Overview of OCTG Threaded Connections
Oil country tubular goods (OCTG), which include casing, tubing and drill pipe, are joined by threaded connections that must carry tensile loads, resist internal and external pressure, and remain leak-tight for the life of the well. Correctly identifying the thread type is essential for inspection, sorting, handling and make-up on the rig, because mismatched threads cannot be run together.
Casing connections are usually described by code names: short round thread (STC), long round thread (LC) and buttress thread (BTC). Some imported products are marked LTC, which stands for long threaded casing. Tubing threads are generally divided into three families: EU (externally upset), NU (non-upset) and premium connections.
Casing Thread Types
| Code | Name | Thread Form | Characteristic |
|---|---|---|---|
| STC | Short round thread | Triangular (V-shaped), rounded crests | Compact length, standard API round thread for most casing sizes |
| LC | Long round thread | Triangular (V-shaped), rounded crests | Longer thread length for higher joint strength |
| BTC | Buttress thread | Trapezoidal cross-section, steep load flank | High tensile strength, used for heavy casing strings |
The buttress thread has a rectangular or trapezoidal cross-section with a nearly flat load flank, which gives it high resistance to axial pull. This is why buttress casing is common in deep and high-load wells.
Tubing Thread Types: EU, NU and Premium
Tubing connections are divided into externally upset (EU), non-upset (NU) and premium types:
EU (externally upset): the pipe end is thickened on the outside before threading. EU joints have a larger outer diameter at the thread area, which keeps the joint strength close to the pipe body strength. EU tubing commonly has 8 threads per inch.
NU (non-upset): the pipe end is not thickened. The threads are cut directly on the pipe body, so the thread area is slightly weaker than the body. NU tubing commonly has 10 threads per inch.
Premium connections: proprietary designs with a trapezoidal thread form and metal-to-metal or pressure-energized seals. They are not thickened externally and are used where gas-tightness is critical.
When inspecting EU-related fittings, three markings are commonly found: EUE (External Upset End) indicates the externally upset end, EUP (External Upset Pin) indicates the external upset male thread, and EUB (External Upset Box) indicates the external upset female thread. NUE is the corresponding marking for the non-upset end.
How to Distinguish Thread Types in the Field
The following checks allow an inspector to distinguish casing and tubing threads without special instruments:
Check thickening: measure the outer diameter of the pipe end. A visible increase in diameter at the end indicates an upset end (EU); a constant diameter indicates non-upset (NU).
Check the thread cross-section: a triangular or V profile indicates a round thread; a rectangular or trapezoidal profile indicates a buttress or premium thread.
Count threads per inch: EU tubing commonly shows 8 TPI, while NU tubing commonly shows 10 TPI. A thread gauge gives a quick reading.
Check the markings: stenciled codes such as EUE, EUP, EUB, NUE, STC, LC and BTC are printed on the pipe body or coupling.
Check the taper and pitch: premium threads usually have a small taper, an equidistant pitch, and a smooth sealing surface machined at the nose.
Premium Connections Explained
Premium connections are proprietary threaded joints that combine a trapezoidal thread form with one or more sealing mechanisms, such as a metal-to-metal radial seal, a torque shoulder, and a thread compound reservoir. They are designed to provide connection strength close to the pipe body and gas-tight sealing under high pressure and temperature. Premium threads are not externally thickened, have a small taper, and are widely used in high-pressure gas wells, HPHT wells, and horizontal wells where standard API round or buttress threads cannot guarantee leak-tightness.
Because premium connection geometry varies by manufacturer, they must always be run with the original make-up equipment and torque specifications supplied by the connection owner.
Common Misconceptions
EU and NU cannot be distinguished by thread appearance alone. Both use the same triangular thread form; the reliable difference is the upset end and the thread count.
Buttress thread is not a premium connection. It is an API standard thread optimized for tension; it does not provide the gas-tight metal-to-metal seal of a premium design.
Not every trapezoidal thread is the same. Different proprietary premium connections have different pitch, taper and seal locations and are not interchangeable.
Thread count alone is not enough to identify a connection. It must be combined with thickening, cross-section and markings.
FAQ
What do STC, LC and BTC mean on casing?
STC means short round thread, LC means long round thread, and BTC means buttress thread connection. They are the three main API casing connection families defined in API 5CT.
What is the difference between EU and NU tubing?
EU (externally upset) tubing has a thickened pipe end so the threaded area keeps strength close to the body; it commonly has 8 threads per inch. NU (non-upset) tubing is threaded directly on the pipe body and commonly has 10 threads per inch.
How can I tell EU from NU by looking at the pipe?
Measure the outside diameter near the end. If the end is visibly thicker than the body, it is EU; if the diameter stays constant, it is NU. The markings EUE, EUP, EUB and NUE also help.
What is a premium connection?
A premium connection is a proprietary threaded joint with a trapezoidal thread, a metal-to-metal seal and a torque shoulder, designed to be gas-tight and as strong as the pipe body.
Why does the thread count matter?
Thread count per inch affects connection strength and sealing. In API round tubing threads, fewer threads per inch mean deeper threads and higher joint strength, which is why EU connections are stronger than NU connections.
What is an upset end?
An upset end is a pipe end that has been locally thickened by forging before threading. The thickening restores the load-carrying area lost to the thread cut, so the joint is not the weak point of the string.