GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, JapanP-We-4

COMPARISON BETWEEN MEASURED VALUE AND SIMULATED VALUE OF MOTION
AND MOORING FORCE IN MITSUBISHI 7MW FLOATING OFFSHORE WIND TURBINE
11122Masao Komatsu , Hideo Mori , Hiroshi Tanaka , Makoto Ohta , Kai Karikomi and Hidekazu Ishii

212

Mitsubishi Shipbuilding Co.,Ltd. Marine Engineering Center, 220-8401, Japan
Mitsubishi Heavy Industries, Ltd. Research and Innovation Center, 851-0392, JapanThe demonstration project of the floating offshore wind farm “ Fukushima FORWARD” is ongoing [1]. Mitsubishi 7MW
floating offshore wind turbine (FOWT) was installed at about 20 km far from seashore on October 2015. This FOWT has
the V-shaped semi-submersible floating structure and is moored by 8 large caliber catenary chains. The motions of floaterand the mooring forces are continuously measured by GPS units, gyro meters and inclinometers specially made for large
mooring chains. The demonstration data in severe conditions have been accumulated, for example, the motion andmooring force data during some severe typhoons. These demonstration data are used to validate the design loads,
motions and mooring forces. In this paper, the stability of FOWT is investigated by the comparison between the measured
value and simulated value, and it is concluded that the V-shaped semi-submersible floating structure is stable andappropriate for large floating offshore wind turbine.
Keywords: renewable energy, FOWT, mooring force, V-shaped semi-submersible floating structure

V-SHAPED SEMI-SUBMERSIBLE FLOATINGSTRUCTURE
The V-shaped semi-submersible floating structure
was developed as the stable floater for 7MW wind turbine,
based on the technology and knowledge of oceanengineering in Mitsubishi Heavy Industries, LTD. (MHI)[2][3]. This floater is designed in such a way that two
rectangular parallelepipeds (lower hull) are arranged inV-shape and three rectangular parallelepipeds (columns)
are arranged at the ends of the lower hull. During theinstallation process of wind turbine, this floater can have ashallow draft, which is suitable for towing from harbor. At
the operation site, this floater should be insemi-submerged condition. Fig. 1 shows the location ofMitsubishi 7MW FOWT and Fig.2 shows the picture of thisFOWT during operation. The principal dimensions areshown in Table 1.

Mitsubishi FOWT

about 20km

Google EarthFig. 1. Location of Mitsubishi 7MW FOWT

Table 1. Principal dimensions
Rotor diameter167m

Hub height from sea surface

105m

Length floating structure

85m

Width floating structure

150m

Column width

14m

Draft

17

m

Displacement

26,000ton

GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japanin August 2016) and the rated operation of 7MW in
December 2017.
Motion analysis programs
The motion analysis program developed by MHI wasused for simulations. In this program, motions of thefloater are derived from solving the equation of motion in
the time domain. The equation of motion consists of theirregular external forces of wind and wave, the fluid forces
calculated by the linearity potential theory, the mass, therestoring force of mooring chains and so on.
Mooring forces in storm conditions

Fig. 2. 7MW wind turbine and V-shaped semi-submersible
floating structure
MEASUREMENT SYSTEMThe position and motion of the floater are measuredby GPS (Global Positioning System), Fiber Optic
Gyroscope, accelerometers and satellite compasses.
Among them, the GPS measurements have the highaccuracy by the high speed communication with the basestations onshore. The stresses at cross sections of thehull structure are monitored by strain gauges. The
inclinations of mooring chains are measured byinclinometer attached to these chains. The inclinometerhas a small inertial sensor inside the pressure resistantcase, and is developed specially for these mooring chains.
The sampling frequency of all equipment is set 20 Hz andthis measurement system has operated since November
2015. The image of sensor arrangement is shown in Fig.
3.

In order to validate the stability of mooring design,
simulations were carried out under storm conditions using
the motion analysis program.
The target storm conditions are the typhoon No.9(Mindulle) and the typhoon No.10 (Lionrock) in 2016. The
highest wind speed during the observation period wasobserved in the typhoon No.9. The highest wave height
was observed in the typhoon No.10. The paths of thetyphoon No.9 and No.10 are shown in Fig. 4. The typhoon
No. 9 had passed about 55km west of Mitsubishi FOWT
and the Typhoon No.10 had passed about 90km east ofMitsubishi FOWT.

No.9No.10

Mitsubishi FOWT

Google EarthFig. 4. Paths of typhoon No.9 and No.10

Fig. 3. Sensor arrangement

The comparison between the maximum mooring
force of measured data and simulation results are shownin Fig. 5 and Fig. 6. In these figures, Uave means the 10
minutes average wind speed, Hs means the 20 minutes
significant wave height and Vcr means the 20 minutes
average current speed.

EVALUATION OF VALIDITY FOR DESIGN
Target conditions to validate design method
Since this measurement system started to operate,some storms with high speed and high wave height haveoccurred. In this paper, we focus on the storm (typhoons

Fig. 5 and Fig. 6, (a) show the time series of Hs andUave at the wind farm during the typhoon No.9 and No.10.
During the typhoon No.9 case, the maximum values ofUave and Hs were measured at around 20:30. (However,there is measurement error for Hs after 20:00.) In thetyphoon No.10 case, the maximum value of Hs was

50

Hs
Uave

840

630

420210000 2 4 6 8 10 12 14 16 18 20 22
Hour [h]

1050

Hs
Uave

840

630

420210000 2 4 6 8 10 12 14 16 18 20 22
Hour [h]Average wind speed [m/s]10

Average wind speed [m/s]Significant wave height [m]measured at around 12:50. Fig. (b) shows the directionsof wind, wave and current. During the typhoon No.9 case,these external forces were acting in nearly same direction,
whereas for typhoon No.10, these forces were acting indifferent directions. (c) shows the comparison result ofmooring forces between measured values and simulatedvalues. The simulation results show the higher value thanthe measured data, hence the simulation can estimate themooring force in safe side.

Significant wave height [m]GRAND RENEWABLE ENERGY 2018 Proceedings
June 17(Sun) - 22(Fri), 2018
Pacifico Yokohama, Yokohama, Japan(a) Time series of Hs and Uave on 2016/8/30(a) Time series of Hs and Uave on 2016/8/22(b) Directions of wind, wave and current

Max. of Mooring force [tf]Typhoon No.10（2016/8/30 12:50）(b) Directions of wind, wave and current

200150100500Max. of Mooring force [tf]Typhoon No.9（2016/8/22 20:30）

Uave = 33.3 m/s
Hs = 6.46 m
Vcr = 0.50 m/s

250
200
150

Uave = 19.9 m/s
Hs = 9.92 m
Vcr = 0.28 m/s

250

Measured [tf]Simulation [tf]

No.1
90.2
83.9

No.2
147.2
154.2

No.3
88.8
69.1

No.4
80.2
75.6

No.5
79.4
73.3

No.6
76.2
58.2

No.7
70.9
58.7

No.8
68.5
74.4

(c) Comparison of mooring forces in storm conditionsFig. 6 Result of analysis for the typhoon No.10

100500No.1
Measured [tf] 182.8
Simulation [tf] 221.4

No.2
74.3
61.7

No.3
59.7
47.4

No.4
55.8
51.4

No.5
58.3
53.0

No.6
118.9
117.1

No.7
137.8
135.2

(c) Comparison of mooring forces in storm conditions
Fig. 5 Result of analysis for the typhoon No.9

No.8
153.0
92.7

In the Fig. 7, the measured floater pitch and rollmotions are compared with the simulation results in thetyphoon No.10. The simulated floater motions werecalculated by the motion analysis program. During thistyphoon, the highest wave height (9.92m) was observed,though, the maximum pitch motion in the typhoon No.10
confirmed to be below 3 degree.

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

Uave = 19.9 m/s Typhoon No.10（2016/8/30 12:50）
Hs = 9.92 m
Vcr = 0.28 m/s

Inclination angle [deg]432
10-1-2-3-4

Max
Simulation
Roll [deg]1.85
Pitch [deg]
2.53

Min
Simulation
-1.60
-2.97

Max
Measured
1.69
2.49

Min
Measured-1.60-2.62

Fig. 7 Comparison of floater motion in the typhoon No.10
Floater motions in 7MW operating conditions
In the Fig. 8, the measured floater pitch and rollmotions are compared with the simulation results in the7MW operating conditions. During the 7MW operation, therange of these motions is within +1 degree to -1 degree.
Considering the floater motion in the 7MW operation
conditions and in the typhoon conditions, the Mitsubishi
FOWT is confirmed to be the stable floater.
Uave = 15.4 m/s 7MW operating（2017/12/8 18:00）
Hs = 3.06 m
Vcr = 0.22 m/s

Inclination angle [deg]43210-1-2Fig. 9 Sample model of dynamic mooring analysis
CONCLUSION
In this paper, the validity of mooring design method
was verified and this V-shaped semi-submersible floating
structure is proved to be stable for both storm conditions
are 7MW operation conditions.
ACKNOWLEDGEMENT”Fukushima floating offshore wind farm demonstration
project (Fukushima FORWARD)”, which is funded by theMinistry of Economy, Trade and Industry, is promoted by aconsortium consisting of Marubeni Corporation (Projectintegrator), the University of Tokyo (Technical advisor) andseveral companies. The authors are grateful to people ofconsortium members striving for realization of thisverification test.

-3-4Max
Simulation
Roll [deg]0.76
Pitch [deg]
0.69

Min
Simulation
-0.23
-0.26

Max
Measured
0.46
0.41

Min
Measured
-0.14
-0.47

Fig. 8 Comparison of floater motion in 7MW operating
conditions
FUTURE WORKS
There are still certain unknown facts about thedynamic effects of mooring force. In order to improve thedesign accuracy of mooring chains we would like toconduct the dynamic analysis in next step.

References[1] http://www.fukushima-forward.jp/index.html[2] M. Komatsu et al., “Development of Offshore Wind
Turbine Floater that Blends into Japanese Waters”,
Mitsubishi Heavy Industries Technical Review Vol. 53 No.
2, 2016, pp.30-39.
[3] M. Ohta et al., “Development of a V-shaped
Semi-submersible Floating Structure for 7MW Offshore
Wind Turbine.”, Proceedings of the International
Symposium on Marine and Offshore Renewable Energy,
2013