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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
an.G
ourv
enec
@uw
a.ed
u.au
ww
w.c
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
ourv
enec
@uw
a.ed
u.au
ww
w.c
ofs.
uwa.
edu.
au
1. Alternative design method
Feng et al., 2014, Géotechnique
Sus
an.G
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@uw
a.ed
u.au
ww
w.c
ofs.
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
ourv
enec
@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
ourv
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
ourv
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
ourv
enec
@uw
a.ed
u.au
ww
w.c
ofs.
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
ourv
enec
@uw
a.ed
u.au
ww
w.c
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
ofs.
uwa.
edu.
au
Increasing # skirts
2M/HB
0
3
6, & solid
• Design charts
Internal skirt spacing
Sus
an.G
ourv
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@uw
a.ed
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
ourv
enec
@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
an.G
ourv
enec
@uw
a.ed
u.au
ww
w.c
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uwa.
edu.
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
an.G
ourv
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@uw
a.ed
u.au
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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
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