obtaining and use of soil parameters for deepwater ... · obtaining and use of soil parameters for...
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Obtaining and Use of Soil ParametersFor Deepwater Pipeline Design
Mark FrancisFugro GeoConsulting (Wallingford)
Jean-Christophe Ballard Fugro GeoConsulting (Belgium)
Date 12th June 2014
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Contents Menu
Introduction Geotechnical input to pipeline design Main soil design parameters Geotechnical models
Site investigation methods Characteristics of deepwater sediments Sampling techniques Field tests Laboratory tests
In-situ measurement of pipe-soil interaction Fugro Smartpipe description Some observations and design implications
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Background
Deepwater pipelines are generally operated under high internal temperature and pressure: end expansion, axial walking and lateral buckling
Pipe/soil interaction forces are key
Soil conditions are challenging and difficult to characterize in top 0,5m: very soft, sensitive soil fabric/structure, low stress level
Considerable cost savings can be made by optimizing pipe-soil interaction forces:– Reduced requirement for stabilisation and anchoring– Reduced need to tolerate end expansion
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Geotechnical input to pipeline design
Vertical, axial and lateral pipe-soil resistances
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Main design soil parameters
Pipeline embedment:– Undrained soil response– Undisturbed and remoulded undrained shear strength profiles Su and Sur
– Submerged unit weight ’
Lateral resistance:– Undrained response– Undisturbed and remoulded undrained shear strength profiles Su and Sur
– Submerged unit weight ’
Axial resistance:– Drained or undrained response– Interface strength (residual):
• Drained ’, ’• Undrained Su, Su,int
• Pipeline coating roughness• Coefficient of consolidation cv
Date
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Main design soil parameters
Accurate undrained shear strength profile at shallow depth is key !
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Pipeline Embedment
Pipeline embedment is a key design parameter
Pipeline embedment is a combination of:– Static penetration due to pipe submerged weight– Installation effects
• Static effects• Dynamic effects
Difficult to predict accurately:work with a range of parameters or statistical distributions
Date
z
White & Cathie (2010)
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Pipeline Embedment
Static penetration resistance
• Randolph & White (2008)
z: Pipe embedmentD: Pipe outside diametersu,inv: Undrained shear strength at pipe invertV: Effective pipe weight
– Expression fitting theoretical solution– Effects of heave and buoyancy can be included
25.0
,
6
Dz
DsV
invu
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Pipe
invert embe
dmen
t z/D
[‐]
Pipeline contact stress V/D [kPa]
Verley & Sotberg (1992)Zhang et al (2002)Randolph et al (2008)
D = 0.8 mSu,inv = 2 kPa
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Pipeline Embedment
Static installation effects– Increased embedment due over-stress at touch-down point– Degree of « over-stress » depends on
• Pipeline bending rigidity• Water depth• Seabed stiffness• Lay tension
– Degree of « over-stress » typically between 1 and 3
Randolph & White (2008)
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Pipeline Embedment
Dynamic installation effects
– Increased penetration due to cyclic pipe movements during pipe-lay
– Typically 2 to 10 times the static embedment
– Depends on• Laying technique• Laying speed• Sea state• Seabed sensitivity
– Use of the remoulded undrained shear strength gives a reasonableestimate of embedment for average lay conditions
Westgate et al (2010)
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Axial Resistance
Simple problem? Depends on sliding velocity, contact stresses , coating roughness and
embedment
White et al (2012)
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Axial Resistance
Effect of axial velocity: drained or undrained response– Typical range of pipe axial velocities: 0 – 100 mm/min– Velocity at which transitions occur depends on soil coefficient of
consolidation cv
White and Cathie (2010)
UndrainedDrained Partially drained
Friction factor
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Axial Resistance
Effect of contact stresses– Typical range of contact stresses: 0 – 10 kPa
Randolph et al (2010)
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Lateral Resistance
Stages of lateral response– First load breakout resistance– First load residual resistance– Cyclic resistance in operation with formation of soil berms
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Lateral Resistance
Breakout lateral resistance– Empirical formula (Bruton et al, 2006; Dendani and Jaeck, 2008) – Theoretical VH yield surfaces (undrained failure)
• Vertical and horizontal resistances are coupled
White & Cathie (2010)
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Contents Menu
Introduction Geotechnical input to pipeline design Main soil design parameters Geotechnical models
Site investigation methods Characteristics of deepwater sediments Sampling techniques Field tests Laboratory tests
In-situ measurement of pipe-soil interaction Fugro Smartpipe description Some observations and design implications
www.fugro.com
Key Issues
Vertical depth control – determining mudline Vertical depth control – sample recovery Sample quality – further laboratory testing
Normally consolidated clay High plasticity Extremely low strength (su) 1kPa to 5kPa Soil water content >200% and submerged unit weights (’) ~3kN/m3
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In-situ testing and Sampling Techniques
Soil Sampling– Box core
• Area ratio = 0.5m x 0.5m x 0.5m, 3mm• Index testing on the front face of the box core
– Piston core • Large (20m to 25m recovery), 105 mm ID, 160mm OD• Smaller piston core (3m to 6m recovery), 75mm ID, 90mm OD
In-situ Testing– Skirted seabed frame with CPT, T-bar or Ball
• Size of skirt, shape of skirt• Seabed frame settlement gauge
– Box core - Mini Tbar, Mini Ball and cyclic testing– SMARTPIPE®
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Dep
th b
elow
Sea
floor
[m]
Sample quality, e / e0
Piston
Boxcore
Sampling - Sample Quality
Inc Oed and CRS Oed data from extremely low strength to low strength clay samples from deep water sites west Africa
Sample quality very good to excellent (<0.03) for both piston core and box corer in the top 2 m BSF.
Very Good to Excellent
Fair to Good
Poor
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Sampling - Vertical Depth Accuracy
Best Estimatex Box core x Piston core
Borehole log of box core data (blue) and piston core data (green) Unit weight (, kN/m3) and undrained shear strength (su, kPa) Piston core may not recover all soil from mudline Box core data (complete representative sample of seafloor) can be
used as a guide.
In this example, piston core recovery missing soil from top 0.25m– Implications for depths of laboratory tests
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In-situ testing - Seabed Frame
T-Bar, CPT or Ball
Skirt
LARS(Launch and
Recovery System)
Frame settlement gauge
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In-situ testing - Box Corer (Mini T-bar)
Mini Tbar testing (D=12mm, L=75mm)– su profile in top 0.5m– Sensitivity– Strength degradation
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In-situ testing - Vertical Depth Accuracy
Mini T-barCPTT-bar Best Estimate DL
Borehole log of box core data – Submerged unit weight (’)– Index testing - undrained shear strength (su) – Mini T-bar - undrained shear strength (su)
Comparison of the su derived from mini T-bar against su derived from CPT and T-bar
su data from the CPT may need to be offset using the settlement gauge
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In-situ testing - Mini T-bar
0.60
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0.00-0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04
T-Bar Bearing Resistance [MPa]
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th B
elow
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line
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Deg
rada
tion
fact
or -
Cycle number
At 0.3m BSF
Undrained shear strength (su) profile Degradation of su with cycling Remoulded su
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SMARTSURF™ System Description
Penetrometer Piston sampler
Folding skirt
Max. water depth 3500m Folding skirt to control settlement Real-time imaging and video
recorders Frame settlement gauge In-situ soil sampling
– 2m piston core In-situ testing
– 3m CPT or T-bar– 1m Mini T-bar
Can be used with SmartPipe®– Direct in-situ measurements
of Pipe soil interaction parameters using a prototype pipe section
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Laboratory Testing
Classification (%) Index strength tests (su, kPa) Thermal conductivity (k, W/mK)
Low-stress consolidation tests– Differential pore water
pressure measurement –Allowing higher precision at lower stresses
– Improved resolution of compression curve, coefficient of consolidation, cv data in low stress range1 kPa to 10 kPa
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Laboratory Testing
Interface testing (friction factor)– Roughness (rough/smooth)– Fast (undrained), ~0.1mm/s– Slow (drained), ~0.0001mm/s– Residual (large displacements)
Low Stress (<50kPa)– Tilt table– CAMshear– Under development, CAMtor
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Tilt table and CAMshear Test
Tilt table– low stress, drained
strength (’, ’)– Soon available in Fugro
Wallingford Laboratory
Cambridge University CAMshear device allows the simulation of axial pipe-soil interaction behaviour by dragging a small sample of extremely low strength clay (0.5 kPa to 5 kPa) over a flat sheet of pipeline coating
Variable rates of shear Variable roughness Limited displacement
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Tests under development - CAMtor
Currently under development with Cambridge University
Torsional interface shear test Simple specimen
arrangement Low normal stress range Pore pressure measurement Variable rates of shear can be
applied
So, r = K0*n
n
Zero radial strain
r=0r=0
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Contents Menu
Introduction Geotechnical input to pipeline design Main soil design parameters Geotechnical models
Site investigation methods Characteristics of deepwater sediments Sampling techniques Field tests Laboratory tests
In-situ measurement of pipe-soil interaction Fugro Smartpipe description Some observations and design implications
www.fugro.com
Description of Smartpipe
Principle: instrumented section of polypropylene-coated steel pipe that measures forces (and pore pressures) while penetrating into the soil and moving laterally and axially
INSTRUMENTED PIPE SECTION
DEPLOYABLE MUDMATS AND DETACHABLE SKIRTS
SETTLEMENT PLATE
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Description of Smartpipe
Instrumented pipe section– Diameter = 225mm (8”)– Overall length = 1200mm (47”)– Instrumented length = 776mm (30”)– Dummy ends to negate end effects– Nine pore pressure transducers– Two tri-ax loadcells– One temperature sensor
Instrumented Pipe Section
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Merits of Smartpipe
Tests performed at the seabed, in undisturbed soil conditions
Tests performed at nearly full scale
Direct pipe/soil resistance measurements accounting for:– Installation effects: strength regain and consolidation effects– Sliding velocity and in-situ drainage properties– Low contact stresses– Coating roughness– Increased confinement due to embedment
Large-scale test (Bruton, 2009)
Direct shear test (White et al, 2011)
Tilt table test
Centrifuge test (Bruton, 2009)
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Some in-situ observations and design implications
A critical design concern is low axial resistance– Extreme end expansion– Excessive axial walking– Hampers reliable buckle formation
Considerable savings can be made by small increases of axial friction factor
Smartpipe® measurements can demonstrate / justify in the field higher axial friction factors for pipeline design
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Some in-situ observations and design implications
Back-analysis of dissipation of excess pore pressures induced by pipe penetration
Cv ~ 100 m²/year
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Some in-situ observations and design implications
Design implications:– Partial drainage for typical pipeline axial velocities– Soil consolidation during hydrotest– Higher axial friction factor
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Some in-situ observations and design implications
Tested range:– Axial velocity: 0,006 to 0,35 mm/s– Embedment: 0,35 to 0,75 D – Contact stresses: 1 to 4 kPa
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Fric
tion
fact
or
= F/V
[-]
Mean normal stress n [kPa]
PeakResidualAxial velocity = 0,006 mm/sAxial velocity = 0,045 mm/sAxial velocity = 0,35 mm/s
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Some in-situ observations and design implications
Effect of pipe unloading on axial friction factor (e.g. light gas pipe after hydrotest)
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tion
fact
or
= F/V
[-]
Pipe unloading ratio Vconsol*/V [-]
PeakResidualAxial velocity = 0,006 mm/sAxial velocity = 0,045 mm/sAxial velocity = 0,35 mm/s
Evidence of strain rate effects in fast tests
Peak friction factor
Residucal friction factor
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Some in-situ observations and design implications
Residual friction factor: evidence of cyclic hardening