In the oil and gas industry, an API Coupling is a short piece of pipe used to connect two joints of threaded casing or tubing.
Because standard pipes (joints) are manufactured with external threads (male) on both ends, a coupling-which has internal threads (female) on both ends-is required to join them together into a continuous "string."
API couplings are governed by the API Spec 5CT standard, which ensures they match the strength, dimensions, and thread requirements of the pipes they are connecting.
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Types of API Couplings
Couplings are categorized based on the type of pipe they connect and the thread profile used:
A. Casing Couplings
STC (Short Thread Coupled): Uses round threads. It is the shortest coupling type and is used for lower-load applications.
LTC (Long Thread Coupled): Also uses round threads but is longer than STC. The extra length provides better tensile strength and is the most common casing connection.
BTC (Buttress Thread Coupled): Uses trapezoidal (buttress) threads. These are designed to handle extremely high pulling forces (tension), making them ideal for deep or heavy casing strings.
B. Tubing Couplings
NUE (Non-Upset End): Used for pipes where the ends are the same thickness as the pipe body.
EUE (External Upset End): This is the industry standard for production tubing. The ends of the pipe are forged thicker (upset) before threading, and the coupling is beefier to provide maximum strength and leak resistance.
coupling specification
| Coupling Type | Pipe Category | Thread Profile | Primary Advantage |
| STC | Casing | Round | Economical, good seal |
| LTC | Casing | Round | Higher strength than STC |
| BTC | Casing | Trapezoidal (Buttress) | Maximum pulling strength |
| EUE | Tubing | Round (Thickened) | Highest reliability for production |
| NUE | Tubing | Round | Slimmer profile for tight spaces |
API 5CT Casing Coupling Size Table
| Pipe OD (Inches) | Coupling OD (W) - Inches | Coupling OD (W) - mm | STC Length (L) | LTC Length (L) | BTC Length (L) |
| 4-½" | 5.000" | 127.00 | 6-¼" | 7" | 8-⅞" |
| 5" | 5.563" | 141.30 | 6-½" | 7-⅜" | 9-⅛" |
| 5-½" | 6.050" | 153.67 | 6-¾" | 7-¾" | 9-¼" |
| 6-⅝" | 7.390" | 187.71 | 7-¼" | 8-¼" | 9-⅝" |
| 7" | 7.656" | 194.46 | 7-¼" | 9" | 10" |
| 7-⅝" | 8.500" | 215.90 | 7-½" | 9-¼" | 10-⅝" |
| 8-⅝" | 9.625" | 244.48 | 7-¾" | - | 10-¾" |
| 9-⅝" | 10.625" | 269.88 | 7-¾" | - | 10-¾" |
| 10-¾" | 11.750" | 298.45 | 8" | - | - |
| 13-⅜" | 14.375" | 365.12 | 8" | - | - |
| 20" | 21.000" | 533.40 | 9" | - | - |
API 5CT Tubing Coupling Size Table
| Pipe OD (Inches) | Coupling OD - EU (W) | Coupling OD - NU (W) | EU Length (L) | NU Length (L) | Thread Type |
| 1.050" | 1.660" | 1.313" | 3-⅝" | 3-⅛" | 10 Round |
| 1.315" | 1.900" | 1.660" | 3-¾" | 3-¼" | 10 Round |
| 1.660" | 2.200" | 2.050" | 4-⅛" | 3-½" | 10 Round |
| 1.900" | 2.500" | 2.200" | 4-⅜" | 3-¾" | 10 Round |
| 2-⅜" (Common) | 3.063" | 2.875" | 4-¾" | 4-¼" | 8 / 10 Round |
| 2-⅞" (Most Common) | 3.668" | 3.500" | 5-¼" | 5-⅛" | 8 / 10 Round |
| 3-½" | 4.500" | 4.250" | 5-¾" | 5-½" | 8 Round |
| 4" | 5.000" | 4.750" | 6-⅛" | 5-¾" | 8 Round |
| 4-½" | 5.563" | 5.200" | 6-⅜" | 6-⅛" | 8 Round |
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FAQ
What material is used in API coupling?
According to API standards, drill pipe couplings are primarily manufactured using the following materials:
Conventional Drill Pipe Couplings:
Commonly utilize AISI 4140H alloy steel (equivalent to Chinese grade 36CrNiMo4), which, after quenching and tempering heat treatment, exhibits high strength and fatigue resistance, meeting API 7/API 5DP requirements for yield strength (≥827 MPa) and impact toughness.
Drill Collars and Large Adapter Subs:
Employ modified AISI 4145H alloy steel, with increased chromium and molybdenum content to enhance hardenability and high-temperature strength, suitable for high-torque applications.
Non-Magnetic Drill Collar Connections:
Use Cr-Ni austenitic stainless steel (e.g., ASTM A182 F316) or Cr-Mn-N austenitic steel to prevent interference with logging-while-drilling tools.
What is api coupling used for
The core purpose of API coupling is to reduce the dependency strength between system modules and achieve a loose coupling architecture.
