What API 5CT Oil Tubing Is
Oil tubing is the inner string through which produced fluids travel from the reservoir to the surface. It is manufactured to API Specification 5CT, which covers casing and tubing for well service and defines steel grades, dimensions, wall thicknesses, end finishes and inspection requirements. Tubing is distinguished from casing by its smaller diameter and by its coupling design, and it is normally supplied in outside diameters from 1.050 in (26.67 mm) to 4-1/2 in (114.30 mm).
Grades and Mechanical Requirements
API 5CT grades are identified by a letter and a number, where the number gives the minimum yield strength in thousand psi. The carbon and carbon-manganese grades cover routine completions, while chromium bearing and higher strength grades are selected for deep wells, carbon dioxide service and sour environments containing hydrogen sulphide.
| Grade | Minimum yield strength | Maximum yield strength | Minimum tensile strength | Typical service |
|---|---|---|---|---|
| H40 | 276 MPa (40,000 psi) | 552 MPa (80,000 psi) | 414 MPa (60,000 psi) | Shallow, low pressure wells |
| J55 and K55 | 379 MPa (55,000 psi) | 552 MPa (80,000 psi) | 517 MPa (75,000 psi) | General well service |
| N80 type 1 and N80Q | 552 MPa (80,000 psi) | 758 MPa (110,000 psi) | 689 MPa (100,000 psi) | Medium depth wells |
| L80 | 552 MPa (80,000 psi) | 655 MPa (95,000 psi) | 655 MPa (95,000 psi) | Sour service, hardness limited |
| C90 and T95 | 621 MPa and 655 MPa | 724 MPa and 758 MPa | 689 MPa and 724 MPa | Sweet and mild sour service |
| P110 | 758 MPa (110,000 psi) | 965 MPa (140,000 psi) | 862 MPa (125,000 psi) | Deep, high pressure wells |
| Q125 | 862 MPa (125,000 psi) | 1034 MPa (150,000 psi) | 931 MPa (135,000 psi) | High strength completions |
Grade L80 is supplied in several types, including a 13 percent chromium variant used in carbon dioxide and mildly sour wells, and a 9 percent chromium variant. All grades are released against tensile testing, and L80 is additionally hardness controlled to resist sulphide stress cracking, with a maximum of 23 HRC.
Common Sizes, Wall Thickness and Length Ranges
Tubing is ordered by outside diameter and nominal mass per unit length, from which the wall thickness follows. Representative sizes are tabulated below; the standard tabulates many intermediate masses for each diameter.
| Outside diameter | Nominal mass | Wall thickness | Usual end finish |
|---|---|---|---|
| 2-3/8 in (60.32 mm) | 4.7 lb/ft (6.99 kg/m) | 4.83 mm | Non-upset or external upset |
| 2-7/8 in (73.02 mm) | 6.5 lb/ft (9.67 kg/m) | 5.51 mm | Non-upset or external upset |
| 3-1/2 in (88.90 mm) | 9.2 lb/ft (13.69 kg/m) | 6.45 mm | External upset |
| 4-1/2 in (114.30 mm) | 12.75 lb/ft (18.97 kg/m) | 6.88 mm | External upset |
Length is ordered in three standard ranges: R1 from 6.10 m to 7.32 m, R2 from 8.53 m to 9.75 m and R3 from 11.58 m to 12.80 m. R2 is the most widely stocked range because it suits most rig configurations and transport limits.
End Finish and Connections
Tubing is supplied with non-upset ends and a coupling, with externally upset ends for higher joint strength, or as an integral joint where the connection is machined directly into the wall. Threads and couplings are made to the gauging requirements of API Specification 5B, so thread form, pitch diameter and stand-off are checked with the specified gauges rather than by visual assessment alone. Premium connections with metal-to-metal seals are available on request for gas tight and high pressure completions, and are qualified by the connection manufacturer's own test protocol.
Inspection, Handling and Storage
Hydrostatic testing is performed on every length unless the purchase order specifies otherwise, and full body non destructive inspection by ultrasonic or electromagnetic methods is available on order.
Drift testing confirms internal diameter, and threads are protected with couplings or thread protectors for transport.
Impact testing is applied for the grades and wall thicknesses required by the standard, and the transition temperature requirement follows the grade grouping.
Pipe should be stacked clear of the ground on timber, with thread protectors left in place until make-up, to avoid mechanical damage and corrosion of the threads.
FAQ
Q: What is the difference between tubing and casing under API 5CT?
Tubing forms the inner production string and is run inside casing, so it has a smaller diameter, a different wall thickness schedule and a coupling designed for repeated make-up and breakout. Casing is the larger outer string that supports the wellbore and is normally cemented in place.
Q: Which API 5CT grade is used for sour service?
L80 is the most common choice because it combines a minimum yield strength of 552 MPa with a hardness limit that reduces susceptibility to sulphide stress cracking. Higher strength grades such as C90 and T95 are also used in sour service, subject to the service conditions agreed with the end user.
Q: What does the Q suffix mean in N80Q?
The Q denotes quenched and tempered heat treatment. N80 type 1 is normally normalised, whereas N80Q is quenched and tempered, which gives more uniform properties and is generally preferred where toughness matters.
Q: How is tubing length specified?
By range: R1 covers approximately 6.10 m to 7.32 m, R2 covers 8.53 m to 9.75 m and R3 covers 11.58 m to 12.80 m. The purchaser states the range and may also state a minimum average length for the string.
Q: Are couplings supplied with the tubing?
Yes, unless the order specifies bare pipe. Couplings are matched to the grade and end finish, and for external upset or non-upset connections they are supplied in the strength class required by the standard or by agreement.
Q: What documentation is normally provided?
Mill test certificates covering heat analysis, tensile properties, hardness for controlled grades and the results of hydrostatic and non destructive testing. Supplementary certificates for impact testing and dimensional verification can be provided for critical completions.
