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Prediction of dynamic response of semi-submersible floating offshore wind turbine using Morison based theory
Shining Zhang , Takeshi Ishihara The University of Tokyo 2015/11/17
1
/12
• Report in Ministry of Environment (2011)
• Japan: 1600GW for offshore.
• But 80% are in deep water (>50m)
Potential WE in Japan and Challenges
2
0
100
200
300
400
500
0-10km 10-20km 20-30km
0-20m20-50m50-200m
Win
d e
nerg
y po
ten
tial (G
W)
Distance from coastline
Water depth
1. In deep water, the best choice is floating type of platform 2. In safe and cost effective design, accurate prediction of
dynamic response of FOWT is necessary
2MW7MW (completed)
• Wave-induced motion • Coupling system
/12
Coupling system
3
1. Morison equation is indispensable. 2. Conventional one is needed to be improved.
Potential theory(FAST) Morison equation(CAsT)
Rigid Elastic
Elastic
Computational efficiency Conservative prediction
Elastic Elastic
Elastic
× No elastic deformation × Bad tension prediction
Consistent coupling system Elastic deformation Accurate prediction
× Time consuming
Pro
s C
on
s
Analytical solution
/12
Learning objectives
4
1. Evaluate hydrodynamic coefficients through numerical simulation and validate them by one water tank test
2. Investigate and improve conventional Morison equation based theory
3. Show effects of dynamic behavior of mooring system on tension prediction
Wind Turbine
Platform
Mooring system
Data base
Response
AQWA
CFD
Data base
/12
Water tank experiment -Evaluate Cm and Cd
5
Towing Exp.
Force vibration
00.20.40.60.8
11.21.41.61.8
2
0 0.2 0.4 0.6 0.8 1
Exp.
CFD
Equiv
ale
nt C
D
Towing speed (m/s)
0
0.5
1
1.5
2
2.5
3
1 1.4 1.8 2.2 2.6 3
Exp.
Eq
uiv
ale
nt C
m
Wave Period T(sec.)
0
0.5
1
1.5
2
2.5
3
1 1.4 1.8 2.2 2.6 3
Exp.
AQWA
Equiv
ale
nt C
m
Wave Period T(sec.)
0
0.5
1
1.5
2
2.5
3
1 1.4 1.8 2.2 2.6 3
Exp.AQWACFD
Eq
uiv
ale
nt C
m
Wave Period T(sec.)
• Validation of numerical simulation
/12
• Evaluation of hydrodynamic coefficients
Part 1-Conclusion for Hydrodynamic coefficients
6
1. Experiment: Accurate, but no distributed Cd and Cm. 2. Potential theory: Acceptable Cm, but no Cd 3. CFD: Accurate and distributed Cm and Cd.
Hydrodynamic Coefficients
Experiment Potential theory
CFD
Cd
Equivalent ○ × ○
Distributed × × ○
Cm
Equivalent ○ ○ ○
Distributed × ○ ○
/12
• hydrodynamic model
Part 2-Conclusion for Hydrodynamic model
7
FAST (NREL)
CAsT (Univ. of Tokyo)
Hydrodynamic model
Theory
Potential theory
Morison equation
Radiation damping ○ ×○
Axial Froude-Krylov force on members
○ ×○
Conventional Morison equation is needed to be improved
/12
• Free decay test
Hydrodynamic model - w.r.t radiation damping
8
Damping ratio Error: -40.5% 5.2% 1
{ } ( 1) ( ) ( )2
H w M w w dC C A F u X + u+ u X u X
Radiation damping is needed to be included
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 5 10 15 20 25 30 35 40
Exp.(3D)FAST
Sw
ay(m
)Time (sec.)
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 5 10 15 20 25 30 35 40
Exp.(3D)FASTCAsT-W/O Radiation
Sw
ay(m
)Time (sec.)
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 5 10 15 20 25 30 35 40
Exp.(3D)FASTCAsT-W/O RadiationCAsT-With Radiation
Sw
ay(m
)Time (sec.)
[ ]RdtnC X
Water tank Exp.
, Regular wave, Irregular wave
AQWA
/12
Response to sea states
9
Regular wave Irregular wave
Results from proposed Morison equation shows good agreement with that from Exp. in dynamic motion
0
0.2
0.4
0.6
0.8
1
1 1.5 2 2.5 3
Pitch
a/
a
Wave period (sec.)
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Po
we
r S
pe
ctr
um
-Hea
ve [m
2s]
Frequency (Hz)
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Po
we
r S
pe
ctr
um
-Pitch
[ra
d2s]
Frequency (Hz)
Pitch
Axial direction: 2 2
4 4
j iz j i
D DF p p
0.4%
0
0.5
1
1.5
2
2.5
3
1 1.5 2 2.5 3
Exp.(3D)FAST
He
ave Z
a/
aWave period (sec.)
0
0.5
1
1.5
2
2.5
3
1 1.5 2 2.5 3
Exp.(3D)FASTCAsT-W/O DynPre
He
ave Z
a/
aWave period (sec.)
0
0.5
1
1.5
2
2.5
3
1 1.5 2 2.5 3
Exp.(3D)FASTCAsT-W/O DynPreCAsT-With DynPre
He
ave
Za/
aWave period (sec.)
Heave
Error:40%
Pitch
Heave
/12
Part 3- Mooring system
-1
-0.5
0
0.5
1
45 50 55 60
Exp.3D(T1)CAsT (T1)
Time (sec.)
Ten
sio
n T
1 (
N)
10
T1
Analytical solution FEM Model
-1
-0.5
0
0.5
1
45 50 55 60
Exp.(T1)CAsT (T1)
Time (sec.)T
ensio
n T
1 (
N)
Analytical solution
1. Amplitude of tension is improved in dynamic model 2. High frequency components are reproduced
/12
Part 3- Mooring system
11
Error: 56%
T1
Analytical solution FEM Model
14% In tension prediction, dynamic modelling shows good agreement with experiment
0
0.2
0.4
0.6
0.8
1
1 1.5 2 2.5 3
Exp. (T1)
CAsT-Quasi-static model(T1)
FAST
T1/(
gA
2)
Wave Period (sec.)
0
0.2
0.4
0.6
0.8
1
1 1.5 2 2.5 3
Exp. (T1)
CAsT-Dynamic model(T1)
T1/(
gA
2)
Wave Period (sec.)
Analytical solution
/12
Prediction of dynamic response of semi-submersible FOWT using Morison equation based theory was conducted in this research. 1. To use Morison equation based theory. AQWA was used to
evaluate radiation damping and Cm. But Cd was evaluated by CFD. Both Cm and Cd were validated by water tank experiment.
2. Conventional Morison equation is improved and proposed Morison equation regarding radiation damping and axial Froude-Krylov force shows good agreement with experiment in dynamic response.
3. Hydrodynamic force on mooring line should be included in tension prediction analysis and dynamic modelling mooring system can give good agreement with experiment .
Conclusions
12
/12 13
Thanks for your attention
Acknowledgment: This research is funded by Ministry of Economy, Trade and Industry, Japan. I wish to express my deepest gratitude to the concerned parties for their assistance and contribution in this research.