OCTG Performance Reference

API 5C3 — Casing Collapse, Burst & Weight Reference Tables

API TR 5C3 / ISO/TR 10400 performance property formulas: four-regime collapse pressure model, grade coefficients for H40 through Q125, Barlow internal yield (burst) pressure, and pipe weight per metre. Includes a Barlow burst reference table and worked examples.

→ Pressure & Weight Calculator → API 5CT Dimension Tables → NACE MR0175 Sour Service Tables

About API TR 5C3 / ISO/TR 10400

API Technical Report 5C3 (now superseded by ISO/TR 10400:2011) defines the formulas for calculating three key OCTG performance properties: collapse pressure, internal yield (burst) pressure, and pipe weight. These are the ratings published in API 5CT Tables and used in casing string design.

The collapse model uses four formulas depending on the D/t (diameter-to-wall) ratio — yield strength collapse for thick-wall pipe and elastic collapse for thin-wall pipe, with plastic and transition formulas covering the intermediate range that governs most production casing and tubing.

Table data verified 2026-07-01 — API TR 5C3 / ISO/TR 10400.

Four-Regime Collapse Model

The governing collapse formula depends on D/t (OD ÷ nominal wall thickness). Apply the formula for the D/t range in which your pipe falls, using the grade-specific coefficients from the tab above. The boundaries (D/t)_YP, (D/t)_PT, (D/t)_TE are tabulated in API TR 5C3 Table 1 and vary by grade minimum yield strength.

1 — Yield Strength Collapse

Applies when: D/t ≤ (D/t)_YP (thick wall)

Pyp = 2Yp × (D/t − 1) / (D/t)²

Wall thick enough that yield stress is reached before buckling. Governed by yield strength only — no grade coefficients needed.

2 — Plastic Collapse

Applies when: (D/t)_YP < D/t ≤ (D/t)_PT

Pp = Yp [A / (D/t) − B] − C

Most production casing falls here. Coefficients A, B, C are grade-specific (see Grade Coefficients tab). C has units of stress.

3 — Transition Collapse

Applies when: (D/t)_PT < D/t ≤ (D/t)_TE

Pt = Yp [F / (D/t) − G]

Intermediate thin-wall range. Coefficients F and G are grade-specific. Pp and Pe formulas converge in this range.

4 — Elastic Collapse

Applies when: D/t > (D/t)_TE (thin wall)

Pe = 323 700 / [(D/t)(D/t − 1)²] MPa

Thin-wall pipe buckles elastically before yield. Formula is independent of grade — yield strength does not appear. In psi: 46,950,000 / [(D/t)(D/t−1)²].

Variable Definitions

API TR 5C3 Collapse Formula Variable Definitions — Symbols, Definitions and Units
Symbol	Definition	Units
D or OD	Pipe outside diameter (nominal)	mm (or in)
t	Nominal wall thickness	mm (or in)
D/t	Diameter-to-thickness ratio (dimensionless)	—
Yp	Minimum yield strength of grade (from API 5CT)	MPa (or psi)
A, B, C	Plastic collapse coefficients per API TR 5C3 Table 1	A, B dimensionless; C same as Yp units
F, G	Transition collapse coefficients per API TR 5C3	Dimensionless
Pyp	Yield strength collapse pressure	MPa (or psi)
Pp	Plastic collapse pressure	MPa (or psi)
Pt	Transition collapse pressure	MPa (or psi)
Pe	Elastic collapse pressure — steel constant = 323 700 MPa (46 950 000 psi)	MPa (or psi)
(D/t)_YP	D/t boundary between yield and plastic regime	—
(D/t)_PT	D/t boundary between plastic and transition regime	—
(D/t)_TE	D/t boundary between transition and elastic regime	—

Approximate D/t Regime Boundaries by Grade

The D/t boundaries shift lower as yield strength increases (higher-grade pipe has a smaller plastic collapse regime). Values below are approximate — use API TR 5C3 Table 1 or ISO/TR 10400 Table B.1 for precise values in casing design.

API TR 5C3 Collapse Pressure Regime D/t Boundaries by Grade — Yield, Plastic, Transition and Elastic Ranges (API TR 5C3 Table 1, Approximate)
Grade	Min Yp (ksi)	Yield regime: D/t ≤	Plastic regime: D/t	Transition regime: D/t	Elastic regime: D/t ≥
H40	40	~16–17	~17 – ~27	~27 – ~43	~43
J55/K55	55	~15–16	~16 – ~26	~26 – ~38	~38
N80/L80	80	~13–14	~14 – ~22	~22 – ~32	~32
C90	90	~13	~13 – ~21	~21 – ~30	~30
T95/R95	95	~12–13	~13 – ~20	~20 – ~29	~29
P110/C110	110	~12	~12 – ~19	~19 – ~27	~27
Q125	125	~11–12	~12 – ~18	~18 – ~25	~25

Most production casing (4½" – 9⅝", standard weights) falls in the plastic or transition collapse regime. Surface casing (13⅜" – 20", lighter walls) often falls in the transition or elastic regime.

API TR 5C3 Collapse Coefficients (USC Units)

Coefficients for the plastic (Pp) and transition (Pt) collapse formulas. Values below are for use with Yp in psi and C in psi; the result is in psi. For SI units (MPa), convert C from psi to MPa by ÷ 145.038. Source: API TR 5C3 Table 1. Verify against current standard (ISO/TR 10400:2011) for engineering use.

API TR 5C3 Collapse Coefficients A, B, C, F, G by Grade for Plastic and Transition Regime Calculations (API TR 5C3 Table 1, USC Units)
Grade	Min Yp (ksi)	Min Yp (MPa)	A (plastic)	B (plastic)	C — psi	C — MPa	F (transition)	G (transition)
H40	40	276	2.950	0.0465	754	5.2	2.063	0.0325
J55/K55	55	379	3.205	0.0541	1,206	8.3	2.038	0.0403
N80/L80	80	552	3.071	0.0667	1,955	13.5	1.998	0.0544
C90	90	621	3.106	0.0706	2,172	15	1.990	0.0575
T95/R95	95	655	3.124	0.0727	2,254	15.5	1.984	0.0584
P110/C110	110	758	3.181	0.0799	2,836	19.6	1.972	0.0609
Q125	125	862	3.239	0.0875	3,561	24.6	1.959	0.0632

Source: API Technical Report 5C3 Table 1 (USC). C110 uses same coefficients as P110 (same minimum yield strength 110 ksi). For updated SI coefficients, refer to ISO/TR 10400:2011 Table B.1.

Worked Example — Plastic Collapse for 7" L80 at 35 ppf

Given: 7" casing, OD = 177.8 mm, wall = 12.65 mm, Grade L80 (Yp = 80,000 psi)

D/t = 177.8 / 12.65 = 14.1

Regime check: D/t = 14.1 is in the plastic regime for L80 (~14 – ~22)

Coefficients (N80/L80): A = 3.071, B = 0.0667, C = 1,955 psi

Pp = 80,000 × [3.071/14.1 − 0.0667] − 1,955
= 80,000 × [0.2178 − 0.0667] − 1,955
= 80,000 × 0.1511 − 1,955
= 12,088 − 1,955
= 10,133 psi (69.9 MPa)

API 5CT published collapse rating for 7" L80 35.0 ppf ≈ 10,180 psi. Small difference due to rounding in D/t.

Elastic Collapse — No Grade Coefficients

The elastic formula Pe = 323,700 / [(D/t)(D/t−1)²] MPa uses only the steel elastic modulus constant — grade yield strength does not appear. At D/t = 28 (thin-wall surface casing): Pe = 323,700 / (28 × 27²) = 323,700 / 20,412 = 15.9 MPa (2,300 psi). This low elastic collapse pressure is why surface casing must be cemented: the formation pressure and cement annulus provide external support.

Barlow Internal Yield Pressure (Burst)

P = 0.875 × (2 × Yp × t) / OD

Equivalently: P = 1.75 × Yp / (D/t)

P = internal yield pressure (MPa if Yp in MPa; psi if Yp in psi)

Yp = minimum yield strength per API 5CT (MPa or psi)

t = nominal wall thickness (mm or in)

OD = outside diameter (mm or in)

0.875 = factor for 12.5% negative wall tolerance (API 5CT)

The Barlow formula gives the theoretical internal pressure at which the pipe body wall first yields. It does not include connection (coupling) burst ratings — always check connection burst separately. Use minimum yield strength (from API 5CT Table) for conservative design.

Barlow Internal Yield Pressure Reference Table (MPa)

API 5C3 Barlow Internal Yield Pressure Reference Table (MPa) — by D/t Ratio and Grade Minimum Yield Strength
D/t	J55 Yp=379 MPa	L80 Yp=552 MPa	T95 Yp=655 MPa	P110 Yp=758 MPa	Q125 Yp=862 MPa
8	82.9 (12,020 psi)	120.8 (17,520 psi)	143.3 (20,780 psi)	165.8 (24,050 psi)	188.6 (27,350 psi)
10	66.3 (9,620 psi)	96.6 (14,010 psi)	114.6 (16,620 psi)	132.7 (19,250 psi)	150.9 (21,890 psi)
12	55.3 (8,020 psi)	80.5 (11,680 psi)	95.5 (13,850 psi)	110.5 (16,030 psi)	125.7 (18,230 psi)
14	47.4 (6,870 psi)	69.0 (10,010 psi)	81.9 (11,880 psi)	94.8 (13,750 psi)	107.8 (15,640 psi)
16	41.5 (6,020 psi)	60.4 (8,760 psi)	71.6 (10,380 psi)	82.9 (12,020 psi)	94.3 (13,680 psi)
18	36.8 (5,340 psi)	53.7 (7,790 psi)	63.7 (9,240 psi)	73.7 (10,690 psi)	83.8 (12,150 psi)
20	33.2 (4,820 psi)	48.3 (7,010 psi)	57.3 (8,310 psi)	66.3 (9,620 psi)	75.4 (10,940 psi)
22	30.1 (4,370 psi)	43.9 (6,370 psi)	52.1 (7,560 psi)	60.3 (8,750 psi)	68.6 (9,950 psi)
25	26.5 (3,840 psi)	38.6 (5,600 psi)	45.9 (6,660 psi)	53.1 (7,700 psi)	60.3 (8,750 psi)

P = 1.75 × Yp / (D/t). Use minimum yield strength (API 5CT). PSI values rounded to nearest 10. For non-tabulated D/t values, use the interactive calculator.

Plain-End Pipe Weight per Metre

W = 0.02466 × t × (OD − t) [kg/m, OD and t in mm]

Equivalent in lb/ft: W = 10.69 × t × (OD − t) / 1000² (OD, t in mm, result in lb/ft — multiply kg/m by 0.6720)

The constant 0.02466 = ρ_steel × π/4 × 2 / 1000² = 7850 kg/m³ × π/4 × 2 / 10⁶

This gives plain-end weight (pipe body only). Nominal weight (as listed in API 5CT) includes thread and coupling allowance and is 1.5–4% higher depending on coupling type. API 5CT Table C.18 lists nominal weight Wt (with coupling) and plain-end weight Wpe side by side.

Weight Verification Table — Selected Casing Sizes

Plain-End Pipe Weight Formula Verification — Calculated (W = 0.02466 × t × (OD − t)) vs API 5CT Table C.18 Values
OD (in)	OD (mm)	Wall (mm)	D/t	W calc. (kg/m)	API 5CT Wpe (kg/m)
4½	114.3	7.37	15.5	19.43	19.87
5	127	9.19	13.8	26.70	27.19
5½	139.7	10.54	13.3	33.57	34.05
7	177.8	10.36	17.2	42.78	43.60
7	177.8	13.72	13	55.51	56.10
7⅝	193.68	12.7	15.3	56.68	58.04
9⅝	244.48	13.84	17.7	78.72	79.62
10¾	273.05	12.57	21.7	80.74	82.59
13⅜	339.72	10.92	31.1	88.54	90.78

W calc. = 0.02466 × t × (OD − t). API 5CT Wpe from Table C.18 (plain-end weight). Values match closely; small differences due to rounding in published tables.

Interactive Calculator

For any OD, wall, and grade combination — enter values directly into the calculator to get burst pressure, pipe weight, and approximate collapse pressure.

→ Open Pressure Calculator

Design Safety Factors

API 5C3 / TR 5C3 performance values are rated capacities, not design limits. Casing string design applies safety factors to each mode:

Casing String Design Safety Factors for Collapse, Burst, Tension and Biaxial Loading (API 5C3 / Industry Practice)
Failure Mode	Typical Design Factor	Meaning	Comments
Collapse	1.0 – 1.125	Working load ≤ rated collapse ÷ DF	Lower DF used for cemented strings; higher for uncemented / worst-case lost returns
Burst	1.0 – 1.25	Working pressure ≤ rated burst ÷ DF	Connection burst often governs over pipe body — verify separately
Tension	1.6 – 2.0	Hanging weight ≤ joint strength ÷ DF	API 5C3 joint strength formula also defined. Include buoyancy, bending, dynamic loads
Biaxial	applies to all	Burst and collapse ratings reduced under axial load	API 5C3 provides biaxial correction. Critical for tension/collapse combined loading in production strings

Biaxial Load Correction

Under combined axial tension and external pressure, the effective collapse resistance is reduced. API 5C3 provides the ellipse of biaxial yield to correct the collapse rating when axial stress is significant:

Pa/Ppure = √[1 − 0.75(σa/Yp)²] − 0.5(σa/Yp)

where:
Pa = biaxially corrected collapse pressure
Ppure = collapse pressure from TR 5C3 (no axial load)
σa = axial stress (tension = positive, psi or MPa)
Yp = minimum yield strength (psi or MPa)

For typical production strings in tension at the top, the biaxial correction reduces effective collapse resistance by 10–25%. Always include biaxial correction for conductor strings and upper sections of long production strings.

Common Pitfalls

Common Pitfalls in Casing Collapse and Burst Calculations — Consequence and Correct Approach (API 5C3)
Pitfall	Consequence	Correct Approach
Using nominal (max) yield strength instead of minimum	Overestimates actual burst rating	Always use minimum yield strength (lower bound) for design
Applying Barlow formula to connection burst	Connection burst is typically 10–20% lower than pipe body	Check connection burst rating separately from coupling manufacturer or API 5C3 table
Ignoring wall tolerance in collapse calculation	Premature collapse; published collapse rating already incorporates -12.5% tolerance	TR 5C3 collapse ratings use nominal (not minimum) wall. No additional tolerance factor needed
Using elastic formula for all D/t ratios	Overestimates collapse for plastic/yield regime (non-conservative if misapplied)	Identify correct regime from D/t; use the appropriate formula
Applying rated collapse to uncemented shallow casing	Insufficient support if trapped annulus pressure is lost	Use safety factors and evaluate worst-case fluid column — consider partial cementing

Frequently Asked Questions

What are the four collapse pressure regimes in API 5C3?

API TR 5C3 defines four collapse pressure regimes based on the D/t (OD-to-wall-thickness) ratio: (1) Yield strength collapse (Pyp) — applies at low D/t (thick wall), where collapse is governed by yielding of the pipe wall; (2) Plastic collapse (Pp) — intermediate D/t range, where the pipe yields before collapse; (3) Transition collapse (Pt) — medium-high D/t; (4) Elastic collapse (Pe) — high D/t (thin wall), where the pipe wall buckles elastically before yielding. The formula to use depends on the D/t ratio compared to grade-specific boundaries published in API TR 5C3 Table 1.

What is the Barlow formula for casing burst pressure per API 5C3?

The Barlow internal yield pressure formula per API TR 5C3 is: P = 0.875 × (2 × Yp × t) / OD, where P is the internal yield pressure (MPa or psi), Yp is the minimum yield strength of the grade (MPa or psi), t is the nominal wall thickness (mm or in), and OD is the outside diameter (mm or in). The factor 0.875 applies the 12.5% negative wall thickness tolerance mandated by API 5CT. Use minimum yield strength (not nominal) for conservative design.

How do I calculate pipe weight per metre for API 5CT casing?

The API 5C3 plain-end weight formula is: W = 0.02466 × t × (OD − t) kg/m, where OD and t are in mm. This gives the plain-end weight (pipe body only). Nominal weight (including thread and coupling) is approximately 1.5–3% higher depending on coupling type and size. For quick lookup, use the casing dimension tables in API 5CT (Table C.18 for casing, Table C.19 for tubing).

How does the D/t ratio determine which collapse pressure regime applies in API 5C3?

API TR 5C3 selects the collapse regime by D/t (OD-to-wall-thickness ratio): Yield Strength Collapse applies for thick-wall pipe (low D/t, typically below ~15); Plastic Collapse covers intermediate D/t where yielding drives collapse; Transition Collapse bridges plastic and elastic behaviour; and Elastic (Lamé) Collapse applies to thin-wall pipe (high D/t, typically above ~30). Regime boundaries shift with grade yield strength — higher-yield grades have narrower plastic collapse ranges. Most production casing falls in the plastic or transition regime. The D/t ratio for common grades and the exact boundary equations are tabulated in API TR 5C3 Table C.1.

What biaxial correction does API 5C3 apply for combined axial and external pressure loading?

When casing carries simultaneous axial tension (from string weight, thermal effects, or pressure loads), API TR 5C3 applies the von Mises yield criterion to reduce the effective collapse resistance. The biaxial correction uses a reduced effective yield strength Ypa = Yp × [√(1 − 0.75(σa/Yp)²) − 0.5(σa/Yp)], where σa is the average axial stress. At 50% of yield in tension, collapse resistance can be reduced by 10–20%. This correction is critical for deviated and extended-reach wells where axial tension in the lower casing sections is significant, and must be accounted for in detailed casing design.

What design safety factors are used for casing burst and collapse per API and ISO?

API 5C3 and ISO 10400 present casing performance ratings as minimum values; safety factors are applied by the well designer, not the standard. Typical industry design factors are: collapse — 1.0 to 1.125 (burst often governs sour-service grades); burst — 1.1 to 1.25 for production strings; tension — 1.6 to 1.8 for surface and intermediate strings; triaxial (von Mises) — 1.25. HPHT and sour-service wells often use higher safety factors and full triaxial analysis. Specific IOC and operator standards (Shell SIEP, Saudi Aramco SAES, etc.) may specify different minimum design factors.

Related Specs & Tools

→ API 5CT Full Spec Tables — Dimensions, Grades & Drift → NACE MR0175 / ISO 15156 Sour Service Tables → API 5L Line Pipe Specification Tables → ASME B36.10 Pipe Schedule Chart → Pressure & Weight Calculator → Sour Service Grade Selector Tool

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