Generated by calculatgeek.com Casing Design Factor Calculator
Casing design is three separate arguments with physics, and the string has to win all of them. Burst is the argument with pressure from inside, the kind a shut-in kick or a casing pressure test applies. Collapse is the argument with pressure from outside, worst during cementing or if the string is ever evacuated. Tension is the argument with gravity, heaviest at the surface where every joint below hangs from the top one. Each argument produces a design factor, the rating divided by the worst credible load, and the string is only as good as its weakest of the three.
The three ratings come from the pipe itself: Barlow’s equation for burst, the API collapse formulas for collapse, and body yield for tension, each of which has its own dedicated calculator on this site that this one is designed to sit on top of. The three loads come from your load cases, which is where engineering judgment lives: what gas gradient do you assume inside for burst, do you design collapse for full evacuation or partial, how much overpull allowance rides on top of buoyed weight. Feed ratings and loads in, and the verdict cards show each design factor against the minimums, which default to the widely used 1.10 burst, 1.125 collapse, and 1.60 tension but are editable because every operator’s policy differs.
These three checks treat each load alone, and real casing feels them together. The interaction that bites most often: tension reduces collapse resistance. A string that passes collapse comfortably in isolation can fail the same check near the top of a long, heavy string where axial stress eats into the collapse rating, which is exactly the correction the API biaxial ellipse applies. Wear from drill pipe rotation, temperature effects on internal pressure, and load changes over the well’s whole life (production, stimulation, plug and abandon) also live outside the simple screen. The right way to use this tool is the way string designers actually work: run the quick uniaxial screen to pick a candidate weight and grade in seconds, then confirm the finalists with the full biaxial treatment before anything is ordered.
The three ratings this consumes come straight from the casing burst calculator, the casing collapse calculator, and the pipe body yield strength calculator. The PetroWiki page on casing design covers load cases and the biaxial corrections.