API Drill pipe joint is a component of drill pipe, divided into male joint and female joint. There are screw buckles (thick buckles) to connect each single drill pipe.
The connection of the thread must meet three conditions.That is, the size of the thread is equal. The type of the thread is the same, and the male and female buckle are matching. Different sizes of drill pipe have different joint sizes. The screw type of drill pipe of the same size is also different. It is difficult for drillpipe manufacturers to use the same type of joint. Therefore, to facilitate the distinction between drillpipe joint and engineering application. API has made uniform requirements for drillpipe joint, forming the API drillpipe joint commonly used in the petroleum industry.
Drill pipe joints
API drill pipe joints are available in both old and new standards. The old API drill pipe joint is a variant of the earlier tapered drill pipe. Which is available in three types: Internal flat (IF), perforated (FH) and normal (REG).
The inner flat joint is mainly using for external upset drill pipe.
Its characteristic is that the inner diameter of drill pipe is equal to the inner diameter of pipe body and the inner diameter of joint. The drilling fluid flow resistance is small, which is conducive to improving the water power of drill bit, but the outer diameter of joint is large and easy to wear.
The perforated joint is suitable for internal thickening drill pipe, which has two inner diameters. The inner diameter of the joint is equal to the inner diameter of the thickening part of the pipe body, but smaller than the inner diameter of part of the pipe body. The resistance of drilling fluid through this joint is greater than that of the inner flat joint, but its outer diameter is smaller than that of the inner flat joint.·
Normal joint is suitable for internal thickening drill pipe. The joint is with a smaller inner diameter than where the drill pipe is upseting. So there are three different diameters of drill pipe with regular joints. Drilling fluid has the greatest resistance through this joint, but it has the smallest outside diameter and the greatest strength. Regular joints are commonly used for small diameter drill pipe and reverse drill pipe, as well as drill bits and fishing tools. The three types of joints all use “V” thread, but the buckle type (indicated by the thread top cutting width) buckle distance, taper and size are very different.
With the rapid development of butt welding pipe, thin pipe has been replaced with the butt welding pipe. The old API tool joint is not very convenient to use. As a result, the American petroleum institute proposed a new type NC series connectors (some call it digital type joint), with a letter and two NC NC type joint digital representation, such as NC50 NC26, NC31, etc.
National Coarse Thread (NC) means National standard Coarse Thread. Two digits represent the size of joint circle diameter of the base surface of the screw. For example, NC26 indicates that the joint is NC type.The diameter of the base surface thread buckle is 2.668in. The NC thread is also “V” thread. With the top of 0.065in flat thread and the bottom of 0.038 round thread. V-0.038r indicates the buckle type, which can connect with the V-0.065 thread. All internal flat (IF) and perforation (FH) in the old API standard are V-0.065 type threads. So it is now considering to abolish the old standard and adopt the NC type joint.
NC joints are more and more widely used in the petroleum industry. But part of the old API standard joints (flat, perforated, normal) are still using in the field. NC joints and old API standard joints have the same pitch diameter, taper, pitch, and can be interchangeably. In drill string,there are all kinds of fitting joint (used to connect different sizes or different buckle type of pipe string), protection joint (protection of pipe string often disassembled on the thread buckle), etc.. In addition, kelly pipes, drill collars, drill bits and other downhole tools are all connected by screw connections.