waves and wind forces with the offshore wind energy converter, which is crucial for the optimal design of offshore wind energy converter.



Conclusion

Numerical resuIts shows that under stormy weather condition, the mean wind velocity is drastically weakened by the thickened atmospheric boundary layer and its accompanying energy loss (over 30%). In addition, the direction of gust does not always comply with the direction of mean wind field, and during a significant portion of the total simulation period [640 s], the gust moves along the opposite direction of the mean direction of mean wind field.




Learning objectives
The problem of excessive initial capital investment can be partially solved by utilizing the stabilizing effect of waves at the design stage by which the redundant rigidity from the substructure of offshore wind energy converter can be removed, and lead to less expensive offshore wind energy converter.