drilling · casing design
Casing Collapse Pressure (API 5C3)
Pcollapse = f(D/t, Yp) — API 5C3 yield / plastic / transition / elastic, regime auto-selected from D/t
click formula to derive ↑
Inputs
ksi
in
in
Description
Computes casing collapse resistance under external pressure across all four API Bulletin 5C3:1994 collapse regimes — yield strength, plastic, transition, and elastic — automatically determining which regime governs from the casing's D/t ratio and yield strength, then applying the correct formula. This supersedes a narrower yield-strength-only check by resolving a real collapse pressure for any D/t instead of warning that the geometry is out of scope.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pcollapse | psi | Collapse Pressure |
| D/t | — | D/t Ratio |
| Yp | ksi | Minimum yield strength of the casing steel grade. 551.6 MPa (80 ksi) corresponds to N-80 grade. |
| t | in | Nominal casing wall thickness. The default pairs a 4-1/2 in tubing OD with a thick wall, landing in the yield-strength regime, but any D/t now resolves to a valid regime and answer. |
| D | in | Specified outside diameter of the casing or tubing (114.3 mm = 4-1/2 in). |
Assumptions
- No axial load, bending, or internal pressure is present — combined loading requires the full API 5C3 biaxial correction (not implemented here)
- Yield strength is the API-specified minimum for the steel grade, in line with how API 5C3's empirical constants were calibrated
- D, t, and Yp fall within the range API 5C3's empirical fits were calibrated against (commercial K55/N80/P110-class OCTG); extreme grades or geometries outside typical casing/tubing sizes are extrapolation
Limitations
- Does not include the biaxial (combined axial + collapse) stress correction from API 5C3 Eq. (14)
- Uses specified minimum yield strength rather than an actual mill-certified value, consistent with API design practice but conservative relative to as-manufactured pipe
- Does not model corrosion, wear, eccentric wear, or out-of-roundness — all of which reduce actual collapse resistance below the nominal API value
- The transition-regime formula (F, G constants) is a curve-fit bridge between the plastic and elastic formulas, not a physically derived equation — unlike the yield and elastic regimes. Kuriyama & Mimaki (1994) independently derived an elasto-plastic collapse formula from thick-wall cylinder stress theory and found API 5C3's yield-region safety margin comparatively small; this is also the exact regime treatment the 2008 ISO TR 10400 / API TR 5C3 ultimate-limit-state revision replaced. Use this calculator's transition-regime result as the established design-basis value, not as the only credible answer.
Use Cases
- → Full-range casing collapse design check: Verify collapse resistance for any candidate casing size, weight, and grade combination — the calculator resolves a correct answer regardless of which of the four API regimes the geometry falls into.
- → Thick-wall tubular collapse check: Verify collapse resistance for heavy-wall casing or tubing strings (e.g., liner tie-backs, thick-wall production casing) that fall in the low D/t yield-collapse regime.
- → Thin-wall surface/conductor casing check: Verify collapse resistance for large-diameter, thin-wall surface or conductor casing that falls in the high D/t elastic-collapse regime, where buckling governs rather than yield.
- → Evacuated-casing collapse risk: Assess collapse risk for casing run empty or partially evacuated during installation, completion, or workover operations.
Related Calculations
Region Notes
Permian Basin
Heavy-wall intermediate casing (e.g., 9-5/8 in, 53.5 lb/ft, P-110) used through high-pressure Wolfcamp intervals often falls near the yield/plastic D/t boundary — the info flag identifies which regime governs, and the result is valid on either side.
Gulf of Mexico
Deepwater production casing and liners are frequently heavy-wall, low-D/t strings selected specifically to stay in the more predictable yield-strength collapse regime under high external (seawater + formation) pressure.
North Sea
Large-diameter, thin-wall surface casing (e.g., 20 in, D/t commonly >35) typically falls in the transition or elastic collapse regime rather than yield — these regimes are governed by buckling instability, not steel yield strength.
Global
API Bulletin 5C3:1994 collapse formulas remain the most widely used design basis industry-wide, despite the 2008 revision (API TR 5C3 / ISO TR 10400) introducing an alternative ultimate-limit-state formulation.
References
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