Post on 14-Feb-2017
Interactions in Offshore Foundation Design
Prof. Guy Houlsby Department of Engineering Science, University of Oxford
Rankine Lecture 2014 Imperial College, 19 March 2014
Rankine Lecture 2014 2
• 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 3
• 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 4
Loads on an offshore turbine
foundation
V
H
V
H
V
HM
H2H1
V2
V1
S
Rankine Lecture 2014 5
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 6
• 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 7
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 8
PISA
PROJECT
Lead partner:
Partners:
Delivery team:
PISA = Pile Soil Analysis
Rankine Lecture 2014 9
Cyclic loading tests
Motor
Reaction Frame
Mass
Mass
Mass
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
Rankine Lecture 2014 10
Approximately 100,000 cycles
data supplied by Abadie
1000 cycles
9000 cycles
90000 cycles
Rankine Lecture 2014 11
Stiffness increases with load cycles
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
zb = 0.20
zb = 0.27
zb = 0.40
zb = 0.53
One-way load cycles
Increasing amplitude
Rankine Lecture 2014 12
Accumulated rotation
zb = 0.20
zb = 0.27
zb = 0.40
zb = 0.53
31.0Nkstatic
LeBlanc, Houlsby and Byrne (Géotechnique, 2010)
Increasing amplitude
Rankine Lecture 2014 13
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
Rankine Lecture 2014 14
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 15
• 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 16
Capacity on tensile loading (sand)
Rankine Lecture 2014 17
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 18
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 19
Key Dimensionless Groups
• Capacity – clay: V/(suDp
2)
– sand: V/(g’Dp3)
• Installation (T = torque) – clay: T/(suDp
3)
– sand: T/(g’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 20
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 21
Compressive capacity Envelope Independent
plates
Rankine Lecture 2014 22
Tension capacity Envelope Independent
plates
Rankine Lecture 2014 23
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 24
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 25
Tension/compression capacity ratio
0
5
10
15
20
25
30
35
0 0,2 0,4 0,6 0,8 1
Pil
e T
ip D
ep
th (
m)
Tension/Compression capacity ratio, Vt /V
Rankine Lecture 2014 26
• 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 27
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 28
• 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
28
Conclusions from Part 3
Rankine Lecture 2014 29
R E M S
Centre for Doctoral Training in Renewable Energy Marine Structures
• Structures by Cranfield and Geotechnics by Oxford
• Training leaders with high level technical expertise
• Cohorts of at least 10 graduate students per year for 5 years
• University based PhD and industry based EngD students
• 4-year research degrees with taught components
• > 15 companies have pledged involvement (inc. Arup, Atkins, Centrica, DNV,
EDF, e.on, Fugro, GE, HRW, Mojo Maritime, Qinetiq, RES, Skanska , Tata Steel, RWE) …
…but we need more
• Contact byron.byrne@eng.ox.ac.uk for more information