interactions in offshore foundation design rankine lecture 2014

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


• Why offshore renewables?

• Challenges and solutions for offshore turbine foundations

– Conventional, unconventional and completely novel solutions

Part 3: Foundation for offshore wind turbines


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


PISA

PROJECT

Lead partner:

Partners:

Delivery team:

PISA = Pile Soil Analysis


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


Stiffness increases with load cycles


zb = 0.20

zb = 0.27

zb = 0.40

zb = 0.53

One-way load cycles

Increasing amplitude


Accumulated rotation

zb = 0.20

zb = 0.27

zb = 0.40

zb = 0.53

31.0Nkstatic


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


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


Capacity on tensile loading (sand)


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/(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


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


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


• 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

28

Conclusions from Part 3


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 [email protected] for more information

mailto:[email protected]

interactions in offshore foundation design rankine lecture 2014

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