Thread Types and Classification of Drill Pipe

Drill pipes are categorized based on their thread types, which play a crucial role in the performance and reliability of drilling operations. The most common classifications include ordinary drill pipe, square-shoulder (square tooth) drill pipe, and weighted drill pipe. Ordinary drill pipe serves as the primary component of the drill string, while square-shoulder drill pipe is typically located at the top of the assembly, designed to transmit torque and support the weight of the entire string. It connects to the drill collar at the bottom, ensuring a secure and efficient transfer of mechanical forces. The most commonly used type is the square-shoulder drill pipe, known for its durability and ease of connection. Weighted drill pipe acts as a transitional section between the ordinary drill pipe and the drill collar. It helps reduce stress concentration at the junctions, improves weight-on-bit (WOB) application, and minimizes wear between the drill string and the wellbore walls. As drilling technologies evolve—such as roller cone drilling, air down-the-hole hammer drilling, and gas lift reverse circulation drilling—the demand for high-performance API-standard components has increased significantly. API (American Petroleum Institute) standards define various types of drill pipe joints, including digital threads (NC), internal flush (IF), through-hole (FH), and regular (RGE) threads. These thread types are essential for connecting different parts of the drill string and ensuring safe and efficient operations. Digital threads (NC) are defined by numerical measurements in inches and tenths of an inch. They feature a V-shaped profile with a flat top and rounded bottom, and are widely used due to their strength and reliability. NC threads come in two taper types: 1:6 and 1:4. They are commonly found in kellys, drill pipes, and stabilizers, and even in some drill bits in fields like Daqing Oilfield. Internal flush (IF) threads are designed to match the inner diameter of the drill pipe, reducing fluid resistance but increasing outer diameter and wear risk. While IF threads were once popular, many have been phased out due to stress concentration issues, replaced by more modern NC threads such as NC50 and NC46. Through-hole (FH) threads allow for equal inner diameters at both ends of the joint, though they are smaller than the inner diameter of the pipe itself. FH threads have three main profiles: V-0.065, V-0.050, and V-0.040. Although widely used in the past, many FH threads are now considered obsolete, with only a few remaining in use. Regular (RGE) threads are primarily used for connecting drill bits. Their design allows for minimal concern about stress concentration at the thread roots, making them ideal for this application. RGE threads use V-0.050 and V-0.040 profiles, with newer versions introduced in recent API specifications. In addition to standard classifications, market variations exist depending on the manufacturer. For example, Qianjiang Machinery produces twist drill pipes, imitation British-style drill pipes, ribbed drill pipes, and others, often customizing thread types and lengths to meet specific customer requirements. As technology advances, the quality of drill pipes continues to improve, and prices are gradually decreasing. Looking ahead, China’s drill pipe industry is expected to grow toward greater technological innovation, better management practices, and higher-quality standards.

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