Different Types of Threads in Oil and Gas – API Coupling

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Buttress Threaded Connection

Threads are a critical component of the oil and gas industry. They are used for all sorts of connections, from those on equipment to those that hold pipes together. There are three types of threading: long-threaded connections (LTC), short-threaded connections (STC), and buttress threaded connections (BTC). Each type has its pros and cons when it comes to safety, cost and  efficiency. 

What Is a Couplings?

A coupling is a piece of steel that slips over pipe to joins two lengths of threaded pipe. Threads are created by cutting or rolling the end sections into long steel tubes to fit them together. One tube is slightly larger than the other in diameter and length to account for different materials (steel vs. polyethylene).. Certain oil and gas production companies require the couplings they will use to meet certain standards and regulations. The most common of these is API (American Petroleum Institute)

API Couplings

API couplings can be used with pipes made from either metal or plastic and come with an anti-rotation feature to not loosen over time when exposed to high pressures like those found in oil wells. The material used is manufactured to the highest standards of API. The certification process is typically done annually unless a customer requests a certain certification to suite their project needs.

What Are Threads? 

Threading is integral to many tasks associated with oil drilling, such as rod handling equipment, tubing connections, wellhead connections, flare stacks on platforms, various types of valves. 

Below are the three types of thread in greater detail:

LTC – Long Threaded Connection

This is found on heavy equipment and is used for most oilfield pipe connections. LTC threads are best to use in situations where some axial load is encountered, but extreme temperature variations are not. Furthermore, these bolts only function within a range of 60-95 Celsius. But they have the following advantages:

  • They can be tightened, loosened, or removed with relative ease. 
  • They are easy to maintain and low-priced.

However, a significant downside of this type is that if one piece has too many vibrations, it may become loose due to poor stability caused by length. 

STC – Short Threaded Connection 

STC is a good choice for shorter length casing applications without high axial loads or bending. It is also a good choice for connecting lightweight rod handling equipment to the wellhead. STC coupling withstands 60-80% of the pipe’s tensile strength, meaning it’s not as strong as LTC but still provides a reasonable degree of strength. They do not, for example, offer as much protection from damage and vibration, exposing them more susceptible to loosening over time when exposed to high pressure like those found in oil wells. 

BTC – Buttress Threaded Connection 

This type of connection is best used with heavy equipment and pipe lengths that exceed the size limits of LTC and STC threads. They have more capacity to withstand axial load and have a broader range of temperature tolerance. That is why this type of connection is commonly used in heavier drilling installations, mainly when the well depths are greater. BTC connections are not as common as the other two types discussed, but they provide greater protection. The main downsides are that the so-called J or blank area may erode or fatigue over time. This means that the threads can wear and lead to loosening or breakage in a higher frequency than other connections.

For all your threading needs, contact us at Lincoln Manufacturing, Inc, which (LMI) provides full-length pipe threading as well as API couplings fabrication to the oil and gas industry. Additionally, LMI manufactures downhole drilling and fracking accessories for gas exploration companies operating in the US. 

For more information, visit LMI’s products and services page on our website.

Lincoln Manufacturing

Lincoln Manufacturing

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