API 5CT Casing and Tubing: Grades, Sizes, Connections and How to Order

Quick Answer

API 5CT is the specification for steel casing, tubing, pup joints and couplings for oil and gas wells; its international counterpart is ISO 11960. Casing is cemented into the well to hold back the formation and contain pressure; tubing runs inside it and carries the produced fluid. Casing runs from 4-1/2 in to 20 in OD and is ordered by OD and nominal weight in lb/ft, which fix the wall, the inside diameter and the drift diameter. The number in a grade is the minimum yield strength in ksi. J55 and K55 (55 ksi, 379 MPa) suit surface and shallow strings and N80 (80 ksi, 552 MPa) deeper sweet wells. L80 Type 1 (80–95 ksi, maximum 23 HRC) is the base sour-service grade. P110 (110 ksi, 758 MPa) is for deep, high-pressure strings and is accepted in sour wells only above about 80 °C (175 °F). The API connections are round threads (STC, LTC) and buttress (BTC), threaded to API 5B; premium connections add a gas-tight metal seal. Ratings come from API TR 5C3: internal yield is 0.875 × 2 × Yp × t / D, where 0.875 covers the −12.5% wall tolerance.

A typical casing enquiry arrives as one line: "9-5/8 casing, N80, 2,000 metres, best price." It looks complete, but it isn't. It gives no weight, and each 9-5/8 in weight has a different wall, drift and pressure rating. It does not say N80 Type 1 or N80Q, or whether the well is sour, in which case N80 may be the wrong grade. It says nothing about thread, range length or inspection. Each gap becomes a question, a delay or an assumption. This guide covers what a complete specification contains.

Casing and Tubing: What Each Does

Casing is cemented into the well to hold back the formation, isolate zones, protect freshwater aquifers and carry the wellhead and blowout preventers. Tubing runs inside the production casing, usually with a packer near the bottom, and carries oil or gas to surface. Casing stays for the life of the well; tubing can be pulled during a workover.

A well is built from several casing strings, each set inside the one before:

Conductors larger than 20 in fall outside the API 5CT size range and are specified separately, often as line pipe; see our guide to API 5L line pipe.

What API 5CT Covers

API 5CT sets the technical delivery conditions for steel casing, tubing, pup joints, couplings, coupling stock and accessory material, from manufacture and heat treatment to dimensions, testing and marking. Pipe may be seamless or electric-welded, with restrictions for some grades.

Threads are not in API 5CT. Thread form, dimensions and gauging are in API 5B, which covers short round thread casing (SC, sold as STC), long round thread casing (LC or LTC), buttress casing (BC or BTC), and non-upset (NU), external upset (EU) and integral-joint (IJ) tubing.

The current eleventh edition of API 5CT lists grades H40, J55, K55, N80 (all types), L80 (all types), C90, R95, T95, P110, C110, C125 and Q125. The standard also defines product specification levels: PSL-1 is the base level, and PSL-2 and PSL-3 add further requirements that the purchaser has to call up in the order.

Grades: What the Name Tells You

The number in a grade name is the minimum yield strength in ksi (thousands of psi): 80 ksi (552 MPa) for N80, 110 ksi (758 MPa) for P110. Every grade also has a maximum yield, and the sour-service grades a maximum hardness, because in H2S service a pipe that is too strong is as much a problem as one that is too weak.

The list also includes R95 and C110. C110 is a high-strength sour-service grade with a 30 HRC hardness limit and sulfide stress cracking testing.

The grades buyers mix up

J55 vs K55. Same yield range, 55–80 ksi, but K55 has a higher minimum tensile strength (95 against 75 ksi). API round-thread joint strength is calculated from tensile strength, so K55 has higher rated joint loads. Both suit surface and shallow strings.

N80 Type 1 vs N80Q. Same strength. Type 1 is normalised; N80Q is quenched and tempered, giving a finer, more uniform structure. Where a programme calls for N80Q, Type 1 is not a substitute.

L80 vs N80. Both have an 80 ksi minimum yield, but L80 is capped at 95 ksi (N80 at 110 ksi), must be quenched and tempered, and is limited to 23 HRC. Those limits make L80 Type 1 a sour-service grade; N80 is not. L80 usually costs more.

L80 Type 1 vs 9Cr and 13Cr. Same strength, different chemistry. Type 1 is a carbon or low-alloy steel for sour service; 9Cr and 13Cr carry about 9% and 13% chromium to resist CO2 corrosion. 13Cr tolerates little H2S (the limit commonly quoted is about 1.5 psi, 0.1 bar, partial pressure), so it does not replace Type 1 in sour wells.

P110. The standard high-strength grade for deep strings. Its 140 ksi maximum yield and lack of a hardness cap rule it out of sour service except at elevated temperature. T95 Type 1 is the high-strength grade accepted in sour service at all temperatures; C110 is a sour-service grade too, but check its ISO 15156-2 temperature limits and qualification requirements for the cooler upper part of the string.

Colour bands

Grades are identified by colour bands painted on the pipe and by coloured couplings. Bands help in the pipe yard; the stencil marking on each length is the definitive identification.

Sour Service: Which Grades and Why

Hydrogen sulfide causes sulfide stress cracking (SSC): hydrogen from corrosion enters the steel and, under tensile stress, cracks it, often below yield and without warning. The defences are limited strength and hardness, a tempered structure and testing.

ISO 15156-2 (NACE MR0175) accepts H40, J55, K55, L80 Type 1, C90 Type 1 and T95 Type 1 at all temperatures. For these grades it relies on the API 5CT hardness limits (23 HRC for L80 Type 1, 25.4 HRC for C90 and T95) rather than the general 22 HRC limit for carbon and low-alloy steels. C90 and T95 are also SSC-tested (NACE TM0177 Method A) in manufacture.

Higher-strength grades are accepted only above a minimum temperature, because SSC is worst near room temperature: about 65 °C (150 °F) for N80Q and R95, 80 °C (175 °F) for N80 Type 1 and P110, and 107 °C (225 °F) for Q125 made from quenched-and-tempered Cr-Mo steel. Check the exact grade type ordered against the current edition of ISO 15156-2. The top of a string is also cooler than the bottom, so upper sections may need a sour grade even in a hot well.

The full framework, with environmental limits and documentation, is in our guide to NACE MR0175 / ISO 15156 sour service materials.

Sizes, Weights, Wall Thickness and Drift

Casing is specified by OD and nominal weight (lb/ft), not by schedule; each listed weight fixes the wall, ID and drift. The nominal weight is a threaded-and-coupled figure, so it is slightly above the plain-end weight: 7 in 26 lb/ft casing weighs 25.69 lb/ft plain-end.

Drift is the guaranteed minimum passage. Every length is drift-tested over its full length with a mandrel slightly smaller than the nominal ID. For the sizes above, the standard drift is 1/8 in below the ID on 5-1/2 in and 7 in casing and 5/32 in below on 9-5/8 in and 13-3/8 in. The drift decides whether the next bit, casing string or tool will pass, and the ID does not. For some sizes and weights API 5CT also lists a larger alternate drift, used when a particular bit size must pass. It has to be ordered explicitly.

Within one OD, heavier weights give higher collapse and burst ratings but a smaller drift; lighter weights pass a larger bit but carry less pressure.

Connections: STC, LTC, BTC and Premium

API threads seal through thread compound in the thread clearances, which is not gas-tight; where gas tightness or high combined loads matter, the programme names a premium connection. Note that API joint strengths are calculated from the ultimate strength of the steel, while many premium connections are rated on yield, so compare ratings on the same basis.

Tubing connections:

• NU (non-upset): threads are cut in the plain pipe wall, so the joint is weaker than the pipe body

• EU (external upset): the pipe ends are thickened before threading, so the joint is at least as strong as the body; widely used for production tubing

• IJ (integral joint): no separate coupling, which halves the number of leak paths and suits tight clearances

Range Lengths

Pipe is ordered in length ranges. For tubing, Range 1 is 6.10–7.32 m (20–24 ft), Range 2 is 8.53–9.75 m (28–32 ft) and Range 3 is 11.58–12.80 m (38–42 ft). Casing joints are generally around 12 m (40 ft) long. State the range on the order, and check the rig's pipe-handling equipment and derrick before choosing the longest range. Pup joints, which are short lengths of the same grade and connection, are used to space out the string at the wellhead.

Performance Ratings: Collapse, Burst and Tension

Casing design rests on three ratings, all calculated with the formulas in API TR 5C3, the successor to Bulletin 5C3:

• Collapse: the external pressure the pipe resists, from four formulas (yield, plastic, transition and elastic collapse) chosen by D/t and yield strength

• Internal yield (burst): the internal pressure at which the inner-wall hoop stress reaches minimum yield; not a rupture pressure

• Body yield and joint strength: the axial load the pipe and the connection can carry

Worked example: 13-3/8 in 68 lb/ft casing

The API internal yield formula is:

P = 0.875 × 2 × Yp × t / D, rounded to the nearest 10 psi

Here Yp is the minimum yield strength, t the nominal wall and D the OD. The 0.875 factor allows for the −12.5% tolerance on wall thickness. For 13-3/8 in 68 lb/ft casing (t = 0.480 in, D = 13.375 in) in P110:

0.875 × 2 × 110,000 × 0.480 / 13.375 = 6,908 psi → 6,910 psi (476 bar)

The same formula gives 6,870 psi for 9-5/8 in 47 lb/ft N80 and 9,440 psi in P110.

Pipe body yield is Fy = π/4 × (D² − d²) × Yp. With d = 12.415 in, the steel area is 19.45 in², so the P110 body yield is 19.45 × 110,000 ≈ 2,139,000 lb (about 9,500 kN).

The example shows two things:

1. Grade raises burst and tension in proportion, not collapse. J55 to P110 doubles the yield strength and the internal yield (3,450 to 6,910 psi), but collapse rises only 20% (1,950 to 2,340 psi). At a D/t of about 28, collapse is governed largely by geometry, and in the elastic range it does not depend on yield strength at all. On large, thin-walled casing, add wall, not grade.

2. Ratings come before design factors. Operators set their own; one published set of minimums is 1.1 for collapse, 1.1–1.25 for burst and 1.4 for tension on yield. A string rated at 5,020 psi is not designed to see 5,020 psi.

Inspection and Documents

API 5CT requires, among other things, hydrostatic testing of each length, full-length drift testing, thread inspection and gauging to API 5B, non-destructive examination of the pipe body to an extent set by grade and PSL (by methods such as electromagnetic inspection or ultrasonic testing), dimensional checks, mechanical tests, stencil and colour-band marking, and SSC testing for C90, T95 and C110.

Buyers often add third-party inspection with witnessed hydrostatic and drift tests, extra NDE and a specific certificate type. Decide the certificate at enquiry: an EN 10204 3.1 mill certificate, or a 3.2 certificate co-validated by the manufacturer's inspection representative and the purchaser's or an official inspector; see our guide to material test certificates.

How to Order: Checklist

1. OD and nominal weight (or wall)

2. Grade and type, for example L80 Type 1 or N80Q

3. Connection (STC, LTC, BTC or a named premium connection) and coupling type

4. Range length and any pup joints

5. Quantity in metres, feet or joints

6. PSL if above PSL-1, and supplementary requirements

7. Drift: standard or alternate

8. Service: sweet or sour, H2S partial pressure, CO2, temperature

9. Inspection and documents: third-party inspection, extra NDE, certificate type, thread protectors

10. Delivery: destination, Incoterm and date

Common Mistakes

1. Ordering by OD and grade without the weight. "9-5/8 N80" describes several different pipes.

   Prevention: State nominal weight or wall; it fixes the drift and every rating.

2. Assuming N80 or P110 is acceptable in a sour well. Below their temperature limits, both can crack.

   Prevention: Get the H2S partial pressure and temperature profile; use L80 Type 1, C90 or T95 where required.

3. Using 13Cr as a sour-service upgrade. 13Cr is a CO2 corrosion grade with low H2S tolerance.

   Prevention: Use L80 Type 1 for H2S; where CO2 and H2S both occur, check the 13Cr limits first.

4. Designing clearance on ID instead of drift. A tool that fits the ID can still stop at drift.

   Prevention: Design to drift; order alternate drift explicitly where a specific bit must pass.

5. Raising grade to fix collapse on large casing. On thin-walled pipe, collapse barely changes with grade.

   Prevention: Check D/t; add wall thickness where collapse governs.

6. Leaving range length and pups unspecified. The rig cannot handle the pipe, or the string cannot be spaced out.

   Prevention: State range length and pup lengths, and check pipe-handling limits.

7. Specifying inspection and certificates after the order. Witness points and certificate types cannot be added once pipe has been tested.

   Prevention: Put third-party inspection, NDE scope and certificate type in the enquiry.

Supply from Kasko Makine

Kasko Mak branded API 5CT casing pipes

• Standard: API 5CT, Specification for Casing and Tubing

• Grades: J55, K55, N80, L80, P110

• Connections: STC (short thread coupling), LTC (long thread coupling), BTC (buttress thread coupling)

• Sizes: 4-1/2 in to 20 in OD

• Testing and inspection: hydrostatic pressure testing per API 5CT, electromagnetic inspection (EMI), thread gauging and drift testing

• Traceability: full material traceability from heat to finished pipe

• Certification: manufactured under API 5CT licence and supplied with mill test certificates

• Third-party inspection: available on request

Engineering support: Send the casing programme or the list of strings (size, weight, grade, connection, range length and quantity) with the service conditions and the inspection and certificates you need. We reply with pricing, lead time and engineering details.

Send the casing size, weight, grade, connection, range length and quantity for a quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• API 5L Line Pipe: Grades, PSL1 and PSL2

• NACE MR0175 / ISO 15156: Sour Service Materials

• Material Test Certificates: EN 10204 3.1 vs 3.2

• Seamless vs Welded Steel Pipe

• Spiral Welded Pipe (SSAW): Manufacturing and Standards

• Industrial Equipment Supply in Kazakhstan

• Industrial Equipment Supply in Iraq

Frequently Asked Questions

Q: What is the difference between casing and tubing?
A: Casing is the larger pipe cemented into the well to hold back the formation, isolate zones and contain pressure; it stays for the life of the well. Tubing is the smaller pipe run inside the production casing to carry oil or gas to surface, and it can be pulled during a workover. Both are specified under API 5CT, and casing runs from 4-1/2 in to 20 in OD.

Q: What does the number in a casing grade mean?
A: The number is the minimum yield strength in thousands of pounds per square inch. J55 has a minimum yield of 55,000 psi (379 MPa), N80 and L80 have 80,000 psi (552 MPa), and P110 has 110,000 psi (758 MPa). Every grade also has a maximum yield strength, and the sour-service grades such as L80 Type 1, C90 and T95 have a maximum hardness as well.

Q: What is the difference between J55 and K55 casing?
A: J55 and K55 have the same yield strength range, 55,000 to 80,000 psi, but K55 has a higher minimum tensile strength of 95,000 psi against 75,000 psi for J55. API round-thread joint strength is calculated from tensile strength, so K55 strings have higher rated joint loads. Both are general-purpose grades for surface and shallow strings and are accepted for sour service.

Q: Which casing grades can be used in sour service?
A: ISO 15156-2 (NACE MR0175) accepts H40, J55, K55, L80 Type 1, C90 Type 1 and T95 Type 1 at all temperatures, relying on the API 5CT hardness limits of 23 HRC for L80 and 25.4 HRC for C90 and T95. N80Q is accepted from about 65 °C, N80 Type 1 and P110 from about 80 °C, and Cr-Mo Q125 from about 107 °C, because sulfide stress cracking is most severe at lower temperatures.

Q: What is casing drift diameter?
A: Drift diameter is the minimum guaranteed passage through a length of casing. Every length is tested by passing a mandrel through its full length, and the standard drift is slightly smaller than the nominal ID: 6.151 in for 7 in 26 lb/ft casing with a 6.276 in ID. Check bit and tool clearance against the drift, not the ID. Some sizes also have a larger alternate drift.

Q: What is the difference between STC, LTC and BTC casing connections?
A: STC and LTC are API round-thread connections with eight threads per inch. LTC has a longer thread, which gives higher joint strength and better sealing than STC. BTC is the API buttress thread, with five threads per inch and asymmetric flanks, and it has much higher axial strength. All three seal through thread compound, so wells that need a gas-tight seal use premium connections with metal-to-metal seals.

Q: How is casing burst pressure calculated?
A: The API internal yield pressure is 0.875 × 2 × Yp × t / D, where Yp is the minimum yield strength, t the nominal wall and D the outside diameter; the 0.875 factor allows for the −12.5% wall tolerance. For 13-3/8 in 68 lb/ft N80 casing it gives 5,020 psi. This is the pressure at which the inner wall reaches yield, not a rupture pressure, and design factors are applied on top.