OCTG, or oil country tubular goods, is the family of pipe products used to drill, complete and produce oil and gas wells. It covers casing, which lines the wellbore and keeps it open, and tubing, which carries the produced fluids to the surface. Both are manufactured and certified to API Specification 5CT, one of the most strictly controlled product standards in the steel industry, because a failure thousands of metres underground is extremely expensive to repair. This article explains the grades, sizes, connections and quality requirements of API 5CT casing and tubing.
Casing vs Tubing: Two Different Jobs
Casing is run into the drilled hole and cemented in place to support the borehole, isolate formations and contain well pressure. It is larger, typically from 4-1/2 inches to 20 inches, and is supplied in long lengths, R3 by default for many sizes. Tubing is the production string inside the casing that transports oil and gas to the wellhead; it is smaller, from 1.66 inches up to 4-1/2 inches, and normally supplied in R2 lengths. Both are available with a range of threaded connections matched to the service.
API 5CT Grades and Mechanical Properties
| Grade | Min Yield Strength (MPa) | Max Yield Strength (MPa) | Min Tensile Strength (MPa) | Typical Use |
|---|---|---|---|---|
| J55 | 379 | 552 | 517 | Shallow wells, moderate pressure |
| K55 | 379 | 552 | 655 | Casing in moderately corrosive wells |
| N80-1 / N80Q | 552 | 758 | 689 | Medium depth and pressure, N80Q is quenched and tempered |
| L80-1 | 552 | 655 | 655 | Sour service casing and tubing |
| C90 | 621 | 724 | 698 | Sour service, higher collapse strength |
| T95 | 655 | 758 | 724 | Sour service, high strength |
| P110 | 758 | 965 | 862 | Deep wells, high pressure, non-sour |
| Q125 | 862 | 1034 | 931 | Very deep and high-load strings |
Grades with the letter C, L and T suffixes are grouped for sour service per API 5CT Annex B, with hardness limits and sulfide-stress-cracking tests added. N80Q, P110 and Q125 are supplied quenched and tempered, while J55 and K55 are normally used in the hot-rolled or normalized condition.
Sizes and Lengths
Casing: 4-1/2, 5, 5-1/2, 7, 7-5/8, 9-5/8, 10-3/4, 13-3/8, 16, 18-5/8 and 20 inches, typically in R3 lengths (about 11-13 m)
Tubing: 1.660, 1.900, 2-3/8, 2-7/8, 3-1/2 and 4-1/2 inches, typically in R2 lengths (about 8-10 m)
Wall thickness and weight per foot follow API 5CT tables for each size and grade
Connections
Casing is most often supplied with BTC (buttress thread coupling) or LTC (long round thread coupled) connections, with STC (short round thread) for lower loads and premium connections where gas tightness is required. Tubing uses non-upset (NUE) and external upset (EUE) threaded and coupled ends, plus premium connections for high-pressure or gas service. Premium connections are machined to tight tolerances and are verified by full-scale testing per the connection supplier's qualification.
Manufacturing and Inspection
API 5CT pipe is produced by seamless rolling or by electric resistance welding for the lower grades, then heat treated where the grade requires it. Every joint is hydrostatically tested, and non-destructive examination with electromagnetic or ultrasonic equipment is applied per API 5CT over the full body. Inspection includes chemical analysis and mechanical testing of each heat, dimensional verification of threads with gauges, and surface inspection of the pipe body. Mills producing API 5CT operate under a licensed API monogram programme with third-party audits, and shipments are documented with material test reports per EN 10204 that list the heat, grade, size and test results.
FAQ
What is the difference between casing and tubing?
Casing lines the wellbore and is cemented in place to support the hole and isolate formations; it is large-diameter pipe from 4-1/2 to 20 inches. Tubing is the production string inside the casing that carries fluids to the surface; it is smaller, from 1.66 to 4-1/2 inches.
What does API 5CT cover?
API 5CT specifies casing, tubing and pup joints for oil and gas wells, including the steel grades, heat treatment, mechanical properties, dimensions, tolerances, connection types, testing requirements and marking. It is the international benchmark, adopted in parallel by ISO 11960.
Which grade is used for sour service?
Grades L80-1, C90 and T95 are the standard sour service choices per API 5CT, with hardness limits and sulfide stress cracking testing. For non-sour high pressure wells, N80Q, P110 and Q125 are typical.
What is the difference between N80-1 and N80Q?
N80-1 is produced to a specified strength range without a mandatory quenching and tempering treatment, while N80Q is quenched and tempered to achieve more uniform microstructure and better toughness at the same strength level.
What connections are available for API 5CT casing?
Standard casing connections are BTC (buttress), LTC (long round thread) and STC (short round thread). Premium gas-tight connections are available where leak resistance under high pressure or gas service is critical.
How is API 5CT quality assured?
Quality is assured by the licensed API monogram programme: each joint is hydrostatically tested, full-body electromagnetic or ultrasonic inspection is applied, threads are gauged, heat analysis and mechanical tests are performed on every heat, and the material test report documents the results.
