When selecting oil well casing, the connection type is just as important as the pipe size, steel grade, and wall thickness. A suitable casing connection helps transfer axial loads between pipe joints and contributes to the integrity of the casing string.
Three commonly used API casing connections are STC (Short Thread and Coupling), LTC (Long Thread and Coupling), and BTC (Buttress Thread and Coupling). Each has a different thread design and is suited to different operating requirements.
Understanding the differences between STC, LTC, and BTC casing connections can help drilling contractors, oilfield equipment distributors, and procurement professionals specify the right products for their projects.
In this guide, we explain how these three connections differ in design, strength, sealing performance, and typical applications.
STC stands for Short Thread and Coupling, commonly referred to as short round-thread casing.
STC is a threaded-and-coupled connection used to join casing joints in oil and gas wells. The pipe ends have external threads, while an internally threaded coupling connects adjacent joints.
STC connections are commonly considered for applications that do not require the greater thread engagement associated with LTC or the buttress thread profile of BTC.
However, STC should not automatically be considered suitable for shallow wells or low-pressure wells. Connection selection must account for the actual well design, pipe dimensions, material grade, and anticipated loads.
LTC stands for Long Thread and Coupling, also known as long round-thread casing.
Like STC, LTC uses a rounded thread profile and a separate coupling. Its longer threaded engagement provides a different connection geometry and can offer greater joint tensile capacity than a comparable STC connection, depending on the specified pipe dimensions and grade.
LTC is often selected for conventional oil and gas well casing when its rated mechanical performance and sealing characteristics satisfy the project requirements.
Although LTC has longer threads than STC, this does not mean it automatically provides gas-tight sealing. Both connection types rely on correct thread engagement, appropriate thread compound, proper make-up, and compliance with the applicable specifications.
BTC stands for Buttress Thread and Coupling, commonly known as buttress-thread casing.
Unlike STC and LTC, which use round threads, BTC uses a buttress-type thread profile designed to provide favorable load-bearing characteristics.
BTC is commonly considered for casing strings where joint tensile strength and resistance to axial loading are important design considerations.
BTC is frequently considered for demanding casing applications, including some deeper wells and wells with substantial axial or bending loads.
However, BTC is not automatically the best choice for every high-pressure or high-temperature well. Standard API BTC connections should not be assumed to provide the same gas-tight sealing performance as qualified premium connections.
The following table summarizes the main differences between these three casing connections.
| Feature | STC | LTC | BTC |
| Full name | Short Thread and Coupling | Long Thread and Coupling | Buttress Thread and Coupling |
| Thread profile | Round | Round | Buttress |
| Typical thread pitch | 8 TPI | 8 TPI | 5 TPI for many standard sizes |
| Thread engagement | Shorter | Longer than STC | Buttress-specific geometry |
| Load-bearing characteristics | Depends on size and grade | Often greater tensile capacity than comparable STC | Designed for favorable axial load transfer |
| Sealing mechanism | Thread engagement and thread compound | Thread engagement and thread compound | Thread engagement and thread compound |
| Typical selection factor | Specified load capacity and cost | Increased joint capacity relative to comparable STC | Axial loading and connection strength requirements |
Important: TPI means threads per inch. Thread pitch, geometry, and connection dimensions must be verified against the applicable standard and ordered product specification. The table describes general characteristics rather than guaranteed performance ratings.
Sealing performance is an important consideration when selecting casing connections for oil and gas wells.
STC and LTC use round-thread designs. Their sealing performance depends on the specified thread geometry, correct make-up, and the use of appropriate thread compound.
BTC uses a buttress thread profile that provides favorable load-bearing characteristics. However, its thread form alone does not guarantee gas-tight performance under every operating condition.
For wells with demanding pressure requirements, gas-tight sealing requirements, or challenging thermal and mechanical cycles, operators may need a qualified premium connection with an appropriate metal-to-metal seal.
The correct selection depends on the required leakage acceptance criteria, pressure conditions, temperature, and applicable qualification or testing requirements.
Selecting a casing connection involves more than comparing thread lengths or counting threads per inch. The following factors should be evaluated.
Determine the expected axial loads on the casing string, including the effects of the casing’s suspended weight and relevant operating conditions.
Where connection tensile capacity is critical, compare the rated joint strength of the available STC, LTC, and BTC options.
The same connection designation does not guarantee the same load capacity across different casing sizes and steel grades.
Confirm the following information before ordering:
Evaluate internal pressure, external pressure, expected temperature, and the required leakage performance.
If the application requires qualified gas-tight performance, verify whether a standard API connection meets the design requirements or whether a suitable premium connection is necessary.
Proper installation is essential for connection reliability. Thread condition, cleanliness, compound selection, make-up procedures, and applicable torque or acceptance criteria must be controlled.
Thread inspection and gauging should follow the applicable API requirements and the manufacturer’s documented procedures.
API Specification 5CT addresses casing and tubing product requirements, including dimensions, material grades, and applicable end finishes. API Specification 5B addresses casing and tubing thread manufacturing, gauging, and inspection requirements.
Always verify the applicable editions, amendments, product specification, and
STC, LTC, and BTC are three widely recognized casing connection types used in oil and gas well construction.
STC features a short round-thread design, LTC provides longer round-thread engagement, and BTC uses a buttress thread profile suited to applications requiring favorable axial load-bearing characteristics.
Choosing the right connection requires consideration of pipe size, steel grade, connection strength, pressure and sealing requirements, installation conditions, and applicable standards.
For casing procurement, always confirm the complete technical specification before ordering to ensure compatibility with the well design and operating conditions.
Looking for API 5CT casing and coupling solutions? Contact our technical team with your required casing size, steel grade, connection type, and quantity to discuss suitable product options.
Baotou, Inner Mongolia – Recently, we welcomed our valued customer delegation from Vietnam to Baotou…
Recently, a delegation of valued clients from Nigeria visited the IMRI manufacturing facility to conduct…
OCTG, short for Oil Country Tubular Goods, is a comprehensive term for specialized steel tubular…
Explore the five mainstream railway rail standards worldwide. Understand the core differences of GB, EN…
Stable mechanical performance is the foundation of rail safety. Core metrics cover tensile strength, yield…
Fusion Bonded Epoxy (FBE) coating is the most practical high-performance anti-corrosion solution for steel pipes.…