Thread Types and Classification of Drill Pipe
Drill pipes are categorized based on their thread types, including standard drill pipe, square drill pipe, and weighted drill pipe. Standard drill pipes form the main body of the drill string, while square drill pipes are typically located at the top of the assembly, connecting to the drill collar at the bottom. The primary function of square drill pipes is to transmit torque and support the weight of the entire drill string. Among these, the square-shoulder (square fang) drill pipe is the most commonly used.
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 between the drill string and the wellbore wall.
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. Particularly, the API thread connections between drill string components play a critical role. 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. All NC threads feature a V-shaped profile with a flat top and rounded bottom, where the radius is 0.038 inches (0.965 mm). NC threads are the preferred type recommended by API and are available in two tapers: 1:6 and 1:4. They are widely used in kellys, drill pipes, drill collars, stabilizers, and even on some drill bits in the Daqing Oilfield.
Internal flush (IF) threads are designed to connect thickened outer or inner pipe ends, ensuring that the inner diameter remains consistent with the drill pipe joint. This design reduces flow resistance for drilling fluids but results in a larger outer diameter, making it 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 corresponding NC threads, they have been phased out due to stress concentration issues, with exceptions like 4 1/2 IF and 4 IF being replaced by NC50 and NC46.
Through-hole (FH) threads are used to connect thickened sections of the pipe, matching the inner diameter of the joint and the thickened end. Though fewer in specifications, FH threads include three types: V-0.065, V-0.050, and V-0.040. These are commonly found in faucets, square drill rods, drill collars, and bits. However, many FH thread types have been retired from the API standards, except for specific sizes like 5 1/2 FH and 6 5/8 FH. The 4 FH thread has also been replaced by NC40.
Regular (RGE) threads were originally designed 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. In the 40th edition of API SPEC 7, new RGE thread types, such as 1REG and 1 1/2 REG with a V-0.055 profile, were introduced.
In the market, drill pipes vary in classification depending on the manufacturer. For example, Qianjiang Machinery produces twist drill pipes, imitation British drill pipes, ribbed drill pipes, and others, with custom thread types and lengths tailored to customer needs. As drilling technology advances, the quality of drill pipes has improved significantly, and prices have decreased. Looking ahead, the Chinese drill pipe market is expected to move toward higher technological standards, better management practices, and enhanced product quality.
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