API 5CT casing pipe

As a key structural component in oil and gas construction, the casing not only stabilizes the wellbore and effectively prevents its collapse, but also isolates the pressure and fluid between different formations, ensuring the safety and efficiency of drilling and completion operations. Among the numerous casing standards, API 5CT specification, with its strict chemical composition control and clear mechanical performance requirements, has become the widely adopted standard in the global oil and gas industry.

What is API 5CT?

API 5CT, whose full name is API SPEC 5CT, is a technical specification standard for oil casing issued by the American Petroleum Institute (API), and is mainly used in the field of oil and gas extraction. This standard specifies technical indicators such as the steel grade system, chemical composition, mechanical properties, and dimensional tolerances for casing and tubing. Strict limits are set on the content of elements such as carbon, manganese and phosphorus, and specific requirements are put forward for mechanical properties such as tensile strength and yield strength.

Steel grade classification of oil casing

In API 5CT, different grades represent the strength, hardness and applicable environmental conditions of the pipe. According to the strength, they can be classified as follows:
Low strength: H40, J55, K55.
Medium strength: N80 (Type 1 / Q), L80 (including 9Cr/13Cr).
High strength: C90, T95, P110, Q125, C110.
For instance, P110 has a relatively high strength and is suitable for deep Wells, with a yield strength of approximately 758-965 MPa and a tensile strength greater than 862MPa.

API 5CT Casing Pipe Dimensions and Wall Thickness Specifications

Outside Diameter (OD)	Wall Thickness Range (in / mm)	Weight (lb/ft)
4½″ (114.3 mm)	0.205–0.337″ / 5.2–8.6 mm	9.5 – 15.1
5″ (127 mm)	0.220–0.362″ / 5.6–9.2 mm	11.5 – 21.4
5½″ (139.7 mm)	0.244–0.750″ / 6.2–19.1 mm	14.0 – 40.5
6⅝″ (168.3 mm)	0.288–0.475″ / 7.3–12.1 mm	~20 – ~32
7″ (177.8 mm)	0.231–0.540″ / 5.9–13.7 mm	~17 – ~57
7⅝″ (193.7 mm)	0.300–0.750″ / 7.6–19.1 mm	~24 – ~47
8⅝″ (219.1 mm)	0.264–0.557″ / 6.7–14.1 mm	~24 – ~49
9⅝″ (244.5 mm)	0.312–0.672″ / 7.9–17.1 mm	~32 – ~75
10¾″ (273.1 mm)	0.279–0.797″ / 7.1–20.2 mm	~32 – ~85
11¾″ (298.5 mm)	0.333–0.489″ / 8.5–12.4 mm	~42 – ~60
13⅜″ (339.7 mm)	~0.300–0.600″	~83
…up to 20″ (508 mm)	Wall thickness up to 16 mm	Wide weight range

The length grades of the casing are generally R1, R2, R3, among which the range of R1 is approximately  4.88-7.62 m; The range of R2 is approximately  7.62-10.36 m; The range of R3 is approximately ≥ 10.36 m.
Standard OD coverage range: from a minimum of 4½ "(114.3 mm) to a maximum of 20" (508 mm).
Allowable deviation of wall thickness: Standard allowable wall thickness -12.5% (minimum value).

Heat treatment and manufacturing methods

The casing can be produced in seamless or resistance welded forms, etc. Among them, the heat treatment methods vary depending on the steel grade of the casing.
High-strength grades such as P110, C90, and T95 require quenching and tempering, with full-body treatment.
N80T, R95 and others also require heat treatment. L80, C90 and T95 have hardness requirements for sour service.
For sleeves produced by welding, it should be noted that the weld seam should be heated to at least 540 °C to remove untempered martensite.

Chemical composition and mechanical properties

API 5CT strictly stipulates the chemical element content indicators of the casing, including carbon content of 0.34-0.43%, manganese content of ≤1.90%, phosphorus content of ≤0.030%, sulfur content of ≤0.030%, etc. Among them, the L80 steel grade contains chromium of 12.0-14.0%, and the P110 steel grade contains molybdenum of 0.15-0.25%.
The minimum yield strength of J55 steel grade is 379MPa, while that of P110 steel grade can reach up to 965MPa.
The elongation requirement is set within the range of 18% to 32% depending on the steel grade.
Hardness indicators such as Brinell hardness of L80 steel grade ≤241HB.

Common connection methods

Casing: Common ones include BTC (buttress), SC (short round tooth), LC (long round tooth), and premium connections (such as HYD).
Tubing: NU (non-upset) and EUE (outward expanded Tubing) ends, with the latter end thickened.

Summary

API 5CT casing tubing standard is widely used in wellbore structure design and construction, covering the entire process from materials, manufacturing, performance testing to connection methods and identification. Different steel grades correspond to different formation pressures and environments (such as H₂S, deep Wells, sand layers, etc.). For instance, P110 is selected for deep Wells, while L80/C90 are used in highly corrosive well sections.