Drill Pipe Connection Types: REG, IF, FH and NC Thread Guide

What is a Drill Pipe?

A drill pipe is a key component of the drill string used in oil and gas drilling. It's a hollow, thick-walled steel tube that connects the drilling rig at the surface to the drill bit at the bottom of the well, transmitting torque and rotation from the rig's rotary table or top drive to the bit, and transporting drilling fluid (mud) down to the bit and back up the annulus to carry cuttings to the surface. The drill pipe structure mainly consists of the pipe body and the tool joints. The pipe body is made from high-strength seamless pipe, while the tool joints, at both ends, are thick threaded connectors that join each section into a continuous string.

These tool joints form the drill pipe connection between sections, and their design directly affects how the string handles torque, axial loads, and drilling fluid flow. Common connection types include REG, IF, FH, and NC.

Types of Drill Pipe Connections

Drill pipe connections are commonly classified into four main types: REG(Regular style connection), IF(Internal-flush style connection), FH(Full-hole style connection); NC(Numbered connection) .

The tool joint is a component of the drill pipe, made up of a pin (male) and a box (female) end, welded onto the two ends of the drill pipe body. To increase connection strength, the pipe wall is thickened at the joint area. Depending on the thickening method, this is done in one of three ways: internal upset, external upset, or internal-external upset.

NC is a newer series that has gradually replaced IF across the API standard, as well as all FH connections except the 5 1/2" FH. Several NC sizes are interchangeable with the older API connections, mainly NC50–2 3/8" IF, NC38–3 1/2" IF, NC40–4" FH, NC46–4" IF, and NC50–4 1/2" IF.

Connection Type	Full Name	Main Features	Typical Application
REG	Regular Connection	Strong and simple design	Conventional drilling
IF	Internal Flush Connection	Large internal diameter, improved fluid flow	Older API drill pipe
FH	Full Hole Connection	High strength and durability	Heavy-duty drilling
NC	Numbered Connection	Modern high-performance connection	Deep and complex wells

1. (Regular Style Connection)

REG connections are used on internally-upset drill pipe: the thread root diameter is smaller than the bore of the upset section, and the upset bore in turn is smaller than the pipe body ID. API originally designed REG mainly for bit connections — since the bit sits at the end of the drill string, thread stress concentration is less of a concern there, which is why API has kept the full range of REG sizes in the standard.

2. IF(Internal-flush style connection)

IF connections are used on EUE (external-upset end) or internal-external upset drill pipe, where the tool joint ID closely matches the pipe body ID and the upset bore, giving a near-uniform flow path through the connection. All IF sizes use the V-0.065 thread form — a flat-crest, flat-root 60° V-thread with a 0.065-inch (1.651 mm) flat width. Because this profile is prone to stress concentration, API has been gradually phasing IF out in favor of NC connections.

3. FH(Full-hole style connection)

FH connections are used on internal-external upset drill pipe, where the tool joint ID matches the upset end diameter, and both are smaller than the pipe body ID. Though the size range is limited, FH is offered in three thread forms — V-0.065, V-0.050, and V-0.040 — which has made it widely used across kellies, drill pipe, drill collars, and drill bits.

4. NC(Numbered Connection) 

For drill pipe with a minimum yield strength of 75,000 psi or greater (high-strength grades), API recommends an additional designation known as the numbered connection, or NC. NC connections pair a pin and box with matching thread designations, and different NC sizes are generally interchangeable as long as the grade codes, thread pitch, and thread types match. Numbered connections range from NC10 to NC77 across various sizes, each with its own inner and outer diameters. As with any connection, it's worth confirming compatibility with the tool joint manufacturer before mixing NC sizes or thread types.

The Function of the Connector:

As the key link between drill pipe sections, tool joints carry significant loads — they must withstand high tension and torque while resisting wear and corrosion.
Several joint types are used in the field, including threaded joints, reversible joints, and non-torque joints, each suited to different operating conditions and selected based on the specific application.
Beyond making up the drill string, connectors also play a role in transmitting data for geological exploration, including temperature, pressure, and flow measurements — functions that are essential to downhole data acquisition.

5. Drill Pipe Connection Types Comparison: REG vs IF vs FH vs NC

Connection	Key Design Feature	Internal Passage	Typical Application
REG	Traditional connection design with a relatively smaller internal diameter	Relatively smaller	Conventional drilling applications
IF	Internal-flush design provides a relatively smooth internal flow passage	Relatively large	Earlier API drill pipe designs and specific applications
FH	Large-bore connection design that provides a larger internal flow area	Large	Applications requiring good flow capacity and a robust connection
NC	Numbered connection system with different sizes and configurations	Depends on NC designation	Widely used for modern drill pipe and tool joints

Note: The actual performance of a drill pipe connection depends on the specific connection designation, tool joint geometry, pipe size, drilling conditions, and applicable specification. Therefore, REG, IF, FH, and NC should not be ranked simply as “high” or “low” performance connections.

How to Connect Kelly Pipe and Drill Pipe?

1. Direct connection

Kelly pipe and drill pipe can be connected using direct connection. This method requires cleaning the surface of the kelly and drill pipe first to ensure the safety of the connection.
The advantage of direct connection is that it is simple and convenient to connect, avoids connection gaps, and can effectively reduce friction loss during rotation of the connection and resistance to sludge discharge. However, direct connection has shortcomings such as low connection strength and easy breakage. It is necessary to pay attention to the strength during connection and inspection after use.

2. Install interface connection

In addition to direct connection, you can also use the installation interface connection method. This method requires the drill pipe and kelly pipe to be installed with respective interfaces and connected through the interfaces.
The advantages of the installation interface connection are high connection strength, high efficiency, and easy replacement of connecting parts. However, installing interface connections requires more preparation work, including installing interfaces, inspecting interfaces, installing drill pipes, etc.

Whether it is direct connection or installation interface connection, you need to pay attention to the following usage precautions:
a. Before connection, the surface finish of the connecting parts and the circular runout error of the connecting parts should be checked.
b. When connecting, the drill pipe should be installed correctly and with appropriate strength, and should not be excessively loose or too tight.
c. Thorough cleaning should be carried out after connection to prevent mud from entering the connection holes.
d. During use, regularly check the wear degree of connecting parts and make necessary replacements.

In short, the connection method between kelly and drill pipe plays a vital role in the safe operation of drilling projects and drilling efficiency. According to actual needs and conditions, selecting appropriate connection methods and strengthening maintenance and inspection can effectively improve project efficiency and reduce risks.

How to Choose the Right Drill Pipe Connection?

Selecting the right drill pipe connection is mainly based on actual drilling conditions and operating requirements.

1. Drilling Conditions

For deep wells or complex formations, higher-strength connections with better fatigue resistance are usually preferred to ensure stable performance during long drilling cycles.

2. Torque Requirements

When higher torque is expected, the connection must have sufficient thread strength and load-bearing capacity to prevent thread damage or failure.

3. Hydraulic Performance

If efficient fluid circulation is important, connections with a larger internal diameter are typically selected to reduce pressure loss and improve flow efficiency.

4. Industry Standards

Most drilling operations follow API standards to ensure compatibility between drill pipes, tool joints, and other drilling components, as well as to maintain consistent performance and safety.

Summary

Drill pipe is a key hollow steel component in oil and gas drilling, used to transmit torque, transport drilling fluid and support the drill bit, composed of pipe body, tool joints and weld zone. Drill pipe joints (male and female) connect drill pipe sections, classified by thread type (REG, IF, FH, NC) and thickening method (internal, external, internal-external). REG is for bit connection; IF is gradually replaced due to stress concentration; FH is widely used in kelly and drill collars; NC (new series) replaces most IF and FH, suitable for high-strength drill pipes. Kelly and drill pipes connect via direct (simple but low-strength) or interface (high-strength but laborious) methods, with relevant precautions. Drill pipes vary in size/length, and common buckles are REG and IF, selected by pipe size.

FAQs

1. What is a drill pipe connection?

A drill pipe connection is a threaded connection used to join individual drill pipes into a complete drill string.

2. What are the common types of drill pipe connections?

Common types include REG, IF, FH and NC connections.

3. What is the difference between NC and IF connections?

NC connections generally provide improved fatigue resistance and are more commonly used in modern drilling operations, while IF connections are older designs.

4. Which drill pipe connection is most commonly used?

NC connections are widely used due to their strength, reliability and performance.

5. How are drill pipe connections inspected?

Connections are inspected through thread inspection, dimensional checks and non-destructive testing to ensure safe operation.

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