The joint is an important part of the drill pipe, on that threads is needed to connect the drill pipe body. In the process of drilling, the joint removed frequently and its surface is subject to considerable bite of the tongs, so the the drill pipe joint need relatively large wall thickness, which can be divided into three forms according to the way of its upset: internal upset(IU), external upset (EU) and internal and external upset(IEU). The outer diameter of the joints should be larger than that of the pipe body and made by alloy steel with high strength, generally 4135.
The smooth connection between thread and tube body must meet the following requirements: the same size, the same thread type, and the same male and female buckle. Please note the screws of the same size of drill pipe may not be the same. In order to distinguish drill pipe joints from engineering applications, API makes uniform provisions on the types of drill pipe joints, that is, API drill pipe joints commonly used in the petroleum industry. There are old and new standards of API drill pipe connector, which are proposed for the early use of fine buckle drill pipe, and are divided into three types: internal flush (IF), full hole (FH) and regular (REG).
The internal flush joint is mainly used for external upset of drill pipe. Its characteristic is that the inner diameter of drill pipe is equal to the inner diameter at the thickening of pipe body and the inner diameter of joint to make a small drilling fluid flow resistance, which is conducive to improving the water power of drill bit. But this kind of joints offer big outside diameter and easy to wear.
Full hole connection is ideal for internal upset drill pipe. It offers two different inner diameter and the inner diameter of the joint is equal tothat of the thickening part of the pipe body but less than the pipe body itself. The resistance of drilling fluid flowing through this joint is greater than that of the internal flat joint, but its outer diameter is smaller than that of the internal flat joint.
Rugular connection is appropriate for IU drill pipe and IEU drill pipe. This joint has the smallest outer diameter, and the inner diameter is smaller than that at the thickening point of the drill pipe. The drill pipe connected by this kind of joint have three kinds of different inner diameters, which all use the "V" shape thread, but thethread type (expressed by the thread top cut level width), the thread distance, the taper and the size and so on all have the very big difference.Regular joints have great strength, and are often used for small diameter drill pipe and back button drill pipe, as well as drill bits, fishing tools and so on.
At present, the fine-buckle drill pipe is gradually replaced by butt welding drill pipe and the old API joint standard can no longer meet the demands. The American petroleum institute proposes a new NC (Coarse Thread) type series connection (digital type connector), represent with letter "NC" and two Numbers, such as NC50 NC26, NC31, etc.The latter two digits referred the diameter of the screw base (the diameter of the pitch circle).For example, NC26 represents an NC-type connector with base screw joint circle diameter of 2.668in.NC thread is also "V" shaped with 0.065 in flat thread top and 0.038 round thread. V-0.038R is used for buckle type, which can be connected with v-0.065 thread.In the old API standard, all internal flat (IF) and full bore (FH) threads were v-0.065.
NC joints are increasingly used in the petroleum industry. At present, NC joints and some old API standard joints are used simultaneously and interchangeably because they have the same pitch diameter, taper, pitch and thread length.In the drill string sysetem, there are a variety of matching joints (used to connect different sizes or buckle type of string) except the drill pipe joint, protection joints (often removed on the protection string screw) and so on.In addition, kelly bar, drill collars, drill bits, and other downhole tools are connected by screws.It should be noted that the screw type of the above mentioned joints and tools is consistent with the standard of drill pipe joints.