16 th ICSMGE Osaka, September 2005 Challenges of Offshore Geotechnical Engineering Randolph, Cassidy, Gourvenec & Erbrich 1 Mark Randolph, Mark Cassidy, Susan Gourvenec Centre for Offshore Foundation Systems The University of Western Australia CENTRE FOR OFFSHORE FOUNDATION SYSTEMS Challenges of Offshore Geotechnical Engineering & Carl Erbrich Advanced Geomechanics, Perth 16 th Int. Conf. on Soil Mechanics and Geotechnical Engineering Osaka, September 2005 16 th ICSMGE, Osaka - September 2005 2 Outline • Background • Trends in offshore design practice • Site Investigation Practice • Strength measurement in soft sediments • Full-flow penetrometers • Pile Foundations • Design approaches in sand • Cyclic loading • Shallow Foundations • Embedded skirts; suction installation • Yield envelopes for combined V-M-H loading • Mobile Drilling Units • Penetration issues • Force-resultant models for foundation response • Anchoring Systems • Suction caissons • Static and dynamic embedment of anchors
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CENTRE FOR OFFSHORE FOUNDATION SYSTEMS …Randolph, Cassidy, Gourvenec & Erbrich 1 ... (onshore) deep basements ... sites with cone (e.g. Gulf of Mexico: N kt ~...
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16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
1
Mark Randolph, Mark Cassidy, Susan GourvenecCentre for Offshore Foundation Systems
The University of Western Australia
CENTRE FOR OFFSHORE FOUNDATION SYSTEMS
Challenges of Offshore Geotechnical Engineering
&Carl Erbrich
Advanced Geomechanics, Perth
16th Int. Conf. on Soil Mechanics and Geotechnical EngineeringOsaka, September 2005
16th ICSMGE, Osaka - September 2005 2
Outline
• Background• Trends in offshore design practice
• Site Investigation Practice• Strength measurement in soft sediments• Full-flow penetrometers
• Pile Foundations• Design approaches in sand• Cyclic loading
• Mobile Drilling Units• Penetration issues• Force-resultant models for foundation response
• Anchoring Systems• Suction caissons• Static and dynamic embedment of anchors
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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16th ICSMGE, Osaka - September 2005 3
Evolution of Platforms
Piles,gravity bases
or suction caissons Piles
Piles or suction caissons
300 m50 m 500 m 1200 m
Courtesy: Minerals Management Services
16th ICSMGE, Osaka - September 2005 4
Evolution of Floating and Subsea Facilities
Courtesy: Minerals Management Services
Suction caissons
SkirtedFoundations
1750 m 2000 m
Piles Drag anchors
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Trends in Offshore Design Practice• General
• Enormous costs of site investigation (> $1 million/borehole)• Focus more on capacity than deformations or stiffness• Strength determination: issues of anisotropy, degradation due to cyclic
loading, sampling challenge in deep water soft clays• Environmental loading more significant than for onshore –yield
envelopes replacing bearing capacity factors• Shallow water
• Driven pipe piles – extension of onshore practice, but high capacity• Carbonate sediments – alternative foundations to driven piles• Skirted ‘gravity base’ foundations instead of (onshore) deep basements• Innovative use of short suction caissons in dense sands
• Deep water• TLPs: Special attention to cyclic loading on tension piles• Anchoring technology (drag anchors, suction caissons etc)• Geohazards and pipelines
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Geohazards, Pipelines (not covered here)
Courtesy: Norwegian Geotechnical Institute
200 m
Scarps
Canyons
800 to 1000 m
Steep slopes
Example continental slope
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Offshore Site Investigation
• Conventional drilling (> 30 to 50 m depth of soil)• Piston samples (typically ~80 mm) • Downhole vane and penetrometer tools (inside 150 mm pipe)
• Seabed frames (< 30 to 50 m depth - soft seabed)• Field vane and wheel-drive penetrometer probes
• Large diameter coring (up to 20 to 30 m long)• Fixded piston jumbo core or STACOR (100 to 150 mm diameter)
• Remote robotics – PROD• Small diameter (44 mm) piston sampling to 125 m• Penetrometer testing to 100 m (2 m strokes)
• Full-flow penetrometers• Extensive use of cylindrical (T-bar) and spherical (ball)
penetrometers for soft sediments
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Seabed Wheeldrive Penetrometer
To control anddata acquisitionTo control and
data acquisition
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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16th ICSMGE, Osaka - September 2005
Resistances Factors For Simple Shear Strength
0
5
10
15
20
25
30
0 5 10 15 20Resistance factors, Nkt-SS and NTbar-SS
Depth(m)
OffshoreWest Africa
OnshoreNorway
OffshoreAustralia
OnshoreAustralia
Open symbols: coneSolid symbols: T-bar
Coefficient of variation of average N values for each site:T-bar: 2 % ; Cone: 14 %
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Comments on Offshore Data
• Vane • Typically more scattered than other data (compensating effects
of disturbance, consolidation and rate effects)
• Cone and T-bar penetrometer data• Significant correction of cone data from measured to net values
• Net cone and T-bar resistances similar but cone resistance increases faster with depth than T-bar
• Correlations with laboratory simple shear data suggest resistance factors of ~12, but greater variance among different sites with cone (e.g. Gulf of Mexico: Nkt ~ 17)
• Field tests: typical strain rates ~105 to 107 higher than for standard laboratory test
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Rate Effects During Undrained Penetration
0.50.60.70.80.9
11.11.21.31.41.5
0.1 1 10 100Rate (mm/s)
Parameter ratio
p/p20 mm/s
Cone: qcnetCone: Du2T-barBall
+ 10%
- 10%
Δu2
qcnet
Hyperbolic sine Logarithmic fit
Rate effect:~13 % per log cycle
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Partial consolidation Undrained
Analysis of Variable Rate T-bar Test
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 10 100 1000 10000V = vd / cv
qTbar ratio: variable/standard
cv = 0.15 mm2/scv = 4.7 m2/yr
Randolph &Hope (2004):
cone resistance
Randolph &Hope (2004):
T-bar resistance
Rate parameter: λ = 0.13
Backbonecurves cv deduced by
matching to backbone curve
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Rate Effects and Softening
• Shear strength dependency on strain rate• strength increase per log cycle: rate (μ) ~ 5 to 20 %
• Expect ~50 to 100 % increase due to rate effects
• Strain-softening or damage• soil gradually remoulded as it passes through T-bar or ball
mechanism
• net (average) strength therefore less than peak strength
Rate and strain-softening effects compensate;quantify using strain path approach
⎟⎟⎠
⎞⎜⎜⎝
⎛γγ
μ+=ouo
u log1ss
&
&
( ) ( ) 95/3remremuus e1s/s ξξ−δ−+δ==ξδ
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T-bar Factors: Strain Rate and Softening
5
10
15
20
25
0 0.05 0.1 0.15 0.2 0.25Rate parameter μ
NTbar ξ95 = 50ξ95 = 25ξ95 = 15
No strainsoftening
Parametersα = δ rem = 0.2ξTbar = 3.85
ξ95 = 10
For typical rate parameter, μ ~ 0.1, NTbar ~ 11 to 13
Corresponding theoretical range for Nball ~ 13 to 16
Strain to 95 % remoulded
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Remoulded Strength and Sensitivity
• Penetrometers ‘measure’ rate-enhanced but partially softened average strength
• Sensitivity gauged by comparing extraction and penetration resistances • Monitor cone resistance during extraction
• Fully remoulded shear strength determined by cycles of penetration and extraction• Full-flow penetrometers (T-bar, ball) superior to cone
• Grouted piles: significant loss of shaft friction due to operational cyclic loading
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Shallow Foundations
• Gravity-base foundations • Large (> 400 m tall, plan area > 15,000 m2)• High environmental loading (cyclic, large moment component)
• Embedded skirts• Serves equivalent purpose as deep excavation, encasing soft sediments• Consolidation within skirts contributes to settlement (long time scale) • Suction-assisted installation and local strengthening of soils• In sands, suction installation cause quasi-piping failure within soil plug
• Yield envelopes for combined V-M-H loading• Design codes gradually replacing factors for inclined and eccentric
loading by yield envelopes in V-M-H space• Uniaxial bearing capacities (Vult, Mult, Hult) need to allow for strength
gradient and embedment
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Troll – Concrete Gravity Base Structure (GBS)
Troll (1995)Total height ~ 450 m; foundation width 150 m, skirt depth 36 m
16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering
Randolph, Cassidy, Gourvenec & Erbrich
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Wandoo Gravity Base – NW Shelf, Australia
1 m of dense carbonate sandoverlying calcareniteCyclic sliding critical