Rankine Lecture 2014 - tgs Lecture 2014 - final...Interactions in Offshore Foundation Design Prof....

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


• Part 1

– Installation of jack-up units

• Part 2

– Performance of jack-up units

• Part 3

– Foundations for offshore wind turbines

Summary


• Jack-up installation as a bearing capacity problem

• Apply probabilistic methods to predictions of performance

Part 1: Installation of jack-up units


Jack-up units 60m

Keppel FELS

Keppel FELS

Maersk


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)


Jack-up installation and preloading

Load on Spudcan

Depth

Light ship load

Preload

Undrained Strength

Depth


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


A deterministic calculation

Geometry Predicted

load Bearing capacity

Strength

su

z

Nc

z

V

z


Best estimate and lower bound Load on Spudcan

Depth

Light ship load

Preload

Undrained Strength

Depth


Freq.

su

su

z

Freq.

r

Nc

z

Freq.

Nc

z

A probabilistic calculation


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%


Predicted range of

behaviour

case record from “InSafeJIP”

(Joint Industry Programme on

Safe Installation of Jack-up Units)

??


Compare to measurements on site

V

z

V

z

Trend correct Trend incorrect


Preload

Light ship

case record from “InSafeJIP”


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)


• Jack-up installation provides data verifying bearing capacity theories

• Learn more about statistics/probability!

18

Conclusions from Part 1


• Interactions with structural engineers

• The importance of “fixity” in structural response

• Application of plasticity theory

Part 2: Performance of jack-up units


Jackup operations

Float to site

Lower legs

Storm Climb to air-gap

and operate

Dump preload

Preload Light ship load


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


• 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


What are the springs?

Structural Engineer’s viewpoint

H

V

2k k

2 legs on windward side

1 leg on leeward side


Moment-rotation test on a spudcan (clay)

from de Santa Maria (1988)

Rotation

Mo

men

t

Yield M

suD3

q


H

Vm

Bearing capacity

H

Vm

Bearing capacity

Interaction diagram

V


Early work on failure surfaces

Roscoe and Schofield, 1957

(Two dimensions)

Butterfield and Ticof, 1979

(Three dimensions)


3-D yield surface

VM

D

M/D

H

V


Testing rig

• Apply controlled

displacements by

stepper motors

• Measure w, q, u

displacements

• Measure V, M, H loads


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


“Swipe” tests to explore yield surface

V

H

M/2R

Swipe paths

M/D

V

H


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)


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


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


Yield surface

0142 2

00

2

0

2

0

vv

hm

mha

h

h

m

mf


6 degree-of-freedom loading rig

D

1

3

2

M3

VQ


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)


Moment-rotation test on a spudcan (clay)

from de Santa Maria (1988)

Rotation

Mo

men

t

Yield M

suD3

q


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)


Multi-surface foundation model

Preloading

Storm loading (windward legs)

M

V

M

V


Multiple yield surface model

Experimental data Theory: multiple surface model


Measured data

data from McCarron and Broussard (BOSS, 1992)

Hull displacement against time

Leg moment against displacement


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)


The framework of plasticity theory (elasticity + yield) is the common language between geotechnical and structural engineers

44

Conclusion from Part 2


• Why offshore renewables?

• Challenges and solutions for offshore turbine foundations

– Conventional, unconventional and completely novel solutions

Part 3: Foundation for offshore wind turbines


Increase of atmospheric CO2

April 2014 – first monthly average over 400 ppmv


Rate of increase of atmospheric CO2


UK oil and gas production

source: DECC


Problem:

• Climate change due to fossil fuel use

• Diminishing supply of hydrocarbons

Solution:

• Nuclear

• Renewables


Water depth Average wind speed

source: DTI Renewable Energy Atlas


Offshore sites

Licensing Round 1 - 2001

(up to 1 GW)


(up to 7 GW)


(up to 32 GW)


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%


2MN

110m

4MN 40m

Loading on wind turbine

Beatrice Wind Farm


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


Loads on an offshore turbine

foundation

V

H

V

H

V

HM

H2H1

V2

V1

S


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

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=TMlGy6qrJNuFcM&tbnid=-yVpxhVZhz8BhM:&ved=0CAUQjRw&url=http://www.londonarray.com/the-project/offshore/foundations/&ei=aMmcUvv9MeO80QX42YGYDg&bvm=bv.57155469,d.ZGU&psig=AFQjCNHMRmSlqcCl4jjp9yfH-c1WoHOIfg&ust=1386093200939158


• Conventional: monopiles

– cyclic loading

• Unconventional: suction caissons

– why?

– challenges: installation, tension capacity

• Novel: screw piles

– solution to the tension problem

Foundations for offshore turbines


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


Cyclic loading tests

Motor

Reaction Frame

Mass

Mass

Mass

LeBlanc, Houlsby and Byrne (Géotechnique, 2010)


Approximately 100,000 cycles

data supplied by Abadie

1000 cycles

9000 cycles

90000 cycles


Accumulated rotation

zb = 0.20

zb = 0.27

zb = 0.40

zb = 0.53

31.0Nkstatic

q

qD


Increasing amplitude


Effect of cycle type


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


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


• 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


Tensile loading of caissons (sand)

Co

mp

ress

ion

Tension


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


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


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


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


Compressive capacity Envelope Independent

plates


Tension capacity Envelope Independent

plates


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


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


• 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


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


• 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


• Learn more statistics

• Use rigorous mathematical methods such as plasticity theory to communicate geotechnical knowledge

• Engage with the energy debate

77

Final remarks:

Interactions in Offshore Foundation Design

Rankine Lecture 2014 - tgs Lecture 2014 - final...Interactions in Offshore Foundation Design Prof....

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