drilling · casing design
Buoyed Casing Weight
Wbuoyed = Wair × (1 − MW/ρsteel)
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Inputs
lb/ft
lb/gal
Description
Computes the effective (buoyed) weight per unit length of a casing string suspended in drilling fluid, accounting for the buoyant support the fluid provides. Buoyed weight — not air weight — is what governs actual hook load, derrick capacity checks, and drag/torque calculations while running casing.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Wbuoyed | lb/ft | Buoyed Weight |
| Wair | lb/ft | Nominal weight per unit length of the casing in air, from the API tubular tally (686 N/m ≈ 47 lb/ft, a common 9-5/8 in casing weight). |
| MW | lb/gal | Density of the drilling fluid the casing is run in, providing buoyant support. |
Assumptions
- Casing is fully submerged in a single, uniform-density fluid (no gas-cut mud or stratified fluid column)
- Steel density of 7850 kg/m³ is representative of standard API casing/tubing steel grades
- Internal fluid (inside the pipe ID) is accounted for separately if different from the annular fluid — this calc considers only the steel displacement buoyancy
Limitations
- Does not account for the additional buoyancy/ballooning effects of trapped air, float equipment, or a different fluid inside vs. outside the casing during running
- Ignores friction/drag forces while running — combine with torque_drag_straight_section for total hook load in a deviated wellbore
- Assumes constant mud weight along the entire string — a real wellbore may have a different fluid density above vs. below a float collar or cement top
Use Cases
- → Casing running hook-load planning: Determine actual hook load while running casing to confirm it remains within rig derrick and drawworks capacity.
- → Casing tension design: Use buoyed weight as the baseline axial load input for tension/burst/collapse combined-load casing design checks.
- → Cementing displacement planning: Estimate the weight change as mud is displaced by heavier cement slurry during a cement job.
Related Calculations
Region Notes
Permian Basin
9-5/8 in, 47 lb/ft intermediate casing run in 1,200-1,400 kg/m³ (10-11.7 ppg) mud typically sees a buoyancy factor around 0.85, reducing string weight by roughly 15%.
Gulf of Mexico
Deepwater riserless casing strings run in seawater (≈1,025 kg/m³) before switching to mud see a buoyancy factor close to 0.87 during the seawater-displacement phase.
Global
As mud weight increases toward 2,400 kg/m³ (20 ppg) in severely overpressured wells, buoyancy factor drops below 0.70 — a substantial hook-load reduction worth confirming against the rig's derrick rating.
References
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