A Coupling is an internally threaded cylinder used to join two lengths of externally threaded pipe.
Purpose: Used in drilling and oil/gas production to extend the length of the string.
Material: Usually the same steel grade as the pipe body (e.g., J55, K55, N80, L80, P110). Sometimes a higher grade is used for the coupling to ensure joint strength.
Manufacturing Standard: Primarily follows API Spec 5CT.
Casing Couplings - Short Thread (STC) Specifications
STC features 8 threads per inch (TPI) with a round thread profile.
| Size (inch) | OD (inch) | OD (mm) | Length NL (mm) | Thread Type |
| 4-1/2 | 5.000 | 127.00 | 158.75 | 8rd |
| 5-1/2 | 6.050 | 153.67 | 171.45 | 8rd |
| 7 | 7.656 | 194.46 | 184.15 | 8rd |
| 9-5/8 | 10.625 | 269.88 | 196.85 | 8rd |
Casing Couplings - Long Thread (LTC) Specifications
LTC has the same thread profile as STC but with a longer thread length, providing better seal integrity and higher tensile strength.
| Size (inch) | OD (inch) | OD (mm) | Length NL (mm) | Thread Type |
| 4-1/2 | 5.000 | 127.00 | 177.80 | 8rd |
| 5-1/2 | 6.050 | 153.67 | 203.20 | 8rd |
| 7 | 7.656 | 194.46 | 228.60 | 8rd |
| 9-5/8 | 10.625 | 269.88 | 266.70 | 8rd |
Casing Coupling - Buttress Thread (BTC) Specifications
BTC threads are trapezoidal (buttress-shaped), designed to handle high axial tension loads.
| Size (inch) | OD (inch) | OD (mm) | Length NL (mm) | Thread Type |
| 5-1/2 | 6.050 | 153.67 | 234.95 | Buttress |
| 7 | 7.656 | 194.46 | 254.00 | Buttress |
| 9-5/8 | 10.625 | 269.88 | 269.88 | Buttress |
| 13-3/8 | 14.375 | 365.12 | 269.88 | Buttress |
Tubing Couplings - External Upset End (EUE) Specifications
EUE is the most common tubing connection. The ends of the pipe are thickened (upset) to make the joint stronger than the pipe body.
| Nominal Size (in) | Coupling OD (mm) | Coupling Length (mm) | TPI | Thread Type |
| 2-3/8 | 73.02 | 123.83 | 8 | Round |
| 2-7/8 | 93.17 | 133.35 | 8 | Round |
| 3-1/2 | 114.30 | 146.05 | 8 | Round |
| 4-1/2 | 141.30 | 155.58 | 8 | Round |
Tubing Couplings - Non-Upset End (NUE) Specifications
NUE threads are cut directly into the pipe body without thickening the ends. Used for shallower or lower-pressure wells.
| Nominal Size (in) | Coupling OD (mm) | Coupling Length (mm) | TPI | Thread Type |
| 2-3/8 | 69.85 | 107.95 | 10 | Round |
| 2-7/8 | 88.90 | 130.18 | 10 | Round |
| 3-1/2 | 107.95 | 142.88 | 10 | Round |
Differences between Casing Coupling and Tubing Coupling
While both serve the same basic function, there are key differences:
| Feature | Casing Coupling | Tubing Coupling |
| Dimensions | Generally larger (4-1/2" to 20") | Smaller (1.050" to 4-1/2") |
| Thread Types | STC (Short), LTC (Long), BTC (Buttress) | EUE (External Upset), NUE (Non-Upset) |
| Threads Per Inch | Usually 8 TPI (Round) or 5 TPI (BTC) | EUE is 8 TPI; NUE is 10 TPI |
| Application | To stabilize the wellbore and prevent collapse | To transport oil/gas to the surface |
| End Form | Rarely uses upset ends | Frequently uses Upset ends for extra strength |
| Wall Thickness | Thicker to resist formation pressure | Thinner, focused on internal pressure/erosion |
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FAQ
What is an API coupling?
An API Coupling is a short, internally threaded pipe used to connect two joints of threaded casing or tubing.
Standard: It is manufactured according to API Spec 5CT (American Petroleum Institute) standards.
Function: It ensures a secure, leak-proof, and high-strength connection between pipes in an oil or gas well.
Consistency: Being "API" means the threads, dimensions, and material grades are standardized globally so that pipes and couplings from different manufacturers are compatible.
What is an API threaded connection?
An API Threaded Connection is a standardized screw thread system used to join oilfield pipes (casing, tubing, and drill pipe).
Standardization: These connections are governed by API Spec 5B, ensuring that pipes and couplings from any manufacturer worldwide will fit together perfectly.
Common Types:
Round Threads (STC, LTC, EUE): Features a rounded "V" shape; easy to stab and run.
Buttress Threads (BTC): Features a trapezoidal shape; designed for high pulling strength (tension).
Purpose: They provide both mechanical strength to hold the heavy weight of the pipe string and a pressure seal to prevent oil or gas leaks.
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