Definition
API 5CT — API 5CT is the American Petroleum Institute specification for casing and tubing, defining the grades, dimensions, mechanical properties and inspection requirements for the pipe used to case and complete oil and gas wells.
Also known as: API 5CT, API Spec 5CT, ISO 11960, casing and tubing spec
What API 5CT is
API Specification 5CT (aligned with ISO 11960) is the manufacturing standard for casing and tubing — the “OCTG” (oil country tubular goods) that line and complete a well. It specifies steel grades, dimensions, wall thickness, mechanical properties, chemical composition, and the testing and inspection each length must pass, along with marking and traceability requirements.
API 5CT defines grade groups by yield strength and service, including common grades such as J55, K55, N80, L80, C90, T95, P110 and Q125. Certain grades — for example L80, C90 and T95 — are controlled for hardness and metallurgy to resist sulfide stress cracking in sour service, tying into NACE MR0175 / ISO 15156.
What it specifies
- Grades — from J55/K55 up to P110 and Q125 by strength and service.
- Dimensions and tolerances — OD, wall, drift and connection interfaces.
- Testing — hydrostatic, hardness, impact and nondestructive inspection.
Because it fixes the manufactured properties of casing and tubing, API 5CT underpins collapse, burst and tension design of the string.
Why it matters for rentals
Workstring tubing and casing accessories must match the specified grade and drift of your program. See our casing accessories and casing running application, or contact us for certificates.