Oil Drill Pipe – Types, Specifications, and Applications

What is oil drill pipe?

Oil drill pipe is a high-strength seamless steel pipe used for oil well drilling. It has a relatively thin wall and light weight per unit length, and mainly serves three core functions: providing a mud passage, transmitting tensile force and torque, and enabling the drill string to be extended as the well deepens.

Note: The drill string consists of Kelly (1 piece) + Drill Pipe (multiple pieces) + Heavy Weight Drill Pipe (multiple pieces) + Drill Collars + Drill Bit. Drill pipe is the longest section of the drill string. (Weighted drill pipe is described in the "Types" section below.)

What are the steel grades of oil drill pipes?

Common oil drill pipe steel grades include E75, X95, G105, and S135. (drill pipe size) The number indicates the minimum yield strength in 1,000 psi (pounds per square inch). 

For example, S135 steel has a minimum yield strength of 135,000 psi. Higher steel grades provide greater strength but also increase cost and require more careful handling in sour (H₂S) environments.

Common Drill Pipe Steel Grades and Minimum Yield Strengths:

Steel Grade	Minimum Yield Strength (psi)	Minimum Yield Strength (MPa)	Typical Application
E75	75,000	517	Shallow wells; conventional vertical sections
X95	95,000	655	edium-depth wells; applications requiring moderate strength
G105	105,000	724	Medium-depth wells; conventional directional wells
S135	135,000	931	Deep wells; ultra-deep wells; extended-reach wells; horizontal wells

What is the length of oil drill pipe?

The most common length of oil drill pipe is about 9.5 meters (R2 range), and the most widely used outer diameter is 5 inches. Other common outer diameters include 3-1/2″, 4″, 4-1/2″, and 5-1/2″.

Oil drill pipes are seamless steel pipes. To strengthen the connection between the pipe body and the tool joints, both ends of the drill pipe body are upset (thickened). According to API standards, the upsetting forms are divided into internal upset (IU), external upset (EU), and internal-external upset (IEU). The upset transition section is typically 20–65 mm long, and the longest can reach 120–130 mm.

Types of oil drill pipes and their differences:

A complete drill string consists of Kelly → Drill Pipe → HWDP → Drill Collars → Bit. Below are the three main types of drill pipe used in this string.

Comparison of Three Main Types of Drill Pipe:

Drill Pipe Type	Position	Cross-sectional Shape	Function	Wall Thickness Characteristics
Kelly	Top of the drill string	Square or hexagonal	Transmits torque; bears the full weight of the drill string	Thick-walled; extremely rigid
Standard Drill Pipe	Below the Kelly, above the HWDP	Circular	Transmits torque; conveys drilling fluid; extends well depth	Relatively thin-walled (approx. 9–11 mm)
Heavy-Weight Drill Pipe (HWDP)	Between standard drill pipe and drill collars	Circular	Provides stiffness transition; prevents fatigue; partially replaces drill collars	Wall thickness is 2–3 times that of standard drill pipe

There are many types of drill pipe. The three most common are: Kelly, Standard Drill Pipe, and Heavy Weight Drill Pipe (HWDP).

1. Kelly

The kelly is located at the top of the drill string and has a square or hexagonal cross-section.

Functions:
① During drilling, the kelly works with the rotary table and bushing to transmit surface rotational torque to the drill string and drill bit.
② It bears the entire weight of the drill string.
③ It serves as a channel for drilling fluid circulation.

Structure:
The kelly consists of upper and lower tool joints and a pipe body. The pipe body is either square or hexagonal (square is more common in oil drilling). The upper joint has a left-hand (reverse) thread to connect to the swivel and prevent back-off during rotation. The lower joint has a right-hand thread to connect to the drill pipe. During operation, the upper end of the kelly is always above the rotary table, while the lower end passes through it.

2. Standard Drill Pipe

Function:
Standard drill pipe is the main body of the drill string. Its primary functions are to transmit torque, transport drilling fluid, and allow continuous wellbore deepening. It is the most numerous component in the drill string by length.

Structure:
Drill pipe is manufactured from seamless steel pipe, with wall thickness typically ranging from 9 mm to 11 mm for standard sizes (e.g., 5″ 19.50 lb/ft), but varying by weight and steel grade. A complete drill pipe consists of two main parts: the pipe body and the tool joint. The pipe body provides the primary conduit and support. The tool joint, which is friction welded to the pipe body, provides the threaded connection between each joint of drill pipe.

With the widespread application of friction welding technology, modern drill pipe has achieved high connection strength and reliability.

3. Heavy Weight Drill Pipe (HWDP)

Heavy weight drill pipe is a thick-wall hollow steel pipe, approximately 9 to 10 meters in length. However, each piece is significantly heavier than standard drill pipe, with a wall thickness two to three times that of standard drill pipe.

Function:
Heavy weight drill pipe is installed between the standard drill pipe and the drill collars. Its main purposes are to smooth the transition in stiffness, prevent fatigue damage caused by abrupt changes in cross-section, and provide additional weight on bit. It can also replace some drill collars in certain assemblies. Its design makes tripping operations easier and helps save time.

Structure:
HWDP features extended, thicker tool joints, increased wall thickness, and external upset (EU) connections. Hardfacing (carbide cladding on the outer surface) extends its service life against wear.

Core Functions of Drill Pipe:

As the longest section of the drill string, drill pipe performs three essential functions that enable modern rotary drilling:

● Transmitting power and torque – It transfers rotational force from the surface rig to the drill bit, allowing it to break rock.
● Circulating drilling fluid – Its hollow interior serves as the conduit for pumping drilling mud down to the bit and carrying rock cuttings back to the surface.
● Extending the wellbore – By adding new joints of drill pipe as drilling progresses, the well can be deepened continuously.

These functions make drill pipe a critical component of the entire drilling system, directly impacting both efficiency and safety.

Additional components:

During drilling operations, the entire drill string consists of numerous components in a specific sequence. These include the drill bit, drill collars, stabilizers, and the kelly, in addition to the drill pipe itself. The coordinated operation of these components ensures efficient and safe drilling.

Note: A drill collar is a thick-wall, heavy pipe used directly above the drill bit to provide weight on bit and stabilize the bottom hole assembly. It is not used to connect drill pipe.

Choosing the right drill pipe type and steel grade is essential for safe and efficient drilling operations. For more detailed specifications or custom requirements, consult your drilling equipment supplier.

Related technical articles you might be interested in:

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● Casing Pipe vs. Drill Pipe: What’s the Difference and Why It Matters

● API 5DP Drill Pipe Specification

● Geological Drill Pipe Selection Guide

● High Performance Drill Pipe