Classification of Drill Pipe Joints
Drill pipe joints are integral components of a drill string, consisting of a pin (male) end and a box (female) end, which are attached to the two ends of the drill pipe body. To enhance the connection strength of the joint, the wall thickness of the pipe body is increased at the joint area. Based on the method of thickening, these can be classified into three types: internal upset (IU), external upset (EU), and internal-external upset (IEU). Threads (coarse threads) are machined onto the joints to facilitate the connection of individual drill pipe sections. The threads on drill pipe joints are tapered pipe threads featuring a sealing shoulder; when tightened, the shoulder faces create a seal, while the threads serve solely to provide the mechanical connection. Different upsetting methods correspond to different thread profiles for the respective joints.
Based on their thread profiles, drill pipe joints are classified into four main types: Internal Flush (IF), Full Hole (FH), Regular (REG), and Numbered Connection (NC).

Conditions for Threaded Connections
A threaded connection must satisfy the following three conditions:
① Equal dimensions;
② Identical thread types;
③ Proper matching between the pin (male) and box (female) threads.

Thread Profiles of Drill Pipe Joints
Internal Flush (IF): Primarily used for externally upset (EU) drill pipes. Its characteristic feature is that the internal diameter remains constant throughout the entire length of the drill pipe, resulting in low flow resistance for drilling fluids. However, it has a relatively large external diameter, making it more susceptible to wear. Full Hole (FH): Primarily used for internally-externally upset (IEU) drill pipes. It is characterized by having two distinct internal diameters within the drill pipe; consequently, the flow resistance for drilling fluids is higher than that of the IF type, though its external diameter is smaller than that of the REG type. Regular (REG): Primarily used for internally upset (IU) drill pipes, as well as for drill bits and fishing tools. Its defining feature is that the internal diameter at the thickened joint section is smaller than the internal diameter of the main pipe body. This results in high flow resistance for drilling fluids-specifically, the highest relative flow resistance among the three types-but offers the smallest external diameter and superior structural strength. All three aforementioned joint types utilize V-shaped threads; however, they differ significantly in terms of thread profile, pitch, taper, and dimensions.
Numbered Connection (NC) Series: This series of joints is based on the U.S. National Standard coarse-pitch thread series. It has since been adopted by the API (American Petroleum Institute) as an international standard. NC threads are also V-shaped threads; certain NC-type joints share identical pitch diameters, tapers, pitches, and thread lengths with older API standard joints, allowing for their interchangeability. Most drill pipe joints utilize either external upset or internal-external upset configurations. Since the outer diameter of the drill pipe joint exceeds that of the pipe body, it is subject to continuous contact and friction against the wellbore wall or casing during drilling operations, resulting in wear. To prevent drilling accidents-such as drill pipe breakage or thread disengagement-caused by joint wear, wear bands must be installed on the box ends of the drill pipe joints.