Géotechnique Lecture 2011 Foundation Design for Offshore Wind

DEPARTMENT OF ENGINEERING SCIENCE

Géotechnique Lecture 2011

Foundation Design for Offshore Wind Turbines

Dr Byron Byrne Oxford University British Geotechnical Association Institution of Civil Engineers Wednesday 9th November 2011

Blades: high strength

composites

Aerodynamics of blades

Forces from

waves and current

Dynamics of tower

Foundation design

Generator

Gearbox

Control of blade pitch

Electrical connections

to shore

November 11, 2011

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Dr Byron Byrne



Water depth Average wind speed

November 11, 2011 Helical Piles for Offshore

Wind Turbines Page 3

Source: DTI Renewable Energy Atlas

Offshore sites

Round 1 - 2001

~ 1 GW

Round 2 - 2003

~ 7 GW

Round 3 - 2010

~ 32 GW

UK Wind Overview

Figures are rated maximum power and not average delivered power

Total UK installed generating capacity is approximately 91GW

November 11, 2011

Page 5

Source: RenewableUK (bwea.com)

Dr Byron Byrne



Status Onshore

Number

Onshore

Power (GW)

Offshore

Number

Offshore

Power (GW)

Operational 296 4.2 14 1.5

Construction 32 1.5 6 2.0

Consent 232 3.6 5 1.6

Planning 314 7.3 4 2.0

Total 874 16.6 29 7.1

96.8% 70% 3.2% 30%

Offshore Wind – Challenges

RenewableUK indicates plans for about 42GW of wind power to be installed, though no time scale indicated

Government announcements : 33GW by 2020

6600 5MW turbines

Over 800 turbines per year to 2020! *Replacement rate about 300 turbines per year indefinitely*

Nearly 500 turbines installed and operating since 2000

Total investment in region of £80bn to £100bn Many (tens of) thousands of jobs in the supply chain

November 11, 2011

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Dr Byron Byrne



Cost Makeup

4% Development and consent 33% Turbine 15% Electrical 22% Support structure 26% Production, integration and installation

Source: Carbon Trust

Foundations and installation part of the last two categories Opportunity to reduce costs by using alternative foundation

concepts and installation processes …as well as by improving design approaches

Costs are of the order say £3m to £3.5m per MW installed

Source UK ERC report Sept 2010

November 11, 2011

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Dr Byron Byrne



Geotechnical Issues

A full range of geotechnical conditions can be found at the various sites - mobile sand banks, dense sand, stiff clays, layered materials, soft clays, rocky strata, boulder clay

Can be considerable variability over a site (turbines are typically spaced more than 500m apart)

A site investigation is important early in the design process and may involve CPTs, Boreholes, vane tests, geophysical surveys

There may also be element testing using samples obtained from the site

November 11, 2011

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Dr Byron Byrne



200MN

25MN

90m to 120m

90m

to

120m

1MN to 2MN

3MN to 8MN

~6MN

November 11, 2011

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Dr Byron Byrne



Loads on an Offshore Turbine Foundation

(b)(a)

V

H

V

H

V

H

M

H2H1

V2

V1

November 11, 2011

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Dr Byron Byrne



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

k

EI

m

L

k

L

EI

Lm

fn 23

3

1

2

1

November 11, 2011

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Dr Byron Byrne



Stiff-Stiff Response

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Dr Byron Byrne



frequency

DAF

1P 3P

1.0

frequency

DAF

1P 3P

1.0

Soft-Stiff Response

November 11, 2011

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Dr Byron Byrne



frequency

DAF

1P 3P

1.0

Effect of Foundation

November 11, 2011

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Dr Byron Byrne



frequency

DAF

1P 3P

1.0

Range of Excitation Frequencies

November 11, 2011

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Dr Byron Byrne



Options for Foundations

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Dr Byron Byrne



Gravity Base Mono-Pile Mono-Caisson Multi-Pile Multi-Caisson

Size and Location of Developments

Wa

ter

de

pth

(m

)

Turbine power (MW)

2 3 4 5

10

20

30

40

Most past

developments

Most future

developments?

Multiple

footings?

Monopods

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Dr Byron Byrne



Other Designs / Possibilities

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Dr Byron Byrne



TLP Spar

Experimental Work

Most of the work presented is based on small scale model tests in the laboratory

High quality sophisticated work carried out

Designed to build up a framework of response

Careful consideration has been given to scaling of the results

Density of sands, strength of clays

Dimensional analysis has yielded dimensionless groups

relevant to each of the problems explored

November 11, 2011

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Dr Byron Byrne



CURRENT DESIGNS PILE FOUNDATIONS

November 11, 2011

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Dr Byron Byrne



Mono-Pile Foundations

A wind turbine monopile is at least 4m diameter and of the order of 25m long

Driving is at the limits of offshore oil-and-gas experience which typically involves smaller diameter (~2m) and longer piles (~100m)

Large diameter drilling is suitable in certain materials

Options:

drive

drill and grout

composite e.g. drive-drill-drive

November 11, 2011

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Dr Byron Byrne



Mono-Pile Multi-Pile

L

D

L

D

s

Mono-Pile Foundations

Average ~89

hours per pile

at North Hoyle

November 11, 2011

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Dr Byron Byrne



Source http://www.rwe.com/web/cms/en/312104/rwe-innogy/sites/wind-offshore/in-operation/north-hoyle/construction-diary/wind-turbine-foundations/

Walney Wind Farm

November 11, 2011

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Dr Byron Byrne



Photos from Dong Energy: Christian LeBlanc Thilsted and Dan Kallehave

Pile Design Issues – Wind Turbine

Stiffness is a key design criterion in addition to capacity.

Various offshore design approaches are based on much more flexible piles and are more concerned with lateral capacity than stiffness.

Typical offshore pile say L/D ~ 30 – 50 or more whilst wind-farm pile L/D ~ 4 - 8.

Are the usual design approaches still appropriate?

Performance under cyclic loading is important but there is very limited guidance for designers (if any at all…)

Accumulated rotations? Stiffness response?

November 11, 2011

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Dr Byron Byrne



November 11, 2011

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Dr Byron Byrne



Structural

configuration

L

D

H

e

D

L

V

Idealisation of

applied forces

H

M = He

V

D

d

L - d

Soil

reactions

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

No

rma

lis

ed

Mo

me

nt

Normalised Horizontal Load

Static Tests

November 11, 2011

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Dr Byron Byrne



Cyclic loading

November 11, 2011

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Dr Byron Byrne



M

0

MR

Real Loading Idealised Loading

Cyclic loading

November 11, 2011

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Dr Byron Byrne



One Way Cycling Two Way

Cycling

M

0

MR

Min = 0Min < Max

Min = Max

November 11, 2011

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Dr Byron Byrne



Motor

Reaction Frame

Mass

Mass

Mass

Testing Equipment

November 11, 2011

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Dr Byron Byrne



Example Cyclic Loading Results

November 11, 2011

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Dr Byron Byrne



-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.002 0.004 0.006 0.008 0.01 0.012

No

rma

lis

ed

Mo

me

nt

Normalised Rotation

Cycle 0 - 10 Cycle 1500 - 1510 Cycle 9050 - 9060

Definitions

Rotation

Min

Max

Moment

0 N

(N)

Normalised Accumulated Rotation

(N)

0

November 11, 2011

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Dr Byron Byrne



0.01

0.1

1

10

1 10 100 1000 10000 100000

No

rma

lis

ed

Ac

cu

mu

late

d R

ota

tio

n

Number of Cycles

Test Results – Accumulated Rotation

November 11, 2011

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Dr Byron Byrne



Analysis of Results

November 11, 2011

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Dr Byron Byrne



0

1

2

3

4

5

-1 -0.5 0 0.5 1

Monotonic

Loading

One Way

Loading

Two Way

Loading

(N)

0

= f1 x f2 x Na

f2

Cyclic loading

November 11, 2011

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Dr Byron Byrne



M

0

MR

Real Loading Idealised Loading

Cyclic loading

November 11, 2011

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Dr Byron Byrne



M

0

MR

Idealised Loading

Real Loading1 + 2 + 3 =

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 500 1000 1500 2000 2500 3000 3500

Ro

tati

on

(R

ad

ian

s)

Number of Cycles

Min

Max

Theory

Load A

Load B

Load C

Experiment – Theory Comparison

November 11, 2011

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Dr Byron Byrne



100

150

200

250

300

350

400

450

500

1 10 100 1000 10000 100000

No

rma

lis

ed

Sti

ffn

es

s

Cycle Number

Test Results – Stiffness

November 11, 2011

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Dr Byron Byrne



Observations – Mono-Pile Foundations

Framework for calculating accumulated rotation Change in stiffness could mean a change in structural natural

frequency – serious factor in the fatigue design Scaling

Larger scale field tests or centrifuge tests

Actual field measurements from installed mono-piles

Larger number of cycles

Current tests only up to 100,000 cycles (i.e. around 7 to 10 days)

Do we need tests in the region of 100m cycles?

Effect of change in load direction

Loading is unlikely to be uni-directional

Is this more onerous?

Frequency effects / Excess pore water pressures

November 11, 2011

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Dr Byron Byrne



**IMPORTANT** Field Monitoring

Data needed from installed piles!

Verify design calculations

Guidance for future designs

Database for the industry?

November 11, 2011

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Dr Byron Byrne



Data and photos from Dong Energy: Christian LeBlanc Thilsted and Dan Kallehave

Measured Strains

November 11, 2011

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Dr Byron Byrne



Data and photos from Dong Energy: Christian LeBlanc Thilsted and Dan Kallehave

0 5 10 15 20 25

Time [s]

Multi-Pile Foundations

Much more like typical oil and gas pile design

…except…

Any cyclic degradation of the axial response must be well understood

Mono-Pile Multi-Pile

L

D

L

D

s

November 11, 2011

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Dr Byron Byrne



Current designs Alpha Ventus

Bard Offshore 1

Beatrice

November 11, 2011

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Dr Byron Byrne



FUTURE DESIGNS? SUCTION CAISSONS

November 11, 2011

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Dr Byron Byrne



Suction Caissons

Flow

Pressure differential

W

Flow

Source: Houlsby, G.T., Ibsen, L.B. and Byrne, B.W (2005) "Suction caissons for wind turbines",

Invited Theme Lecture, Proc. International Symposium on Frontiers in Offshore Geotechnics, Perth,

Australia, 19-21 September, Taylor and Francis, pp 75-94, ISBN 0415 39063 X

November 11, 2011

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Dr Byron Byrne



Caisson Foundations

Design Issues

Suction installation

Combined Loading (mono-caisson)

Vertical Loading (multi-caisson)

Research

Laboratory testing

Field scale testing

Theoretical investigations

Numerical modelling

Focus here is on work at Oxford

Mono-Caisson Multi-Caisson

L

Ds

L

D

November 11, 2011

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Dr Byron Byrne



Installation

Theoretical calculations for design (ICE Proceedings)

Separate calculations for sand and for clay

Self weight calculation and suction installation

In sand seepage gradients are important.

Beneficial reduction of the end bearing resistance

Penetration possible in very dense sand

Guidance on the limiting aspect ratios for both cases

More recent work on installation in layered soils

November 11, 2011

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Dr Byron Byrne



0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35

Dis

pla

ce

me

nt (m

m)

Suction (kPa)

Field Installation - 3m diameter

November 11, 2011

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Dr Byron Byrne



Statoil’s Sleipner T (14m diameter)

0

1

2

3

4

5

6

0 20 40 60 80 100

Dep

th h

(m)

Required suction s (kPa)

Predicted suction

Range of measured suction

November 11, 2011

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Dr Byron Byrne



COMBINED LOADING MONO-CAISSON STRUCTURE

November 11, 2011

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Dr Byron Byrne



Mobile met mast (Denmark, 2009) Source: LeBlanc, C. (2009). Design of offshore wind turbine support structures; selected

topics in the field of geotechnical engineering. PhD Thesis, Aalborg University.

November 11, 2011

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Dr Byron Byrne



*Breaking News Oct 2011* Dogger Bank Met Masts

November 11, 2011

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Dr Byron Byrne



Source: Forewind website

Experimental equipment

November 11, 2011

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Dr Byron Byrne



Moment Loading at Low Vertical Load

-100

-80

-60

-40

-20

0

20

40

60

80

100

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Rotational Displacement, 2R (mm)

Mo

men

t L

oad

, M

/2R

(N

)

November 11, 2011

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Dr Byron Byrne



Conventional approach:

effective area and inclination factors

November 11, 2011

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Dr Byron Byrne



M

V

H

B

Q

B

e

V, M, H Q, e,

Source: G.T. Houlsby and R. Butterfield Géotechnique Lecture 2001

Q

B

e

Q

B

e

B' = B - 2e

“Hardening Plasticity” Models

The plasticity model requires

A yield surface to define

allowable load combinations

Hardening rule to define yield

surface expansion

Flow rules to define plastic

movements at yield

Elasticity expressions to define

pre-yield movements

M

V

H

2R

November 11, 2011

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Dr Byron Byrne



Source: G.T. Houlsby and R. Butterfield Géotechnique Lecture 2001

Moment Loading at Low Vertical Load

-100

-80

-60

-40

-20

0

20

40

60

80

100

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2


Mo

men

t L

oad

, M

/2R

(N

)

November 11, 2011

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Dr Byron Byrne



0

0.05

0.1

0.15

0.2

0.25

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Dim

en

sio

nle

ss

Mo

me

nt,

M/(

'D4)

Dimensionless Vertical Load, V/( 'D3)

Range of potential designs for

monopod structures

Yield Points and Design Curves 1 (Sand)

November 11, 2011

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Dr Byron Byrne



-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

No

n-D

ime

nsio

nal M

om

en

t, M

/(D

4)

No

n-D

ime

nsio

nal I

nc

rem

en

tal R

ota

tio

n,

Non-Dimensional Horizontal Load, H/ D3

Non-Dimensional Incremental Horizontal Displacement, u/D

Range of potential monopod designs

Yield Points and Design Curves 2 (Sand)

November 11, 2011

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Dr Byron Byrne



-100

-80

-60

-40

-20

0

20

40

60

80

100

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Mo

me

nt

Lo

ad

, M

/2R

(N)


Comparison of Theory to Experiment

November 11, 2011

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Dr Byron Byrne



Field Testing

Bothkennar

(clay)

Luce Bay

(sand)

November 11, 2011

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Dr Byron Byrne



Field Testing at Bothkennar

November 11, 2011

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Dr Byron Byrne



Moment loading

4000 4000 6000

30001500

H

H

HB

CC

A

RR

W

V

A

B

C

V

LL

L

L L LL

42

00

November 11, 2011

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Dr Byron Byrne



Moment tests at small and large rotations

Small

Large

November 11, 2011

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Dr Byron Byrne



-30

-20

-10

0

10

20

30

-0.00005 -0.000025 0 0.000025 0.00005

Mo

men

t (k

Nm

)

Rotation (radians)

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

-0.03 -0.02 -0.01 0 0.01 0.02 0.03

Mo

men

t (k

Nm

)

Rotation (Radians)

0

2

4

6

8

10

12

14

16

0.00001 0.0001 0.001 0.01 0.1

G (

MP

a)

(radians)

Analysis of Results (Clay)

November 11, 2011

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Dr Byron Byrne



Moment Test Results (Clay)

November 11, 2011

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Dr Byron Byrne



VERTICAL LOADING MULTI-CAISSON STRUCTURE

November 11, 2011

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Dr Byron Byrne



Multiple caissons: Draupner E platform Source: Andersen, K.H., Jostad, H.P. and Dyvik, R. (2008) "Penetration resistance of offshore skirted foundations

and anchors in dense sand", Proc ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol 134, No 1,

pp 106 - 116.

November 11, 2011

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Dr Byron Byrne



Tripod or tetrapod?

Tower

Tower

Tower

(a) (b) (c)

November 11, 2011

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Dr Byron Byrne



Vertical Loading Tests (in a Pressure Vessel)

November 11, 2011

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Dr Byron Byrne



-400

-200

0

200

400

600

800

1000

1200

1400

200 210 220 230 240 250 260 270

Ve

rtic

al s

tre

ss (

kP

a)

Displacement (mm)

Cyclic Vertical Loading (Sand)

November 11, 2011

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Dr Byron Byrne



Vertical Loading Tests (Sand)

Com

pre

ssio

n

Tension

November 11, 2011

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Dr Byron Byrne



-450

-400

-350

-300

-250

-200

-150

-100

-50

0

150 160 170 180 190 200 210

Ve

rtic

al s

tre

ss

(k

Pa

)

Displacement (mm)

Direction of movement

SLOW

FAST

FAST + PRESSURE

Capacity on tensile loading (Sand)

November 11, 2011

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Dr Byron Byrne



Observations – Caisson Foundations

Significant body of work available for development of designs

Both mono-caisson and multi-caisson structures can make contributions to offshore wind

Suitable to a range of soil conditions

…but not all

Initial structures must be monitored

To enable a better understanding of the foundation performance

To understand the limitations

To reduce conservatism

November 11, 2011

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Dr Byron Byrne



Concluding Comments

Mono-piles will continue to be used in the short-term

Better understanding of long term cyclic loading needed

Better understanding of stiffness of response also needed?

Suction caissons could be used for offshore wind turbines

A new technology will help to drive down costs

No pile-driving noise to worry about!

Field monitoring of structures and foundations is essential

Instrumentation relatively inexpensive

Valuable information will lead to better design guidance

…and more confidence in new and improved designs

November 11, 2011

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Dr Byron Byrne



Acknowledgements

The co-authors of the various papers covered in this lecture

Prof Guy Houlsby (Oxford University)

Dr Christian LeBlanc Thilsted (Dong Energy)

Dr Richard Kelly (Coffey Geotechnics, Australia)

John Huxtable (formerly Fugro, now at Doosan)

Dr Chris Martin collaborated on the shallow foundation work A number of Colleagues at Oxford and elsewhere

Sponsors included EPSRC, DTI, Royal Society Industry participants for the caisson work included SLP

Engineering, Fugro, Garrad Hassan, GE Wind, NEG Micon, Shell Renewables, HR Wallingford.

Dong Energy for information and photos related to Walney

November 11, 2011

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Dr Byron Byrne



Géotechnique Papers

1. Response of stiff piles to random two-way lateral loading.

Géotechnique 60 9:715-721.

2. Response of stiff piles to long term cyclic loading.

Géotechnique 60 2: 79-90.

3. Transient vertical loading of model suction caissons in a pressure chamber.


4. A comparison of field and laboratory caisson tests in sand and clay.


5. Field trials of suction caissons in sand for offshore wind turbine foundations.


6. Field trials of suction caissons in clay for offshore wind turbine foundations.

Géotechnique 55 4: 287-296. Contact [email protected] for further information.

November 11, 2011

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Dr Byron Byrne



Blyth Scroby Sands

Barrow

North Hoyle

November 11, 2011

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Dr Byron Byrne



Walney

Source – Various websites

Documents

Géotechnique Lecture 2011 Foundation Design for Offshore Wind