API BTC Coupling is an internally threaded connector manufactured according to API Spec 5CT. Unlike round threads (STC/LTC), the BTC thread profile is trapezoidal, featuring a nearly flat load face.
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API Trapezoidal Thread (BTC) Coupling Specifications
This table outlines the technical standards and quality requirements for GNEE's BTC couplings.
| Feature | Technical Details (API 5CT) |
| Common Name | Buttress Thread Coupling (BTC) |
| Standard | API Spec 5CT (Latest Edition) |
| Steel Grades | J55, K55, N80-Q, L80, C90, T95, P110, Q125 |
| Thread Density | 5 TPI (5 Threads Per Inch) |
| Thread Profile | Trapezoidal (Sawtooth) – 3° load flank / 10° stab flank |
| Surface Treatment | Tin-Plating (Highly recommended for BTC) or Phosphating |
| Testing & QC | 100% Magnetic Particle Inspection (MPI), Hydrostatic Test, API 5B Gauging |
| Traceability | Full MTC (Mill Test Certificate) with Heat Number tracking |
API Trapezoidal Thread (BTC) Coupling Size & Dimension Chart
Dimensions are critical for ensuring the coupling fits within the wellbore (casing clearance).
| Casing size (OD) | Thread Type | Coupling OD - W (in/mm) | Coupling Length - L (in/mm) | Counterbore Dia - Q (in/mm) |
| 4-1/2" | BTC | 5.000 (127.00) | 8.875 (225.43) | 4.635 (117.73) |
| 5" | BTC | 5.563 (141.30) | 9.125 (231.78) | 5.135 (130.43) |
| 5-1/2" | BTC | 6.050 (153.67) | 9.250 (234.95) | 5.635 (143.13) |
| 6-5/8" | BTC | 7.390 (187.71) | 9.625 (244.48) | 6.760 (171.70) |
| 7" | BTC | 7.656 (194.46) | 10.000 (254.00) | 7.135 (181.23) |
| 7-5/8" | BTC | 8.500 (215.90) | 10.375 (263.53) | 7.760 (197.10) |
| 8-5/8" | BTC | 9.625 (244.48) | 10.625 (269.88) | 8.760 (222.50) |
| 9-5/8" | BTC | 10.625 (269.88) | 10.625 (269.88) | 9.760 (247.90) |
| 13-3/8" | BTC | 14.375 (365.13) | 10.625 (269.88) | 13.510 (343.15) |
| 20" | BTC | 21.000 (533.40) | 10.625 (269.88) | 20.135 (511.43) |
Comparison: API Round Thread vs. Buttress Thread (BTC)
| Feature | API Round (STC/LTC) | API Buttress (BTC) |
| Thread Shape | Circular / V-shaped | Trapezoidal / Sawtooth |
| TPI | 8 | 5 |
| Primary Strength | Sealing & Ease of Use | High Tensile Load Resistance |
| Seal Type | Interference Fit + Thread Dope | Interference Fit + Thread Dope |
| Typical Depth | Shallow to Medium Wells | Deep and Ultra-Deep Wells |
Main Technical Parameters of API Standard Threads
| Parameter | Technical Description | Significance |
| 1. Threads Per Inch (TPI) | The number of thread crests in one inch of axial length. <br>• Round Thread (STC/LTC): 8 TPI <br>• Buttress Thread (BTC): 5 TPI | Defines the "density" of the thread. Higher TPI (Round) allows for easier "stabbing" and alignment. |
| 2. Pitch (P) | The axial distance between adjacent thread crests. <br>• Pitch = 1 / TPI | Ensures the male and female threads mesh perfectly without overlap or gaps. |
| 3. Taper | The change in diameter per unit of length along the pipe axis. <br>• Standard API Taper: 0.0625 in/in (1:16) | Allows the coupling to tighten as it is screwed on, creating the interference fit necessary for sealing. |
| 4. Thread Angle | The angle between the thread flanks. <br>• Round Thread: 60° (V-shape) <br>• Buttress Thread: 3° load flank / 10° stab flank | Determines how the connection handles tension. BTC's trapezoidal angle prevents "pull-out" under heavy loads. |
| 5. Thread Height (h) | The distance between the thread crest (top) and root (bottom), measured perpendicular to the axis. | Proper height ensures sufficient mechanical interlocking between the pipe and coupling. |
| 6. Lead | The axial distance a thread advances in one complete turn (360°). | Essential for synchronized makeup between the two components. |
| 7. Crest & Root Radius | The specific rounding of the thread tops and bottoms (for API Round Threads). | Rounded crests and roots reduce stress concentration and prevent thread "galling" (seizing). |
| 8. Stand-off (A) | The axial distance between the end of the coupling and the hand-tight plane on the pipe. | Used during inspection to verify that the threads are machined within tolerance. |
Applications of API Trapezoidal Thread (BTC) Casing Couplings
API BTC couplings are engineered for environments where standard round threads (STC/LTC) reach their physical limits. Because the trapezoidal thread profile offers nearly 100% joint strength efficiency, they are the "go-to" choice for the following scenarios:
1. Deep and Ultra-Deep Well Construction
In wells exceeding 3,000–5,000 meters, the weight of the casing string itself becomes a massive load.
Why BTC: The sawtooth trapezoidal design creates a mechanical lock that prevents the pipe from "pulling out" (joint separation) under its own extreme axial weight.
Application: Used primarily in production and intermediate casing strings for deep-hole exploration.
2. Offshore and Subsea Drilling
Offshore platforms, especially in the Gulf of Guinea (Ghana) or the North Sea, face variable stresses from currents and heavy casing loads.
Why BTC: BTC provides the structural integrity required for subsea wellheads and long casing runs from the platform to the seabed.
Application: Critical for offshore structural casing and high-tension conductor pipes.
3. Horizontal and Directional Drilling
Modern drilling often involves bending the pipe to reach oil pockets horizontally. This puts immense torque and bending stress on the joints.
Why BTC: The rugged trapezoidal threads handle bending and high-torque "make-up" better than round threads, reducing the risk of thread deformation during directional changes.
Application: Shale gas extraction and complex directional oil wells.
4. High-Pressure Oil and Gas Reservoirs
Wells with high bottom-hole pressures (BHP) require connections that won't fail under internal pressure.
Why BTC: When paired with high-quality API thread lubricant (dope), the interference fit of the BTC thread provides a robust seal capable of handling high-pressure environments.
Application: High-pressure production casing.
5. Large Diameter Surface Casing
Surface casing (often 13-3/8" or 20") acts as the foundation of the entire well, supporting all subsequent casing strings.
Why BTC: Because surface casing must hold the weight of everything hung below it, the high-load capacity of BTC is essential for the top sections of the well.
Application: Critical infrastructure for wellbore stability.
6. Geothermal Well Completion
Geothermal wells experience extreme temperature fluctuations, causing the steel to expand and contract.
Why BTC: The trapezoidal thread profile is more resistant to the "telescoping" effect caused by thermal expansion/contraction compared to standard V-threads.
Application: Steam and hot water extraction wells.
API Trapezoidal Thread casing Couplings test equipmen
API Trapezoidal Thread casing Couplings factory
FAQ
What are the three types of coupling?
In the oil and gas industry (specifically under the API 5CT standard), the three main types of casing couplings are:
1. STC (Short Round Thread Coupling)
Thread Profile: Circular/Round thread.
Best For: Shallow wells and low-tension applications.
Key Feature: Shorter thread length; the most economical choice.
2. LTC (Long Round Thread Coupling)
Thread Profile: Circular/Round thread (same as STC).
Best For: Medium-to-deep wells.
Key Feature: Longer thread length provides higher joint strength and better leak resistance than STC.
3. BTC (Buttress Thread Coupling)
Thread Profile: Trapezoidal (sawtooth) thread.
Best For: Deep wells, high-pressure, and high-tension environments.
Key Feature: The trapezoidal shape creates a mechanical lock that prevents the pipes from pulling apart under extreme weight.
What does API thread stand for?
API Thread stands for American Petroleum Institute thread.
It refers to the standardized threading specifications established by the American Petroleum Institute for the oil and gas industry.
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