Virtual Anchor Length - KBR

Gas Speak Colloquium 2011

Determination of Soil Restraint Properties and Calculation of Virtual Anchor Lengths in Buried Pipelines Daniel Tian Mechanical Engineer KBR

Introduction

• What is Pipe Pipe-Soil Soil Interaction? – Behavior of buried pipeline in the surrounding soil – Pipe and soil together form the engineered system

• Soil Restraint Properties p – Relationship between soil resistance and pipe movement

• Virtual Anchor Length – Pipe section moving relative to the soil near directional change

Why now? • Risk based pipeline design approach (AS2885) • Challenges facing pipeline design in Australia – Higher pressures – Larger diameters

Telfer Pipeline

Brooklyn Lara Brooklyn-Lara Pipeline

Iona Orford Iona-Orford Pipeline

Queensland CSG Pipeline

Size (DN)

250

500

450

1050

Length g (km) ( )

443

65

101

Over 1000

MAOP (MPag)

10.2

10.2

15.3

10.2 / 15.3

WT (mm)

4.7

7.9

9.1

14.1 / 18.7

Material

API 5L X60

API 5L X70

API 5L X70

API 5L X70

Onshore Pipeline Uplift, Uzbekistan

Subsea Oil Pipeline Failure, Brazil, 2000

Purpose of Calculating Pipe-Soil Interaction • Soil restraint input into Pipe Stress Analysis (AutoPipe or Caesar II) • Virtual anchor length to define model boundary – Above-Ground Facilities

- Buried Pipeline p Sections (soil restraint is the fundamental part)

Compressor Station

Major Water Crossing

Determination of Soil Restraint Properties Longitudinal

Actual

Vertical Upward

Transverse

Vertical Downward

Elastic-Plastic Soil Spring p g

Determination of Soil Restraint Properties Soil and Pipe Properties:

Elastic-Plastic Soil Springs:

• • • • •

• •

Unit Weight () Undrained Shear Strength (Su) Angle of Internal Friction () Pipe Diameter (D) Depth of Cover (H)

ALA AutoPipe Peng

Soil Stiffness (K1) Ultimate Soil Resistance (P1)

ALA Method for Calculating Pipe-Soil Interaction • Widely recognised and applied • Advanced soil modeller in Caesar II • Based on laboratory and field experimental i t l iinvestigations ti ti • Formulas available for design factors • Limitation: – Under-estimate vertical upward resistance

(Appendix B)

AutoPipe Method for Calculating Pipe-Soil Interaction • Detailed in AutoPipe Technical Reference Manual • Based on laboratory and field experimental i l iinvestigations i i • Limitations: Li it ti – Design factors need to be determined from tables and charts (Pipe-Soil Appendix)

Peng’s Method for Calculating Pipe-Soil Interaction • First published in 1978 • Basic Soil Modeller in Caesar II • Limitations: – Preliminary P li i estimation ti ti – Based on theoretical soil mechanics

(Chapter 10)

KBR Pipe-Soil Interaction Calculation Sheet

• Each of the 3 established methods has strengths and weaknesses. • Compare results from 3 methods and choose which method is suitable for a particular application • Calculate input data for stress software package

Typical Soil Properties Soil Type

Soft Clay y

Normal Clay y

Stiff Clay y

Loose Sand

Medium Sand

Dense Sand

Dry Unit Weight (kg/m3)

1600

1800

2000

1600

1800

2000

Undrained Shear Strength (kP ) (kPa)

5

25

100

0

0

0

Internal Friction Angle (°) ()

0

0

0

25

30

40

• To be used when soil data is not available for critical locations • Soft clayy – worst design g case for unknown soil

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods Vertical Upward

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods Transverse

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods Longitudinal

Vertical Downward

Calculation of Virtual Anchor Length in Buried Pipeline • Distance from bend, tee or A/G-U/G transition to the point where pipe axial strain is completely suppressed by soil.

La Virtual Anchor Length

La = Co A [ε E + (0.5 - v) SHP] / Plong

Elastic Factor

Stress Thermal Expansion

Stress Pressure Elongation

Longitudinal Soil Resistance

Virtual Anchor Lengths under Different Soil Conditions DN1000, 1200mm Cover, 55°C DT, 10.2MPag DP

Applications Anchor Block Requirement at Pig Trap Major W t Water Crossing

Depth of Cover Change

Overbend at Top of Hill

Pipeline Design Engineer’s Responsibility • Know how to use the right engineering design tools • Do some research and understand pipe-soil interaction • Make sound engineering judgement

Questions?

Disclaimer These materials contain information of a general nature and are provided for discussion p p purposes p only. y Theyy do not in anyy way represent engineering advice and KBR does not warrant the accuracy, completeness or currency of the information in these materials. Any person who uses or relies on these materials does so entirely at their own risk.