Thread Types and Classification of Drill Pipes
Drill pipes are categorized based on their thread types, which include standard drill pipe, square-shoulder drill pipe, and heavy-weight drill pipe. Standard drill pipes form the main body of the drill string, while square-shoulder drill pipes are positioned at the top of the assembly, designed to transmit torque and support the weight of the entire string. These are typically connected to the drill collar at the bottom. Square-shoulder drill pipes are commonly used in drilling operations due to their durability and efficiency.
Heavy-weight drill pipes serve as a transitional section between standard drill pipes and drill collars. They help reduce stress concentration at the junction, improve weight-on-bit (WOB) transmission, and minimize wear between the drill string and the wellbore wall. This type is especially useful in high-stress drilling environments where equipment longevity is critical.
With the advancement of drilling technologies such as roller cone drilling, down-the-hole hammers, and gas lift reverse circulation, API-standard drill pipe components have become widely adopted. The API thread system plays a vital role in connecting various parts of the drill string. Common thread types include numerical (NC), internal flush (IF), through-hole (FH), and regular (RGE) threads.
Numerical (NC) threads are defined by their pitch diameter in inches and fractions, featuring a V-0.038R triangular profile with a rounded root. NC threads are preferred by API and come in 1:6 and 1:4 tapers. They are commonly used in kellys, drill pipes, and stabilizers, and even appear on drill bits in certain fields like Daqing Oilfield.
Internal Flush (IF) threads are designed to match inner or outer thickened sections, allowing for smooth flow of drilling fluid. However, they tend to cause stress concentration due to their flat-top design. API has phased out most IF threads, replacing them with NC versions like NC50 and NC46.
Through-Hole (FH) threads ensure that the inner diameter of the joint matches the thickened end, though it remains smaller than the pipe’s inner diameter. FH threads come in different profiles such as V-0.065, V-0.050, and V-0.040, and were once widely used in various drilling tools. Many FH threads are now considered obsolete, with only a few still in use.
Regular (RGE) threads are primarily used for connecting drill bits. Since the drill bit is at the end of the string, stress concentration at the thread root is less of a concern. RGE threads are still used in some applications and feature V-0.050 and V-0.040 profiles. Newer versions like 1REG and 1½REG with V-0.055 profiles have also been introduced.
In the market, drill pipes are classified differently depending on the manufacturer. For example, Qianjiang Machinery offers twist drill pipes, imitation British-style drill pipes, ribbed drill pipes, and others, all customized according to client requirements. As technology improves, the quality of drill pipes continues to rise, and prices are gradually decreasing. In the future, China's drill pipe industry is expected to evolve toward higher technological standards, better management practices, and improved product quality.
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