Geotechnical Lectures Evening

Delft University of Technology

Faculty of Civil Engineering & Geosciences, TU Delft

*Formerly School of Civil Engineering, University College Dublin

Scour

Lecture

Scour on Offshore Wind Turbines

Luke J. Prendergast PhD

15th November 2016

Scour Erosion - Introduction

2

1. Introduction to Scour 2. Research Approach 3. Experimental Analysis 4. Numerical Modelling 5. Full-Scale Turbine Modelling 6. Summary

Scour Erosion - Introduction

3

Wake Vortices

Bridge Pier

Scour Hole

Downflow

Horseshoe Vortices

Source: http://www.nortek-as.com/images-repository-1/scour/the-scour-monitor/image_preview

Wind Turbines – Dynamically Sensitive

4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

Pow

er

Spectr

al

Densit

y

1P 3P

Frequency (Hz)

Soft -

Stiff

Stiff -

Stiff

Rotor Spinning Blade Passing

Wave Spectrum

Soft -

Soft

Scour Erosion – Research Method

5

Establish effect of scour on dynamic characteristics of

monopile supported turbines

EXPERIMENTAL

Flexible Pile L/D~20 Stiff Pile L/D~5

NUMERICAL

Develop Winkler model using site specific soil data

NUMERICAL

Investigate scour effect on full-scale

turbine

Scour Erosion - Experimental

6

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Scour Erosion - Experimental

7

0 0.5 1 1.5 2-100

-50

0

50

100

Time (s)

Acc

eler

atio

n (

m s

-2)

10 20 30 40 50 600

50

100

150

200

250

300

Frequency (Hz)

Mag

nit

ude

Scour Level -6 Scour Level -6

Scour Level -4

(a) (b)

0 0.5 1 1.5 2-100

-50

0

50

100

Time (s)

Acc

eler

atio

n (

m s

-2)

10 20 30 40 50 600

50

100

150

200

250

300

Frequency (Hz)

Mag

nit

ude

Scour Level -6 Scour Level -6

Scour Level -4

(a) (b)

Impact with hammer

Calculate Frequency

using Fourier Analysis

Change in Natural Frequency

between two scour depths

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Scour Erosion - Experimental

8

0 5 10 15 20 25 30 35 40-7

-6

-5

-4

-3

-2

-1

0

Frequency (Hz)

Sco

ur

dep

th (

m)

Experimental frequency

Fixed cantilever frequency

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Blessington Sand

Initial Level

22

60

65

00

Scour Level -1

Scour Level -2

Scour Level -3

Scour Level -4

Scour Level -5

Scour Level -6

Scour Level -7

Scour Level -8

Scour Level -9

Scour Level -10

Scour Level -11

Scour Level -12

Base Level

500

Pile

500

500

10

00

A A

Section A-A

R170

R157Accelerometer 4

Accelerometer 3

Accelerometer 2

Accelerometer 1

87

60

Scour Erosion - Experimental

9

0.8

m2.2

m

Blessington

sand

Scour depth -8

Scour depth -7

Scour depth -6

Scour depth -5

Scour depth -4

Scour depth -3

Scour depth -2

Scour depth -1

Initial Level 0.2

m

Monopile

3 m

R 0.17 m

R 0.156 m

1.8

m

0.3

0.3

0.0

75 m

Accelerometer

Reference accelerometer

0.3

Scour depth -9

A A

Section A-A

0.8

m2.2

m

Blessington

sand

Scour depth -8

Scour depth -7

Scour depth -6

Scour depth -5

Scour depth -4

Scour depth -3

Scour depth -2

Scour depth -1

Initial Level 0.2

m

Monopile

3 m

R 0.17 m

R 0.156 m

1.8

m

0.3

0.3

0.0

75 m

Accelerometer

Reference accelerometer

0.3

Scour depth -9

A A

Section A-A

0 10 20 30 40 50 60-6

-5

-4

-3

-2

-1

0

Frequency (Hz)

S/D

pil

e

+/- freq,expt

Experimental results

Scour Erosion – Numerical Model

10

0.8

m2.2

m

Blessington

sand20 S

pri

ng-B

eam

Ele

men

ts

7 Beam Elements

R 0

.17 m

R 0.156 m

Section A-A

A A kS,1

kS,2

kS,N

tF

tF

tF

tx

tx

tx

K

tx

tx

tx

C

tx

tx

tx

M

N

2

1

N

2

1

G

N

2

1

G

N

2

1

G

0 20 40 60 80 100 120

-2

-1.5

-1

-0.5

0

Shear modulus (G0) (MPa)

Depth

(m

)

G0 from shear wave

G0 from CPT q

c

Small-strain soil stiffness from site used to derive soil spring stiffness

0 , 11

11,,

isis kkis,K

Euler-Bernoulli Beam Elements

Winkler Spring Elements

Scour Erosion – Numerical Model

11

0 0.5 1 1.5 2-2

0

2

Acc

eler

atio

n (

g)

15 20 25 30 35 400

0.01

0.02

0 0.5 1 1.5 2-2

0

2

15 20 25 30 35 400

0.01

0.02

FR

F (

g N

-1)

15 20 25 30 35 400

0.01

0.02

0 0.5 1 1.5 2-2

0

2

15 20 25 30 35 400

0.01

0.02

0 0.5 1 1.5 2-2

0

2

Time (s)

15 20 25 30 35 400

0.01

0.02

Frequency (Hz)

0 0.5 1 1.5 2-2

0

2

EXPT

BIOT

EXPT

VESIC

EXPT

M&B

EXPT

K&G

EXPT

SELVADURAI

EXPT

BIOT

EXPT

VESIC

EXPT

M&B

EXPT

K&G

EXPT

SELVADURAI

(a) (b)

24.9 Hz

23.93 Hz

26.12 Hz

27.83 Hz

24.66 Hz

20.26 Hz

20.26 Hz

20.26 Hz

20.26 Hz

20.26 Hz

108.0

2

4

0

2

0

)1()1(

95.0

EIv

DE

vD

Ek

ss

s

12/14

0

2

0

)1(

65.0

EI

DE

vD

Ek

s

s

)1( 2

0

s

svD

Ek

)1(

2 0

s

svD

Ek

)1(

65.02

0

s

sv

E

Dk

Subgrade Reaction Models

[1]

[2]

[3]

[5]

[4]

Scour Erosion – Numerical Model

12

Two methods used to derive site-specific soil data

0 10 20 30 40 50

-8

-7

-6

-5

-4

-3

-2

-1

0

Cone tip resistance qc (MPa)

Dep

th (

m)

150 200 250 300

-8

-7

-6

-5

-4

-3

-2

-1

0

Shear wave velocity (m s-1)

Dep

th (

m)

Shear wave

velocityMax qc

(a) (b)

Min qc

Average qc

[1] Correlation to Cone Penetration Test data

[2] Correlation to Shear Wave Velocity data

2

0 svG

cqG 60

Scour Erosion – Numerical Model

13

0 5 10 15 20 25 30 35 40-7

-6

-5

-4

-3

-2

-1

0

Frequency (Hz)

Sco

ur

dep

th (

m)

Experimental frequency

Numerical (shear wave) frequency

Numerical (CPT) frequency

Numerical (API) frequency

Fixed cantilever frequency

Pile with L/D ~ 20

0 10 20 30 40 50 60 70 80-6

-5

-4

-3

-2

-1

0

Frequency (Hz)

S/D

pil

e

+/- freq,expt

Experimental results

Shear wave numerical model

CPT numerical model

Pile with L/D ~ 5

12/14

0

2

0

)1(

EI

DE

vD

Ek

s

s

Scour Erosion – Turbine Model

14

Ks,1

Ks,Ns

MTop, J

MTransition

Am, Im

A1t, I1t

ANt, INt

Section A-A

R2.91 m

R3 m

A A

Mud line

Mean sea level

Monopile

Tower

Nacelle & blades

30 m

30 m

15 m

75 m

70 m

Taper

ed

Nsp

ring

s =

60

Np

ile

= 1

50

Nto

wer

= 1

40

0 50 100 150 200-30

-25

-20

-15

-10

-5

0

G0 profile (MPa)

Dep

th (

m)

0 10 20 30 40-30

-25

-20

-15

-10

-5

0

Cone tip resistance qc (MPa)

Dep

th (

m)

Loose qc

Medium dense qc

Very dense qc

Loose G0

Medium dense G0

Very dense G0

(b)(a)

Representative qc profiles

created

Loose sand

Med dense sand

Dense sand

Scour Erosion – Turbine Model

15

0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31

-1.5

-1

-0.5

0

Frequency (Hz)

S /

Dp

ile

Loose sand freq

Med dense sand freq

Very dense sand freq

0.86 0.88 0.9 0.92 0.94 0.96 0.98 1

-1.5

-1

-0.5

0

F = F/FNo scour

S /

Dp

ile

Loose sand F

Med dense sand F

Very dense sand F

1P frequency

Design scour depth (1.3Dpile

) = 7.8 m

Design scour depth (1.3Dpile

) = 7.8 m

Summary

16

- Scour has a significant effect on the natural frequency of a pilot-scaled monopile

- Offshore wind turbines are dynamically sensitive therefore scour may represent a significant risk to stability

- Scour can be combatted at design stage by allowing for an increased effective monopile length however the accurate specification of operational soil stiffness is imperative to the safe operation

- There is still uncertainty surrounding cyclic loading effects on operational stiffness

- As well as dynamic stability, scour has a significant effect on lateral ultimate capacity, not covered in this discussion

Acknowledgements

- Prof. Kenneth Gavin, Subsurface Engineering, TU Delft

- The Geotechnical Research Group at University College Dublin

Thank you for your attention!

17