Threads are used as mechanical means to hold the neighboring joints together during axial tension or compression. For casing pipe, most popular threading type is LTC (Long thread coupling), STC (Short thread coupling) and BTC (Buttress thread coupling).For all casing sizes, the threads are not intended to be leak resistant when made up.The design of those threading types you can find them in API 5CT or API 5B.

LTC (Long thread)
Long threads have obvious advantages that are widely used in casing pipe connections. For example, it is easy to process, good tightness, strong connection strength, simple maintenance, low price. The LTC thread is the same as the STC thread in the shape and can be connected, but it is generally not allowed to connect because the maximum torque they withstand is different. Casing pipe with long threads is the commonly used in the oil and gas industry.

STC (Short threading) 
STC coupling can withstand strength of the tubular 60~80% of the pipe tensile, BTC coupling can hold more tensile but it’s sealing performance is poor under circumstance like high internal pressure.

BTC (Buttress threading)
This thread is designed to improve axial tensile or axial compressive load capacity and to provide leakage resistance.LTC, STC and BTC are the same in appearance, how to distinguish them that depends the outside diameter and application.

What’s the use of LTC, STC and BTC?

Connection strength and tightness are two main coupling technical indicators.Both LTC and STC couplings in generally use lead, zinc, copper, graphite or silicone oil composition to improve seals, but those seals only work under temperature of 60-95 Celsius. Also for STC coupling, if under huge external pressure and axial tensile stress, when heavy impact or bent happens, the threading will have more chance to slippage. Both of those two type of threading are not suitable for heavy oil thermal recovery, ultra-deep well and heavy corrosion environment.

API standard threading coupling primarily seal the pipe by thread grease, metal plating and thread engagement of interface. STC thread root to tip clearance is 0.152mm, BTC threading gap is the whole height of tooth. Bottom to tip is 0.178 for pipe under 193.7mm, and 0.229mm for pipe above 219.1mm.