A drill pipe thread is a vital tool machined at each end of the drill pipe tool joint that connects different sections of the drill pipe and other downhole tools, forming a drill string. These threads provide a tight and secure seal to contain drilling fluids in the drill string and prevent leakages while maintaining adequate pressure levels in the wellbore.
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They help transfer torque to the drill bit while withstanding immense rotational stress, axial loads, and other mechanical stresses. Accordingly, the right threaded connections play a pivotal role in the safe and successful function of the drill string, ensuring operational efficiency on the oilfield.
Thread TypeOuter Diameter Range (inches)CompatibilityAPI Regular (REG)2.375- 6.625Compatible with REG threadsAPI Internal Flush (IF)3.5- 6.625Compatible with IF threads.API Full Hole (FH)4.5- 6.625Compatible with FH threads.API Numbered Connection (NC)2.375- 6.625Compatible with NC threads VAM (Premium)2.375- 6.625Compatible with VAM threadsXT (Premium)2.375- 6.625Compatible with XT threadsDrill pipe thread types are an integral part of drill pipe specifications. These specifications are critical for ensuring compatibility, performance, and safety in drilling operations, enhancing efficiency while reducing the risk of failure and nonproductive downtime.
A drill pipe includes an external male thread (pin) on one end that connects to the internal female thread (box), creating a robust connection.
There are two types of drill pipe tool joint threads- threads specified by the American Petroleum Institute (API) and non-API threads, also known as premium threads that are custom-made by various manufacturers as per specific requirements. API threads are generally not compatible with premium connections due to different design specifications..
API threads are standardized and offer compatibility across different manufacturers, allowing for interchangeability. They are the most common threads used in the drilling industry.
The regular type thread connects the inner upset drill pipe and is mainly used in bit connections. As the drill bit is located at the end of the drilling string, the regular api connection is less prone to failure and more reliable as there is minimal stress concentration.
Regular style threads are suitable for a wide range of drilling operations. They typically have a specific, V-shaped profile that balances ease of assembly and disassembly while having the ability to withstand axial and torsional loads, ensuring a powerful and reliable connection.
The internal flush thread type features a design where the internal diameter of the threaded connection is similar to the internal diameter of the pipe body. Its main benefit is that it ensures smoother fluid flow through the drill pipe, helping maintain high fluid flow rates while reducing internal wear, such as during directional drilling.
A full hole thread in drill pipes indicates that the threads on the pipe's connections extend fully through the wall thickness of the pipe. That ensures a robust and secure connection between the drill pipes, which is crucial for maintaining the integrity and strength of the drill string.
The larger inside diameter enables better fluid flow, reducing pressure losses and improving drilling efficiency. It features three thread specifications, V-0.065, V-0.05 and V-0.040.
Numbered connection threads are designed for drill pipe featuring yield strength of a minimum 75,000 PSI or higher. They come in 17 different types, each designated by a number, ranging from NC-10 to NC-77. The kind of NC used depends upon specific drilling requirements, such as deep well drilling, high torque and strength, etc.
NC threads are standardized, meaning they are made to be compatible with other NC threads of the same type from another manufacturer. So, an NC 10 thread from one manufacturer is compatible with an NC 10 thread from another, provided the thread dimensions, like outside diameter and pin and box ends, match.
Premium threads are designed in varying shapes and sizes and are typically equipped with a double shoulder. These proprietary connections are not standardized but suitable for demanding drilling conditions, as they provide enhanced features like superior sealing capabilities, extra torque shoulder, and more durability to endure higher pressures, stresses, and corrosive conditions.
Their compatibility typically requires matching threads from the same manufacturer and series. Some of the common premium connections include XT, Vam, and H-90.
Drill pipe thread compatibility is crucial because it ensures a secure connection between different drill pipe sections.
With various types of threads available, compatible threads are vital to maintaining drill pipe integrity and preventing it from breaking or disconnecting during drilling operations. They keep the drill string intact under the high pressures and tensions encountered during drilling operations.
Accordingly. that helps avoid equipment failures, reducing the risk of costly nonproductive time and major safety hazards like blowouts.
One of the most crucial aspects of compatible threads is that they create a tight, leak-proof seal, ensuring smooth drilling fluid circulation while preventing fluids from spilling and harming the environment.
Mismatched threads can cause excessive wear and damage to the connections, leading to more frequent repairs and replacements. Compatible threads help extend the lifespan of the drill pipe and drill collar, saving the time and costs associated with replacements.
Lastly, compatible threads make it easier to assemble and disassemble the drill pipe, enhancing drilling efficiency on the oilfield while saving time.
Matching drill pipe threads is at the core of safe and efficient drill operations. The threads must meet the dimensional requirements specified by industry standards, so the correct dimensions are essential for proper function.
Verify the nominal size of the threads. The outer diameter determines the size and shape of the threads of the pipe. It should be consistent with the tool joint dimensions or match between the pin and box connections to endure the drilling stresses and prevent leaks while forming strong connections.
Likewise, the inner diameter is paramount for the smooth flow of the drilling fluids, so its dimensions and tolerances must adhere to industry standards.
The pitch diameter is another vital dimension that must be uniform between the connecting parts as per the API or manufacturer specifications. That helps form a secure fit and a tight seal while ensuring equal torque distribution across the threaded connection.
Thread pitch is the distance between two threads, usually measured in threads per inch. Ensuring the pin and box connections have the same thread pitch is critical for proper alignment and compatibility between threads. It is indispensable to creating a tight and secure seal and preventing thread damage by equally distributing the loads between threads.
As mentioned in the drill pipe classification chart, the taper is the rate of change of the thread diameter along its length. The API specifies taper for drill pipe connections, and the right thread taper accurately measured against the set industry standards is critical for thread compatibility and forming secure connections.
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With over three decades of experience dealing in oilfield equipment, we realize the importance of drill pipe thread compatibility for safe and efficient drilling. Therefore, our team of experts will be happy to assist you in choosing compatible drill pipe threads, minimizing the risk of failures associated with compromised threads.
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Activities
Published 30.09.
At Odfjell Well Services we constantly hear from people who ask us why certain drill pipes don't go together. Sometimes these are experienced drillers who have been in the industry for years. So here comes a post with the need-to-know basic stuff about drilling connection types. Hopefully it will be of use to you ' whether you're a rookie or a veteran.
It can be tough getting the terminology right, so let's nail it down right now. In this post I will deal with the drill string connections only. I won't be talking about connections for casing or tubing.
To narrow it down further we are going to look at Rotary Shouldered Connections (RSC). RSC's come in two sub-categories:
Study the schematic for drill pipe tool joints below to gain a better understanding of what we are talking about:
> Read also: How to choose the right drill pipe
API connections go under many names:
These are single shoulder connections: They only have one torque shoulder on both pin and box.
Premium connections are like cell chargers. They come in different shapes and sizes from the many competing factories. Some examples are:
Most premium connections have a double shoulder. That means that they have an extra torque shoulder in the nose of the pin and in the bottom of the box. This provides a bigger shoulder area, thus allowing a higher makeup torque.
As is common for all threaded connection types, RSC's are designed with a single purpose in mind, namely drilling. They must be strong enough to endure different loads and combinations of tension and torsion.
What might come as a surprise to many is that no API RSC is pressure rated with regards to gases and liquids. The drill string data sheets tell us what the pipe can endure internally in the drill string before it bursts or collapses. Yet, this applies to the pipe, and NOT to the connections.
RSC's can endure high pressures under favourable conditions. But take the necessary precautions. You don't want an unexpected shoulder separation and an uncontrollable leak when drilling a deviated well..
But why all these connection types? Why can't we as an industry just pick the best one and go with it? That is a legitimate question. The short answer is competition. Manufacturers compete for the best connection, and that will probably continue for some time.
The wide range of connections naturally brings up the question about compatibility. Can we screw type A together with type B, etc.? Yes, some connections go together with other types under given circumstances. Let's take a look at how this works.
Some premium connections are compatible with API connections, even though one of them has a double shoulder and the other not. These premium connections have the same thread profile as the corresponding API connections. Examples of this are Vam Eis, DSTJ, GPDS, etc.
If there is space enough for a secondary shoulder in the API box, this can be a solution. In these scenarios the API box typically has a bore back (BB).
Before you try to screw different connections together, do the following. Ask your drilling equipment provider whether they permit it. If they do, take care to use the makeup torque values for the weakest connection.
Only in rare circumstances can you screw together two different types of premium connections . In order for this to work the thread profiles need to match each other. Besides, the dimensions (diameter and length) of the secondary shoulder need to allow it.
Feel free to use this compatibility matrix as a guide when trying to figure out what goes together with what:
So until a genius invents the perfect quick connection, we just have to accept the status quo and live with it. As Voltaire so eloquently put it: 'One day everything will be well, that is our hope. Everything's fine today, that is our illusion'.
Topics: Drilling Equipment
By: Frode Bjørheim
Frode Bjørheim is a specialist in drill pipe standards and lifting equipments with more than 30 years' experience in the oil & gas industry, both offshore and onshore.
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