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
πΉπ΅ β Buoyancy Force (lbf)
π·π β Pipe outside diameter including coating (in)
πΎπ€ β 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
ππ β Weight of bare pipe in air (lbf)
π·ππ β Pipe outside diameter (in)
π‘ β 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
ππ β Weight of coating in air (lbf)
πΎπ β 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}
πππ β Weight of product in pipe (lbf)
πΎππ β 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
πΉπππ‘ β Net controlling force (lbf)
ππΉ = Safety Factor
Downward Force of the Concrete Weight
F_{wt}=V_{wt}(\gamma_{wt}-\gamma_w)
F_{wt}=V_{wt}(\gamma_{wt}-\gamma_w)
ππ€π‘ β Weight of concrete (ft3)
πΎπ€π‘ β Unit weight of concrete (lb/ft3)
πΎπ€ β 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
- Select the Buoyancy Analysis & Concrete Weight Spacing application from the Design & Stress Analysis Module
- To create a new case, click the βAdd Caseβ button
- Enter Case Name, Location, Date and any necessary notes.
- Fill out all required Parameters.
- Make sure the values you are inputting are in the correct units.
- Click the CALCULATE button to overview results.
Input Parameters
- Nominal Pipe Size (in): (0.625β β 48β)
- Wall Thickness (in): (0.068β- >2β)
- Pipe grade: (24000psi-80000psi) (if unknown use Grade A 24000)
- Corrosion Coating Thickness (in): (1mil β 50mils)
- Unit Weight of Pipe in Air: (21 β 1750) (lbs/ft)
- Unit Weight of Water: (59 β 64) (lbs/ft3)
- Unit Weight of Corrosion Coating: (50 – 100) (lbs/ft3)
- Unit Weight of Concrete: (103 β 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
- If you need to modify an input parameter, click the CALCULATE button after the change.
- To SAVE, fill out all required case details then click the SAVE button.
- To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
- To generate a REPORT, click the REPORT button.
- The user may export the Case/Report by clicking the Export to Excel icon.
- To delete a case, click the DELETE icon near the top of the widget.
Results
- Weight of Pipe in Air: (21 β 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 β Gas Transmission and Distribution Piping Systems
- API 5L, API 5LS and API 5LX β Specification of Pipe Grade
- ASTM β Various β 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 β Theory of Elasticity Anchor Force
FAQ
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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
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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
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