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Fatigue Failure Accident of Wind Turbine Tower in
Taikoyama Wind Farm
Yin LIU1, Takeshi ISHIHARA2
1,2Department of Civil Engineering, School of Engineering, the University of Tokyo, Tokyo, Japan
Abstract
One of the wind turbine nacelles at Taikoyama wind farm collapsed due to the fatigue failure of high tension
bolts. Strain gauges and accelerometers were installed on the wind turbine to verify the aerodynamic model.
Furthermore a FEM model was built in order to find out the relationship between tower tube and high tension
bolts at the position of flange joint, where the fracture occurred. When the bolt’s pre-tension force decreases,
its stress range increases. Less the pretension force left, the larger the stress range will be. Hence when pre-
tension force is 0%, the fatigue life is left for only a few days. On the other hand when 17 bolts are damaged,
the turbine tube stress is three times larger than the stress when all the bolts are in good condition. Hence the
fatigue evaluation shows that the life time rapidly decreases to less than two months compared with that of the
normal life time which is 20 years.
Key Words: Fatigue failure, pre-tension force, high tension bolt, nacelle collapse.
1. Introduction
The Taikoyama wind farm is located at the top of
Taikoyama Mountain, Kyoto Prefecture, Japan,
which is surrounded by the Tango peninsular and
faces north to the Sea of Japan. The construction
cost is approximately 12.5 million dollars and it
reduces nearly 5900 tons of carbon dioxide every
year. The wind farm information is summarized in
Table 1.
In March 2013 the nacelle of No.3 wind turbine
collapsed[1] and the accident scene and schematic
diagram of the wind turbine is shown in Fig. 1.
1 Presenting and corresponding author, PhD candidate, E-mail: liuyin@bridge.t.u-tokyo.ac.jp
Table 1 Summary of Taikoyama wind farm
Name
Operating time
Manufacturer
Unit
Max power output
Taikoyama Wind Farm
15th, November, 2001
Lagerwey
6×750kW
4500kW
Performance
Cut-in wind speed
Rated wind speed
Cut-out wind speed
Resistant wind speed
3m/s
12m/s
25m/s
60m/s
Rotor
Diameter
Generation rotor speed
Number of blades
Hub height
50.5m
13~33rpm
3
50m
Tower Height
Material
46m
SM400 (steel)
Flange connection
high-tension bolts F10T M24
Nacelle Dimensions
Material
W5.6×L3.3×H6.5m
SS400, GFRE
Wind direction
control Control method Active yaw control
Rated power
output control Control method Pitch control
(a) Collapsed nacelle
(b) Fracture section (c) Vertical cross section
Fig. 1 Accident scene and schematic diagram
The detailed structure is shown in Fig. 2.
(a) Flange joint (b) Fracture section in detail
Fig. 2 Detail drawing of fracture section
The field investigation indicates that the wind
condition satisfied the construction requirement
based on the IEC 61400-1[2] including annual wind
speed, turbulence intensity and flow inclination angle.
By observing the fracture section of the tower tube,
we found that the material strength was strong
enough, but evidence of fatigue crack propagation
was detected at the inner surface of the tube.
Furthermore, 17 broken bolts were found during the
field investigation and fatigue cracks were also
detected. By comparing the two aspects, fracture is
considered to be preceded by a certain degree of
fatigue damage caused by the reduction of bolts pre-
tension force up to 30%~100%.
The wind turbine collapsed very early in 12 years,
where the expected life period was 20 years.
Moreover, the accident happened only three months
after the periodical inspection was carried out.
Additionally, there are more than 120 wind turbines
in service of the same type across Japan. Therefore,
it is necessary and urgent to understand the cause
of this accident, so that this kind of accident can be
prevented in the future.
This paper proceeds as follows: 1) Field
measurement; 2) Aerodynamic modelling and
verification; 3) Clarify the fracture section’s
aerodynamic characteristics; 4) Explain the
relationship between nominal stress, local stress and
bolt stress using FEM model; 5) Evaluate the fatigue
life of both high-tension bolt and tower tube, and
reveal the reason for the failure.
2. Field measurement
2.1 Wind condition investigation
All the data were measured from Feb. 2nd 2015 to
Feb. 28th 2015.
Fig. 3 Occurrence frequency Fig. 4 Average wind speed
Fig. 3 and Fig.4 indicate the occurrence frequency
and average wind speed respectively. The
occurrence frequency of dominate wind direction
WSW, W and WNW is 9%, 27% and 15%
respectively.
Since the SCADA data contains only maximum wind
speed and average wind speed in a time scale of 1
minute, we calculated the turbulence intensity
according to reference [3] in equation (1).
𝐼𝑝 =𝑈𝑚𝑎𝑥 𝑈𝑚𝑒𝑎𝑛−1⁄
𝑃, 𝑃 =
1
2𝑙𝑛
𝑇
𝑡 (1)
0%
10%
20%
30%N
NNE
NE
ENE
E
ESE
SE
SSES
SSW
SW
WSW
W
WNW
NW
NNW
02468
1012
NNNE
NE
ENE
E
ESE
SE
SSES
SSW
SW
WSW
W
WNW
NW
NNW
50m
45.94m
0m
Fracture section
Local structur
Flange
Welding 10mm below flange
Fracture section
The maximum wind speed Umax and average wind
speed Umean are derived from the 10min SCADA data,
the peak factor P is evaluated by a time scale T of
600 seconds and average time t of 1 second.
Consequently 1m/s bin average is calculated. Fig. 5
shows the field turbulence intensity.
Because of the insufficient high wind speed data
(>17m/s) during the measurement period, the high
wind speed turbulence intensity is extrapolated
assuming the normal turbulence intensity in
reference[2], and it is described as equation (2)
𝜎1 = 𝐼𝑟𝑒𝑓(0.75𝑉ℎ𝑢𝑏 + 𝑏), 𝑏 = 3.8 (2)
Iref is the expected value of hub-height turbulence
intensity at a 10 min average wind speed of 15m/s,
Vhub is the wind speed at hub height and 𝜎1 is hub-
height longitudinal wind velocity standard deviation.
As a result for aerodynamic simulation, a combined
turbulence intensity is used: measurement value for
low wind speed ( ≪ 17m/s ) and the extrapolated
value for high wind speed respectively (>17m/s).
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20 25
MeasurementExtroplatedbin average
Tu
rbu
lence in
ten
sity
Wind speed (m/s)
Fig. 5 Turbulence intensity in the direction of WSW+W+WNW
For the turbulence spectrum, the Kaimal model is
used. The lateral and vertical turbulence intensity
component are considered as 0.8 𝜎1 and 0.5 𝜎1
according to reference [2].
2.2 Moment measurement
Strain gauges with sampling frequency of 20Hz were
installed in eight directions in order to get the
moment at the height of 12.6m above tower base. Fig.
6 shows the strain gauges installment.
The nacelle was forced to rotate one circle without
operating for the estimation of the strain gauges’
installment error, and the compensation value can be
calculated by the amplitude of the sin curve.
Fig. 6 Strain gauges installment
Fig. 7 Moment calculation schematic diagram
The measurement moment was calculated following
the method by Ishihara and Phuc[4]. According to Fig.
7, the East-West moment and South-North moment
were given in equation (3) and (4) respectively.
Where M and ε is the moment and strain at
corresponding direction, EI is the stiffness of tower tube
and D is the inner diameter.
𝑀𝐸𝑊 = 𝐸𝐼𝜀
𝐷= 𝐸𝐼
𝜀𝐸−𝜀𝑊
𝐷 (3)
𝑀𝑆𝑁 = 𝐸𝐼𝜀
𝐷= 𝐸𝐼
𝜀𝑆−𝜀𝑁
𝐷 (4)
The total moment is given in equation (5). If the
direction of total moment is opposite to the nacelle
direction, then the total moment will be positive,
otherwise it is negative.
𝑀𝑡𝑜𝑡𝑎𝑙 = √𝑀𝐸𝑊2 + 𝑀𝑆𝑁
2 (5)
The average bending moment, maximum bending
moment and standard deviation of bending moment
are plotted in Fig. 8.
ε𝐸
ε𝐸
ε𝐸
ε𝐸ε𝐸
Nacelle direction ε𝑊
ε𝑠
ε𝐸
ε𝑁
M𝐸𝑊
M𝑆𝑁
M𝑡𝑜𝑡𝑎𝑙
α θ
-1000
0
1000
2000
3000
4000
5000
0 5 10 15 20 25
measurement
bin average
Mo
men
t (k
Nm
)
Wind speed (m/s)
-1000
0
1000
2000
3000
4000
5000
0 5 10 15 20 25
measurement
bin average
Mo
men
t (k
Nm
)
Wind speed (m/s)
(a) Average moment (b) Maximum moment
-1000
0
1000
2000
3000
4000
5000
0 5 10 15 20 25
measurement
bin average
Mo
men
t (k
Nm
)
Wind speed (m/s) (c) Standard deviation of moment
Fig. 8 Comparison of measurement and bins average moment
3. Aerodynamic analysis and
fatigue life investigation
3.1 Aerodynamic modelling
Aerodynamic model is built to simulate the dynamic
performance by GL’s Bladed wind turbine modelling
tool[ 5 ]. The tower section refers to the real
engineering drawings. For commercial confidentiality,
the blade profile is not available from manufacturer.
As a result we selected airfoils from NREL’s airfoil
family, which are S818 for root section, S830 for
primary section and S831 for tip section[ 6 ], and
thickness/chord ratio, Reynolds number, lift
coefficient Cl and draft coefficient Cd were
determined.
For control method, some adjustment had been
applied. In case of the high turbulence intensity in the
mountainous area, the wind turbine encounter over
speed at times. Once it exceeds the maximum rotor
speed of 33 rpm, it stops suddenly and starts to
operate again when the rotor speed drops below the
maximum value which causes frequent downtime.
Hence the manufacturer modified the maximum rotor
speed and power output to decrease the downtime.
Since the details were commercial confidentiality, we
adjust rated power output and maximum rotor speed
according to the measurement data. Moreover a five
degrees pitch angle error is considered to eliminate
the error in pitch control. With the adjustment above
the power output, rotor speed and pitch angle are
now close to the measurement data as shown in Fig.
9.
0
100
200
300
400
500
600
700
800
0 5 10 15 20 25
measurement
simulation modified
simulation default
Po
wer
ou
tpu
t (k
W)
Wind speed (m/s)
0
5
10
15
20
25
30
35
0 5 10 15 20 25
measurementsimulation modifiedsimulation default
Ro
tor
spee
d (
rpm
)
Wind speed (m/s)
(a) Power output (b) Rotor speed
0
5
10
15
20
25
30
0 5 10 15 20 25
measurementsimulation modifedsimulation default
Pit
ch a
ngle
(deg
)
Wind speed (m/s)
(c) Pitch angle
Fig. 9 Comparison of power output, rotor speed and pitch angle
The proportional gain KQP and integral gain KQI for
torque control, and proportional gain KSP and integral
gain KSI for pitch control were calculated based on
Guidelines for Design of Wind Turbine Support
Structures and Foundations, JSCE[7 ],and optimal
mode gain Kopt was modified to validate the dynamic
simulation results with measurement results.
Some key parameters for Bladed modelling are
summarized in Table 2.
Table 2 Key parameters for Bladed modelling
Optimal mode
gain Kopt
Demanded generator
toque (Nm)
Rated Power
generation (kW)
Rotor speed
(rpm)
Error in Pitch
angle (degree)
Torque
control Pitch control
Default 22583.5 216450 750 33rpm 0 KQP=789139
KQI=516780
KSP=0.458180
KSI=0.847957
Modified 23340.2 231387 630 26rpm 5 KQP=461249
KQI=176551
KSP=0.492799
KSI=0.771005
A field test was carried out to measure the natural
frequency of the tower. The damping ratio of the 1st
order frequency was applied as 0.5% based on the
field inspection [1]. The natural frequency is shown
in Table 3, which is consistent with the aerodynamic
simulation result.
Table 3 Comparison of tower natural frequencies
Tower natural frequencies Measurement Simulation
1st order (fore-art) 0.515Hz 0.533
1st order (side-side) 0.518Hz 0.533
2nd order (fore-art) 3.838Hz 3.685
2nd order (side-side) 3.832Hz 3.578
Finally, Fig. 10 shows the measurement and
simulation results for moment at 12.6m above tower
base were in good agreement, and the aerodynamic
model is verified to be correct.
-1000
0
1000
2000
3000
4000
5000
6000
0 5 10 15 20 25
measurementsimulation modifiedsimulation default
Mom
en
t (k
Nm
)
Wind speed (m/s)
-1000
0
1000
2000
3000
4000
5000
6000
0 5 10 15 20 25
measurementsimulation modifiedsimulation default
Mom
ent
(kN
m)
Wind speed (m/s)
(a) Average moment (12.6m) (b) Std of moment (12.6m)
-1000
0
1000
2000
3000
4000
5000
6000
0 5 10 15 20 25
measurementsimulation modifiedsimulation default
Mom
ent
(kN
m)
Wind speed (m/s)
(c) Maximum moment (12.6m)
Fig. 10 Comparison of moment
3.2 Characteristics of fracture section
Fig. 11 (a) and Fig. 11 (b) show simulated axial force
N and bending moment M at the tower fracture
section (45.94m) at different wind steps respectively
according to simulation result.
Hence the nominal stress can be calculated from
equation (6), where A is the sectional area and Z is
the sectional resistance moment.
σ𝑛 =𝑁
𝐴−
𝑀
𝑍 (6)
-800
-700
-600
-500
-400
-300
0 5 10 15 20 25
MinAverageMax
Ax
ial
forc
e (
kN
)
Wind speed (m/s)
-1000
-800
-600
-400
-200
0
0 5 10 15 20 25
MinAverageMax
Mo
men
t (k
Nm
)
Wind speed (m/s)
(a) Axial force N (45.94m) (b) Bending moment M (45.94m)
-15
-10
-5
0
5
10
15
20
25
0 5 10 15 20 25
MaxAverageMin
No
min
al
stre
ss (
N/m
m2)
Wind speed (m/s)
c) Nominal stress (45.94m)
Fig. 11 Aerodynamic characteristics at the fracture section
As shown in Fig. 11 (c), the nominal stress σ𝑛
changes and varies with the increase of the wind
speed. The minimum stress turns into negative value
when the wind speed is above 18 m/s.
3.3 FEM modelling
The fracture section is very close to the top flange
welding position, and according to the field
investigation the fatigue failure propagated at the
inner surface of the tower tube, so the stress
concentration and spatial effect may influence the
local stress σ𝑙𝑜𝑐𝑎𝑙 significantly. A 3D FEM model is
Thrust force by wind
built to clarify the relationship between nominal
stress σ𝑛 , local stress σ𝑙𝑜𝑐𝑎𝑙 and bolt get pre-
tension force before and after the bolts damaged.
The relationship of nacelle weight, thrust force and
top flange is illustrated in Fig. 12. The nacelle weighs
53.3t and it is rigidly connected to the yaw bearing.
The stress concentration factor of welding geometric
profile was proposed by Caccese[8]. The case for
Taikomaya wind turbine is as shown in Fig. 13. Solid
element is used for the modelling of yaw bearing, top
flange and bolts, and shell element is used for tower
tube modelling. Furthermore, contact element is
considered for the contact surface of yaw bearing
and top flange and the friction factor is 0.2. The bolts
are rigidly connected to the yaw bearing.
Fig. 12 Force applying position relationship
Fig. 13 FEM detail at top flange position
3.4 Investigation of the tower tube
fatigue life
As for the tower tube, Fig. 14 shows the cases when
17 bolts broken.
Fig. 14 Diagram of the damage area
Thrust force is considered in seven cases from 0kN
to 250kN to simulate different wind loading. Fig. 15
shows an example of the local stress σ𝑙𝑜𝑐𝑎𝑙 before
and after 17 bolts are damaged at wind speed of
16m/s.
Fig. 15 (a) implies that the cause of maximum tensile
stress happens at the inner tube because of the law
of lever, which is consistent with the observation of
fracture face. According to Fig. 15 (b), the local
stress is much larger when 17 bolts are broken.
(a) Bolts normal
(b) 17 Bolts broken
Fig. 15 Comparison of the local stress (16m/s)
The relationship between nominal stress and local
stress considering the welding stress concentration
[7] is now given as following respectively:
Bolts normal
𝜎𝑙𝑜𝑐𝑎𝑙 = −3.05 + 2.65𝜎𝑛 (7)
17 bolts broken
𝜎𝑙𝑜𝑐𝑎𝑙 = −10.6 + 6.35𝜎𝑛 + 0.16𝜎𝑛2 (8)
Equation (7) and (8) are plotted in Fig. 16. When 17
W
N
S
C
L
5
m
m
Contact
element
Yaw bearing
Top flange
Tower tube
Shell element
Fracture section
Yaw bearing
Flange
Tower tube
Welding
Local stress
Bolts
Yaw bearing
Flange
Tower tube
Welding
Local stress
Bolts
Nacelle opposite side
E (0°)
Edge of damaged
Bolts (53.6°)
Top flange
Hub Height (=GL+50.0m) Nacelle weight (53.3t)
Thrust force by wind
Nacelle center
of gravity
4000m
1525mm
Yaw bearing
Lee wind side (180°)
Rotor side (0°)
bolts are broken, the local stress is more than three
times larger than bolts at normal condition.
-100
-50
0
50
100
150
200
-20 -10 0 10 20
Bolts normal17 Bolts broken
Lo
cal
stre
ss
local
(N/m
m2)
Nominal stress n (N/mm2)
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0
1 7 b o l t s b r o k e n
B o l t s n o r m a l
- 5 0
0
50
100
150
200
Lo
cal str
ess
local (N
/mm
2)
time (s) Fig. 16 Local stress vs. Fig. 17 Time history of local
nominal stress stress (22m/s)
With a time period of 10 minutes, the time series
simulation result is available for each wind speed
combining aerodynamic model with equation (7) and
(8). When the wind speed is low, the tensile stress
predominates. However with increase in wind speed,
compressive stress occurs and the stress amplitude
increases. The case of wind speed at 22m/s is shown
in Fig.17.
With the time history of bolt pre-tension stress, we
can investigate its fatigue life. Rain flow counting
algorithm is used for fatigue analysis in order to
reduce the spectrum of varying stress into a set of
simple stress reversals. Goodman relation as shown
in equation (9) is used to quantify the interaction of
mean and alternating stresses.
𝜎𝑎 = 𝜎𝑤(1 − 𝜎𝑚/𝜎𝐵) (9)
𝜎𝑎 is the alternating stress from rain flow counting
result, 𝜎𝑚 is the mean stress, 𝜎𝑤 is the fatigue limit
for comple`tely reversed loading and 𝜎𝐵 is the
ultimate tensile strength of the material, which is
493Mpa for SM400 steel.
By using the fatigue limit for completely reversed
loading 𝜎𝑤, S-N curve based on GL wind 2005 with
a detail category of 71[ 9 ], and Miner’s rule, the
accumulative fatigue damage D in 10 minutes is
given in Equation (10), and failure is reached when
D equals to 1.
∑𝑛𝑖
𝑁𝑖
𝑘𝑖=1 = 𝐷 (10)
Frequency distribution of the wind speed is based on
Rayleigh distribution with a mean annual wind speed
of 8.5m/s.
The fatigue life of tower tube is shown in Fig. 18.
When the bolts are in normal condition the fatigue life
is 27.5 years, which is in agreement with the design
requirement. However, if 17 bolts are broken, the
fatigue life decreases dramatically to 0.09 years,
approximately one months. It is in accordance with
the time interval between the last periodical
inspection and the accident.
0
5
10
15
20
25
30
17 bolts brokenBolts normal
Tow
er
tube
fa
tig
ue life (
ye
ar)
27.5
0.09
17 bolts broken Bolts normal
Fig. 18 Tower tube fatigue life
3.5 Investigation of the high tension
bolts fatigue life
Based on the field investigation [1], six bolts at
nacelle’s opposite side were found to have reduction
pre-tension force reduced as shown in Fig. 19.
Fig. 19 Bolts pre-tension force decreasing
In order to recreate the real situation, blots pre-
tension force is set in six different cases which were
100%, 80%, 60%, 40%, 20% and 0% of the design
pre-tension force corresponding to 850kNm torque.
The relationship between the nominal stress and bolt
pre-tension stress is given as shown in Fig. 20. With
the nominal stress increasing, the gradient increases
as pre-tension decreases, and it is much more
obvious when the pre-tension force decreases. The
larger the gradient the larger the bolt stress range will
be, and the bolt’s fatigue load. Since the nominal
E W
N
S
Nacelle’s opposite side
stress ranges mainly between -5N/mm2 to 25 N/mm2
according to Fig. 11(c), the stress range may vary a
lot especially when the bolts pre-tension stress drops
to 0% as illustrated in Fig. 20.
-400
-200
0
200
400
600
800
-30 -20 -10 0 10 20 30 40 50
0%20%40%60%80%100%
Bo
lt p
re-t
en
sio
n str
ess (
N/m
m2)
Nominal stress
-5 25
0
50
100
150
200
250
300
0 100 200 300 400 500 600
Pre-tension 20%Pre-tension 0%
Blo
t p
re-t
en
sio
n s
tress (
N/m
m2)
Time (s)
Fig. 20 Nominal stress Vs. Fig. 21 Time history of bolt
bolt pre-tension stress pre-tension stress (14m/s)
Fig.21 shows one example of the time history of the
bolt pre-tension stress at the wind speed of 14m/s. It
is clear that when the pre-tension force drops the
stress range increases significantly.
The fatigue life investigation follows the rules
mentioned in Section 3.4. The ultimate tensile
strength of FT10 bolts is1000Mpa and the detail
category is 36.
The bolts fatigue life is shown in Fig. 22.
0
0.001
0.01
0.1
1
10
100
1000
020406080100
Bo
lts f
atig
ue
life
(Y
)
Bolt pre-tension force(%)
20 years
0.22 days
6.95 years
318.25 years
Fig. 22 Bolts fatigue life vs. bolt pre-tension percentage
As we can see that when the pre-tension force is
over 40%, the life time does not decrease. However
when the pre-tension force is below 40% the fatigue
life time drops dramatically as only a few days left,
when the pre-tension force is 0%.
4. Conclusions
This research is based on the collapse accident of
Taikoyama wind farm No.3 turbine. The field
measurement of tower model frequency, SCADA
data and strain gauge data were measured. At the
same time the aerodynamic model was built. In
addition, the tower top FEM model was built to
evaluate the high-tension bolts and tower tube
fatigue life.
The cause of the collapse of the wind turbine is
discussed and the following conclusions were drawn:
1) Due to high turbulence intensity at site, the control
of the wind turbine was modified by manufacturer.
Power output and maximum rotor speed were
adjusted according to measurement data, and a five
degree of pitch error was applied. With this control
method the simulation results show good agreement
with measurement results;
2) For the high tension bolts, by considering the
nonlinear phenomenon and stress concentration
closed to welding zone, when the pre-tension force
decreases, the stress range increases, especially
when pre-tension force is 0% it is 30 times larger. The
less the pre-tension force left, the larger its range is.
As a result, when the pre-tension force is below 40%
the fatigue life time drops drastically and it is only a
few days when the pre-tension force is 0%;
3) Similarly, the FEM model shows that with 17 bolts
broken the local stress at fracture section increases
more than three times compared with the case of
bolts at normal condition. This phenomenon
accelerated the fatigue initiation and propagation
and the fatigue life of the fracture section decreases
dramatically to 1/200 of its life time.
4) The reason for the Taikoyama wind farm accident
is now clearly understood in a detailed manner. It is
not the matter of design or material, but was due to
the fatigue failure caused by the reduction of high
tension bolts’ pre-tension force.
For the Taikoyama wind turbines’ high tension bolts,
according to the service manual the temporary
torqueing and final torqueing was applied. And at the
time of 500 hours after bolt changing, the re-
torqueing must be applied. However at the time of
periodical bolt changing operation, the re-torqueing
was not applied. The wind turbine is a rotating
machine system, in which the contact surface and
the bolt itself plasticity deforms accompany with the
wind turbine operation, and therefore the pre-tension
force reduces.
Moreover, according to the service manual, 5% of the
bolts should be inspected per year, which means
only three bolts were inspected. We should check at
least 16 bolts per year in order to cover the bolts in
all wind direction.
Besides, during the year from 2005 to2008, the
workers only conducted the method of counter mark
inspection to make sure the torque was enough.
It is a serious problem between manufacturer and
operator that expertise technique is not transferred
accurately and efficiently. Clear rules must be made
even after guarantee periods, or it may lead to
devastating accident.
Reference
[1] Kyoto fu, Report of the accident in Taikoyama wind farm No.3 wind turbine, Kyoto, 2013.
[2] International Electrotechnical Commission, (2005). IEC 61400-1, 3rd edition, Part 1: Design requirements. Geneva.
[3] Ishizaki, H.(1983) Wind profiles, turbulence intensities and gust factors for design in typhoon-prone regions. Journal of Wind engineering & Industrial Aerodynamics, 13: 55-66.
[4] T. Ishihara, P.V. phuc, Yozo Fujino. A Field Test and Full Dynamic Simulation on a Stall Regulated Wind Turbine. The sixth Asia-Pacific Conference on Wind Engineering, Seoul, September 2005: 599-612.
[5] Garrad Hassan Bladed, version 4.4, DNV-GL, 2013.
[6] Tony Burton, David Sharpe, Nick Jenkins. Wind Energy Handbook. John Wiley & Sons Ltd, Chichester, 2001.
[7] Japan Society of Civil Engineers, (2010). Guidelines for Design of Wind Turbine Support Structures and Foundations. Task Committee on Dynamic Analysis and Structural Design of Wind Turbine Committee of Structural Engineering, Tokyo.
[8] V. Caccese, P.A. Blomquist, K.A. Berube. Effect of weld geometric profile on fatigue life of cruciform welds mad by laser/GMAW processes. Marine Structures, 2006, 19: 1-22.
[9] Germanischer Lloyd WindEnergie GmbH (2005), Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd WindEnergie, Hamburg.
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