API Coupling and Threads for Tubing & Casing Pipe Connection

Introduction to API coupling and thread:

API coupling:
API coupling refers to the steel coupling used to connect casing pipe and tubing. Also known as oil well pipe coupling, it is usually made of seamless steel pipe, the material grade is the same as the pipe body (API 5CT K55, J55, N80, L80, P110, etc.), the same as the standard pipeline (PSL) or higher than the required grade. Unless otherwise required, the same heat treatment process can also be used.

API thread:
API thread refers to the thread manufactured and inspected in accordance with API 5B standard. The coupling shall not be expanded to achieve the required thread taper according to API specifications.
(Note: Due to insufficient pressure between the coupling and the pin, the leakage resistance of the coupling with API thread may not reach the internal yield pressure of the pipe body.)

Casing and casing string:
Oil well casing, a seamless or welded pipe made of high-quality steel, both ends of which are finely processed to form tapered threads. In practical applications, these casings are usually connected to each other through casing couplings to form a casing string. This columnar structure is widely used to seal the exposed rock part of the well wall to ensure the safety and stability of the oil well.

API 5CT is a standard for oil casing formulated by the American Petroleum Institute, which defines a series of technical requirements for oil casing, including connection methods. There are two main connection methods for API 5CT oil casing and tubing: threaded connection and non-threaded connection. This article mainly discusses the threaded connection method of API 5CT oil casing and tubing.

Casing thread connection:

API thread connection is one of the most common connection methods. It is achieved by cutting threads between the pipe end of the casing and the drill pipe. Common thread types are external threads and internal threads. Choose the appropriate thread type according to different needs and use environments. Threaded connection has the advantages of simple connection, easy disassembly, and high reliability, and is widely used in oil production and downhole operations.

API 5CT Tubing and Casing Pipe Connection Type:

API 5CT oil tubing and casing connection methods mainly include the following types, each of which is suitable for different working conditions and performance requirements:

1. Short Thread Coupling (STC)

Features: Short thread length, easy installation, low cost.
Applicable scenarios: Generally used in shallow wells or medium-deep wells, where sealing and tensile strength are not required.

2. Long Thread Coupling (LTC)

Features: The thread part is longer than STC, providing better sealing and connection strength.

Applicable scenarios: Applicable to medium-deep wells that require higher sealing and tensile strength.

3. Buttress Thread Coupling (BTC)

Features: The thread is trapezoidal, has stronger tensile and compressive resistance, better sealing than round thread, and is suitable for high-pressure environments.
Applicable scenarios: Applicable to deep wells, ultra-deep wells and high-pressure environments, such as shale gas extraction. It is also suitable for working conditions that require high torque resistance and vibration resistance.

4. Premium Connection

Features: It adopts non-API standard design, usually has advanced structures such as metal-to-metal sealing and multiple threads, and has extremely high air tightness and fatigue resistance.
Applicable scenarios: extreme working conditions (such as high temperature and high pressure, hydrogen sulfide environment) or high-demand complex well conditions (horizontal wells, large displacement wells).

5. Direct connection (X-line, XC)

Features: No coupling is required, the casing is directly connected through special threads, reducing the outer diameter, increasing the diameter, and improving the fluid flow capacity.
Applicable scenarios: small hole drilling or well conditions that require maximum inner diameter.

6. External Upset End (EUE)

Features: The pipe end is thickened to enhance the connection strength, usually used for tubing rather than casing.
Applicable scenarios: high-pressure oil wells or working conditions that require high torsional strength.

7. Plain End

Features: No threads, welding or clamp connection required.

Applicable scenarios: Special customization requirements or occasions requiring on-site welding.

The main parameters of API standard threads include:

(1) Thread length, (except for trapezoidal thread): the length from the starting point (pipe end) of the thread to the vanishing point;
(2) Thread height: the distance between the top of the thread and the bottom of the tooth;
(3) Pitch: the distance from any point of the thread to the corresponding point of the adjacent tooth in the axial direction;
(4) Thread taper: the change in thread diameter per inch of thread length expressed in inches;
(5) Close distance: the distance measured in the axial direction from the end face of the pipe or coupling to the tightening position of the ring gauge or plug gauge;
(6) Thread tail taper (only partial ladder thread): The rapid retreat of the cutting tool causes a steep slope at the end of the thread.

Selection basis:

Well depth: Shallow well (STC), deep well (BTC or special buckle).
Pressure: BTC or special buckle is required for high pressure environment.
Corrosiveness: Special buckle or sulfur-resistant material (such as L80-13Cr) is recommended when containing H₂S or CO₂.
Cost: STC is the most economical, and special buckle is the most expensive.

The specific thread parameters and inspection standards of each connection method must comply with API 5B specifications.

Casing threadless connection:
Threadless connection is an emerging connection method in recent years, mainly to solve the problems of easy wear and leakage of traditional threaded connection. Threadless connection is achieved by welding or pressing the metal surfaces of the casing end and the drill pipe end together. Threadless connection has the advantages of simple structure, good sealing and long service life, which can reduce casing oil leakage and pollution caused by thread wear.

Conclusion:

API casing & tubing threaded connection and non-threaded connection have their own characteristics. Choose the appropriate connection method according to different needs and usage conditions. Whether it is threaded connection or non-threaded connection, it needs to comply with the relevant requirements of API 5CT standard to ensure the quality and reliability of the connection. This can ensure the normal use of oil casing in oil production and underground operations.

Read more: Steel Grades of OCTG Casing or API 5CT Steel Casing and Tubing Chart