Drill Pipe Connection Threads

The drill pipe connection threads are the weakest point in the drill string. To overcome this weakness, the drill pipe end is usually thickened (internal or external thickening) to compensate for the weakening of the pipe wall caused by thread machining. The thickened drill pipe must then undergo heat treatment (quenching + high-temperature tempering) to eliminate heat-affected zones and restore and optimize mechanical properties.

This article covers drill pipe thread types, materials and heat treatment, kellys, failure prevention, thread grease, torque control, friction welding connections, surface hardening, and standard systems. It can be used for technical training, field operation reference, and procurement selection.

1. Types and Standards of Drill Pipe Threads

Drill pipe threads are mainly classified into the following types according to API SPEC 7-2 standards:

Thread Type	Applicable Scenarios	Characteristics
Numerical Threads	Mainstream standards for oil drill pipes	Such as NC26, NC31, NC38, NC46, NC50, numbered by pitch circle diameter, good versatility
API Trapezoidal Threads	Deep wells, high torque scenarios	Strong load-bearing capacity, but limited gas sealing performance
API Round Threads	Conventional wells, shallow and medium-depth wells	Simple processing, low cost, weaker torsional resistance
Special Threads	High-pressure gas wells, horizontal wells	Metal-to-metal seal, good gas tightness, resistance to bending fatigue

Drill pipe bodies are usually made of alloy seamless steel pipes, with alloying elements including manganese, silicon, boron, molybdenum, etc. Oil drill pipes follow API SPEC 5DP standards, and the content of harmful elements phosphorus and sulfur is generally controlled below 0.04%. The higher the drill pipe steel grade (e.g., G-105, S-135), the greater its minimum yield strength.

2. Kelly

The kelly is located at the uppermost end of the drill string. It is driven by the rotary table, connected above to the swivel for supplying drilling fluid, and connected below to the drill string to transmit torque and axial load.

Its cross-section may be square, hexagonal, or double‑keyway shaped, with relatively large dimensions to ensure rigidity. The kelly is typically 4.5 meters or 6 meters in length, which must be greater than the length of a single drill pipe to allow for making connections through the rotary table opening when adding a single.

3. Thread Failure Modes and Prevention

Threads are a weak point in the drill string and mainly exhibit the following failure modes:

a. Sticking: Insufficient lubrication or excessive torque on the thread surface.
Prevention: Use high-quality thread grease and control the sticking torque.

b. Fatigue Fracture: Alternating stress leads to crack propagation.
Prevention: Optimize the thread root radius design and perform regular flaw detection.

c. Wear: Repeated turning and friction within the well.
Prevention: Surface hardening treatment and regular inspection.

d. Leakage: Sealing structure failure.
Prevention: Select appropriate thread grease or a special sealing structure.

To prevent stress concentration, the thread root is designed with a rounded transition. Threads must withstand alternating stress during frequent tightening, therefore they must have sufficient strength and wear resistance. Simultaneously, the drill pipe must withstand high pressure from flushing fluid, requiring a reliable sealing structure at the joint end face.

4. Function and Use of Thread Grease

Thread grease is an indispensable material in drill pipe threaded connections. Its functions include:
● Lubrication: Prevents sticking and reduces the coefficient of friction
● Sealing: Fills thread gaps, forming a sealing barrier
● Rust Prevention: Protects the thread surface and prevents corrosion

Usage Requirements:
● Clean the thread surface before threading
● Apply an appropriate amount of thread grease evenly
● Select the appropriate type of thread grease according to the working conditions (API standard grease, high torque grease, special environment grease)

5. Threading Torque Control

Threading torque is a core control parameter for threaded connection quality:

Too low torque: Loose connection, seal failure, easy stripping;
Too high torque: Thread yielding, sticking, joint cracking. On-site, a power clamp and torque monitoring system are used for threading operations, and the threading curve is recorded to determine the connection quality.

6. Connection between Drill Pipe and Tool Joint

Modern drill pipes use inertial friction welding to connect the pipe body to the tool joint. Friction welding is a solid-state welding process, and the weld strength is no less than that of the base material of the pipe body, replacing traditional flash butt welding or direct threaded connection methods.

After welding, tempering heat treatment is performed to ensure stable performance in the joint area.

7. Surface Hardening Treatment
Because the drill string rubs against the borehole wall during rotation, the drill pipe surface is usually subjected to high-frequency quenching to improve wear resistance. However, the depth of the hardened layer is generally controlled within the range of 0.5-1.5mm to avoid affecting the core toughness.

To ensure the overall performance of the pipe body, rolled steel pipes need to undergo normalizing, tempering, or quenching and tempering treatment.

8. Relevant Standards

ISO 10424-2:2007: Oil and Gas Industry — Rotary Drilling Equipment — Part 2: Machining and Measurement of Rotary Shoulder Threaded Connections. The core international standards for drill pipe threads specify the requirements for three types of rotary shoulder threaded connections: NC, REG, and FH, including dimensional requirements, measurement methods, gauge specifications, and inspection methods.
EN ISO 10424-2: Machining and Measurement of Rotary Shoulder Threaded Connections

API SPEC 5DP: Drill Pipe Specification for the Oil and Gas Industry (Internationally Applicable)
API SPEC 7-2: Specification for Rotary Shoulder Threaded Connections

GB/T 20656: Drill Pipe for the Oil and Gas Industry (Chinese National Standard)

GB/T 22512.2-2008: China's current national standard, based on ISO 10424-2, with some technical adjustments made to suit my country's national conditions, such as adding requirements for longitudinal sludge trap structures. This standard applies to NC, REG, and FH type rotary shoulder threaded connections.

Read more: Drill Pipe Structure and Common Specifications