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

𝐹_𝐵 − Buoyancy Force (lbf)
𝐷_𝑜 − Pipe outside diameter including coating (in)
𝛾_𝑤 − 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

𝑊_𝑃 − Weight of bare pipe in air (lbf)
𝐷_𝑜𝑝 − Pipe outside diameter (in)
𝑡 − 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

𝑊_𝑐 − Weight of coating in air (lbf)
𝛾_𝑐 − 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}

𝑊_𝑝𝑟 − Weight of product in pipe (lbf)
𝛾_𝑝𝑟 − 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

𝐹_𝑛𝑒𝑡 − 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)

𝑉_𝑤𝑡 − Weight of concrete (ft³)
𝛾_𝑤𝑡 − Unit weight of concrete (lb/ft³)
𝛾_𝑤 − 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

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

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