Pipeline-Soil Interaction

Shawn Kenny, Ph.D., P.Eng.Assistant ProfessorFaculty of Engineering and Applied ScienceMemorial University of [email protected]

ENGI 8673 Subsea Pipeline Engineering

Lecture 15: Pipeline/Soil Interaction

2 ENGI 8673 Subsea Pipeline Engineering – Lecture 15© 2008 S. Kenny, Ph.D., P.Eng.

Lecture 15 Objective

to examine engineering models to analysegeotechnical loads, pipeline/soil interaction and structural load effects for offshore pipelines


Overview

Geotechnical LoadsSoil mechanical behaviour

Pipeline/Soil InteractionLoad transfer mechanisms

Structural Load EffectsPipeline mechanical response


Design ConsiderationsInstallation

Pipeline embedmentOn-bottom roughness

• Mechanical response, free spans

InterventionPre-sweep, clearanceTrenching

• Natural in-fill, mechanical backfillRock dump

OperationsThermal expansionLateral and upheaval bucklingOn-bottom stability

Ref: Langley (2005)


Geotechnical Loads – Soil Mechanics

Seabed SurveysRemote sensingIn-situ testing and sample recoveryIndex and laboratory testing

Key IssuesSoil typeStrengthparametersLoad-displacementbehaviour

Ref: BCOG (2001)


Pipeline/Soil InteractionEngineering Tools

Guidance documents• ALA, DNV, NEN

Numerical models• Structural• Continuum

Physical models• Full-scale• Large-scale• Centrifuge

Key IssuesLoad transfer mechanismsStress or strain based designModel uncertainty

Ref: C-CORE


Structural Load Effects

Design ChecksLimit States• SLS• ULS

Stress• Combined loading

criteriaStrain• Rupture• Local buckling


Pipeline/Soil Interaction Analysis

Structural Finite Element Procedures

Standard toolRigid pipeline/structureSoil load-displacement


Soil Load-Displacement Relationships

Axial

Transverse Lateral

Vertical Upward

Vertical DownwardRef: ALA (2001)


Trench Effects

Engineering ModelsLoad-Displacement

CentrifugemodelsLarge-scalephysicalmodelsContinuum FEA Ref: Phillips et al. (2004)


Buried Performance

ThermalFlow assurance

MechanicalUplift, flotation, subsidence during pipe layUpheaval buckling during operations

Ref: C-CORE


Example 15-01

Calculate the virtual anchor point, axial strain and end deflection due to thermal expansion for a buried pipeline

Design conditionPartial restraint• Shore approach• Platform tie-in

EN 8673 Subsea Pipeline Engineering Lecture 15Example 15-01

Winter 2008

Example 15-01

Calculate the anchor point, axial strain and end deflection due to thermal expansion for a buried offshore pipelinelocated outside the 500m excursion limit.

DEFINED UNITS

MPa 106Pa:= kPa 103Pa:= GPa 109Pa:= C K:= kN 103N:=

PIPELINE SYSTEM PARAMETERS

Nominal Outside Diameter Do 273.1mm:=

Initial Selection Nominal Wall Thickness (Sec.5 C203 Table 5-3) tnom 9.525mm:=

External Corrosion Protection Coating Thickness tcpc 0mm:=

Fabrication Process (Sec.7 B300 Table 7-1) [SMLS, HFW, SAW] FAB "SMLS":=

Corrosion Allowance (Sec.6 D203) tcorr 3mm:=

Elastic Modulus E 205GPa:=

Specified Minimum Yield Stress (Sec.7 B300 Table 7-5) SMYS 450MPa:=

Speciifed Minimum Tensile Stress (Sec.7 B300 Table 7-5) SMTS 535MPa:=

Coefficient of Thermal Expansion αT 1.15 10 5−⋅ C 1−

:=

Poisson's Ratio ν 0.3:=

Pipeline Route Length Lp 25km:=

Linepipe Density ρs 7850kg m 3−⋅:=

Concrete Coating Thickness tc 50mm:=

Concrete Coating Density ρc 3050kg m 3−⋅:=

OPERATATIONAL PARAMETERS

API Gravity API 38:=

Product Contents Density

ρcont 1000 kg⋅ m 3−⋅

141.5131.5 API+

⋅:= ρcont 835 m 3− kg⋅=

Design Pressure (Gauge) Pd 10MPa:=

Safety Class (Sec.2 C200-C400) [L, M, H] SC "M":=

Design Pressure Reference Level href 5m:=

Temperature Differential ΔT 50 C⋅:=

Maximum Water Depth hl 0m:=

Seawater Density ρw 1025kg m 3−⋅:=

Hydrotest Fluid Density ρt 1025kg m 3−⋅:=

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GEOTECHNICAL PARAMETERS

Undrained Shear Strength Cu 25kPa:=

Adhesion Factor αsoil 0.25:=

DNV OS-F101 PARTIAL FACTORS AND DESIGN PARAMETERS

System Operations Incidental/Design Pressure Factor (Sec.3 B304) γinc_o 1.10:=

System Test Incidental/Design Pressure Factor (Sec.3 B304) γinc_t 1.00:=

Material Resistance Factor (Sec.5 C205 Table 5-4) γm 1.15:=

Safety Class Resistance Factor (Sec.5 C206 Table 5-5) γSC 1.138:=

Material Strength Factor (Sec.5 C306 Table 5-6) αU 0.96:=

Maximum Fabrication Factor (Sec.5 C307 Table 5-7)

αfab 1.00 FAB "SMLS"=if

0.93 FAB "HFW"=if

0.85 FAB "SAW"=if

:= αfab 1.00=

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Diameter Fabrication Tolerance(Sec.7 G200 Table 7-17)

ΔDo max 0.5mm 0.0075 Do⋅, ( ) FAB "SMLS"= Do 610mm≤∧if

0.01 Do⋅ FAB "SMLS"= Do 610mm>∧if

min max 0.5mm 0.0075 Do⋅, ( ) 3.2mm, ( ) FAB "HFW"= Do 610mm≤∧if

min 0.005 Do⋅ 3.2mm, ( ) FAB "HFW"= Do 610mm>∧if

min max 0.5mm 0.0075 Do⋅, ( ) 3.2mm, ( ) FAB "SAW"= Do 610mm≤∧if

min 0.005 Do⋅ 3.2mm, ( ) FAB "SAW"= Do 610mm>∧if

:= ΔDo 2.048 mm⋅=

Wall Thickness Fabrication Tolerance(Sec.7 G307 Table 7-18)

tfab 0.5mm FAB "SMLS"= tnom 4mm≤∧if

0.125 tnom⋅ FAB "SMLS"= tnom 4mm>∧if

0.125 tnom⋅ FAB "SMLS"= tnom 10mm≥∧if

0.100 tnom⋅ FAB "SMLS"= tnom 25mm≥∧if

3mm FAB "SMLS"= tnom 30mm≥∧if

0.4mm FAB "HFW"= tnom 6mm≤∧if

0.7mm FAB "HFW"= tnom 6mm>∧if

1.0mm FAB "HFW"= tnom 15mm>∧if

0.5mm FAB "SAW"= tnom 6mm≤∧if

0.7mm FAB "SAW"= tnom 6mm>∧if



:= tfab 1.191 mm⋅=

Material Derating (Sec.5 C300 Figure 2)

ΔSMYS 0MPa ΔT 50C<if

ΔT 50 C⋅−( )30MPa50 C⋅

⎛⎜⎝

⎞⎟⎠

⋅⎡⎢⎣

⎤⎥⎦

50 C⋅ ΔT< 100C<if

30MPa ΔT 100 C⋅−( )40MPa100 C⋅

⎛⎜⎝

⎞⎟⎠

⋅+⎡⎢⎣

⎤⎥⎦

otherwise

:= ΔSMYS 10.00 MPa⋅=

ΔSMTS 0MPa ΔT 50C<if

ΔT 50 C⋅−( )30MPa50 C⋅

⎛⎜⎝

⎞⎟⎠

⋅⎡⎢⎣

⎤⎥⎦

50 C⋅ ΔT< 100C<if

30MPa ΔT 100 C⋅−( )40MPa100 C⋅

⎛⎜⎝

⎞⎟⎠

⋅+⎡⎢⎣

⎤⎥⎦

otherwise

:= ΔSMYS 10.00 MPa⋅=

fy SMYS ΔSMYS−( ) αU⋅:= fy 422 MPa⋅=

fu SMTS ΔSMTS−( ) αU⋅:= fu 504 MPa⋅=

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Winter 2008

ENGINEERING ANALYSIS

PIPELINE GEOMETRIC PROPERTIES

Inside Pipeline Diameter (Operations Case)

Di_o Do 2. tcorr⋅− 2. tfab⋅−:= Di_o 264.72 mm⋅=

Inside Pipeline Radius (Operations Case)

Ri_o 0.5 Di_o⋅:= Ri_o 132.36 mm⋅=

Effective Outside Pipeline Diameter

De Do 2. tcpc⋅+ 2. tc⋅+:= De 373.10 mm⋅=

Pipeline Steel Area

Astπ

4Do

2 Do 2 tnom⋅−( )2−⎡

⎣⎤⎦⋅:= Ast 7.89 103

× mm2⋅=

Concrete Area

Acπ

4Do 2 tc⋅+( )2 Do

2−⎡

⎣⎤⎦⋅:= Ac 5.08 104

× mm2⋅=

Effective Outside Pipeline Area

Aeπ

4Do 2 tc⋅+( )2

⋅:= Ae 1.09 105× mm2

⋅=

Inside Pipeline Area

Aiπ

4Di_o

2⋅:= Ai 5.50 104

× mm2⋅=

BUOYANCY FORCE (per meter basis)

BF g m⋅ ρw Ae⋅ ρc Ac⋅− ρs Ast⋅−( )⋅:= BF 1.03− kN⋅=

Buoyancy Force Check

BFchk "NEGATIVE BUOYANCY" BF 0<if

"FLOTATION" otherwise

:= BFchk "NEGATIVE BUOYANCY"=

External Hydrostatic Pressure

Pe ρw g⋅ hl⋅:= Pe 0.00 MPa⋅=

HOOP STRESS (THIN WALL THEORY)

σhPd Di_o⋅ Pe Do⋅−

2 tnom tcorr− tfab−( ):= σh 248.13 MPa⋅=

SOIL RESISTANCE PARAMETERS

Soil Axial Restraint per Unit Length (Eqn B-1 ALA 2001

f π De⋅ Cu⋅ αsoil⋅:= f 7.33 103× N m 1−

⋅⋅=

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Winter 2008

Distance to Virtual Anchor Point - Assumes constant temperature (conservative) - Equation 9 of Palmer and Ling (1981) OTC4067

zπ Pd⋅ Ri_o

2⋅

f1 2 ν⋅−

2 tnom⋅

Pd Ri_o⋅E⋅ αT⋅ ΔT⋅+

⎛⎜⎝

⎞⎟⎠

⋅:= z 157.51 m=

Virtual Anchor Length Check

zchk "VIRTUAL ANCHOR OK" z 0.5 Lp⋅<if

"RECALCULATE" otherwise

:=zchk "VIRTUAL ANCHOR OK"=

COMBINED STRESS STATE

Axial End Displacement

δendPd Ri_o⋅

2 E⋅ tnom⋅1 2ν−( )⋅ αT ΔT⋅+

⎡⎢⎣

⎤⎥⎦

z⋅f z2

⋅

4 π⋅ E⋅ Ri_o⋅ tnom⋅−:= δend 56 mm⋅=

Axial End Displacement [Equation 12 - Palmer and Ling (1981) OTC 4067]

δPalmerπ Ri_o⋅ E⋅ tnom⋅ αT ΔT⋅( )2

⋅

f1

Pd Ri_o⋅12

ν−⎛⎜⎝

⎞⎟⎠

⋅

E tnom⋅ αT⋅ ΔT⋅+

⎡⎢⎢⎣

⎤⎥⎥⎦

2

⋅:= δPalmer 56 mm⋅=

Axial Stress (For X < Z)

x75 0.75 z⋅:=

σl_75Pd Ri_o⋅

2tnom

f2 π⋅ Ri_o⋅ tnom⋅

x75⋅−:= σl_75 39.77− MPa⋅=

x1 1.00 z⋅:=

σl_1Pd Ri_o⋅

2tnom

f2 π⋅ Ri_o⋅ tnom⋅

x1⋅−:= σl_1 76.19− MPa⋅=

AXIAL STRESS (FOR X >= Z)

σl νPd Ri_o⋅

tnom⋅ E αT⋅ ΔT⋅−:= σl 76.19− MPa⋅=

EQUIVALENT STRESS CHECK

σeq σh2

σh σl⋅− σl2

+:= σeq 293.73 MPa⋅=

σeqchk "EQUIVALENT STRESS OK" σeq 0.9 SMYS⋅<if

"INCREASE WALL THICKNESS" otherwise

:=σeqchk "EQUIVALENT STRESS OK"=

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Reading Listhttp://www.fugro.com/survey/offshore/gcs.asp

ALA (2001). Guideline for the Design of Buried Steel Pipe. July 2001, 83p.[2001_ALA_Design_Guideline.pdf]

Cathie, D.N., Jaeck, C., Ballard, J.-C. and Wintgens, J.-F. (2005). “Pipeline geotechnics – state-of-the-art.” Frontiers in Offshore Geotechnics, ISFOG, ISBN 0 415 39063 X, pp.95-114[2005_Cathie_PSI.pdf]

Palmer, A.C. and Ling, M.T.S. (1981). “Movements of Submarine Pipelines Close to Platforms.” Proc., OTC, OTC 4067, pp.17-24.

Palmer, A.C., Ellinas, C.P., Richards, D.M. and Guijt, J. “Design of Submarine Pipelines Against Upheaval Buckling.” Proc., OTC, OTC 6335, pp.551-560.


Referenceshttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Soil_mechanicsBCOG (2001). BC Offshore Oil & Gas Technology Update, JWEL Project No. BCV50229, October 19, 2001DNV (2007). Submarine Pipeline Systems. Offshore Standard, DNV OS-F101, October 2007, 240p.Langley, D. (2005). “A Resourceful Industry Lands the Serpent”, Journal of Petroleum Technology, 57(10), 6p.Phillips, R. A. Nobahar and J. Zhou (2004). “Trench effects on pipe-soil interaction.” Proc. IPC, IPC 04-0141, 7p.

Pipeline-Soil Interaction

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