State of the art subsea foundation design- supporting industry needs

Professor Susan GourvenecCentre for Offshore Foundation Systems, University of Western AustraliaARC Centre for Geotechnical Science and EngineeringEnergy and Minerals Institute

SUT Technical Meeting, 26 February 2014

Subsea development

Sus

an.G

ourv

enec

@uw

a.ed

u.au

ww

w.c

ofs.

uwa.

edu.

au

Julimar field, Apache. www.apachecorp.com

Subsea structure

Pazfloor, Angola. Image courtesy of Subsea 7

B = 5 m, L = 10 md = 1 mV ~ 500 ‐ 600 kNIdeally small and light enough to be installed by pipe‐laying vessel

Sus

an.G

ourv

enec

@uw

a.ed

u.au

ww

w.c

ofs.

uwa.

edu.

au

Subsea foundation - VH2M2T• Subsea mudmats subjected to loading in 6 dof

• Self‐weight, biaxial horizontal loads (from thermal expansion of pipelines)

• Vertical and horizontal eccentricity of loads =  biaxial moments and torsion

Jumper

Pipeline Mudmat

Hx

Hy V

Sus

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@uw

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u.au

ww

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ofs.

uwa.

edu.

au

The geotechnical design issue• Classical bearing capacity theory

– qult = ( + 2)su x lots of reduction factors, e.g. sc, dc, ic, B’, L’, F,

– VHM, no torsion, and HM poorly predicted

– Output = single allowable vertical load or bearing pressure

– No indication of effect of different load components on proximity to failure.

• Conventional design method• Too large for pipelaying 

vessel to install. • Require second vessel on 

site = $$$ 

• Optimized design method• Small enough for 

pipelaying vessel to install. •

Sus

an.G

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@uw

a.ed

u.au

ww

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uwa.

edu.

au

1. Alternative design method

Feng et al., 2014, Géotechnique

Sus

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u.au

ww

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uwa.

edu.

au

• Failure envelope design method– Explicitly account for effect of applied loads, V, Hx, Hy, Mx, My, T

– Explicitly account for effect of geometry , B/L, d/B 

– Explicitly account for shear strength profile, kB/su0 , surface crust

– Determine how individual variables affect the design outcome.

– Indicate displacements at failure.  

Caveats

Sus

an.G

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@uw

a.ed

u.au

ww

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uwa.

edu.

au

• Be wary of oversimplification.

• Choice of appropriate su is central to calculation –affected by various conditions, inc. cyclic loading.

• Assumptions of undrained load response.

But …• Neat tool for preliminary sizing.

• Indicates influence of design input independent variables.

• Augment with more detailed analysis for detailed design.

• Tool demonstrates how improved site investigation data can have significant impact on design.

Industry impact

Sus

an.G

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enec

@uw

a.ed

u.au

ww

w.c

ofs.

uwa.

edu.

au

2. Hybrid subsea foundation • Mudmat with corner pin piles

– Increase in 6 dof load capacity over mat alone, in particular lateral and torsional resistance.

– Smaller footprint sizes. 

Sus

an.G

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enec

@uw

a.ed

u.au

ww

w.c

ofs.

uwa.

edu.

au

Gaudin et al., 2011, SUT LondonDimmock et al., 2013, ASCE JGGE

2. Hybrid subsea foundation • Mudmat with corner pin piles

Sus

an.G

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@uw

a.ed

u.au

ww

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uwa.

edu.

au

Gaudin et al., 2011, SUT LondonDimmock et al., 2013, ASCE JGGE

3. Internal skirt spacing• Optimal spacing of internal shear keys

– Prevent shearing within the confined soil plug. 

Sus

an.G

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@uw

a.ed

u.au

ww

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ofs.

uwa.

edu.

au

00.20.40.60.8

11.21.41.61.8

2

-1.5 -1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5

Mom

ent,

My(

x)/A

Bs u

0

Horizontal load, Hx(y)/Asu0

B0L0, B1L1, B2L2, B4L4, B5L5, Solid

• Failure envelopes & failure mechanisms

Internal skirt spacing

Sus

an.G

ourv

enec

@uw

a.ed

u.au

ww

w.c

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uwa.

edu.

au

Increasing # skirts

2M/HB

0

3

6, & solid

• Design charts

Internal skirt spacing

Sus

an.G

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@uw

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u.au

ww

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uwa.

edu.

au

0

1

2

3

4

5

6

7

0 0.05 0.1 0.15 0.2

Crit

ical

num

ber o

f int

erna

l ski

rts

Equivalent embedment ratio

0 ≤ κ ≤ 88 ≤ κ ≤ 1010 ≤ κ ≤ 2020 ≤ κ ≤ 100κ > 100

Feng & Gourvenec, OMAE, 2013Mana et al., ASCE JGGGE, 2012

VH2M2TV/Vu ≤ 0.5 = kB/su0or kL/su0

d/B or d/L

4. Consolidated shear strength• Increase in in situ shear strength under foundation and structure self‐

weight prior to in‐service loading.

Sus

an.G

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@uw

a.ed

u.au

ww

w.c

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uwa.

edu.

au

1

1.2

1.4

1.6

1.8

2

2.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Preload: Vp/Vu

Gai

n:V

u,co

ns/V

u

H

V

M

Fully

con

solid

ated

gai

n

Preload, vp/vu

Consolidated shear strength• Gain can be scaled linearly as a function of the degree of consolidation

Sus

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@uw

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u.au

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au

Gourvenec et al, Géotechnique, 2014

Prop

orti

on o

f ful

ly

cons

olid

ated

gai

n

Degree of consolidation, U = w/wf

5. Mobile foundations

Sus

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@uw

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u.au

ww

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uwa.

edu.

au

• Federal funding for a 3 year project to develop a geotechnical design framework for mobile foundations.

Toolbox for optimized design

1. 6 dof failure envelope design methodology and calculation spreadsheet

2. Pin piles – ‘hybrid subsea foundation’

3. Skirt spacing

4. Consolidated shear strength

5. Mobile foundations

Sus

an.G

ourv

enec

@uw

a.ed

u.au

ww

w.c

ofs.

uwa.

edu.

au

Acknowledgements:• Colleagues at COFS, in particular Mark Randolph, Christophe Gaudin, Xiaowei Feng, Divya Mana, Cristina Vulpe and Michael Cocjin

• Australian Research Council• Our industry partners, in particular Subsea 7• EMI for sponsoring this evening• SUT for the invitation to talk

Thank you for your attention! Please feel free to contact me: Susan.Gourvenec@uwa.edu.au 

www.cofs.uwa.edu.au, www.emi.uwa.edu.au