Connection method of drill pipe

Drill pipe is a steel pipe with a thread at the end, used to connect the surface equipment of the drilling rig to the drilling and grinding equipment or bottom hole device at the bottom of the drilling well. Generally, seamless  pipes are used.The purpose of the drill pipe is to transport drilling mud to the drill bit and to raise, lower or rotate the bottom hole device together with the drill bit. Drill pipe must be able to withstand tremendous internal and external pressure, twisting, bending and vibration. In the process of oil and gas extraction and refining, drill pipe can be used multiple times. So how do drill pipes connect? Common connection methods of drill pipe are threaded connection, clamp connection, hook connection, friction welding connection, threaded bonded male and female connectors and welded male and female connectors.

Drill Pipe Structure

A complete drill pipe consists of three parts:

Drill Pipe Body: The middle section, typically a thin-walled steel tube.

Joint: The thickened ends with threads.

Welded Zone: The area where the joint is butt-welded to the drill pipe body.

Drill Pipe Classification

Drill pipes are classified into three types: kelly, standard drill pipe, and heavy weight drill pipe.

The connection sequence is: angular drill pipe (1 piece) + standard drill pipe (n pieces, determined according to well depth) + heavy-duty drill pipe (n pieces, determined according to the drill string assembly design).

1. Kelly

Connects the rotary table to the drill string, transmitting torque.

2. Standard Drill Pipe

Constitutes the main drill string, transmitting torque and drilling mud.

3. Heavy Weight Drill Pipe (HWDP)

Transitions stress, reducing stress concentration.

Drill Pipe Connection Methods

1. Threaded Connection

Threaded connection is one of the most common ways of connecting drill pipes. It threads two drill pipes tightly together and tightens them by turning. Threaded connections are different from ordinary threads.

The thread shape and size are more precise to ensure a secure connection and reduce leakage. Threaded connections can be subdivided into two types: internal threads and external threads.

Internal threaded connections are generally used to connect the upper part of the drill pipe, and external threads are generally used to connect the lower part of the drill pipe.

Common threaded connections include API threads, IF threads, and REG rotary drill pipe threads.

Advantages:

The advantages of threaded connections are tight and convenient connections, simple structure, and wide application range. It can connect various types of drill pipes and pipe strings, and is suitable for use in drilling wells with smaller depths and smaller diameters.

Disadvantages:

The disadvantage of threaded connections is that the tightening process is cumbersome and requires corresponding tools, and the connection process is prone to wear and fatigue and has a relatively short service life.

2. Locking Connection/Clamp Connection

The locking connection uses a metal clamp with high-strength material properties to connect two pipes. The pipes are fixed together by compressing the clamp to achieve the connection. Due to its high connection strength, the locking connection has extremely high Robustness and reliability.

Locking connections are suitable for large diameter drill pipes and ultra-deep wells. Locking connections come in various designs, but the most common is the cylinder-shaped clamp, which consists of several lugs that allow the tightness of the connection to be adjusted as needed.

Advantages:

The advantages of the locking connection are fast, simple, stable, high reliability, easy to carry and use, and long life.

Disadvantages:

The disadvantage of the clamp connection is that the length of the connected pipe is longer, resulting in shallower depth and smaller diameter.

There are problems with use in drilling operations. In addition, locking connections have poorer sealing properties than threaded connections.

3. Hook Connection

Hook head connection is a way to connect drill pipes together through hook heads. It is a common quick connection method and is suitable for quick disassembly and installation of drill pipes.

Advantages:

The advantage of the hook head connection is that it is quick and convenient to connect and disassemble.

Disadvantages:

The disadvantage of the hook head connection is that the connection strength is relatively small and is not suitable for working environments that bear large tensile or compressive forces.

4. Triangular Pyramid Type High-efficiency Friction Welding Connection

This connection method uses a triangular pyramid-shaped rod body and connects the drill pipe joints together through friction welding. This method embodies the characteristics of energy saving and high efficiency.

5. Threaded Bonded Male and Female Connectors and Welded Male and Female Connectors

Both methods are to weld or bond male and female thread joints at both ends of the drill pipe body to achieve the connection of the drill pipe.

How to Choose the Appropriate Drill Pipe Connection Method?

When choosing a drill pipe connection method, the following should be considered comprehensively:

Well depth and well type (vertical/directional/horizontal well);

Torque and tensile load;

Mud discharge and pressure;

Compliance with API/ISO standards;

Maintenance and replacement costs.

In the oil and gas drilling field, API standard threaded connections remain the irreplaceable preferred solution.

FAQ

1. Which drill pipe connection method is the most reliable?

Threaded connections are currently the most commonly used, mature, and reliable drill pipe connection method in oil and gas drilling, especially API REG, IF, FH, and NC series threads that conform to API 7-2/API 5DP standards, which are widely used in conventional wells, deep wells, and ultra-deep wells.

2. What is the difference between API drill pipe threads and ordinary industrial threads?

API drill pipe threads are high-precision tapered load-bearing threads, offering the following advantages compared to ordinary industrial threads:

Higher torque capacity;

Better airtightness and hydraulic tightness;

Specialized tooth profile design reducing stress concentration;

Strict tolerance and inspection requirements;

Ordinary industrial threads are not suitable for drilling conditions.

3. Why are NC (Numbered Connection) drill pipe connections more suitable for deep and ultra-deep wells?

NC threads possess:

Higher torque resistance;

Longer engagement length;

Better fatigue life.

Therefore, they are more widely used in deep wells, horizontal wells, and high-speed drilling.

4. What are the most common failure modes of drill pipe threads?

Galling: Scratching of the thread surface, metal adhesion.

Wear: Blunt tooth profile, reduced thread height.

Slippage: Reduced load-bearing capacity, inability to lock.

Seal failure: Mud leakage at the joint shoulder.

Fatigue cracking: Fracture at the thread root or joint.

Summarize

Drill pipe is a very important tool in the drilling operation. The stability and reliability of its connection method are crucial to the safety and efficiency of the drilling operation. Threaded connection, locking connection and hook head connection are common drill pipe connection methods. When selecting a connection method, it is necessary to comprehensively consider factors such as drilling depth, compression force, and tensile force to select the most suitable method.

Read more: How Many Types of Drill Pipe Threads Are There? or API Drill Pipe Thread Types