Drillstring Mechanics
Drillstring load maths — buoyancy factor and buoyed weight, margin of overpull, drill-collar weight for weight-on-bit, and pipe stretch under tension. Field and metric units, fully offline.
Open Drillstring Mechanics →What is the drillstring mechanics calculator?
A drillstring load calculator that runs entirely in your browser. It works out the buoyancy factor of a mud and the buoyed hanging weight of a string, the margin of overpull available before the maximum allowable pull, the length of drill collars needed to carry a given weight on bit, and the elastic stretch of a pipe body under tension. Field units (ppg, feet, pounds) and metric units (SG, metres, kilograms) are both supported, and every result updates as you type. Nothing is uploaded — the maths uses the standard Lapeyrouse/IADC relationships and stays on your device, so it works on a rig laptop with no signal.
How to use Drillstring Mechanics
- Choose your units — Use the Field / Metric switch at the top. Field is ppg, feet and pounds; metric is specific gravity (SG), metres and kilograms.
- Pick a calculation — Select a tab — Buoyancy, Overpull, Collars for WOB or Stretch — for the value you need.
- Enter the known values — Type the mud weight, weights, collar figures or tension and area. Optional fields such as air weight and inclination refine the result. Everything calculates immediately.
- Read the result — The main figure is your answer; the smaller line shows a helpful equivalent, such as the buoyed weight, the buoyancy factor used, or the stretch in feet.
Frequently asked questions
Which buoyancy constant does it use?
The buoyancy factor is 1 − mud weight ÷ 65.5 in field units (ppg) and 1 − mud weight ÷ 7.85 in metric (SG). These denominators are the density of steel expressed in the matching units.
How is the collar length worked out?
It carries the weight on bit times your design factor on the buoyed weight of the collars, adjusted for hole inclination: length = (WOB × SF) ÷ (collar weight × BF × cos inclination). This keeps the neutral point inside the collars.
How is stretch calculated?
By Hooke's law using the pipe body cross-sectional area. In field units the stretch in inches is (pull × length) ÷ (area × 2,500,000); the metric form gives millimetres. It assumes uniform steel — sum non-uniform strings section by section.
Does it work offline?
Yes. There are no dependencies and no network calls, and your last inputs are saved on your device.
Tips
- The margin of overpull compares your current hookload with the maximum allowable pull — the pipe tensile capacity reduced by a design factor.
- Leave the inclination at zero for a vertical hole; entering a deviation reduces the effective collar weight and lengthens the collars needed.
- Stretch uses the cross-sectional steel area of the pipe body, not the bore area — take it from the pipe data sheet for the grade and weight you are running.