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Foundations for Offshore Wind Turbines
NCTU, Hsinchu, Taiwan, 17 December 2014
Prof. Guy Houlsby
Department of Engineering Science, University of Oxford
Part 3: Foundation for offshore wind turbines
• Why offshore renewables?
• Challenges and solutions for offshore turbine
foundations
– Conventional, unconventional and completely novel solutions
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2
Increase of atmospheric CO2
April 2014 – first monthly average over 400 ppmv
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Rate of increase of atmospheric CO2
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UK oil and gas production
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source: DECC
5
Problem:
• Climate change due to fossil fuel use
• Diminishing supply of hydrocarbons
Solution:
• Nuclear
• Renewables
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Average wind speed
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Water depth
source: DTI Renewable Energy Atlas
7
First
round
sites
Blyth (2)
Barrow (30)
Scroby Sands (30)
North Hoyle (30)
Kentish Flats
(30)
8
Second
round
sites
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Greater
Wash
10
Offshore sites
Licensing Round 1 - 2001
(up to 1 GW)
Licensing Round 2 - 2003
(up to 7 GW)
Licensing Round 3 - 2010
(up to 32 GW)
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Total and offshore installed wind capacity
(approx. end of 2013)
Total installed
capacity (MW)
Offshore installed
capacity (MW)
China
US
Germany
Spain
India
UK
Italy
France
Canada
Denmark
75234
60007
31315
22785
18412
8292
8144
7564
6201
4162
390
0
520
0
0
3681
0
0
0
1271
…rest of world
40471
1090
16%
282587
6952
100%
Country
Total
6%
7%
53%
18%
sources: IEA Wind 2012 Annual Report
EWEA statistics 2013
4Coffshore
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Taiwan
K2 Management
“The project is part of the
government’s plan to set up 600
offshore wind turbines capable of
generating 3,000 megawatts by the
end of 2030”
Taipei Times
4COffshore
13
Blades: high strength
composites
Generator
Aerodynamics of blades
Gearbox
Control of blade pitch
Dynamics of tower
Forces from waves
and current
Foundation design
Electrical connections to
shore
14
Loading on wind
turbine
2MN
110m
4MN
40m
Beatrice Wind Farm
15
Loading on wind
turbine
H = 2MN + 4MN = 6MN
wind wave
M = 2 x (110 + 40) + 4 x 40
wind
wave
=
=
10MN
460MNm
6MN
300 + 160
460 MNm
V = 10MN
Beatrice Wind Farm
16
Foundation stiffness
• The main excitation
frequencies are 1P (the
rotational frequency) and 3P
(the blade-passing frequency)
• These must be avoided
• The flexibility of the
foundation reduces the
natural frequency
m
L
EI
k
1
fn 
2
1
 L3

2
m
 kL 
 3EI



Stiff-stiff design
DAF
1.0
1P
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3P
frequency
18
Soft-stiff design
DAF
1.0
1P
3P
frequency
Effect of foundation
DAF
1.0
1P
3P
frequency
Range of excitation frequencies
DAF
1.0
1P
3P
frequency
Loads on an
offshore turbine
foundation
H
H
V
V
V2
V1
M
H
H1
H2
V
S
22
Options for offshore foundations
• Single footing options
– Monopiles
– Monopod gravity bases
– Monopod caissons
• Multiple footing options
– Tripod or tetrapod?
– Piled foundations
– Caisson foundations
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Options for foundations
(a)
(b)
(c)
(d)
(e)
L
D
L
D
s
24
60
Foundation
type related
to size and
depth
Beatrice
Beatrice
Water depth (m)
London Array
50
Most future
developments?
40
Monopiles
30
20
10
Past
developments
Thanet
Barrow
Teesside
North Hoyle
Blyth
Scroby
Robin
Rigg
Walney
Sheringham
London
Ormonde
Lynn
Dowsing
Lincs
Gunfleet
Rhyl
Gunfleet 3
Burbo
Kentish
2
Walney 2
Gabbard
3
4
5
6
Turbine power (MW)
7
25
Nysted gravity foundations
• Simple
• Seabed
preparation
necessary
• Ballasting
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Installing a “monopile” foundation
Average 89
hours per pile
at North Hoyle
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Design issues for Monopiles
photo: Anholt Offshore Wind Farm
photo: Ciscon
• Oil and gas
Length: 30m - 80m
Diameter: 1m - 2m
L/D approx. 30 - 60
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• Offshore wind monopile
Length: approx. 30m
Diameter: 4m to 6m
L/D approx. 5 to 7
28
Delivery team:
PISA
PROJECT
Lead partner:
Partners:
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Walney Wind Farm
Photos from Dong Energy: Christian LeBlanc Thilsted and Dan Kallehave
30
Full Scale Results – Standard Systems
• Basis for developing modified design methodologies
Standard API p-y
formulation
Modified p-y
formulation
Data and from DONG Energy: Dan Kallehave and Christian LeBlanc Thilsted
31
PISA Project Overview
Figure from Christelle Abadie, PhD student funded by EDF; Photo of pile load test from Dr Ken Gavin, University College Dublin
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H
API/DNV vs. FE comparison - Long
180
FE
160
3
API/DNV
140
120
2.5
100
2
H (MN)
H (MN)
3.5
80
1.5
60
1
40
20
0.5
0
0
0
1
2
vground (m)
𝜂𝑢𝑙𝑡 = 47%
3
0
0.005
vground (m)
0.01
𝜂𝑠𝑑 = 31%
33
H
20
18
16
14
12
10
8
6
4
2
0
H (MN)
H (MN)
API/DNV vs. FE comparison – Short
0
2
4
vground (m)
𝜂𝑢𝑙𝑡 = 34%
6
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
FE
API/DNV
0
0.0005
vground (m)
0.001
𝜂𝑠𝑑 = 9%
34
Cowden Field Test Site
35
Borehole Campaign Cowden
36
Cowden CPT Measurements
0
5
CPT qc (MPa)
10 15 20 25
30
0
35
6
8
10
M3
S2
M2
M4
L2b
M5
2
4
Depth , z (m)
Depth , z (m)
4
600
0
0
2
CPT fs (kPa)
200
400
6
M3
S2
M2
M4
L2b
M5
8
10
12
12
14
14
16
16
37
Fibre optic strain gauges
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Cyclic loading tests
Reaction Frame
Motor
Mass
Mass
Mass
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
39
Approximately 100,000 cycles
1000
cycles
9000
cycles
90000
cycles
data supplied by Abadie
40
Stiffness increases with load cycles
One-way load cycles
zb = 0.20
zb = 0.27
zb = 0.40
zb = 0.53 Increasing
amplitude
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
41
Accumulated rotation
zb = 0.53
zb = 0.40
zb = 0.27
zb = 0.20
Increasing
amplitude

 static
 k  N 0.31
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
42
Effect of cycle type
Tb
Tc
One-way cycling

 static
M
1.0
MR
 Tb  Tc  N 0.31
Symmetric
cycling
M
MR
0.75
0.5
0.5
0.0
0.25
-0.5
0
0
-1.0
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
43
Flow
Pressure
differential
W
Suction caissons
Installed by:
1. Self weight
2. Suction
Advantages:
• Less expensive equipment
for installation
• No pile driving noise
Flow
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photo: Universal Foundation A/S
44
Multiple foundation solutions
45
Tripod or tetrapod?
Tower
Tower
Tower
(a)
(b)
(c)
46
Piling arrangements
47
Multiple piles: Beatrice structures
48
Multiple caissons: Europipe platform
49
Main issues for suction caissons
• Can they be installed?
OK except:
– Very stiff or fissured clays
– Very coarse-grained soils
– Layered and other nonhomogeneous soils
Wind and
wave
• Tensile capacity
Tension
• Cyclic loading
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Compression
Tensile loading of caissons (sand)
Tension
51
Screw piles
• Small diameter shaft (D)
• Large diameter helical plates (Dp)
• Installed by twisting motion from
hydraulically driven torque-motor
• Some downward vertical load helps
installation
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Screw piles
Onshore:
• Used regularly for light
construction
• Quick and easy to install
Offshore:
• Why?
– Tension capacity
– Silent installation
– Torque measurement
helps confirm capacity
• Challenges:
– Scale up to much larger
sizes and capacities
– Develop installation
equipment
photograph: FLI
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Key Dimensionless Groups
• Geometry: Dp/D, s/Dp, N
V/(suDp2)
V/(g’Dp3)
• Installation (T = torque)
– clay:
– sand:
Vt
T
• Capacity
– clay:
– sand:
V or
Dp
s
T/(suDp3)
T/(g’Dp4)
D
• Key ratios: VDp/T , Vt/V
(not V/T as often currently used onshore)
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Summary data of screw pile experience
(model tests and onshore)
Source
Test type
Soil
Tsuha et al (2010)
Centrifuge
Sand
Rao et al (1991)
Sakr (2009)
Livneh and El Naggar (2008)
Ghaly et al (1991)
Cerato and Victor (2009)
Perko (2009)
Laboratory
Field
Field
Laboratory
Field
Various
Soft Clay
Oil Sand
Clayey Silt
Sand
Layered soil
Various
VtDp/T
Min Mean Max
6.0 8.3 12.5
6.4
3.2
2.6
1.6
5.2
8.0
5.0
14.4
8.5
Vt/V
0.64
0.52
10.9
6.1
23.3
24.6 0.8-0.96
(implied)
Tensile capacity x Diameter / Torque
55
Compressive capacity
Independent
plates
Envelope
56
Tension capacity
Independent
plates
Envelope
57
Compression and tension capacity
Total Bearing Load, kN
0
5000
10000
15000
20000
25000
30000
0
Pile Tip Depth (m)
5
10
Minimum - Compression
Independent - Compression
Interacting - Compression
Tension
15
20
25
30
35
58
Dimensionless torque ratio
Torque Ratio, VtDp/T
0
2
4
6
8
10
12
0
Pile Tip Depth (m)
5
10
15
20
25
30
35
59
Tension/compression capacity ratio
Tension/Compression capacity ratio, Vt /V
0
0.2
0.4
0.6
0.8
1
0
Pile Tip Depth (m)
5
10
15
20
25
30
35
60
Maplin Sands Lighthouse
(1838)
• Foundation designed by
Alexander Mitchell
•
•
•
•
•
9 screw piles into sand
1.2m (4 ft) diameter
0.125m (5 inch) shaft diameter
7m (22 ft) depth below mudline
Operated till 1931
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illustrations provided by Alan Lutenegger
61
Whether this broad spiral flange, or ‘Ground Screw’, as it
may be termed, be applied to the foot of a pile to support a
superincumbent weight, or be employed as a mooring to
resist an upward strain, its holding power entirely depends
upon the area of its disc, the nature of the ground into which
it is inserted, and the depth to which it is forced beneath the
surface.
The proper area of the screw should, in every case, be
determined by the nature of the ground in which it is to be
placed, and which must be ascertained by previous
experiment.
Mitchell “On Submarine Foundations”, 1848
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Conclusions
• Offshore wind will be a key element of the UK’s
energy mix
• Larger structures in deeper water will see a transition
from monopiles/monopods to multiple footing
structures
• We need innovative solutions to drive costs down:
helical piling is an old solution to a new problem
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