Foundations for Offshore Wind Turbines - Durham … · 6 Walney Wind Farm Photos from Dong Energy:...

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DEPARTMENT OF ENGINEERING SCIENCE

September 4, 2013

Foundations for Offshore Wind

Turbines

Byron Byrne and Guy Houlsby SUPERGEN Wind 7th Training Event

Overview

This presentation focuses on foundation design for offshore wind energy applications

Refers to the interaction between the structure and the ground

Loading and structural design also very important

Many turbines for each site (say 30 to 100 or more) over a large site area

Foundation significant fraction of the installed cost

Different problem to bespoke oil and gas foundation design

Site conditions can vary markedly across a site (e.g. factor 10 on

strengths in some areas, layered materials, sand waves etc)

Good characterisation of the site early on very important for

developing realistic designs (but minimal coverage likely)

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Foundations for Offshore Wind Turbines

Byron Byrne and Guy Houlsby

SUPERGEN Wind 7th Training Event

September 4, 2013

2

90m to 120m

90m

to

120m

1MN to 2MN

3MN to 8MN

~6MN

200MN

25MN

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September 4, 2013

Loads on an Offshore Turbine Foundation

(b)(a)

V

H

V

H

V

H

M

H2H1

V2

V1

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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

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September 4, 2013

frequency

DAF

1P 3P

1.0

Structural Response

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Options for foundations

sD

LD

L

(g) (h) (i)(f)(e)(d)(c)(b)(a)

(j)

(k)

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September 4, 2013

CURRENT DESIGNS PILE FOUNDATIONS

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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

Mono-Pile Multi-Pile

L

D

L

D

s

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September 4, 2013

“Monopile” Foundations

Average 89

hours per pile

at North Hoyle Image/Notes from www.foundationworld.org.uk

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6

Walney Wind Farm

Photos from Dong Energy: Christian LeBlanc Thilsted and Dan Kallehave

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September 4, 2013

Pile Design Issues – Wind Turbine

Stiffness is the over-riding design criterion rather than capacity.

Various offshore design approaches (API, DNV) 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

Accumulated rotations?

Stiffness response?

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Experiment

Motor

Reaction Frame

Mass

Mass

Mass

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September 4, 2013

Research Results two way

one way

h

L

D

-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

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Summary of Research Output

Framework developed for calculating accumulated rotation Results indicate change in pile stiffness with cycle number

Effect on natural frequency of structure and implications for fatigue

Limitations include scaling, cycle numbers, loading direction…

Additional laboratory testing addressing these areas

Ongoing research…

Monitoring of structures in the field

Assessment of actual loading conditions and foundation response

(e.g. moment distribution)

Large scale field tests

Better basis for design methodology development

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September 4, 2013

Basis for developing modified design methodologies

Full Scale Results – Standard Systems

Standard API p-y

formulation

Modified p-y

formulation Data and photos from DONG Energy: Christian LeBlanc Thilsted and Dan Kallehave

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Field Monitoring

Data currently being gathered from installed piles

Verify design calculations

Guidance for future designs

Database for the industry?

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

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September 4, 2013

Measured Strains

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

0 5 10 15 20 25

Time [s]

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Large Scale Pile Testing – PISA Project

Figure from Christelle Abadie, PhD student funded by EDF; Photo of field pile load test from Ken Gavin, University College Dublin

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September 4, 2013

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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

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September 4, 2013

Current designs Alpha Ventus

Bard Offshore 1

Beatrice

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Source: Various websites

September 4, 2013

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FUTURE DESIGNS? SUCTION CAISSONS

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September 4, 2013

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

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Suction Caissons

Flow

Pressure differential

W

Flow

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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

September 4, 2013

Mobile met mast application (DONG, 2009)

Source: C.L. Thilsted, PhD Thesis

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September 4, 2013

14

M

2R

Lt

Research Methodology

Laboratory model tests

Theoretical analyses

Calibrate theory

against tests to

develop design

methods

Field trials

Validated

design

methods

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September 4, 2013

Field Installation - 3m diameter

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)

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)

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“Hardening Plasticity” Models

Plasticity models require

Yield Surface

Hardening Rule

Flow Rule

Elasticity Expressions

Suitable for coupling with structural calculations

M

V

H

2R

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Experimental equipment

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16

Field Testing at Bothkennar

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September 4, 2013

Moment Test Results (Clay)

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-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)

Rotational Displacement, 2Rq (mm)

-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)

Rotational Displacement, 2Rq (mm)

Comparison of Theory to Experiment

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September 4, 2013

Loads on an Offshore Turbine Foundation

(b)(a)

V

H

V

H

V

H

M

H2H1

V2

V1

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Tripod or tetrapod?

Tower

Tower

Tower

(a) (b) (c)

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September 4, 2013

Multiple caissons: Europipe platform

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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.




September 4, 2013

19

Vertical Loading Tests (Sand)

Com

pre

ssio

n

Tension

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September 4, 2013

Summary of Research Outputs

Caisson design approaches supported by a range of experimental and field scale data

Installation

Cyclic loading

Monotonic loading

Sand / Clay and layered soils

Stiffness / Capacity

Issues of scalability need to be investigated further

Only by larger scale trial offshore…

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September 4, 2013

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Other possibilities - Helical Screw Piles

(a) (b) (c) (d) (e)

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September 4, 2013

Helical Screw Piles Characteristics

Good tension capacity

Applicable to a range of soil conditions

Reusable - piles can be “unscrewed”

Installation

Torque motors smaller than pile drivers

Over water or under water possibilities

Quick and quiet installation process

Design approaches

Pile design verified during the SI phase

CPT correlations with pile design

Further SI during pile installation

Torque data can verify capacity

M

V

H

(f) (g)

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September 4, 2013

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Scale of Design for Offshore

Small onshore turbine – 50kW V = 50 kN, H = 60 kN, M = 1.4 MNm 2m by 2m grillage system with 9 screw piles

helical flights approximately 0.5m diameter

installed with torque of 60kNm

Large offshore turbine – 3 MW turbine in 19m water depth V = 6 MN, H = 6.5MN, M = 160 MNm 30m by 30m tetrapod structure with screw piles at each corner

helical flights approximately 2.0m

installed with torque of 1.5MNm

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September 4, 2013

Summary

Offshore wind energy will require significant geotechnical design efforts going forward

Monopile designs are currently used though there are concerns about design approaches and the effects of cyclic loading

More innovative suction caisson foundations could be used and the next step is a larger scale offshore trial

Other more radical solutions might involve helical screw piles but this would involve significant upscaling from onshore designs

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September 4, 2013

Foundations for Offshore Wind Turbines - Durham … · 6 Walney Wind Farm Photos from Dong Energy:...

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Transcript of Foundations for Offshore Wind Turbines - Durham … · 6 Walney Wind Farm Photos from Dong Energy:...