API 5CT is a technical specification standard for oilfield casing published by the American Petroleum Institute (API), primarily used in the oil and gas extraction industry. This standard specifies the steel grade system, chemical composition, mechanical properties, dimensional tolerances, and other technical indicators for casing and tubing. API 5CT casing pipe is one of the key drilling and completion equipment in the oil and gas industry, mainly used in the casing structure of oil or gas wells to protect the wellbore, prevent well collapse, isolate fluids from different formations, and ensure the safety and efficiency of drilling operations. According to the API 5CT standard, casing can be classified into several types according to steel grade, application, and manufacturing process, including different steel grades such as J55, K55, N80, L80, C90, T95, and P110. Each steel grade has different yield strength, tensile strength, and chemical composition requirements to adapt to different formation pressures, temperatures, and corrosive environments.
What is API 5CT Casing Pipe?
API 5CT is a technical specification standard for oil well casing and tubing published by the American Petroleum Institute (API). It standardizes the design, manufacture, inspection, and testing requirements for steel tubing used in oil and gas wells. This standard covers the technical conditions for casing, tubing, pup joints, coupling blanks and materials, and related accessories, ensuring reliable mechanical properties and durability in oil well environments.The latest API Spec 5CT (11th Edition) was released in 2023, and further refined its quality and performance requirements in 2025 through the API Addendum, particularly emphasizing enhanced requirements for extreme conditions such as high collapse pressure.
API 5CT casing is typically classified into the following categories according to its application:
1. Casing pipe: Used as the outer casing layer in oil or gas wells to withstand formation pressure, prevent wellbore collapse, and isolate different formation fluids.
2. Tubing: Used in the oil and gas production stage to transport fluids from the bottom of the well to surface equipment.
3. Casing Couplings: Used to connect casing sections, ensuring the overall strength and sealing of the pipeline.
API 5CT casing undergoes strict control over dimensional tolerances, thread connection standards, and mechanical properties during design and manufacturing to ensure long-term reliability under high pressure, high temperature, and corrosive environments. Manufacturing processes include hot rolling, cold rolling, and seamless pipe manufacturing, with different processes meeting the requirements of different well depths and operating conditions.
Furthermore, casing can be selected from special steel grades with corrosion resistance, sulfide stress cracking resistance (SSC), and high temperature and high pressure (HTHP) resistance, depending on environmental conditions, and is widely used in extreme conditions such as offshore drilling, deep wells, and ultra-deep wells.
Chemical and mechanical properties of API 5CT casing pipe
The standard establishes a complete parameter system, including the following core elements:Chemical Composition: Specifies elemental indicators such as carbon content 0.34-0.43%, manganese content ≤1.90%, phosphorus content ≤0.030%, and sulfur content ≤0.030%. L80 steel grade contains 12.0-14.0% chromium, and P110 steel grade contains 0.15-0.25% molybdenum.
Mechanical Properties: J55 steel grade has a minimum yield strength of 379 MPa, while P110 steel grade reaches a maximum of 965 MPa. Elongation requirements are set between 18-32% depending on the steel grade.
Hardness indicators, such as L80 steel grade Brinell hardness ≤241 HB.
| Grade | C≤ | Si≤ | Mn≤ | P≤ | S≤ | Cr≤ | Ni≤ | Cu≤ | V≤ | Als≤ |
| API 5CT J55 | 0.34-0.39 | 0.20-0.35 | 1.25-1.50 | 0.02 | 0.015 | 0.15 | 0.2 | 0.2 | / | 0.02 |
| API 5CT K55 | 0.34-0.39 | 0.20-0.35 | 1.25-1.50 | 0.02 | 0.015 | 0.15 | 0.2 | 0.2 | / | 0.02 |
| API 5CT N80 | 0.34-0.38 | 0.20-0.35 | 1.45-1.70 | 0.02 | 0.015 | 0.15 | / | / | 0.11-0.16 | 0.02 |
| API 5CT L80 | 0.15-0.22 | 1 | 0.25-1.00 | 0.02 | 0.01 | 12.0-14.0 | 0.2 | 0.2 | / | 0.02 |
| API 5CT J P110 | 0.26-035 | 0.17-0.37 | 0.40-0.70 | 0.02 | 0.01 | 0.80-1.10 | 0.2 | 0.2 | 0.08 | 0.02 |
API 5CT casing pipe size specifications
The API 5CT standard provides detailed requirements for the geometric dimensions of casing pipes to ensure proper fit, structural integrity, and safe operation in oil and gas wells.| Parameter | Specification / Range | Tolerance / Deviation |
| Outer Diameter ≤ 101.60 mm | ≤ 101.60 mm | ±0.79 mm |
| Outer Diameter ≥ 114.30 mm | ≥ 114.30 mm | -2 |
| Wall Thickness Deviation | Nominal wall thickness | 0 / -12.5% |
| Casing Pipe Length Classification | Short: 4.88 – 7.62 m | — |
| Medium: 7.62 – 10.36 m | — | |
| Long: 10.36 – 14.63 m | — | |
| Standard Tubing Length | 6.10 – 9.75 m | — |
The standard specifies detailed requirements for pipe end treatment: Processing methods are divided into three types: flat end (PL), thickened end (EU), and threaded end (NU). Thread parameters include: taper requirement 1:16; pitch tolerance ±0.051mm; tooth height deviation ±0.051mm. Coupling types include five categories: standard couplings, special intermittent couplings, etc.
Conclusion
The API 5CT casing standard is the core specification for oil and gas casing and tubing manufacturing, covering all key technical requirements from steel grade selection and sizing to inspection and testing. Whether you are a drilling engineer, product purchaser, or manufacturer, a thorough understanding of the API 5CT requirements will help ensure the reliability and service life of your casing products in complex operating conditions.The API 5CT casing standard is the core specification for oil and gas casing and tubing manufacturing, covering all key technical requirements from steel grade selection and sizing to inspection and testing. Whether you are a drilling engineer, product purchaser, or manufacturer, a thorough understanding of the API 5CT requirements will help ensure the reliability and service life of your casing products in complex operating conditions.
