GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan

P-We-5LEARNING FROM FIELD TEST REGARDING DAMPING OF A FLOATER MOTION-2MW FOWT “FUKUSHIMA MIRAI”1

Hiroshi Yamaguchi and Akihiko Imakita

212

Akishima Laboratoies(Mitusi Zosen)Inc., Akishima, Tokyo, 196-0012 Japan
Mitsui Engineering and Shipbuilding Co., Ltd, Chuo-ku, Tokyo, 103-0027 JapanIn Fukushima Demonstration Project, 2MW FOWT “Fukushima Mirai” has produced electricity for four years sinceNovember 2013 without major trouble. During the demonstration, field test data have been obtained and analyzed. In WTemergency shutdown test the floater was in free roll condition and decay of motion was recorded. The data was used tomodify a numerical model. In this pater damping of floater motion is compared between water tank test and field test.
Discussion on damping was given and the design based on the tank test is found to be in safe side.
Keywords: floating wind turbine, 2MW downwind type, damping, Fukushima

INTRODUCTION
A 2MW floating wind turbine was installed at 20km offcoast of Fukushima Pref., Japan, in August 2013[1]. It
started producing electricity in November 2013. Since
then it has been four years and meaningful field data havebeen obtained.
Interesting data is decay motion time history obtainedin WT shutdown test. In a case of floating offshore
WT(FOWT) roll and pitch motion is important for safetyand protection of WT. Identification of damping characteris especially important in design to predict floater
response. Viscous damping is usually investigated in tanktest but scale effect between tank test and real structure
has been reported in few papers.
In this paper comparison between field and tank test
data regarding to damping is shown and discussion isgiven on scale effect.
PARTICULARS OF FUKUSHIMA MIRAI
Fukushima Mirai is a 2MW floating wind turbine.
Table 1 shows principle particulars of the floater. Figure 1
shows Fukushima Mirai installed 20 km off the coast of
Fukushima pref., Japan. This floater has the following
features.
1)

2)
3)

It has a center column, three corner columns.
The three corner columns are connected to thecenter column with pontoons, braces and upperdeck beams. The heave plates are installed at
the keel of the corner columns and the pontoons
to reduce heave, pitch and roll motions of floater.
The wind turbine is 2MW downwind type (HitachiHTW2.0-80). It is positioned on the top of thecenter column.
The floater is moored to the seabed using
catenary 6 lines. Six mooring lines spreadequally 120 degrees around the platform.

Table 2.

Figure 1. “Fukushima Mirai”
Table 1. Principle Particular
Floater type
Length57.2m

Width64.2m

Depth32.0m

Design draft16.0m

ClassNippon Kaiji KyokaiWater depth

120m

Table 2. Mooring system
Mooring chains
Diameter

132mm

GradeR3S

Minimum Breaking ForceAnchor Type

14,500kN
drag anchor

MODEL EXPERIMENTS
Experimental setup

The particulars of the mooring system are listed inSemi-submersible

GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, JapanThe water tank test was conducted at the Akishimalaboratory in Tokyo. The scale of the model is 1/60. Figure
2 shows the experimental setup. In extreme condition, theturbine is in waiting and the blade is fully feathered. So theturbine does not generate electricity, a steady wind load
was applied at the tower of which height was adjusted tobe equal to the overturning moment.

Surge
Xa/ζa(m/m)

2.0Model test
Bladed

1.5

1.0
0.5
0.0
05

1015
wave period[s]

2025

2025

2025

Heave
Za/ζa(m/m)

2.0Model test
Bladed

1.5

1.0
0.5
0.0
05

Figure 2. Experimental setup

Pitch

The numerical analyses of motion were carried out byTM
using Bladed . Bladed is the fully coupled dynamic
analysis for floating offshore wind turbines, including
aero-hydro-servo-elastic interactions. The hydrodynamicforces due to heave plate can be expressed using aMorison formulation. The added mass coefficient Ca anddrag coefficient Cd were determined from experimentalresults.

For example, Figure 3 shows time series of theheave and pitch motion during the free decay test. The
results calculated by Bladed are also plotted. These
comparison results show a good agreement of thedamping force applied to the numerical model.
Examples of the response amplitude operators(RAOs) are shown in Figure 4. It is obvious that thecalculation results are in good agreement with modelexperiments.
Heave20-4.0304050[s]

Pitch

10.0

Model test
Bladed1015
wave period[s]Procedure for free decay test
Emergency shutdown test was conducted on October
2nd, 2017. Before shutdown the floater had an inclination
due to WT thrust force. After shutdown the floater began
oscillating in free. During shutdown test the sea was very
calm. Therefore floater motion due to wave can beneglected. Figure 5 shows free motion in roll.
Roll decay tests for Fukushima mirai
6.04.02.00.0-2.0-4.0-6.0

Powerproduction

Emergency
stopPowerEmergency
production stop

TimeFigure 5. Time series of the rolling decay tests

5.0[deg]

5Figure 4. Comparison of the experimental andsimulated RAOs of surge, heave and pitch of floater

[deg]

[m]

0.0
10

Model test
Bladed0Model test
Bladed

2.00

3.0
2.5
2.0
1.5
1.0
0.5
0.0

IDENTIFICATION OF NON-LINEAR DAMPING FROMFREE DECAY TEST

Experimental results-2.0

θa/ζa [deg/m]Numerical analysis

4.01015wave period[s]0.0-5.0-10.001020304050

Comparison of extinction property[s]

Figure 3. Comparison of the experimental and simulated
free-decay motions of heave and pitch

Figure 6 is the extinction property obtained in tanktest and shutdown test at sea. The decay curve is fittedusing a two-degree polynomial:

GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan∆θ = a𝜃𝑚 + 𝑏𝜃𝑚 2(1)The coefficients a and 𝑏 shown in Table 3, arecalled decay coefficients. The influence of drag forces
appear in the quadratic coefficient𝑏. Seeing Table 3, thecoefficient 𝑏 of full scale is about 1.8 times larger thanthe one in water tank test.

Amplitude difference[deg]Exctinction curve
0.8
Experimental data(model test)

0.6

measured data(at sea)

Other than these factors, damping forces due to theWT or chain may have some effects. However theaerodynamic damping is small when the turbine is shut
down. Also, when observing mooring lines attached thefloater in free roll motion, amplitudes of each part of thelines are small.
CONCLUSION

0.4
0.2

0.0
0.0

that the thinner plate could provide more damping. In thetank test, the plate like a heave plate is relatively thicker
than full-scale in order to have sufficient strength. Itseems that the thickness ratio of heave plate has
influenced the damping coefficient.

0.5
1.0
Average amplitude [deg]1.5

The hydrodynamic damping properties of “FukushimaMirai” have been investigated with tank test and shutdown
test at sea. It has been confirmed that the drag force
obtained in full-scale floater is larger than the tank test.
Here, we discussed the factors of this difference, but it will
be necessary to continue examining them more. However,it can be said that the floater design based on tank test
becomes in safe side because of smaller drag coefficient.

Figure 6. Comparison of extinction property
ACKNOWLEDGEMENT
Table 3. Decay coefficientsa𝑏full scale

0.025

0.236

tank test

0.023

0.130

The project is supported by Agency for Natural
Resources and Energy, Ministry of Economy, Trade andIndustry Japan. Authors thank all related governmentorganizations and members of Fukushima FORWARDconsortium.
REFERENCESDISCUSSION
The heave plate increases the hydrodynamic
damping arising from vortex shedding at the edges andthe flow separation. The principal parameterscharacterizing the flow around the heave plate are theKeulegan-Carpenter number (KC) and Reynolds number(Re). They are defined as:
KC = 2𝜋𝑎⁄𝐷(2)Re = 2𝜋𝑎𝐷⁄𝜈𝑇(3)where 𝐷 is the diameter of plate, 𝑎 and 𝑇 are theamplitude and period of the oscillation, 𝜈 is thekinematic viscosity of the fluid. The frequency parameter(β) given by the ratio of Re and KC as:
β = 𝑅𝑒⁄𝐾𝐶(4)In the tank test and shutdown test at sea, the both ofthese KC numbers are in the same range of 0.16 to 0.3.
Also the damping coefficient of heave plate is not sosensitive for frequency parameter β of some fixed KC[2].
Therefore, drag coefficient difference may not come fromeffect of KC number nor β.
On the other hand, Tao[3] and Xin Li et al.[4] show

[1] A. Imakita, “Design, Construction and Installation of
2MW Floating Wind Turbine Fukushima Mirai”, Grand
Renewable Energy. 2014[2] T. Xin-liang et al., “Effects of Reynolds Number on theHydrodynamic Characteristics of Heave Damping Plate”,
Journal of Ship Mechanics, 17, 2013, pp.1392-1401.
[3] L. Tao et al., “Heave motion suppression of a spar witha heave plate”, Ocean Engineering, 2004
[4] T. Xin-liang et al., “Hydrodynamic coefficients ofoscillating flat plates at 0.15< KC < 3.15”, Journal ofMarine Science and Technology, 2016