Understanding Piping Stress Analysis

Analyzing Buried Branch Connections Using Caesar II

Zachary SwartzJune 10, 2015

Agenda

1) Introduction to Pipe Stress Analysis2) Buried Tee Analysis

a) Preliminary Resultsb) Determining the problem

3) Solution4) Lessons Learned

Introduction

Pipe Stress Analysis:• Computer modeling software simulates how

piping system will react under real-world conditions:

Weight ǁ Temperature ǁ Pressure

• Piping will expand and contract in response to these conditions

• Piping movement:o creates stress in the pipeo applies a force to the pipe restraints and connecting

equipment• Stress levels are checked to ensure they remain

within allowable limits• Forces on restraints/equipment are checked to

ensure they are not excessive

Caesar II: Pipe stress analysis software

Introduction

Existing buried 24” Pipeline

New connection to existing 24” pipeline

New buried 16” Piping

Grade

Above Ground

Below Ground

Above Ground

Below Ground

Grade

Caesar II Stress Model

Modeling Soils in Caesar II• Caesar II simulates the soil surrounding the

pipeline using springs of varying stiffness

Fig. 1: Caesar II modeling the soil as springs

Stiff SpringWeak Spring

Dense SoilLoose Soil

• 2 options for calculating soil springs in Caesar II:1) Basic Soil Model 2) American Lifelines Alliance (ALA) Soil Model

• Basic Soil Model initially selected:o Simpler model requiring limited inputso Relatively simple theory: “hand calculations” o Geotechnical report provided soil data to match

Basic Soil Model inputs

Modeling Soils in Caesar II

IMPORTANT! TO BE DISCUSSED

Basic Soil Model• Soil surrounding the pipeline is modeled using

the Basic Soil Model

Fig. 2: Sample Geotechnical Report

Fig. 3: Basic Soil Model Input Screenshot

Receive Geotechnical

ReportExtract Soil Data Input Data into

Soil Model

Caesar II calculated springs are applied to the model to simulate the surrounding soil

Grade

Above Ground

Below Ground

Springs modeled in the lateral (sideways/vertical) and axial directions

Caesar II Stress Model

Preliminary Results

Buried Tee is overstressed(Node 770)

INITIAL DIAGNOSIS:• Pipeline expands, drags the vertical piping with it• The vertical piping is resisted by the soil packed around it• Piping breaks at the Tee!

Pipeline Expansion

Soil around piping resists movement

Preliminary Results

There’s a stress problem!

Let’s fix it!

a) Make the Tee stronger!b) Increase wall thickness!c) Soft padding at the Tee!

$$$$$$$$$$

http://thesalesblog.com/wp-content/uploads/2013/11/Screen-Shot-2013-11-11-at-9.38.40-PM.png

On second thought, let me take another look and get back to you…

My Boss

Preliminary Results

Wait a second…Detailed stress results at Tee:

o Bending stress > 1 Gpa (1,000,000 kPa)

o Unrealistic! Impossible order of magnitudeoMust be error in the stress model

Fig. 4: Caesar II Stress Results

Tee

Reviewing the Model• Review the soil springs calculated by the Basic

Soil Model:

• Axial spring is much weaker relative to the lateral springs (Side/Up/Down)

Axial Soil Resistance WEAKLateral Soil Resistance STRONG

Soil Spring Spring Stiffness (N/cm/mm)

Axial 3

Side 297

Up 297

Down 297

CAUTION!

Fig. 5: Basic Soil Model Soil Spring Summary

Fig. 6: Pipeline expansion and Caesar II springs sketch

Reviewing the Model• Axial resistance (red arrows) = WEAK• Lateral resistance (blue arrows) = STRONG

Fig. 6: Pipeline expansion and Caesar II springs sketch

Reviewing the Model

• Pipeline expands in the +ve X direction

• Weak axial resistance = large pipeline expansion

• Tee moves from original position χ χ’

24” pipeline: • only weak axial springs

affectedNo issues

16” branch:• Pipeline movement pulls

branch in the +ve X direction

Reviewing the Model

Fig. 6: Pipeline expansion and Caesar II springs sketch

• strong lateral springs engaged

• “Unstoppable force vs. immovable object”

• Branch must deflect, but to deflect against strong lateral springs, a huge force is required

F = k x force = Stress

Reviewing the Model

Fig. 6: Pipeline expansion and Caesar II springs sketch

• Basic Soil Model:o Theory adequate for traditional pipeline analysis

(typically, a single line in a 2D plane)o Theory translates poorly to unique geometry of

buried branch connection

• Limitation of the software

More realistic soil model is required

More realistic stress results obtained

Secondary Diagnosis

Fig. 7: Caesar II Basic Soil Model Fig. 8: Caesar II ALA Soil Model

• American Lifelines Alliance (ALA) soil model used instead

• Different theory to calculate soil springs• Additional inputs and information required

ALA Soil Model

• More balanced axial/lateral springs obtained

• Recall: lateral springs on branch are a problem• ALA lateral springs are 27x less stiff than Basic

model

ALA Soil Model

Soil Spring Basic SoilSpring Stiffness (N/cm/mm)

ALA SoilSpring Stiffness (N/cm/mm)

Axial 3 11

Side 297 11

Up 297 2

Down 297 28

27x reduction

F = k x force = Stress

x27x27

• Visual comparison of 2 soil models:Basic Soil Model ALA Soil Model

Basic Model vs. ALA

AXIAL

SIDE

VERTICAL

SIDE

AXIAL

VERTICAL

Stress level OK: 25% of allowable (12x reduction from previous 294%)

• Less stiff lateral springs vertical branch can deflect into the soil with less force required

STRESS IS ACCEPTABLE

ALA Results

https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRskVcSS2Z_SyCYae9zAgQtspvH8dVxPtxkIXnEmdFotzPicEEa

My boss

• Caesar II Basic Soil Model was initially used:1) Simpler model2) Easy to procure and input data for Basic Model3) Geotech report provided soil inputs in Basic

Model format

• Simple communication issue (or lack thereof) Client can provide us inputs for either format,

we just have to ask!

Conclusion

• ALA Soil Model used after further examinationMore realistic soil spring values

• Costly (potentially impossible) modifications were avoided

Conclusion

Lessons Learned1) Software is an aid, but NOT A REPLACEMENT for

engineering judgement

Engineer must constantly analyze the results and think, “does this make sense?”

Engineering is critical thinking, not “plug and place”

http://www.build-the-body.com/muscle-confusion.html

2) Imperative for the engineer to have a deeper understanding of the software

Must understand the math occurring behind the scenes

Software should never be a calculation “black box”

http://www.chemistry-blog.com/2012/05/04/the-source-code-debate/

Lessons Learned

3) Good reminder that every tool has its limits Software can only be as accurate/helpful as

the engineer utilizing it

“Garbage in, then garbage out”

http://left.mn/2014/02/polymet-knew-now-knew/#sthash.GxGAUAcl.dpuf

Lessons Learned