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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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Axial Resistance

Effect of pipeline coating roughness

Hill et al (2012)

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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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Sampling - Box Corer

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Sampling - Piston Core

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Dep

th b

elow

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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

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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

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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

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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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tion

fact

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[-]

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

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= 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

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