Buoyancy Analysis & Concrete Coating Requirements – 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.

The calculation does not account/allow for depth of water or burial. PLTB input is unit weight of water.

Determine Bare Pipe Weight

W_{pipe}=10.68(D-t)t\,[lbs/ft],\,where

W_{pipe}=10.68(D-t)t\,[lbs/ft],\,where

𝐷 − Pipe outside diameter (in)
𝑡 − Pipe wall thickness (in)

Determine Total Volume of Pipe in Air Including Corrosion and Concrete Coating

V_{total}=\frac{\pi(D+2(t_{corr}+t_{con}))^2}{4(144)}[ft^3],\,where

V_{total}=\frac{\pi(D+2(t_{corr}+t_{con}))^2}{4(144)}[ft^3],\,where

𝑡_{𝑐𝑜𝑟𝑟} = Corrosion coating thickness (in)
𝑡_𝑐𝑜𝑛 = Concrete coating thickness (in)

Determine Volume of Corrosion Coating

V_{corr}=\frac{\pi (D+2t_{corr})^2}{4(144)}-\frac{\pi D^2}{4(144)}[ft^3],\,where

V_{corr}=\frac{\pi (D+2t_{corr})^2}{4(144)}-\frac{\pi D^2}{4(144)}[ft^3],\,where

Determine Volume of Concrete Coating

V_{con}=V_{total}-V_{corr}-\frac{\pi D^2}{4(144)}[ft^3]

V_{con}=V_{total}-V_{corr}-\frac{\pi D^2}{4(144)}[ft^3]

Determine Total Weight of Pipe in Air, Including Weight of Corrosion and Concrete Coating

W_{total}=W_{pipe}+V_{corr}\rho_{corr}+V_{con}\rho_{con}\,[lb/ft^3],\,where

W_{total}=W_{pipe}+V_{corr}\rho_{corr}+V_{con}\rho_{con}\,[lb/ft^3],\,where

𝜌_{𝑐𝑜𝑟𝑟} = Corrosion coating specific weight (lb/ft³)
𝜌_𝑐𝑜𝑛 = Concrete coating specific weight (lb/ft³)

Determine Weight of Displaced Water

W_{water}=V_{total}\rho_{w}\,[lbs/ft],\,where

W_{water}=V_{total}\rho_{w}\,[lbs/ft],\,where

𝜌_𝑤 = Specific weight of water (lb/ft³)

Determine the Difference

F=W_{total}-W_{water}[lbs/ft]

F=W_{total}-W_{water}[lbs/ft]

Determine Bulk Specific Gravity

B=\frac{W_{total}}{W_{water}}

B=\frac{W_{total}}{W_{water}}

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)
• Pipe length (ft/joint)
• Water Density: (59 – 64) (lbs./ft³)
• Concrete Coating Density: (50- 170) (lbs./ft³)
• Corrosion Coating Density: (50 – 100) (lbs./ft³)
• Design Specific Gravity Ratio
• Corrosion Coating Thickness (in): (1mil – 50mils)

Part 2: Outputs/Reports

Results

• Pipe weight (lbs/ft)
• Total Volume (ft³)
• Corrosion Coating Volume (ft³)
• Concrete Coating Thickness: (0.25” – 8.5”) (in)
• Weight of Pipe in Air: (21 – 1750) (lbs/ft)
• Weight of Water Displaced: (3 – 1600) (lbs/ft)
• Weight of Submerged Pipe: (2l – 250) (lbs/ft)
• Weight per Joint: (800 – 70000) (lbs/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