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

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• 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

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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

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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

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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

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• 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 × ○ ○

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• 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

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• 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

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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

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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

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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

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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

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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.