Well design · casing · field units

Casing design basics: burst, collapse and tension

Casing design is the check that every casing string can survive three loads over the life of the well: burst, when pressure inside the pipe exceeds the pressure outside; collapse, when pressure outside exceeds the pressure inside; and tension, from the weight of the string hanging below each joint. For each load, the pipe's published rating divided by the worst expected load must be at least the design factor in the operator's policy, commonly about 1.1 for burst, 1.0 to 1.125 for collapse and 1.6 to 1.8 for tension.

Reference material only. Casing designs are produced and approved by the operator's well engineers using the company's load cases, design factors and qualified software. Ratings must come from API TR 5C3 / ISO 10400 tables and the connection manufacturer's data sheets for the exact pipe and connection on location.

1. The casing design formulas

Yp is the minimum yield strength of the grade in psi (80,000 psi for N-80 and L-80, 110,000 psi for P-110), D the outside diameter, d the inside diameter and t the nominal wall thickness, all in inches. The 0.875 in the burst formula allows for the −12.5% manufacturing tolerance on wall thickness. A, B and C are API empirical constants for each grade (for N-80: A = 3.071, B = 0.0667, C = 1,955). Pressures at depth come from the hydrostatic pressure formula, and the buoyed weight from the buoyancy factor.

2. The three loads, and where each one is worst

Because burst is worst at the top and collapse at the bottom, a string is often built in sections: heavier or higher-grade pipe where a load peaks and lighter pipe where it does not. Combined loading matters too. Under the biaxial criterion in API TR 5C3 / ISO 10400, axial tension reduces collapse resistance, so pipe near the top of a long string in tension has a lower effective collapse rating; triaxial (von Mises) checks with a design factor of about 1.25 cover burst, collapse and axial load acting together.

3. API collapse regimes

Collapse ratings are not one formula. API TR 5C3 / ISO 10400 uses four regimes chosen by the grade and the D/t ratio (outside diameter over wall thickness):

The practical point: thin-wall casing loses collapse resistance quickly, and raising the grade does nothing for pipe in the elastic regime. Only a thicker wall helps there.

4. Worked example: ratings of 9-5/8 in, 47 lb/ft N-80

This casing has an outside diameter of 9.625 in and a nominal wall of 0.472 in (11.99 mm), so the inside diameter is 9.625 − 2 × 0.472 = 8.681 in. N-80 has a minimum yield strength of 80,000 psi.

• Burst: 0.875 × 2 × 80,000 × 0.472 ÷ 9.625 = 6,865 psi, which API rounds to 6,870 psi.
• Pipe body yield: 0.7854 × (9.625² − 8.681²) = 13.57 in² of steel × 80,000 psi = about 1,086,000 lbf.
• Collapse regime: D/t = 9.625 ÷ 0.472 = 20.4, which falls in the N-80 plastic range (13.38 to 22.47).
• Plastic collapse: 80,000 × (3.071 ÷ 20.39 − 0.0667) − 1,955 ≈ 4,757 psi, matching the published rating of 4,750 psi.

These hand calculations only show where the table values come from. Use the published table for the exact weight, grade and connection; joint strength in particular depends on the connection type and is not given by the pipe body formula.

5. Worked example: checking the string against simple load cases

The 9-5/8 in, 47 lb/ft N-80 casing is set at 8,000 ft TVD in 10.0 ppg mud. The next hole section goes to 12,000 ft, where the highest expected pore pressure is 0.50 psi/ft. The design factors in this illustration are 1.1 burst, 1.125 collapse and 1.6 tension.

• Burst, gas to surface: pore pressure at 12,000 ft = 0.50 × 12,000 = 6,000 psi. With a 0.1 psi/ft gas column to surface, pressure inside the casing at the wellhead = 6,000 − 0.1 × 12,000 = 4,800 psi, with no backup outside at surface. DF = 6,870 ÷ 4,800 = 1.43, above 1.1.
• Burst at the shoe: inside = 6,000 − 0.1 × 4,000 = 5,600 psi; assuming a 0.465 psi/ft salt-water backup outside, outside = 0.465 × 8,000 = 3,720 psi; net 1,880 psi. DF = 6,870 ÷ 1,880 = 3.65. In practice the inside pressure at the shoe cannot exceed what the formation below the shoe can hold, which is why burst designs are often capped at the shoe fracture pressure; see MAASP and leak-off test.
• Collapse, full evacuation: outside at the shoe = 0.052 × 10.0 × 8,000 = 4,160 psi, inside empty. DF = 4,750 ÷ 4,160 = 1.14, just above 1.125. A heavier mud behind the pipe or a deeper setting depth would fail this check, and a design that uses a higher external gradient would need a heavier weight or higher grade at the bottom.
• Tension: air weight = 47 × 8,000 = 376,000 lbf. Buoyancy factor in 10.0 ppg mud = 1 − 10.0 ÷ 65.5 = 0.847, so buoyed weight ≈ 318,600 lbf. Using the air weight as a conservative load, body DF = 1,086,000 ÷ 376,000 = 2.89. The connection's joint strength must also clear 1.6 × 376,000 ≈ 601,600 lbf, and many operators also require a minimum overpull margin (for example 100,000 lbf) set by company policy.

This string passes all three simple checks, with collapse the tightest. Real designs add more load cases (cementing, pressure testing, drilling-ahead kicks, production and injection loads, temperature effects) and check combined triaxial stress, but the method is the same: worst load at each depth against the rating, divided out to a design factor.

6. Design factors in practice

A design factor is not a statement of how safe the pipe is: it depends on how severe the load cases are. A string designed to 1.0 against full evacuation can be more conservative than one designed to 1.125 against partial evacuation. That is why the factors and the load cases are always set together in one policy.

7. Common mistakes

• Using nominal weight to pick ratings without confirming grade and connection on the mill certificates and tally.
• Checking tension against pipe body yield when the connection is weaker.
• Ignoring the effect of tension on collapse near the top of long strings, and of wear on wall thickness in long drilling sections.
• Assuming mud behind the casing keeps its density for years; long-term designs often use pore pressure or a salt-water gradient for the external column.
• Forgetting that a higher grade does not raise collapse resistance for thin-wall pipe in the elastic regime.

Casing loads also feed the rest of the well plan: the setting depth decides the shoe strength and the kick tolerance of the next section, and the string weight decides the hook load and buoyancy margins while running it. Cementing the string is covered in cement volume calculation.

8. In DrillX

DrillX brings daily drilling reports with IADC-style time logs, a rig fleet view, NPT insights and engineering calculators into one tool. The calculators cover hydrostatic pressure, ECD, annular pressure loss, triplex pump output, annular capacity and velocity, buoyancy and the kill sheet. This page explains how casing burst, collapse and tension checks work; it does not describe a DrillX casing design calculator. DrillX is in final testing; email info@drillx.app for a demo or early access.

Sources and further reading

• API TR 5C3 / ISO 10400, Petroleum and natural gas industries: equations and calculations for the properties of casing, tubing, drill pipe and line pipe used as casing or tubing: internal yield, pipe body yield, collapse regimes and the biaxial collapse criterion.
• SPE PetroWiki, Strength of casing and tubing: the 0.875 wall-tolerance factor, axial strength as area × yield, the four API collapse regimes and the reduction of collapse resistance with axial stress.
• Drilling Manual, casing design safety factors: burst 1.1, collapse 1.125, tension 1.6, triaxial 1.25, compression 1.2.
• A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986), chapter 7: casing design load cases.
• H. Rabia, Well Engineering and Construction, Entrac Consulting (2002): casing design factors and load cases.

Reference summary only. Worked examples are illustrative arithmetic, not a casing design or recommended design factors.