Thread type and classification of drill pipe
Thread Types and Classification of Drill Pipe
Drill pipes are categorized based on their thread types, which include standard drill pipe, square tooth drill pipe, and weighted drill pipe. Standard drill pipe is the core component of the drill string, while square tooth drill pipe is typically located at the top of the assembly, connecting to the drill collar at the bottom. Its main function is to transmit torque and support the weight of the entire drill string. The most commonly used type is the square fang drill pipe. Weighted drill pipe serves as a transitional section between the standard drill pipe and the drill collar, helping to reduce stress concentration at the junction, improve 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 in-well components and downhole tools have become widely adopted. Among these, the 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) threads, inner flat (IF) threads, through-hole (FH) threads, and regular (RGE) threads.
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The classification of tested oil drill pipe joint threads is based on the type of thread used.
Numerical (NC) threads are defined by measurements in inches and tenths of an inch, with a V-0.038R triangular profile featuring a rounded root. This design is preferred by API and comes in two tapers: 1:6 and 1:4. NC threads are widely used in kellys, drill pipes, drill collars, stabilizers, and even on drill bits in Daqing Oilfield, such as NC50.
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Inner flat (IF) threads are designed to connect thickened or non-thickened drill pipes, ensuring that the inner diameter of the joint matches that of the pipe body. This minimizes flow resistance for drilling fluids, but the larger outer diameter makes them more prone to wear. IF threads use a V-0.065 flat-top and flat-bottom profile. While most IF threads are interchangeable with NC threads, they are susceptible to stress concentration and have been phased out by API, replaced by NC50 and NC46 in many cases.
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Through-hole (FH) threads are used to connect thickened ends of drill pipes, making the inner diameters equal at both ends. They come in three types: V-0.065, V-0.050, and V-0.040. These threads are commonly found in faucets, square drill rods, drill pipes, collars, and bits. However, most FH threads are now considered obsolete, except for specific sizes like 5 1/2 FH and 6 5/8 FH, which still use the V-0.050 profile. The 4 FH has been replaced by NC40, similar to the transition from IF to NC threads.
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Regular (RGE) threads are used primarily for connecting inner thickened drill pipes, where the inner diameter of the joint is smaller than that of the thickened end. These threads were originally designed for drill bit connections, as stress concentration at the thread root is less critical at the end of the drill string. API retains all RGE specifications, using V-0.050 and V-0.040 profiles. In the 40th edition of API SPEC 7, new types like 1REG and 1 1/2 REG with a V-0.055 profile were introduced, expanding the range of available thread options.
Drill pipe classifications vary among manufacturers. For example, Qianjiang Machinery produces twist drill pipe, imitation British drill pipe, ribbed drill pipe, and others, with custom thread types and lengths to meet customer requirements. As research and development in drill pipe technology advance, quality has improved significantly, and prices have dropped. Looking ahead, China's drill pipe market is expected to grow toward a future driven by technological innovation, efficient management, and high-quality standards.
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