Proceedings of the International Symposium on Marine and Offshore Renewable Energy
Oct 28-30, 2013, Tokyo, Japan

Applicability of fatigue solutions to floating wind turbine structures
Y. Kayamori, T. Inoue
Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation
T. Okawa
Oita R&D Laboratory, Nippon Steel & Sumitomo Metal Corporation
S. Nishimura
Plate Div., Nippon Steel & Sumitomo Metal Corporation
T. Ishihara
Department of Civil Engineering, The University of Tokyo
floating large offshore wind farms. This project plans todevelop three floating wind turbines and one floating
power sub-station off the coast of Fukushima, and various
subjects in floating offshore wind turbine technologies
have been discussed by the 11-entry consortium.
Fatigue is one of the subjects because variable loadingby external forces such as ocean winds and sea waves canlead to the fatigue of floating wind turbine structures.
Another one is weight reduction for constructing a lot oflarge offshore wind turbines efficiently, and materials withhigh strength can contribute to weight reduction. However,the fatigue strength of steel welded joints is independent of
the static tensile strength of the steel because of tensile
residual stress, high stress concentration andmicrostructure change. Fatigue solutions are usefultechnologies for the improvement of fatigue strength insteel welded structures, where it is expected that the use ofhigh tensile strength steel plates will result in high fatigue
strength.
Post weld toe shape improvement methods such as burr
grinding and TIG dressing are conventional fatigue
solutions, and have been used for welded structures.
Peening is another solution, and its effect on fatigue ishigher than that of toe shape improvement only because
peening induces compressive residual stress as well assmooth toe shape. Ultrasonic impact treatment, UIT, is akind of peening methods, and has advantages in treatment
speed, vibration, noise, etc. UIT also gives a good fatigue
performance in welded joints [2-3], and the details of afatigue property improvement have been investigated inrecent years [4-6]. However, UIT has not applied tofloating wind turbine structures, and it is unclear whether
UIT improves the fatigue strength of welded joints infloating wind turbines. In this study, the effect of UIT on
the fatigue properties of welded large-scale structuralmodels, which simulated a welded connection of a tower toa floating body, was examined by using the 2.5MN large
fatigue test machine. High strength YP460 steel plates
were used for manufacturing the large-scale structuralmodels.
In addition to the welded joints, base metal plates
should have high fatigue strength in case the plates areused for machined and worked structural members. In a

Abstract - Fukushima floating offshore wind farmdemonstration project has started since 2012, andvarious subjects in floating offshore wind turbine
technologies have been discussed by Fukushima
floating offshore consortium in the project. The fatigueof floating wind turbine structures is one of the subjects
because variable loading by external forces such as
ocean winds and sea waves can lead to fatigue, and twosolutions to the fatigue were investigated in this study.
One solution is ultrasonic impact treatment, UIT,
and the effect of UIT on the fatigue properties oflarge-scale welded models was examined by using the
2.5MN large fatigue test machine. YP460 TMCP steel
plates were used for manufacturing the large-scale
structural models, and UIT significantly improved
fatigue life particularly in the low stress range and longfatigue life region.
The other solution is steel for plasma arc cutting,which is used for a process of fabricating large-scalewelded structures such as wind turbines. Notchedfatigue specimens were sampled by using plasma arc
cutting from two kinds of TMCP steel plates, YP460
and KD36, and their fatigue properties were
investigated. In each TMCP steel plate, fatigue strength
was not degraded on the plasma arc cut surface.
I. INTRODUCTION
The Great East Japan Earthquake struck thenortheastern area of Honshu in 2011, and many local
industries were damaged by the earthquake and tsunami. A
clue to an industrial recovery from the damage ispotentially new industrial development that creates new
employment in the area. After the post-tsunami shutdown
and catastrophic damage of Fukushima Daiichi nuclear
power plant, a possible industry is renewable energy. The
Agency for Natural Resources and Energy, ANRE, in the
Ministry of Economy, Trade and Industry in Japan, METI,
organized Fukushima Floating Offshore Consortium, andFukushima Floating Offshore Wind Farm Demonstration
Project, Fukushima FOWARD, was launched in 2012 [1].
The aim of this project is to establish a business model of1

EsonicsTM 27 UIS made by Applied Ultrasonics was usedin this study. The operating frequency of an ultrasonic
generator was 27kHz, and the diameter of an indenter pin
was 3mm in the equipment. The weld toes of the UIT
specimens were treated until the original weld toes
disappeared, and the maximum indent depth was about
0.4mm after the treatment, as shown in Fig. 2. Residual
stress distributions near the toe and UIT lines were
measured in the specimen longitudinal direction by an
X-ray stress analyzer, Xstress 3000, using the X-ray
diffraction (sin2ψ) method, where the collimator size wasset at 3mm. Compressive residual stress was dominant near
the UIT lines although tensile residual stress was spreadnear the as-welded toe, as shown in Fig. 3.
Structural model fatigue tests were conducted by using
the 2.5MN large fatigue test machine in air at room
temperature, and the load was applied with sinusoidal
waveform at the frequency of 0.5 to 1.0Hz. The nominal
stress range, Δσ, was simply obtained in the longitudinal
direction by the four point bending calculation of an
I-section beam, and the stress ratio was set at 0.1. The local
strain range at 5mm away from the center of a UIT line or
the weld toe was measured in the longitudinal direction
through strain gauges. The number of cycles to crack
initiation, Ni, was defined as the fatigue life when the local
strain range dropped by five percent of the initial local
strain range. Fatigue crack initiation was also detected bydye penetrant testing.

process of fabricating large-scale welded structures such as
wind turbines, the plates are often cut by plasma arc
cutting machines, where the fatigue strength of a quenchedand hardened cut surface is probably affected by thechemical composition of steel [7]. Accordingly, anotherfatigue solution is steel for plasma arc cutting, and thethermomechanical control process, TMCP, is available formaking the steel. In this study, two kinds of TMCP steel
plates, YP460 and KD36, were used for notched plasmaarc cut specimens, and their fatigue properties were
investigated.
II. EXPERIMENTAL AND ANALYTICAL METHODS
A. Materials
Two kinds of high strength steel plates were used for
fatigue tests in this study. One was YP460, whose
specified minimum yield stress, SMYS, was 460MPa, andthe other was KD36, which was based on the rolled steel
plates for shipbuilding ruled by the Japan shipclassification society, ClassNK, and its SMYS was355MPa. Both steel plates were manufactured by TMCP.
Tables 1 and 2 show the chemical compositions andmechanical properties of the plates. YP460 was used forlarge-scale structural fatigue specimens, whose plate
thicknesses were 25mm and 50mm, while two kinds ofsteel plates, YP460 and KD36, were used for notched
plasma arc cut specimens, which had the identical plate
thickness of 25mm.

C. Plasma Arc Cut Surface Fatigue Tests
Notched plasma arc cut specimens were sampled in theplate longitudinal direction by using a CNC plasma arc
cutting machine, and its condition is shown in Table 3. A
plasma arc penetrated through the plate thickness, and theplasma arc moved along the outline of the specimens. Five
specimens were cut out from each steel plate. Fig. 4
illustrates a notched plasma arc cut specimen used in thisstudy. The side notch radius was 190mm, and the stressconcentration factor, Kt, was 1.1.
Plasma arc cut surface fatigue tests were carried out by
using a 1.0MN fatigue test machine in air at room
temperature, and the load was applied with sinusoidal
waveform at the frequency of 3 to 10Hz. The net section
nominal stress range, Δσ, was calculated in thelongitudinal direction, and the stress ratio was 0.1. The
number of cycles to failure, Nf, was defined as the fatiguelife.

B. Structural Model Fatigue Tests
Fig. 1 shows a schematic floating wind turbine and alarge-scale structural fatigue specimen. Most of floating
wind turbine structures have brackets between a tower anda floating body, and a bracket end was selected as thelocation of fatigue crack initiation in the specimen. The
loading type of the specimen was four point bending, andthe total length of the specimen was 5,500mm. Flux coredarc welding, FCAW, was performed with wires of SF-60(JIS Z3313 T59J1T1-1CA-N2M1- UH5, AWS A5.29
E81T1-G) using CO2 as the shielding gas. The number ofpasses was three, and the leg length was about 15mm. Fourlarge-scale structural specimens were manufactured; twowere examined for welds with UIT, and the others were foras-welded toes for comparison. As UIT equipment,

Table 1 YP460 steel plates used for large-scale structural fatigue specimens.
Chemical compositions (%)

Thickness
(mm)
25
50

C

Si

Mn

P

S

0.09
0.09
0.10

0.29
0.29
0.05

1.51
1.51
1.42

0.013
0.012
0.009

0.002
0.002
0.004

Yield stress
(MPa)
564
584
527

Mechanical properties
Tensile strength
(MPa)
623
657
620

Elongation
(%)
20
19
19

Table 2 Steel plates used for plasma arc cut surface fatigue specimens.
Steel

Thickness

YP460
KD36

(mm)
25
25

C

Chemical compositions (%)
Si
Mn
P

0.09
0.13

0.29
0.22

1.51
1.18

0.013
0.011

2

S
0.002
0.003

Yield stress
(MPa)
564
411

Mechanical properties
Tensile strength
Elongation
(MPa)
(%)
623
20
519
23

As-welded or
UIT bracket end
200

390

As-welded or UIT bracket end

600

50t

25t

15

2,450

410

200

15

250

1,400

2,200

360

5,500

(Unit:mm)

Fig. 1 A schematic floating wind turbine and a large-scale structural fatigue specimen.

Plasma arc cut

Kt=1.1

Fig. 2 Bracket end sections measured usinga laser displacement sensor.

700
t=25 (Unit:mm)

500

Fig. 4 Shape and size of notched plasma arc cut
fatigue specimen.

On a flange plate of an as‐welded specimen

400
Residual stress (MPa)

50

150

R1

90

Table 3 Plasma arc cutting condition.
Machine
CNC plasma arc cutting machine
Tip bore dia.
2.5mm
Plasma gas flow rate
O2:32.5L/min
Assist gas flow rate
O2:17.5L/min, Air: 17.5L/min
Plasma current
260A
Piercing height
12.0mmTorch height
5.0mm
Cutting speed
1350mm/min

300
200
On a rib plateof an UIT
specimen

100
0

MSC Marc 2010r2. A quarter of a large-scale structural
specimen was modelled considering the symmetry
conditions, as shown in Fig. 5, and the load of 1kN was
applied to the inner two points of the model. Four-nodeisoparametric quadrilateral shell elements were used forthis modeling. The structural hot spot stress, σhs, wasdetermined in the vicinity of the bracket end according to
the IIW fatigue design recommendations [8]. Two kinds ofsurface stress extrapolation methods were employed; one
was fine mesh Type-a using two reference points of 0.4t
and 1.0t (t is the plate thickness) for the weld toe on theflange plate, and the other was fine mesh Type-b using
three reference points of 4mm, 8mm and 12mm for theweld toe at the rib plate edge. Meshing around the bracket
ends was performed considering the surface stressextrapolation methods of IIW [8].

-100
-200
On a flange plateof an UIT specimen

-300
-400
-500
0

5
10
15
20
Distance from an UIT line or a weld toe (mm)Fig. 3 Residual stress distribution.
D. Finite Element Analysis
Three-dimensional elastic finite element analysis was
conducted using a commercial finite element analysis code,3III. RESULTS
A. Structural Model Fatigue Tests
Fig. 6 demonstrates the initial local strain changes of
two structural specimens for Δσ=100MPa. Local plastic
deformation around the bracket end was larger in the
as-welded specimen than that in the UIT specimen, and theload versus local strain relation was almost linear in the
UIT specimen.
Fig. 7 shows the relationships between Δσ and Ni,
where two data points were obtained for each UIT
specimen because a cracked UIT line was mended byrepair welding and the other survived UIT line wasre-tested in the UIT specimen. However, two or four data
points were not sufficient to fit a regression S-N curve. The
slope, m, of a S-N curve was accordingly determined to be
the same values as those of DNV-RP-C203 [9], m=3 for
as-welded joints and m=5 for hammer peened welded
joints. The S-N curves were fitted using the following
equation, where logܽത is the intercept of the logNi-axis by
the S-N curve.
logNi = logܽത − m·logΔσ.

Fig. 5 Modelling and meshing of a large-scale structural
fatigue specimen.
700
600

All data points of UIT specimens were located in thehigher fatigue strength and longer fatigue life region thanas-welded specimens.
Fig. 8 shows bracket end appearances after the fatigue
tests. Fatigue cracks initiated from indented surfaces at rib
plate edges of the UIT specimens although fatigue cracks
initiated from weld toes on flange plates of the as-welded
specimens.
Analytical results were demonstrated in Fig. 9 andFig.10, where the maximum principal stress distributions areshown. The extrapolation of surface stress along the rib plategave a higher structural hot stress than that on the flange
plate, and the relationships between Δσhs and Ni are shownin Fig. 11, where a fatigue design S-N curve against thestructural hot spot stress of bracket ends, FAT 100 in the
IIW fatigue design recommendations [8], is also indicatedjust for reference. All data points of UIT specimens werelocated in the higher fatigue strength and longer fatigue life
region than as-welded specimens.

500

Load (kN)(1.1)

Δσ = 100MPa, R=0.1, In air at R.T.
UIT specimen

400
300
200
As‐welded specimen

100
0
0

500

1,000

1,500

Local strain (×10−6)

Fig. 6 Local strain changes of two large-scale structural
specimens for Δσ=100MPa.

Nominal stress range, Δσ (MPa)200

B. Plasma Arc Cut Surface Fatigue Tests
The relationships between Δσ and Nf are shown in Fig.
12. Four of five specimens were fractured by fatigue crack
growth from the notched plasma arc cut surface, and one
specimen with s in the figure of each TMCP steel plate wasfractured by fatigue crack growth from the original platesurface. The fatigue strength of steel base metal is
proportional to the static tensile strength of the steel unlessthe cut surface is not deteriorated. Accordingly, S-N dataof two machine cut plates, SM58Q [10] and SM50B [11],were referred to for comparison. The static strengths of
SM58Q and SM50B are almost equivalent to those of
YP460 and KD36, respectively. S-N data of plasma arc cut
YP460 were closely located to those of machine cut
SM58Q. In addition, the fatigue strength of plasma arc cut
YP460 was higher than that of machine cut SM58Q in thelong fatigue life region. This relation was identical inplasma arc cut KD36 and machine cut SM50B.

UIT specimens
m=5

100
80
60

m=3

40

20
4
10

Structural models
YP460
R=0.1 in air at R.T.

As-welded
specimens

5

10

10

6

Number of cycles to crack initiation, Ni
Fig. 7 Relationships between Δσ and Ni
for large-scale structural specimens.

4

Hot spot stress range, Δσ hs (MPa)

1000
800
600
400

Crack
Crack
(a) UIT specimen(b) As-welded specimen

Fig. 8 Bracket end appearances after fatigue testing.

IIW
UIT specimens
m=5

FAT100
m=3

200
100
80
60
40
20

m=3
As-welded
specimens
Structural models
YP460
R=0.1 in air at R.T.

10
4
10

5

6

10

10

Number of cycles to crack initiation, Ni
Fig. 11 Relationships between Δσhs and Ni
for large-scale structural specimens.

Nominal stress range, Δσ (MPa)

700
600
500
s

400
Base metal specimens
R=0.1 in air at R.T.

300
4
10

Fig. 9 Principal stress contour diagram.

YP460 / plasma arc cut
SM58Q / machine cut(NRIM FDS No.11)

5

6

10

7

10

10

Number of cycles to failure, Nf(a) YP460 and SM58Q

600
Nominal stress range, Δσ (MPa)

0.6
0.5

σ1 (MPa)

0.4

IIW fine mesh type‐b
0.3
0.2

IIW fine mesh type‐a
0.1

0

10

20

30

40

50

400
s

300
Base metal specimens
R=0.1 in air at R.T.

200
4
10

0
60

Distance from a weld toe (mm)KD36 / plasma arc cut
SM50B / machine cut(NRIM FDS No.5)

500

5

10

6

10

Number of cycles to failure, Nf(b) KD36 and SM50B

Fig. 10 Principal stress extrapolation.

Fig. 12 Relationships between Δσ and Nf
for base metal specimens.

5

7

10

IV. DISCUSSION

V. CONCLUDING REMARKS

A. Effect of UIT on Fatigue Property
As shown in Fig.3, UIT induced compressive residual
stress near the treated bracket end, and as shown in Fig. 6,
the compressive residual stress reduced local plastic
deformation near the bracket end in the low stress range
region. The S-N curve of UIT specimens was consequently
located in the higher fatigue strength and longer fatigue life
region than that of as-welded specimens, and the effect of
UIT on fatigue property improvement was particularly
large in the low stress range and long fatigue life region, as
demonstrated in Fig. 7. For example, the extrapolated
fatigue strength of UIT specimens at 2×106 cycles is twice
as high as that of as-welded specimens.
The location of fatigue crack initiation changed fromweld toes on flange plates in as-welded specimens to rib
plate edges of UIT specimens, as shown in Fig.8. This ispossibly caused by difference in residual stress and in hot
spot stress.
UIT is effective in the improvement of fatigue
properties as far as this study shows. However, actualloading conditions of floating wind turbine structures arepotentially different from those in this study. The effect ofstress ratio, average stress, preload, etc. on fatigue property
improvement should be investigated for floating windturbine structures with UIT in the future.

Fukushima floating offshore wind farm demonstration
project has started, and two fatigue solutions, UIT and TMCP
steel for plasma arc cutting, were investigated in the project.
Large-scale welded models of floating offshore windturbine structures were manufactured using YP460 TMCP
steel plates, and the bracket ends of the models were treatedwith UIT equipment. The effect of UIT on fatigue properties
was examined by using the 2.5MN large fatigue test machine,and UIT significantly improved fatigue life particularly in the
low stress range and long fatigue life region.
In addition, notched fatigue specimens were sampled byplasma arc cutting from two TMCP steel plates, YP460 and
KD36, and their fatigue properties were investigated. In each
TMCP steel plate, fatigue strength was not degraded on theplasma arc cut surface.
ACKNOWLEDGEMENTS
This study was conducted in Fukushima Floating Offshore
Wind Farm Demonstration Project, and supervised by the
Ministry of Economy, Trade and Industry in Japan. Useful
discussions in the project are greatly acknowledged.
REFERENCES

B. Steel for Plasma Arc Cutting
In each TMCP steel plate, YP460 or KD36, fatigue
strength was not degraded on the plasma arc cut surface,
and it was equivalent to or higher than that of the machine
cut surface, as shown in Fig. 12. In addition, S-N data
points of YP460 and KD36 are plotted in Fig. 13. The
plasma arc cut surface of YP460 had higher fatigue
strength than that of KD36. This suggests that the highstrength of TMCP steel plates is advantageous to the highfatigue strength of the plasma arc cut surface even in the
as-cut condition.

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Demonstration Project - Current state and prospective,”
J. Japan Wind Energy Association, vol.36, No.4, pp.
553-556, 2013, (in Japanese).
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methods for increase in fatigue strength of welded
joints,” IIW Doc., XII-1668-97, 1997.
[3]P. J. Haagensen, E. S. Statnikov and L. Lopez-Martinez,“Introductory fatigue tests on welded joints in highstrength steel and aluminium improved by various
methods including ultrasonic impact treatment (UIT),”IIW Doc., XIII 1748-98, 1998.
[4] T. Nose and H. Shimanuki, “Experiment and analysis
of influence of ultrasonic peening on fatigue life in pad
welded joint,” Transactions of the Japan Society of
Mechanical Engineers, Series A, vol.74, No737,
pp.166-167, 2008, (in Japanese).
[5] T. Mori, H. Shimanuki and M. Tanaka, “Effect of UIT
on fatigue strength of web-gusset welded joints
considering service condition of steel structures,”
Welding in the world, vol.56, pp.141-149, 2012.
[6] T. Okawa, H. Shimanuki, Y. Funatsu. T. Nose and Y.
Sumi, “Effect of preload and stress ratio on fatigue
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impact treatment,” Welding in the world, vol.57,
pp.235-241, 2013.
[7] H. Yajima, Y. Kho, T. Ishikawa, M. Minagawa, K.
Hirota and T. Kubo, “A study on fatigue strength in airof plasma arc cut steel plate,” Transactions of theWest-Japan Society of Naval Architects, vol.105, 2003,
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[8] A. Hobbacher, "Recommendations for fatigue design ofwelded joints and components," WRC Bulletin 520,
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1254r4-07, 2009.

Nominal stress range, Δσ (MPa)700
YP460 / plasma arc cut
KD36 / plasma arc cut

600
500

s

400
s

300

Base metal specimens
R=0.1 in air at R.T.

10

4

5

10

6

10

7

10

Number of cycles to failure, Nf
Fig. 13 S-N data points of YP460 and KD36.

6

[9] Det Norske Veritas (DNV), "Fatigue design of offshore
steel structures," DNV-RP-C203, 2011.
[10]National Research Institute for Metals (NRIM), "Datasheets on fatigue properties of butt welded joints of
SM58Q rolled steel for welded structure – Effect ofspecimen size–,” NRIM fatigue data sheet No.11, 1979,(in Japanese).
[11]National Research Institute for Metals (NRIM), "Datasheets on fatigue properties for butt-welded joints of
SM50B high tensile structural steel plates,” NRIM
fatigue data sheet No.5, 1978, (in Japanese).

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