Interactions in Offshore Foundation Design
Prof. Guy Houlsby Department of Engineering Science, University of Oxford
Rankine Lecture 2014 Imperial College, 19 March 2014
Repeated at TGS, Taipei, 15 December 2014
Rankine Lecture 2014 2
Christelle Abadie Angelo Amorosi Charles Augarde Ross Bell Alan Bloodworth Peter Bransby Britta Bienen Boon Chia Weng Chris Brocklehurst Harvey Burd Roy Butterfield Byron Byrne John Carter Mark Cassidy Lily Chow Ian Collins Oliver Cotter James Doherty Dong Yuepeng Itai Einav Keith Evans Guido Gottardi Jim Graham Gue See-Sew Ross Hitchman Neil Houlsby Mike Hoyle
Acknowledgements John Huxtable Jimmy James Richard Jewell Richard Kelly Tatsuya Koumoto Christian LeBlanc Suched Likitlersuang Liu Gang Jerry Love Jan Mangal Chris Martin George Milligan Chiara Modenese Giuseppe Mortara David Muir Wood Shandry Nageswaran Ron Nedderman Luan Ngo-Tran Nguyen Dinh Giang Nguyen-Sy Lam Nigel Nutt Julian Osborne Brooks Paige Miguel Peña John Pickhaver Sotiris Psomas Sasha Puzrin Mark Randolph
Eduardo Rojas Brendan Ruck Cesar Sagaseta Paulo de Santa Maria Fernando Schnaid Radhey Sharma Gilliane Sills Alessandro Simoni Scott Sloan Mike Smith Mike Sweeney Teh Cee-Ing Teh Kar-Lu Richard Thompson Tor-Inge Tjelta Stefano Utili Felipe Villalobos Adam Wheeler Simon Wheeler Martin Williams Nick Withers Frank Wood Peter Wroth Mitsuhiro Yao Yu Hai-Sui Tao Zhao Zhu Bin
Rankine Lecture 2014 3
• Part 1
– Installation of jack-up units
• Part 2
– Performance of jack-up units
• Part 3
– Foundations for offshore wind turbines
Summary
Rankine Lecture 2014 4
• Jack-up installation as a bearing capacity problem
• Apply probabilistic methods to predictions of performance
Part 1: Installation of jack-up units
Rankine Lecture 2014 6
Jack-up operations
Float to site
Lower legs
Storm Climb to air-gap
and operate
Dump preload
Preload Light ship load
sketches after Poulos (1988)
Rankine Lecture 2014 7
Jack-up installation and preloading
Load on Spudcan
Depth
Light ship load
Preload
Undrained Strength
Depth
Rankine Lecture 2014 8
Spudcan penetration as a bearing capacity problem
AhsNV uc
h
V
0,,,
uc
s
D
D
hfN
Cone angle Rate of increase of strength
Depth Base roughness
D
su
z
suo
Average
Rankine Lecture 2014 9
A deterministic calculation
Geometry Predicted
load Bearing capacity
Strength
su
z
Nc
z
V
z
Rankine Lecture 2014 10
Best estimate and lower bound Load on Spudcan
Depth
Light ship load
Preload
Undrained Strength
Depth
Rankine Lecture 2014 12
Vertical load V
Depth
Vertical load V
Depth
Cumulative frequency, %
100
75
50
25
Vertical load V
Depth
Cumulative frequency, %
100
75
50
25
95%
5%
25%
50%
75%
Vertical load V
Depth
50%75%
95%
25%
5%
Rankine Lecture 2014 13
Predicted range of
behaviour
case record from “InSafeJIP”
(Joint Industry Programme on
Safe Installation of Jack-up Units)
??
Rankine Lecture 2014 16
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000
De
pth
(m
)
Load (kN)
5th25thMedian75th95thData
0
5
10
15
20
25
30
35
40
0 20000 40000 60000 80000 100000 120000
De
pth
(m
)
Load (kN)
5th25thMedian75th95th
“Prior” distribution based on SI data
“Posterior” distribution based on Prior + observed
penetrations
Observations
analysis by Brooks Paige
Probabilistic Programming (Bayesian inference)
Rankine Lecture 2014 18
• Jack-up installation provides data verifying bearing capacity theories
• Learn more about statistics/probability!
18
Conclusions from Part 1
Rankine Lecture 2014 19
• Interactions with structural engineers
• The importance of “fixity” in structural response
• Application of plasticity theory
Part 2: Performance of jack-up units
Rankine Lecture 2014 20
Jackup operations
Float to site
Lower legs
Storm Climb to air-gap
and operate
Dump preload
Preload Light ship load
Rankine Lecture 2014 21
2V HL V HL
H H
V V
2H/3 H/3
w
L
3 w-
3 w+ 2V HL
2H/3
3 2w- V HL
H/3
3 2w+
HL/3 HL/6
Pinned or fixed?
Factor of 2
Pinned Fixed
Rankine Lecture 2014 22
• Changes in foundation vertical load halved
• Maximum moment in leg halved (Moment at spudcan increased, with implications for fatigue)
• (Quasi-static) lateral displacement reduced by factor of 4
Benefits of fixity
0
1
2
3
4
5
0 1 2
Dyn
amic
Am
plif
icat
ion
Fa
cto
r (D
AF)
Excitation Frequency/Natural Frequency
• First natural frequency doubled
– Moves resonance further away from wave excitation frequency
Rankine Lecture 2014 23
What are the springs?
Structural Engineer’s viewpoint
H
V
2k k
2 legs on windward side
1 leg on leeward side
Rankine Lecture 2014 24
Moment-rotation test on a spudcan (clay)
from de Santa Maria (1988)
Rotation
Mo
men
t
Yield M
suD3
q
Rankine Lecture 2014 26
Early work on failure surfaces
Roscoe and Schofield, 1957
(Two dimensions)
Butterfield and Ticof, 1979
(Three dimensions)
Rankine Lecture 2014 28
Testing rig
• Apply controlled
displacements by
stepper motors
• Measure w, q, u
displacements
• Measure V, M, H loads
Rankine Lecture 2014 29
Constant V
Constant vertical
displacement
Vertical loading V
M/D
or
H
Load paths in tests
Constant V
Constant vertical
displacement
Vertical loading V
M/D
or
H
Rankine Lecture 2014 31
Horizontal swipe tests (sand)
Gottardi, Houlsby, and Butterfield (Géotechnique, 1999)
0 200 400 600 800 1000 1200 1400
V (N)
0
50
100
150
200
250
0
50
100
150
200
250
H (N)
2 4 6
u (mm) V (N) u (mm)
h (N)
Rankine Lecture 2014 32
Normalised results: (v, q) plane
0.0 0.2 0.4 0.6 0.8 1.0 v
0.0
0.2
0.4
0.6
0.8
1.0
1.2
q
Best Fit
GG03
GG04
GG05
GG06
GG28
GG29
GG07
GG08
GG10
GG12
vvq 14
Gottardi, Houlsby and Butterfield (Géotechnique, 1999)
oooo
ooo
hm
amh
h
h
m
mq
V
Hh
DV
Mm
V
Vv
2
,,
2
2
2
2
v
q
Rankine Lecture 2014 33
Normalised results: (M/D, H) plane
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
m
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
hn
n
Best fit
GG03
GG04
GG05
GG06
GG28
GG29
GG07
GG08
GG10
GG12
vv
hhn
14
Gottardi, Houlsby, and Butterfield (Géotechnique, 1999)
mn
hn
vv
mmn
14
Rankine Lecture 2014 36
6 degree-of-freedom swipe tests
01
2
21 2
0
2
012
2
00
0000
32232
00
32
00
22
00
32
00
2
V
V
V
V
DVq
Q
DVmVh
MHMHa
DVm
M
DVm
M
Vh
H
Vh
Hf
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 0.2 0.4 0.6 0.8 1
H/V
o,
M/D
Vo,
Q/D
Vo
V/Vo
Horizontal Tests
Rotational Tests
Twisting Tests
see Bienen, Byrne, Houlsby and Cassidy (Géotechnique, 2006)
Rankine Lecture 2014 37
Moment-rotation test on a spudcan (clay)
from de Santa Maria (1988)
Rotation
Mo
men
t
Yield M
suD3
q
Rankine Lecture 2014 38
Moment tests at small and large rotations
0
10
20
30
40
50
60
70
80
90
100
0.000001 0.00001 0.0001 0.001 0.01 0.1
Dq (radians)
G (
MP
a)
Jacking
SEMV
Hyperbolic curve fit
Houlsby, Kelly, Huxtable and Byrne (Géotechnique, 2006)
Seca
nt
stif
fnes
s
Amplitude (log scale)
Rankine Lecture 2014 39
Multi-surface foundation model
Preloading
Storm loading (windward legs)
M
V
M
V
Rankine Lecture 2014 40
Multiple yield surface model
Experimental data Theory: multiple surface model
Rankine Lecture 2014 42
Measured data
data from McCarron and Broussard (BOSS, 1992)
Hull displacement against time
Leg moment against displacement
Rankine Lecture 2014 43
Pisa Tower
0
0.02
0.04
0.06
0.08
0.1
0.12
0 0.2 0.4 0.6 0.8 1
M/D
V0
V/V0
Initial loads
Apply lead weights
Soil removal
Remove lead weights
Vertical load
Mo
me
nt
data from Marchi (2008)
Rankine Lecture 2014 44
The framework of plasticity theory (elasticity + yield) is the common language between geotechnical and structural engineers
44
Conclusion from Part 2
Rankine Lecture 2014 45
• Why offshore renewables?
• Challenges and solutions for offshore turbine foundations
– Conventional, unconventional and completely novel solutions
Part 3: Foundation for offshore wind turbines
Rankine Lecture 2014 46
Increase of atmospheric CO2
April 2014 – first monthly average over 400 ppmv
Rankine Lecture 2014 49
Problem:
• Climate change due to fossil fuel use
• Diminishing supply of hydrocarbons
Solution:
• Nuclear
• Renewables
Rankine Lecture 2014 51
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)
Rankine Lecture 2014 52
Total and offshore installed wind capacity (approx. end of 2013)
sources: IEA Wind 2012 Annual Report EWEA statistics 2013
4Coffshore
Country Total installed
capacity (MW) Offshore installed
capacity (MW)
China 75234 390 6% US 60007 0
Germany 31315 520 7%
Spain 22785 0
India 18412 0 UK 8292 3681 53%
Italy 8144 0 France 7564 0
Canada 6201 0 Denmark 4162 1271 18%
…rest of world 40471 1090 16%
Total 282587 6952 100%
Rankine Lecture 2014 54
Loading on wind turbine
H = 2MN + 4MN = 6MN wind wave
M = 2 x (110 + 40) + 4 x 40 wind wave = 300 + 160 = 460 MNm
V = 10MN
10MN
460MNm 6MN
Beatrice Wind Farm
Rankine Lecture 2014 56
Wat
er d
epth
(m
)
Turbine power (MW)2 3 4 5
10
20
30
40
Past developments
Most future developments?
Monopiles
6 7
60
50
Beatrice
Barrow
Blyth
Burbo
Gabbard
Gunfleet 3Gunfleet
Dowsing
Kentish
Lincs
London
Lynn
North Hoyle
Ormonde
Rhyl
Robin Rigg
Scroby
Sheringham
Teesside
Thanet
Walney
Walney 2
Foundation type related to size and depth
Beatrice
London Array
Rankine Lecture 2014 57
• Conventional: monopiles
– cyclic loading
• Unconventional: suction caissons
– why?
– challenges: installation, tension capacity
• Novel: screw piles
– solution to the tension problem
Foundations for offshore turbines
Rankine Lecture 2014 58
Monopiles
• Oil and gas
Length: 30m - 80m
Diameter: 1m - 2m
L/D approx. 30 - 60
• Offshore wind monopile
Length: approx. 30m
Diameter: 4m to 6m
L/D approx. 5 to 7
photo: Anholt Offshore Wind Farm
photo: Ciscon
Rankine Lecture 2014 59
Cyclic loading tests
Motor
Reaction Frame
Mass
Mass
Mass
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
Rankine Lecture 2014 60
Approximately 100,000 cycles
data supplied by Abadie
1000 cycles
9000 cycles
90000 cycles
Rankine Lecture 2014 61
Accumulated rotation
zb = 0.20
zb = 0.27
zb = 0.40
zb = 0.53
31.0Nkstatic
q
qD
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
Increasing amplitude
Rankine Lecture 2014 62
Effect of cycle type
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
0.25
M
0
MR
0.5
0.75
1.0
Tb
One-way cycling
Symmetric cycling
-1.0
M
0
MR
-0.5
0.0
0.5
Tc
31.0NTT cbstatic
q
qD
Rankine Lecture 2014 63
Flow
Pressure differential
W
Flow
Suction caissons
photo: Universal Foundation A/S
Installed by: 1. Self weight 2. Suction
Advantages: • Less expensive equipment
for installation • No pile driving noise
Rankine Lecture 2014 64
• Can they be installed?
OK except:
– Very stiff or fissured clays
– Very coarse-grained soils
– Layered and other non-homogeneous soils
• Tensile capacity
• Cyclic loading
Main issues for suction caissons
Wind and wave
Tension
Rankine Lecture 2014 66
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
Rankine Lecture 2014 67
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
Rankine Lecture 2014 68
Key Dimensionless Groups
• Capacity – clay: V/(suDp
2)
– sand: V/(’Dp3)
• Installation (T = torque) – clay: T/(suDp
3)
– sand: T/(’Dp4)
• Key ratios: VDp/T , Vt/V
(not V/T as often currently used onshore)
• Geometry: Dp/D, s/Dp, N
s
D
Dp
V
T
Vtor
Rankine Lecture 2014 69
Summary data of screw pile experience (model tests and onshore)
Source Test type Soil VtDp/T Vt/V
Min Mean Max
Tsuha et al (2010) Centrifuge Sand 6.0 8.3 12.5
Rao et al (1991) Laboratory Soft Clay 0.64
Sakr (2009) Field Oil Sand 5.2 0.52
Livneh and El Naggar (2008) Field Clayey Silt 6.4 8.0 10.9
Ghaly et al (1991) Laboratory Sand 3.2 5.0 6.1
Cerato and Victor (2009) Field Layered soil 2.6 14.4 23.3
Perko (2009) Various Various 1.6 8.5 24.6 0.8-0.96
(implied)
Tensile capacity x Diameter / Torque
Rankine Lecture 2014 72
Compression and tension capacity
0
5
10
15
20
25
30
35
0 5000 10000 15000 20000 25000 30000
Pil
e T
ip D
ep
th (
m)
Total Bearing Load, kN
Minimum - Compression
Independent - Compression
Interacting - Compression
Tension
Rankine Lecture 2014 73
Dimensionless torque ratio
0
5
10
15
20
25
30
35
0 2 4 6 8 10 12
Pil
e T
ip D
ep
th (
m)
Torque Ratio, VtDp/T
Rankine Lecture 2014 74
• 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
Maplin Sands Lighthouse (1838)
illustrations provided by Alan Lutenegger
Rankine Lecture 2014 75
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
Rankine Lecture 2014 76
• 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
76
Conclusions from Part 3
Top Related