lecture scour on offshore wind turbines
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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
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