API 5CT
What is API 5CT?
API 5CT is a standard published by the American Petroleum Institute (API) that defines requirements for seamless and welded casing and tubing used in oil wells. This standard ensures high-quality materials, proper manufacturing processes, and stringent testing procedures to meet the demands of extreme drilling environments.
API 5CT classifies casing and tubing into different steel grades, each designed for specific good conditions. These grades differ in mechanical properties, chemical composition, and performance under high pressure, high temperature (HPHT), and corrosive environments.
| Parameter | Description |
| Standard | API 5CT |
| Product | Casing & Tubing |
| Grades | J55, K55, N80 type 1 and N80Q, L80, C90, T95, P110, and Q125 |
| Manufacturing Process | Seamless / ERW |
| Outer Diameter (OD) | 4.5 in to 20 in (114.3 mm to 508 mm) |
| Casing dimensions | 114.3 mm (4.5 in) to 406.4 mm (16 in) |
| Tubing dimensions | 26.7 mm (1.05 in) to 114.3 mm (4.5 in) |
| Wall Thickness (WT) | 1.0 mm to 30 mm |
| Length Range | R1 (4.88–7.62m), R2 (7.62–10.36m), R3 (10.36–14.63m) |
| Ends | Plain End (PE), Threaded & Coupled (T&C), Buttress Threaded (BTC), Extreme-Line (XC) |
| Surface Coating | Black Paint, Anti-Rust Oil, Phosphating, Galvanized |
| Application | Oil & Gas Well Drilling, Casing Protection, Fluid Transmission |
API 5CT Standard Products
API 5CT Casing
API 5Ct Tubing
API 5CT Pipe
API 5CT Dimensions
- Outer diameter: 4.5 in to 20 in (114.3 mm to 508 mm)
- Wall thickness: 1.0 mm to 30 mm
- Casing dimensions: 114.3 mm (4.5 in) to 406.4 mm (16 in)
- Tubing dimensions: 26.7 mm (1.05 in) to 114.3 mm (4.5 in)
API 5CT Common Grades
• J55/K55
• N80/N80Q/L80
• C90
• R95/T95
• P110/C110
• Q125
API 5CT J55, K55, N80 type 1 and N80Q, L80, C90, T95, P110, and Q125 Chemical Composition
| Grade | C (Max) | Mn | P (Max) | S (Max) | Si (Max) | Mo (Max) | Cr (Max) | Ni (Max) | Cu (Max) | V (Max) |
| H40 | 0.22 | 0.30-1.00 | 0.03 | 0.03 | 0.45 | – | – | – | – | – |
| J55 | 0.26 | 0.30-1.35 | 0.03 | 0.03 | 0.45 | – | – | – | – | – |
| K55 | 0.26 | 0.30-1.35 | 0.03 | 0.03 | 0.45 | – | – | – | – | – |
| N80 | 0.34 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.2 | 0.5 | 0.5 | 0.5 | 0.08 |
| L80 | 0.43 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.25 | 0.75 | 0.75 | 0.35 | 0.08 |
| C90 | 0.45 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.35 | 1 | 0.5 | 0.5 | 0.1 |
| T95 | 0.45 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.35 | 1 | 0.5 | 0.5 | 0.1 |
| P110 | 0.4 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.35 | 1 | 0.5 | 0.5 | 0.1 |
| Q125 | 0.5 | 0.30-1.50 | 0.03 | 0.03 | 0.45 | 0.5 | 1.5 | 0.5 | 0.5 | 0.1 |
API 5CT J55, K55, N80 type 1 and N80Q, L80, C90, T95, P110, and Q125 Mechanical Properties
| Grade | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) |
| H40 | ≥276 | 414-552 | ≥21 |
| J55 | ≥379 | 517-758 | ≥19 |
| K55 | ≥379 | 517-758 | ≥19 |
| N80 | ≥552 | 689-862 | ≥15 |
| L80 | ≥552 | 655-880 | ≥15 |
| C90 | ≥621 | 689-862 | ≥10 |
| T95 | ≥655 | 724-896 | ≥10 |
| P110 | ≥758 | 862-1034 | ≥10 |
| Q125 | ≥862 | 931-1207 | ≥10 |
Differences Between Casing and Tubing
While both casing and tubing are essential for oil well construction, they serve different purposes:
- Casing: Provides structural integrity to the well and prevents fluid migration
- Tubing: Facilitates the extraction of oil and gas to the surface
- Size: Casing has a larger diameter than tubing
- Installation: Casing is cemented into place, while tubing is removable for maintenance
- Mechanical Properties: Tubing must withstand dynamic pressure and fluid flow, while casing primarily supports the wellbore
API 5CT Specifications and Requirements
1. Manufacturing Processes
API 5CT allows for both seamless and welded pipes:
- Seamless Pipes: Manufactured by extruding a solid billet, offering superior strength and uniformity
- Welded Pipes: Produced by welding steel plates or strips, typically used in less demanding applications
2. Dimensional Specifications
API 5CT defines pipe dimensions, including outer diameter, wall thickness, and length. Common sizes range from 4 ½ inches to 20 inches for casing and 1.9 inches to 4 ½ inches for tubing.
3. Connections and Threads
Proper connections ensure secure joining of casing and tubing. API 5CT specifies different types of threads:
- API Round Thread (STC, LTC) – Standard thread types for moderate well conditions
- API Buttress Thread (BTC) – Provides a stronger connection for deep wells
- Premium Connections – Used in high-pressure and corrosive environments
4. Testing and Quality Control
API 5CT mandates rigorous testing, including:
- Hydrostatic Testing – Ensures pipes can withstand high internal pressure
- Non-Destructive Testing (NDT) – Identifies defects using ultrasonic or electromagnetic methods
- Hardness and Impact Testing – Evaluates material strength and resistance to brittle fracture
- Corrosion Resistance Testing – Assesses durability in harsh environments
Benefits
Enhanced Well Integrity – Prevents well collapse and fluid contamination
High Strength and Durability – Withstands extreme pressures and temperatures
Corrosion Resistance – Protects against aggressive chemical environments
Standardized Quality Control – Ensures consistency and reliability
Improved Operational Efficiency – Reduces downtime and maintenance costs
Application
- Onshore and offshore oil wells – Supporting both land-based and deep-sea drilling operations
- Conventional and unconventional wells – Suitable for traditional reservoirs and shale gas/fracking applications
- High-temperature, high-pressure wells – Designed for extreme drilling conditions
- Corrosive environments – Grades like L80 and T95 are resistant to hydrogen sulfide (H2S) and other corrosive elements
