Drill Pipe Structure and Common Specifications

Drill pipe is the core component of the drill string in drilling engineering. It is typically made of seamless steel pipe and consists of two key parts: the drill pipe body and the drill pipe joint. The drill pipe is the most numerous and longest part of the drill string system, primarily used for transmitting power, circulating drilling fluid, and bearing the weight of the drill string. Its structural design and specification selection vary depending on the application scenario (such as oil drilling or geological exploration) and directly affect the safety and efficiency of drilling operations.

Drill Pipe Structure: 

A complete drill pipe consists of three main parts: the tubing body, the tool joint, and the welded area.

1. Pipe Body

Commonly made of high-strength alloy steel. Its characteristic is that the wall thickness of the tubing body is relatively thin, aiming to reduce the overall weight while ensuring strength. It also has a certain degree of elasticity to adapt to the bending of the wellbore.

Function: The main load-bearing structure for transmitting torque and transporting drilling fluid.

2. Tool Joint

Made of a higher grade alloy steel than the pipe body, and undergoes special heat treatment, resulting in higher strength, hardness, and wear resistance. Its characteristics include a significantly larger outer diameter and wall thickness than the pipe body to withstand enormous connection stress and wear. Both ends are machined with threads (external and internal threads).

Functions:
Connection: Connects single drill pipes into a drill string thousands of meters long.
Load Bearing: Bears most of the tensile, compressive, and torsional loads.
Wear Resistance: The joint is usually coated with a wear-resistant strip to resist frictional wear against the wellbore.

Drill Pipe Joint Classifications: Internal flat type, full-hole type, regular type, digital thread (NC), special joints, etc.

3. Welding Zone 

This is the area where the tool joint is butt-welded to the pipe body using friction welding.

Requirements: The strength of the weld zone must be no less than the strength of the pipe body itself; this is one of the most critical technological aspects of the entire drill pipe process.

Drill Pipe Connection Methods:

There are two main methods for connecting the drill pipe body and the connector: one is a fine-thread connection, where both ends of the pipe body are machined with fine male threads, which connect to the fine female thread at one end of the connector. This type of drill pipe is called a fine-thread drill pipe. The other is to butt-weld the pipe body and connector together via friction welding. This type of drill pipe is called a friction-welded drill pipe.

It is worth noting that drill pipe connections mainly use fine-thread and friction welding methods, with friction-welded drill pipes currently being the mainstream. Fine-thread drill pipes have been largely phased out, and most drill pipes produced globally today are friction-welded drill pipes.

Friction welding of drill pipes involves butt-welding the pipe body and connector together through frictional heat and pressure. This welding method has advantages such as high connection strength and stable process, and is currently the mainstream connection method in drill pipe production. During friction welding, welding parameters must be strictly controlled to ensure weld quality.

To further improve the stability of the connection between the pipe body and the connector, the ends of the pipe body are usually thickened. Common thickening methods include internal thickening, external thickening, and internal and external thickening. 

Common Drill Pipe Specifications:

Drill pipe specifications are primarily defined by API 5DP standards. Core parameters include outer diameter, weight (wall thickness), and steel grade.

1. Common Outer Diameters
Drill pipes have a range of commonly used outer diameters, typically 2-7/8 inches to 6-5/8 inches (73.0 mm to 168.3 mm).

Specifically:
2-7/8": Nominal outer diameter of 73.0 mm, suitable for shallow wells and well workovers (maintenance, treatment, and modification of existing wells to extend their lifespan).
4-1/2": Nominal outer diameter of 114.3 mm, very versatile, used in medium to deep wells.
5": Nominal outer diameter of 127.0 mm, the most widely used size globally, used in deep wells.
5-1/2": Nominal outer diameter of 139.7 mm, suitable for deep and ultra-deep wells.

2. Weight Rating
The weight rating represents the wall thickness of the drill pipe. Multiple options are available for the same outer diameter, expressed in pounds per foot (lb/ft).

Example: 5-inch drill pipe
Standard weight: 19.50 lb/ft
Heavyweight weight: 25.60 lb/ft

Selection criteria: Thicker walls result in greater weight per unit area, higher tensile and torsional strength, but a smaller inner diameter.

3. Steel Grade
The steel grade represents the yield strength of the drill pipe material. The number represents the minimum yield strength. Increasing the steel grade is an effective way to enhance the strength of the drill pipe.

The steel grade of the drill pipe, i.e., the grade of the steel, is classified according to its minimum yield strength. In the API standard, drill pipe steel grades are divided into four types: E75, X95, G105, and S135. X, G, and S grades are all in the high-strength category. A higher steel grade means a greater yield strength of the pipe material, resulting in improved tensile and torsional strength of the drill pipe. In the strength design of drill strings, it is generally recommended to enhance the strength of the drill string by increasing the steel grade, rather than simply increasing the wall thickness.

Differences in Application Scenarios:

Common oil drilling operations and geological exploration have drastically different requirements for drill pipes, mainly in terms of load, well depth, and equipment size.

1) Oil Drilling Drill Pipe

Oil drill pipe is designed to achieve efficient and safe deep drilling under harsh conditions. It requires high strength, high torque, and wide channels. Its working environment is mostly deep wells, ultra-deep wells, high torque, and heavy hook loads. It needs to be able to transmit huge torque, withstand huge tensile forces, and deliver high-pressure drilling fluid.

Common Specifications:
Main Specifications: 3-1/2 inch, 4 inch, 4-1/2 inch, 5 inch.

Ultra-deep Well Specifications: 5-1/2 inch, 5-7/8 inch.

Main Steel Grades: G-105, S-135.

The 5-inch drill pipe is the most widely used size in global onshore deep well drilling. S-135 steel grade (minimum yield strength 135,000 psi) is standard for modern deep and ultra-deep wells, providing extremely high tensile and torsional strength. For very deep wells, even V-150 steel grade is used.

Structural Design Features:
Thickened Tool Joints: The outer diameter of the joint is much larger than the pipe body, forming a "dumbbell shape" structure to ensure connection strength.
Abrasion-Resistant Bands: Abrasion-resistant material is sprayed onto the joint surface to resist friction with the wellbore.
Internal Coating: High-grade drill pipe has an anti-corrosion and anti-friction coating on the inner wall to protect the pipe body and improve drilling fluid flow.

How to Choose?

Deep/Ultra-Deep Wells: 5" S-135 or 5-1/2" S-135.
Medium-Deep Wells: 4-1/2" G-105 or 5" G-105.
Directional/Horizontal Wells: High-grade steel and high-performance special threads are preferred for drill pipes to withstand greater friction and torque.

2) Geological Exploration Drill Pipes

The core task of geological exploration drill pipes is to obtain uncontaminated subsurface core samples; therefore, their design revolves around "coring." They require lightweight construction, high portability, and the ability to meet coring requirements. Their working environment is mostly shallow to medium-deep holes with relatively low torque and tensile forces. They need to ensure core recovery rates, handle frequent tripping operations, and adapt to complex terrain.

Common Specifications: Geological exploration drill pipe specifications differ from those of oil drilling pipes, typically expressed as outer diameter * inner diameter or a code.

AQ (outer diameter approx. 48 mm, core diameter approx. 27 mm) - Used in shallow holes and complex formations.
BQ (outer diameter approx. 60 mm, core diameter approx. 36 mm) - Widely used.
NQ (outer diameter approx. 75 mm, core diameter approx. 47 mm) - Mainstream specification, balancing strength and core size.
HQ (outer diameter approx. 96 mm, core diameter approx. 63 mm) - For deeper holes or projects requiring large-diameter cores.
PQ (outer diameter approx. 122 mm, core diameter approx. 85 mm) - For deep hole exploration.

Materials and Manufacturing Process:
Materials: Typically made of high-strength alloy steel, but with lower strength and wall thickness than oil drill pipe.

Connection Type: Threaded connections are widely used, but "wired coring" designs are also common to reduce weight and improve efficiency. This means the drill pipe itself acts as a double-walled tube, with an inner tube channel for deploying and retrieving the inner tube containing the core. Cores can be retrieved without lifting the entire drill string, resulting in extremely high efficiency.

How to Choose?

Solid Mineral Exploration: The NQ and HQ series are the absolute mainstays, providing appropriately sized, high-quality cores.
Hydrogeological/Engineering Geological Exploration: BQ, NQ, or smaller AQ series to meet shallow sampling and logging needs.
Deep Mineral Exploration: HQ and PQ series to meet depth and coring size requirements.

Drill Pipe Length and Classification:

According to the American Petroleum Institute (API) standards, drill pipe lengths are classified into three categories: R1, R2, and R3, each corresponding to a different length range.

R1: Range 5.49 m - 6.71 m (18 - 22 ft), now rarely used, only found in older drilling rigs or specific limited applications.
R2: Range 8.23 m - 9.14 m (27 - 30 ft), the most common and mainstream range.
R3: Range 11.58 m - 13.72 m (38 - 45 ft), used for applications requiring high drilling efficiency, reducing downhole connections, but with higher transportation and operational requirements.

Note: Within R2, approximately 9.5 m (close to 31 ft) is the most common single drill pipe length in the field.

Read more: API SPEC 7-1 Drill Pipe or Casing Pipe and Drill Pipe Differences