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
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