Thread Types and Classification of Drill Pipes
Drill pipes are categorized based on their thread types, including 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 located at the top of the assembly, connecting to the drill collar at the bottom. These square-threaded pipes 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, helping to reduce stress concentration in the transition area and improving wear resistance between 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 and downhole tools have become widely adopted. Among these, API thread connections play a critical role in ensuring reliable performance. The drill pipe joint 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 numerical measurements in inches and tenths of an inch along the pitch diameter. They feature a V-shaped profile with a flat top and rounded bottom, with a radius of 0.038 inches (0.965 mm). NC threads are the preferred option recommended by API and come in two taper types: 1:6 and 1:4. They are commonly used in kellys, drill pipes, drill collars, stabilizers, and even on drill bits in certain oil fields like Daqing.
Internal flush (IF) threads are designed to connect thickened outer or inner ends of drill pipes, ensuring that the inner diameter remains consistent with the rest of the drill string. This reduces fluid flow resistance but increases the risk of external wear due to larger outer diameters. IF threads use a V-0.065 profile with flat tops and bottoms. While most IF threads are interchangeable with NC threads, they are prone to stress concentration and have been gradually phased out by API, replaced by NC50 and NC46 threads.
Through-hole (FH) threads are used to connect thickened sections of drill pipes, making the inner diameter of the joint match the thickened end. However, this inner diameter is still smaller than the pipe's inner diameter. FH threads come in three forms: V-0.065, V-0.050, and V-0.040, each with different tooth profiles. Although once widely used in various drill components, many FH thread sizes are now considered obsolete, with only a few remaining in use.
Regular (RGE) threads were originally designed for drill bit connections, where stress concentration at the thread root was less of a concern. These threads use V-0.050 and V-0.040 profiles, and newer versions like 1REG and 1½REG with V-0.055 profiles have been added in recent editions of API SPEC 7.
In the market, drill pipes are often classified differently depending on the manufacturer. For example, Qianjiang Machinery produces twist drill pipes, imitation British-style drill pipes, ribbed drill pipes, and others, all customized to meet specific customer requirements. As research and development in drill pipe technology continue to advance, the quality has significantly improved, and prices have also dropped. Looking ahead, the Chinese drill pipe market is expected to evolve toward greater technological innovation, better management practices, and higher-quality standards.
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