The effect of breaking wave inducedCurrent on an Offshore Wind Turbine Foundation

Sung-Jin Choi Ove T. Gudmestad

University of Stavanger Stavanger NorwayUniversity of Stavanger, Stavanger, NorwayPresentation to Wind Power R&D seminar on 21 January 2011

ContentsContents

1 Introduction

2 Application of 3D numerical analysis

3 Determination of wave plus current forces

on an offshore wind turbineon an offshore wind turbine

4 Conclusions4 Conclusions

IntroductionIntroduction• For the design of an offshore wind turbine installed on a flat bottom, Morison

Equation utilizing a wave theory like Stream function theory has generally beenEquation, utilizing a wave theory like Stream function theory, has generally been

employed to determine wave forces acting on the structure for a given design

wave conditionwave condition.

Hd = ?T = ?HP=19.2 m

T=15 sec

HHWL

T = ?U = ?

F = ?58.6 m

M = ?Fig. 1 An offshore wind turbine installed on flat bottom

---------------------HP : Wave height at deepwater, Hd : Wave height at structural positionT : Wave period, F and M : Wave force and moment, U : Current velocity at structural position1 / 21

IntroductionIntroduction• In the case where an offshore wind turbine is installed nearby a submerged

shoal the waves may show unsymmetrical shapes or breaking patternsshoal, the waves may show unsymmetrical shapes or breaking patterns.

• Calculations of wave forces can be beyond the applicable range of Morison

equation and Stream function wave theory.q y

Hd = ?

HP=19.2 mT=15 sec

F = ?

T = ?U = ?

HHWL

M ?

F = ?

58.6 m

13.1 m

M = ?

Fig. 2 An offshore wind turbine installed nearby a submerged shoal

---------------------HP : Wave height at deepwater, Hd : Wave height at structural positionT : Wave period, F and M : Wave force and moment, U : Current velocity at structural position2 / 21

Introduction• Chun et al. (1999) performed three-dimensional hydraulic model tests to measure

the wave plus current forces and wave heights nearby a submerged shoal.

Introduction

• The directions of the waves were adopted for four cases (NNW,SSW, S and SE).

ve makerHp= 0.205 m

Waves from NNW-direction

N

B

NNWWave

m

Wave guider

pT= 1.37 sec

B'S

SSW

SE

48

48 m Wave absorber

Waves from S-direction

Structural position

B'

HHWL

0.489 m 0.363 m

B B'Eardo

Fig. 3 Plane layout of experiments (scale : 1/120) Fig. 4 Photograph of experiments3 / 21

---------------------- 3D hydraulic model tests were performed at a small scale (1/120) in a wave tank at the Korea Institute of Construction Technology.

IntroductionIntroduction• Waves from the NNW-direction were the only case where breaking waves occurred

before the waves propagated over the structural positionbefore the waves propagated over the structural position.

• The measured signals of wave forces showed irregular shapes which tended to

one side either positive or negativeone side, either positive or negative.

0.0 40.0 80.0 120.0

20 0

0.0

20.0

Waves from NNW-direction-20.0

0.0 40.0 80.0 120.0

-10.0

0.0

10.0

20.0

a es o d ect o

0.0 40.0 80.0 120.0

0.0

10.0

0.0

20.0

Breaking wavePositive

0.0 40.0 80.0 120.0

10 0

0.0

10.0

0.0 40.0 80.0 120.0

-20.0

0.0

Negative

-10.0

Fig. 5 Time histories of measured data from wave sensors and wave force meter : Waves from NNW-direction

4 / 21

IntroductionIntroduction• In spite of that the wave heights nearby (on lee side of) the submerged shoal

d t b ll d ith th h i ht fl t b tt thappeared to be small compared with the wave heights on a flat bottom, the

measured wave forces rather exceeded the wave forces on a flat bottom which

were calculated by SACS

SACS (peak)

were calculated by SACS.

15.0

20.0

Exp (peak): AExp (peak): GWaves from NNW-direction

10.0

15.0

Wav

e fo

rce

(N)

Breaking wave

0.0 20.0 40.0 60.0 80.0 100.0

0.0

5.0

WOrientation of structure (Deg.)

Fig. 6 The horizontal wave forces from SACS and experiment : Waves from NNW-direction---------------------SACS : Structural Analysis Computer System (EDI,1995)5 / 21

IntroductionIntroduction• The results suggested that the breaking waves might have induced the strongcurrent forcescurrent forces.

• For an offshore wind turbine is installed nearby a submerged shoal, the use ofwaves only may result in an underestimated design of the structure.

The objectives of the present research,

1. The presence of breaking wave induced current will be clarified.

2 3D numerical analysis will be carried out to quantify the wave height and2. 3D numerical analysis will be carried out to quantify the wave height and

current velocity at the structural position.

3. The wave plus current forces, wave forces without current and wave forces

on a flat bottom will be calculated and compared.

4. The design wave forces acting on the structure will be determined.

6 / 21

Application of 3D Numerical analysispp y• Computational domain

- Sponge layers are located to the left right upper and lower sides with aSponge layers are located to the left, right, upper and lower sides with athickness of 2L.

- The internal wave generator is located in front of the sponge layer which is

located to the upper part of the computational domain.

55.0

2L

2L

44 m2L

50.0

40.0

35 0

45.0

-0.489m-0.489m

2L

5LInternal wave generator

N-direction

Y (m

) 35.0

25.0

20.0

30.08 m

5.0

10.0

15.0 -0.109m-0.109m8L

2L

Fig. 7 Computational domain for 3D modelX (m)

55.045.0 50.040.035.030.025.020.015.010.05.00.00.0

2L

Sponge layer---------------------

: Wave lengthL7 / 21

Application of 3D Numerical analysis• Input conditions

T bl 1 I t diti f 3D d l

pp y

(m) (sec)(m)

/(m) (m)

pH Tbh th

hx ty

Table 1 Input conditions for 3D model

0.16 1.37 - 0.489 - 0.109 0.1 / 0.1 0.02

55.0

35.0

40.0

45.0

50.0

0 24-0.22-0.2-0.18-0.16-0.14-0.12-0.1

0 24-0.22-0.2-0.18-0.16-0.14-0.12-0.1

15.0

20.0

25.0

30.0

Y (m

)

0 46-0.44-0.42-0.4-0.38-0.36-0.34-0.32-0.3-0.28-0.26-0.24

-0.46-0.44-0.42-0.4-0.38-0.36-0.34-0.32-0.3-0.28-0.26-0.24

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0

X (m)

0.0

5.0

10.0 -0.5-0.48-0.46

-0.5-0.48

X (m)

Fig. 9 3D perspective for 3D modelFig. 8 Depth contour for 3D model---------------------

, : Incident wave height and Wave period , : Grid spacing (distance), : Grid spacing (time), : Water depth at the bottom and top of submerged shoalbh th

TpH x ty8 / 21

Application of 3D Numerical analysis• Wave heights

pp y

Fi 10 h th di t ib ti f th h i ht l th h i t l li

0.30Section (B – B’)

- Fig. 10 shows the distribution of the wave heights along the horizontal lines.

- The wave heights are continuously reduced after breaking waves take place.

45.0

50.0

55.0

0 210.22

Wave Direction

N

40.00

0.10

0.20

Z (m

)

( )

30.0

35.0

40.0

(m)

0 110.120.130.140.150.160.170.180.190.200.21

B'B

C C'

0.0 10.0 20.0 30.0 40.0 50.0 60.0X (m)

0.20

0.30

Z (m

)

Section (C – C’)

15.0

20.0

25.0Y (

0.020.030.040.050.060.070.080.090.100.11

D D'0.0 10.0 20.0 30.0 40.0 50.0 60.0

X (m)

0.00

0.10Z

0.30

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.00.0

5.0

10.0-0.010.000.01

0 00

0.10

0.20

Z (m

)

Section (D – D’)

X (m) 0.0 10.0 20.0 30.0 40.0 50.0 60.0X (m)

0.00

DecreasingFig. 10 Horizontal section of wave heights along sections B – B’, C – C’ and D – D’ 9 / 21

Application of 3D Numerical analysis• Surface elevation

pp y

After the waves propagate over the top of the submerged shoal the wave

E

- After the waves propagate over the top of the submerged shoal, the wave

transformation occurs by breaking waves.

45.0

50.0

55.0Wave Direction

N

4

30.0

35.0

40.0

(m) 0.05

0.10

0.15

m)

15.0

20.0

25.0Y (

50 0 40 0 30 0 20 0 10 0 0 0

-0.15

-0.10

-0.05

0.00

h (m

0 0 5 0 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 00.0

5.0

10.0-50.0 -40.0 -30.0 -20.0 -10.0 0.0

Y (m)

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0

X (m) E'

Fig. 11 Surface image of the wave propagation and wave surface elevation10 / 21

Application of 3D Numerical analysis• Breaking wave induced current

pp y

- A strong current (0.391 m/s) is induced by breaking waves.

Current Velocity (m/s)

- The strong current exists in the form of a circular flow in the vicinity of theleft and right sides behind the submerged shoal.

Wave Direction

50.0

55.0

Current Velocity (m/s)

0.52 F

N

4

0.4

0.5

)

U = 0.391 m/s

35.0

40.0

45.0

4

0.1

0.2

0.3

U (m

/s)

20.0

25.0

30.0

Y (m

)

-50.0 -40.0 -30.0 -20.0 -10.0 0.0Y (m)

0.0

5.0

10.0

15.0

Fig. 12 Vector plot of breaking wave induced current

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0

X (m)

0.0

F'

11 / 21

Application of 3D Numerical analysis• Fig. 13 shows the variation of the determined wave heights and current velocities

at structural positions.

pp y

• After the breaking wave takes place, the wave heights continuously decrease ;

however, the current velocities continuously increase.

0.380m

1 32

0.3 0.5Point 1 Point 2 Point 3

Hd

36m 28m 20m

Description Point 1 Point 2 Point 3

0.2

Hd (

m)

0 2

0.3

0.4

(m/s

)

HU

Description Point 1 Point 2 Point 3

Hd (m) 0.139 0.067 0.027

0.0

0.1H

0.0

0.1

0.2 U

U (m/s) 0.023 0.391 0.2460.0

-36.0 -34.0 -32.0 -30.0 -28.0 -26.0 -24.0 -22.0 -20.0

Y (m)

0.0

Fig. 13 Variation of the determined wave heights and current velocities at the structural positions

---------------------Hd : Wave height at structural positionU : Current velocity at structural position

12 / 21

Determination of wave plus current forceete at o o a e p us cu e t o ce• The model structure is selected as a vertical cylinder (D = 0.025 m).

• The structural positions are adopted at three locations (point 1 point 2 and• The structural positions are adopted at three locations (point 1, point 2 and

point 3) over the submerged shoal.

45.0

50.0

55.0

-0.489

Wave Direction

D=0.025m

30.0

35.0

40.0

(m)

Point 1

Point 2

A

A'A

Point 3Point 2Point 120m28m36m

0.380m

10 0

15.0

20.0

25.0Y

0 109

Point 3

A'

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0

X (m)

0.0

5.0

10.0 -0.109

Fig. 14 Model structure and structural positions

X (m)

13 / 21

Determination of wave plus current forceete at o o a e p us cu e t o ce• Input conditions

Hd=0 139m Hd=0.067m Hd=0.027mHd=0.139mU=0.023m/s U=0.391m/s U=0.246m/s

- 0.109 m

Point 3Point 2

Point 1

- 0.489 m

Fig. 15 Input conditions for calculating wave plus current forces on the model structure

Table 2 Input conditions for calculating wave plus current forces on the model structure

Point 1 Point 2 Point 3

g p g p

Point 1 Point 2 Point 3

Hd (m) 0.139 0.067 0.027

hd (m) - 0.489 - 0.109 - 0.109

( )T (sec) 1.37 1.37 1.37

U (m/s) 0.023 0.391 0.246

D (m) 0.025 0.025 0.025

CD / CM 1.2 / 2.0 1.2 / 2.0 1.2 / 2.0---------------------Hd : Wave height at structural position, hd : Water depth at structural position, T : Wave periodU : Current velocity at structural position, D : Diameter, CD : Drag coefficient, CM : Inertia coefficient

14 / 21

Determination of wave plus current force (Point 1)

• For point 1, the wave plus current forces increased by about 12 % compared with

f

p ( )

the wave forces without current.

0.025m

0 380Hd=0.139mU=0 023m/s

Point 1

0.380m Structural positionU=0.023m/s

2.0Without CurrentWith C t 12%

0.8

1.2

1.6

F (N

)

With Current 12%

0.0 0.5 1.0 1.5 2.0 2.5 3.0

t (sec)

0.0

0.4

t (sec)Fig. 16 Comparison of the wave plus current forces and

wave forces without current at point 1 ---------------------Hd : Wave height at structural positionU : Current velocity at structural position15 / 21

Determination of wave plus current force (Point 2)

• For point 2, the wave plus current forces increased by about 245 % compared with

p ( )

the wave forces without current.

0.025m

Point 2

Hd=0.067mU=0.391m/s

Point 2Structural position

1 6

2.0Without CurrentWith Current 245%

0.8

1.2

1.6

F (N

)

245%

0.0 0.5 1.0 1.5 2.0 2.5 3.0

t (sec)

0.0

0.4

t (sec)Fig. 17 Comparison of the wave plus current forces and

wave forces without current at point 2 ---------------------Hd : Wave height at structural positionU : Current velocity at structural position16 / 21

Determination of wave plus current force (Point 3)

• For point 3, the wave plus current forces increased by about 218 % compared with

f

p ( )

Hd=0.027m

the wave forces without current.

0.025m

Point 3

U=0.246m/s

Point 3Structural position

0.4

0.5Without CurrentWith Current 218%

0.2

0.3

0.4

F (N

)

t Cu e t 218%

0.0 0.5 1.0 1.5 2.0 2.5 3.0

t (sec)

0.0

0.1

t (sec)Fig. 18 Comparison of the wave plus current forces and

wave forces without current at point 3 ---------------------Hd : Wave height at structural positionU : Current velocity at structural position17 / 21

Determination of wave plus current force• In spite of that the wave height at point 2 appeared to be small compared with

the wave height on a flat bottom, the wave plus current force increased by about

ete at o o a e p us cu e t o ce

g , p y

45 % compared with the wave force on a flat bottom.

• Forces in breaking wave situation only briefly estimated.

Hd=0.139mU=0.023m/s U=0.391m/s

Hd=0.067mU=0.246m/sHd=0.027m

Point 3Point 2

Point 1 45%

4.0

6.0

N)

Flat BottomWith CurrentWithout Current

45%

0 0

2.0F (N

0.0

Point 1 Point 2 Point 3Fig. 19 Comparison of the wave plus current forces, wave forces

without current and wave forces on flat bottom at all points---------------------Hd : Wave height at structural positionU : Current velocity at structural position18 / 21

Determination of design wave forceete at o o des g a e o ce

• The results show that the wave plus current forces greatly increased comparedith th f ith t t M th l t fwith the wave forces without current. Moreover, the wave plus current forces

rather exceeded the wave forces on a flat bottom.

• For the determination of the design wave forces on the structure which is• For the determination of the design wave forces on the structure which is

installed in the vicinity of the submerged shoal, the maximum wave forces

have to be selected after comparison of the wave plus current forces, wave

forces without current and wave forces on a flat bottom.

Table 3 Determined design wave forces on the model structures

DescriptionPoint 1 Point 2 Point 3

F (N) M (N-m) F (N) M (N-m) F (N) M (N-m)

Wave forces and 1 31 0 46 1 16 0 41 1 29 0 48Wave forces andmoment on flat bottom 1.31 0.46 1.16 0.41 1.29 0.48

Wave forces andmoments without current 1.28 0.45 0.49 0.05 0.11 0.01

Wave plus currentforces and moments 1.43 0.50 1.69 0.16 0.35 0.02

19 / 21

ConclusionsCo c us o s

• Three dimensional numerical analysis showed that a strong current (0.246 ~ y g (

0.391 m/s) can take place in the vicinity of the submerged shoal due to

radiation stress differentials given by the breaking waves.

• Comparison of the total forces on the structure without the current and with the

current showed that the wave plus current forces in this area increased by anp y

average of 200 % to 250 % compared with the wave forces without current.

• In spite of that the wave heights at point 2 appeared to be small compared with

the wave height on a flat bottom, the wave plus current force increased by about

45 % compared with the wave force on a flat bottom.p

• This can be attributed to the combined effect of waves and current which can be

induced by breaking waves.

20 / 21

ConclusionsCo c us o s

• For an offshore wind turbine installed on the lee side of a submerged shoal, g

the use of waves only (i.e., without current velocity) could result in the under-

estimated design of the structure.

• For the determination of the design wave forces on the structure which is

installed on the lee side of the submerged shoal, the maximum wave forcesinstalled on the lee side of the submerged shoal, the maximum wave forces

have to be selected after comparison of the wave plus current forces, wave

forces without current and wave forces on a flat bottom.

21 / 21

Th kThank you