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Buoyancy Analysis & Concrete Weight Spacing – Gas

Introduction

The buoyancy of a pipeline depends upon the weight of the pipe, the weight of the volume of water displaced by the pipe, the weight of the liquid load carried by the pipe and the weight of the backfill. As a conservative analytical practice, consider the pipeline empty for two reasons; so, the weight of the liquid will be considered as an additional safety factor and the possibility of the pipeline not being in use during a period.

This calculation is used to determine the buoyance of the specified underwater pipe based on the required thickness of concrete coating to counter the buoyancy forces.

Buoyancy Force

F_B=\frac{\pi}{4}(\frac{D_o}{12})^2\gamma_w,\,where

F_B=\frac{\pi}{4}(\frac{D_o}{12})^2\gamma_w,\,where

𝐹𝐡 βˆ’ Buoyancy Force (lbf)
π·π‘œ βˆ’ Pipe outside diameter including coating (in)
𝛾𝑀 βˆ’ Unit weight of fluid/water (lb/ft3)

Weight of Steel Pipe in the Air

W_P=10.68(D_{op}-t)t\,[lbf/ft],\,where

W_P=10.68(D_{op}-t)t\,[lbf/ft],\,where

π‘Šπ‘ƒ βˆ’ Weight of bare pipe in air (lbf)
π·π‘œπ‘ βˆ’ Pipe outside diameter (in)
𝑑 βˆ’ Pipe wall thickness (in)

Weight of Pipe Coating in the Air

W_c=[ \frac{\pi}{4} (\frac{D_o}{12})^2-\frac{\pi}{4} (\frac{D_{op}}{12})^2 ]\gamma_c\,[lbf/ft],\,where

W_c=[  \frac{\pi}{4} (\frac{D_o}{12})^2-\frac{\pi}{4} (\frac{D_{op}}{12})^2 ]\gamma_c\,[lbf/ft],\,where

π‘Šπ‘ βˆ’ Weight of coating in air (lbf)
𝛾𝑐 βˆ’ Unit weight of coating (lb/ft3)

Weight of Product in the Pipe

W_{pr}=\frac{\pi}{4}(\frac{(D_{op}-2t)}{12})^2\gamma_{pr}

W_{pr}=\frac{\pi}{4}(\frac{(D_{op}-2t)}{12})^2\gamma_{pr}

π‘Šπ‘π‘Ÿ βˆ’ Weight of product in pipe (lbf)
π›Ύπ‘π‘Ÿ βˆ’ Unit weight of product in pipe (lb/ft3)

Downward Force of the Pipe

F_P=W_P+W_c+W_{pr}\,[lbf],\,where

F_P=W_P+W_c+W_{pr}\,[lbf],\,where

Net Controlling Force

F_{net}=(F_B-F_p)SF\,[lbs/ft],\,where

F_{net}=(F_B-F_p)SF\,[lbs/ft],\,where

𝐹𝑛𝑒𝑑 βˆ’ Net controlling force (lbf)
𝑆𝐹 = Safety Factor

Downward Force of the Concrete Weight

F_{wt}=V_{wt}(\gamma_{wt}-\gamma_w)

F_{wt}=V_{wt}(\gamma_{wt}-\gamma_w)

𝑉𝑀𝑑 βˆ’ Weight of concrete (ft3)
𝛾𝑀𝑑 βˆ’ Unit weight of concrete (lb/ft3)
𝛾𝑀 βˆ’ Unit weight of fluid/water (lb/ft3)

Concrete Weight Spacing

L=\frac{F_{wt}}{F_{net}}\,[ft]

L=\frac{F_{wt}}{F_{net}}\,[ft]

Case Guide

Part 1: Create Case

  1. Select the Buoyancy Analysis & Concrete Weight Spacing application from the Design & Stress Analysis Module
  2. To create a new case, click the β€œAdd Case” button
  3. Enter Case Name, Location, Date and any necessary notes.
  4. Fill out all required Parameters.
  5. Make sure the values you are inputting are in the correct units.
  6. Click the CALCULATE button to overview results.

Input Parameters

  • Nominal Pipe Size (in): (0.625” – 48”)
  • Wall Thickness (in): (0.068”- >2”)
  • Pipe grade: (24000psi-80000psi) (if unknown use Grade A 24000)
  • Corrosion Coating Thickness (in): (1mil – 50mils)
  • Unit Weight of Pipe in Air: (21 – 1750) (lbs/ft)
  • Unit Weight of Water: (59 – 64) (lbs/ft3)
  • Unit Weight of Corrosion Coating: (50 – 100) (lbs/ft3)
  • Unit Weight of Concrete: (103 – 150) (lbs/ft3)
  • Unit Weight of Product in the Pipe: (50-100) (lbs/ft3)
  • Volume of Concrete weight (ft3)
  • Safety Factor

Part 2: Outputs/Reports

  1. If you need to modify an input parameter, click the CALCULATE button after the change.
  2. To SAVE, fill out all required case details then click the SAVE button.
  3. To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
  4. To generate a REPORT, click the REPORT button.
  5. The user may export the Case/Report by clicking the Export to Excel icon.
  6. To delete a case, click the DELETE icon near the top of the widget.

Results

  • Weight of Pipe in Air: (21 – 1750) (lbf/ft)
  • Buoyancy Force (lbf/ft)
  • Weight of Coating in Air (lbf/ft)
  • Weight of Product in Pipe (lbf/ft)
  • Downward Force of the Pipe (lbf/ft)
  • Net Controlling Force: (-4 – -255) (lbf/ft)
  • Downward Force of the Concrete Weight (lbf)
  • Concrete Weight Spacing (ft)

References

  • ASME B31.8 – Gas Transmission and Distribution Piping Systems
  • API 5L, API 5LS and API 5LX – Specification of Pipe Grade
  • ASTM – Various – Weld Joint Factor
  • CFR Code Part 192
  • USDA-SCS Modified (Permissible Velocity of Water and Soil Erodibility)
  • FHWA-HEC
  • Pipeline Rules of Thumb Handbook
  • Timoshenko, S – Theory of Elasticity Anchor Force

FAQ

  • Restrained versus Unrestrained Pipe (Difference in Gas vs. Liquid)?

    ASME B31.4 liquid and B31.8 gas codes include calculations for the net longitudinal compressive stress that must be applied only for a restrained line that equates to a low (less than 2%) longitudinal strain. This stress status is characteristic to underground pipelines located some distance away from above ground piping facilities.

    Unrestrained lines means those above ground sections of piping without axial restraint as with buried pipe with soil.Β  Β In others words the soil exerts substantial axial restraint, but not fully restrained. Check Out

  • What is the Maximum Span Length of rev1?

    Regarding span factors with and without water are based on bending stress and deflection.Β  Larger diameter pipe spansΒ  require saddles for stability. Many standards that require pipes to be filled with water are based on bending and shear stresses not to exceed 1,500 psi and a deflection between supports not exceed 0.1 inches. Check Out

  • What is the model used for Thrust at Blow-Off?

    HUBPL uses the equation from the DOT Inspectors Handbook:Β 

    TF = 0.5042 * G*Q^2/(P*D^2)

    Check Out


Updated on December 15, 2023

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