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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
Kai Karikomi Mitsubishi Heavy Industries, Ltd., Japan
Co-authors:
Kai Karikomi (1) F P Takuya Koyanagi (1) Makoto Ohta (1) Akihiro Nakamura (1) Satoshi Iwasaki (1) Yoshiyuki Hayashi (1) Akihiro Honda (1)
(1) Mitsubishi Heavy Industries, Ltd., Nagasaki, 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

KARIKOMI Kai has been working in the wind industry for 11 years. He is currently a senior researcher at the research & development center of Mitsubishi Heavy Industries, Ltd. He holds a Ph.D in Mechanical Engineering from Nagoya University. His work at the company covers wide range of technical topics of wind turbine technology such as the site assessment, the rotor aerodynamics and the aeroelasticity. His recent research is focused on simulations of a floating offshore wind turbine.


Abstract

Wind Tunnel Testing on Negative-damped Responses of a 7MW Floating Wind Turbine

Introduction

For pitch-controlled Floating Offshore Wind Turbines (FOWTs),
the instability problem might occur by the coupling of the blade pitch control
and the floater pitch motion, which leads to adverse effect on the loads on the FOWTs.
There exist a number of papers which deal with the instability problem
by using aeroelastic codes for FOWTs. To the best knowledge of the authors,
however, only few experimental studies are published and presented
in which the unstable motion of FOWT due to the negative damping is realized
with a tank or wind tunnel testing using a scaled FOWT model.


Approach

In this paper, we proved experimentally the unstable
motion of FOWT and validated an enhanced control
method to suppress the negative damped response of the platform.
In order to realize the negative-damped responses and investigate effects of parameters
for the rotor speed control, we performed a series of wind tunnel tests using a 1/64 scaled FOWT model,
which is equipped with the blade pitch actuator to control the rotational speed of the rotor
during full load operation. On a floor of the wind tunnel, a simplified water tank is deployed
in order to generate wind and wave simultaneously.


Main body of abstract

The floating platform used for the experiment is categorized into the spar-buoy type, whose motion in pitch direction is
lower damped than other floater concepts like the tension leg and the semi-submersible platform.
To simulate the negative damped responses experimentally, it is necessary
to achieve the following two points in terms of the similarity with the full scale turbine :
a) the similarity of aerodynamic characteristics and b) the similarity of control algorithm.
Regarding a), the Froude similarity rule was applied to this experiment then
the Froude rule suggested an aerodynamic problem caused by the 1/512 scalded Reynolds number
compared to the full scale turbine; this very low Reynolds number with the order of 10^4
results in definitely poor rotor performance and loads. In order to ensure
the similarity of the test model’s power and thrust with the real turbine,
rotor blades were re-designed using thinner
aerofoil and wider chords than the rotor blades of the full scale prototype.
Regarding b), this FOWT model has a control system enable to regulate
the rotor speed with control demands of the blade pitch angle and the generator torque,
in analogy with the real turbine. Three blades of the rotor can be pitched collectively
using a servo motor equipped in the nacelle.
By these two features of this model, in the full load operation
above the rated wind speed, the power
and rotor speed were regulated to be constant and the characteristics of thrust force
shows a negative slope against increasing wind speeds.


Conclusion

We investigated effects of the gain parameters of the rotor speed control
on the behavior of the floater pitch motion.
The instability of the FOWT response in pitch motion depends on controller bandwidth.
The negative damped response, that is, the pitch fluctuation increasing with time,
was measured for the cases with high controller bandwidth.
Even for the high controller bandwidth, we validated an enhanced control method to suppress
the negative damped response of the platform.


Learning objectives
After this presentation, delegates will be able to learn about:
1. The instability problem about FOWT
2. Technique for wind tunnel testing of FOWT
3. How to stabilize the instability by control