Buoyed String Weight

Method: Archimedes’ principle applied to a steel string in drilling fluid — a widely used oilfield shortcut expresses the resulting buoyancy factor directly from mud weight, using 65.5 ppg as the equivalent density of steel (specific gravity ≈ 7.85).

Formula basis: BF = 1 − (MW ÷ 65.5). Buoyed weight = nominal (air) weight per foot × length × BF. At MW = 0 (air), BF = 1 and the buoyed weight equals the air weight, as expected.

Scope and limitations: This is static buoyed weight for a single uniform section — it does not include friction/drag, shock loads while running, a tapered or multi-section string (calculate and sum each section separately), or the effect of fluid inside versus outside the pipe when they differ (e.g. running empty pipe versus pipe full of a different-density fluid). For hook-load planning on a real job, cross-check against your rig’s own weight-indicator calibration and torque-and-drag model.

For preliminary screening only. Not a substitute for a full torque-and-drag or well-specific engineering analysis.

Want the worked example and why buoyancy matters more than it looks? Read Why Your String Weighs Less Downhole.