Thread Types and Classification of Drill Pipes
Drill pipes are categorized based on their thread types, which include standard drill pipes, square-threaded drill pipes, and weighted drill pipes. Standard drill pipes form the main body of the drill string, while square-threaded drill pipes are typically positioned at the top of the assembly, connecting to the drill collar at the bottom. These square-threaded joints are responsible for transmitting torque and supporting the weight of the entire drill string. The most commonly used type is the square fang drill pipe.
Weighted drill pipes serve as a transitional section between standard drill pipes and drill collars. They help reduce stress concentration at the junction between the drill collar and the drill pipe, provide weight on bit (WOB), and minimize wear on the drill string and wellbore walls.
With the advancement of drilling technologies such as roller cone drilling, air down-the-hole hammer drilling, and gas lift reverse circulation drilling, API-standard components have become widely used in well operations. Among these, API drill joint threads play a crucial role in connecting various parts of the drill string. These threads can be classified into four main types: numerical (NC), internal flush (IF), through-hole (FH), and regular (RGE).
Numerical (NC) threads are defined by their pitch diameter in inches and tenths of an inch. They feature a V-0.038R profile with a rounded bottom radius of 0.038 inches (0.965 mm). NC threads are preferred by API and come in two taper types: 1:6 and 1:4. They are commonly used in kellys, drill pipes, drill collars, and stabilizers, and even on drill bits in some oil fields like Daqing.
Internal flush (IF) joints are designed to connect thickened outer or inner ends of drill pipes, ensuring that the inner diameter remains similar to that of the drill pipe itself. This minimizes flow resistance for drilling fluids but results in larger outer diameters that are more prone to wear. IF threads use a V-0.065 profile with a flat top and bottom. While most IF threads are interchangeable with NC threads, they are susceptible to stress concentration due to their design. As a result, API has phased out several IF sizes, replacing them with NC threads like NC50 and NC46.
Through-hole (FH) joints are used to connect thickened ends of drill pipes so that the inner diameter of the joint matches that of the thickened portion. However, this inner diameter is smaller than that of the pipe body. FH threads come in three profiles: V-0.065, V-0.050, and V-0.040. They are widely used in various tools such as kellys, square drill rods, and drill bits. However, most FH threads are now considered obsolete, with only a few still in use. For example, 4 FH has been replaced by NC40.
Regular (RGE) threads are used primarily for connecting inner-thickened drill pipes. Their design allows for a smaller inner diameter at the joint compared to the thickened end, making them ideal for drill bit connections. Since the drill bit is at the end of the drill string, stress concentration at the root of the thread is less of a concern. RGE threads use V-0.050 and V-0.040 profiles, with newer versions like 1REG and 1½ REG introduced in the 40th edition of API SPEC 7.
In the market, drill pipes are also classified differently depending on the manufacturer. For instance, Qianjiang Machinery produces twist drill pipes, imitation British drill pipes, ribbed drill pipes, and others, all customized according to customer requirements. With continuous improvements in manufacturing technology, the quality of drill pipes has significantly improved, and prices have dropped. Looking ahead, China’s drill pipe market is expected to evolve toward greater technological innovation, better management practices, and higher product quality.
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