Find the buoyancy factor for a given mud weight, and the buoyed (effective) weight your string will hang at in that fluid.
When you run a pipe string into a fluid-filled hole, the mud pushes up on it. The string weighs less in the hole than it does in air — and the buoyancy factor is the fraction of its air weight that it actually hangs at. Multiply the air weight by the buoyancy factor to get the effective weight the derrick, blocks and top drive actually carry.
It matters for hook load, for picking the right pipe grade so you don't exceed tensile limits, and for weight-on-bit calculations. Heavier mud means more buoyancy, so a lower factor and a lighter effective string.
The 65.4 is the weight of a gallon-equivalent of steel — steel has a density of about 65.4 ppg (65.5 is also used). Buoyed weight is then:
This assumes a steel string and the pipe is fully submerged in a single fluid. For dual-density or partially filled strings, calculate section by section.
A string weighs 150,000 lbs in air and is run in 12.0 ppg mud. Buoyancy factor = 1 − 12.0/65.4 = 0.8165. Effective hanging weight = 150,000 × 0.8165 ≈ 122,477 lbs. The string effectively "loses" about 27,500 lbs to buoyancy.
Completion and workover engineering support — tubular selection, load cases and more.