Geological Drill Pipe Threads

Drill pipe threads are crucial components connecting the drill pipe and drill bit, responsible for transmitting drilling power and load. The quality of the thread directly determines drilling efficiency, connection reliability, and the service life of the entire drilling tool set.

In applications such as geological exploration, water well drilling, and mining drilling, threads typically need to operate for extended periods under high torque, high impact, and high wear environments. This article will detail the classification, tooth profile, dimensions, and API specifications of geological drill pipe threads.

Classification of Geological Drill Pipe Threads

From an industry application perspective, geological drill pipe threads can be mainly divided into two systems:

1. API Standard Threads (Mainstream, Highly Versatile)

Following standards such as API Spec 5DP / API Spec 7-1, these threads offer good interchangeability, are globally applicable, and are widely used in petroleum, geological, water well, and mining drilling.

2. Non-API Threads (Custom or Industry-Specific)

Commonly found in mining and engineering drilling systems, usually custom-made for factory standards or specific projects. Advantages include optimization for specific working conditions, but interchangeability is poor.

Tooth Profiles of Geological Drill Pipe Thread

Geological drill pipes come in various thread types according to tooth profile, each suitable for different drilling needs and conditions.

1. Triangular Threads

Triangular threads are a common type, with a triangular tooth profile. They offer good sealing and self-locking properties, effectively preventing drilling fluid leakage.

Suitable for scenarios with high sealing requirements, such as drilling in geological environments with water or other fluids.

2. Trapezoidal Threads

Trapezoidal threads have a trapezoidal tooth profile. They have strong load-bearing capacity and can transmit large torques and axial forces.

Suitable for drilling depths requiring large power transmission, widely used in drilling through hard rock formations.

3. Rectangular Threads

Rectangular threads have a rectangular tooth profile. They offer high transmission efficiency and low friction between threads.

Suitable for scenarios requiring rapid drilling and frequent disassembly, but due to its poor self-locking performance, it needs to be used in conjunction with other locking devices.

4. Others

In addition, there are some special types of threads, such as sawtooth threads, which combine the advantages of trapezoidal and rectangular threads, can withstand larger unidirectional axial forces, and play a role in specific working conditions.

Geological Drill Pipe Thread Sizes

1. Thread Outer Diameter

The thread outer diameter, also known as the "tooth diameter," is the standard size for the maximum diameter of the drill pipe thread. Its value is usually equal to or slightly smaller than the drill pipe diameter.

Common thread outer diameters specified by API standards include 2-3/8 inches, 2-7/8 inches, 3-1/2 inches, 4 inches, 4-1/2 inches, and 5 inches.

2. Pitch

The pitch is the distance between two adjacent teeth on the thread helix, and is an important parameter describing the degree of thread engagement.

Common API thread pitches include 4, 4.5, 5, and 5.5. Generally, a larger pitch results in a larger engagement area and contact pressure, and a relatively smaller volume and weight.

4 TPI: Deep fit, high load capacity

4.5 TPI: Balanced type

5 / 5.5 TPI: Faster assembly and disassembly, lighter weight

3. Nominal Diameter

Nominal diameter, also known as "finger diameter," is the distance between the thread peak line and the thread axis, usually corresponding to the diameter of the middle of the thread unit.

Common API thread nominal diameters include 2.063 inches, 2.469 inches, 2.875 inches, 3.5 inches, and 4 inches.

4. Nominal Length

Nominal length, also known as "thread length," is an important parameter describing the thread length, representing the effective length of the fit within the thread unit.

Common nominal lengths specified by API standards include 2.4 inches, 2.7 inches, and 3.1 inches.

API Geological Drill Pipe Thread Types

Common API thread types include STC, LTC, BTC, NC, and REG.

Causes of Damage to Geological Drill Pipe Threads

1. Improper Installation

Forcing tightening without proper thread alignment or using excessive tightening force during drill pipe connection can damage and deform the thread profile, even leading to stripping.

2. Wear

Increased drilling depth and duration result in frequent friction between threads. The presence of rock cuttings, mud, and other impurities accelerates thread wear, reducing sealing performance and connection strength.

3. Material Issues

Insufficient material strength or defects in the drill pipe thread material can easily lead to thread breakage and chipping under high torque and pressure.

4. Harsh Geological Conditions

Encountering hard rock layers or complex geological structures can subject drill pipes to severe impacts and vibrations, causing additional damage to the threads and resulting in thread failure.

Geological Drill Pipe Thread Selection Principles

1. Select the appropriate thread type, outer diameter, and length based on the operating environment.

2. Use standardized API threads whenever possible to facilitate thread interchange and connection.

3. Pitch and thread length should be balanced according to requirements to obtain optimal thread parameters that meet operational needs.

FAQ

1. Can API threads and non-API threads be mixed?

Generally, mixing them is not recommended.

Different thread standards have differences in thread profile angle, taper, pitch, and tolerances. Mixing them can easily lead to: insufficient fit, seal failure, premature thread wear, or stripping.

On-site, threads of the same standard and series should always be used together.

2. What is the angle of geological drill pipe threads?

Different thread types have different angles:

Triangular thread (V-type): Typically 60°

Trapezoidal thread: Commonly 30°

Sawtooth (BTC) thread: Asymmetrical angle, one side is close to the perpendicular bearing surface.

The specific angle should be based on API standards or manufacturer technical drawings.

3. Is a larger pitch always better for drill pipe threads?

Not necessarily.

Larger pitch: Faster assembly and disassembly, relatively lighter weight.

Smaller pitch: Larger contact area, stronger load-bearing capacity.

In deep wells and high-torque conditions, pitch combinations with higher load-bearing capacity are usually preferred over simply pursuing assembly and disassembly efficiency.

Summary

In summary, geological drill pipe threads are not only a connection structure but also a key component determining drilling safety and economy. Appropriate selection of thread type, size, and standard can significantly improve drilling efficiency and effectively reduce risks such as pipe breakage and stripping.