Development of drill string composition and connection type

Table of Contents

The drill string is generally composed of kelly, drill pipe segment and BHA. With the development of top drive technology, kelly pipe has been slowly phasing out. The drill pipe section includes the drill pipe and the joint, sometimes with a reamer. The BHA consists mainly of drill collars but may also be fitting with stabilizers, shock absorbers, tremors, reamers and other special tools. The specific composition of drill string varies with different purposes and requirements.

1. Drill pipe

Drill pipe is the basic component of drill string. It is made of seamless steel pipe (wall thickness is generally 9~11mm). Its main function is to transfer torque and transport drilling fluid. and the hole is deepening by the gradual lengthening of drill pipe. Therefore, drill pipe plays a very important role in oil drilling.

(1) Drill pipe structure and specifications

The drill pipe is composed of two parts of the drill pipe body and the drill pipe joint.

There are two ways to connect the drill pipe body joint:

one is connecting with the wire buckle, that is the pipe body has a fine male buckle at both ends of the car, and is connecting with the female fine buckle at one end of the joint.

This drill pipe is calling the drill pipe with a fine buckle. The other is the pipe body and joint with friction welding butt welding together, called the butt welded drill pipe. With the basic elimination of fine buckle drill pipe.

The entire production or import of drill pipe in our country is welding drill pipe (without fine buckle drill pipe).

drill string
API drill string


In order to strengthen the connection between the tube body and the joint, the tube body is thickening at both ends. Commonly used thickening forms are internal thickening, external thickening, internal and external thickening three. According to the American Petroleum Institute (API), drill pipe is dividing into three categories by length:


The first type, 5.486-6.706m (18-22ft);

Second, 8.230~9.144m(27~30ft);

The third type, 11.582~13.716m(38~45ft);

The most commonly used drill pipe sizes are 88.9(3.5in), 114.3(4in), and 127.0mm(5in).

drill string
drill string

(2) Steel grade and strength of drill pipe

The steel grade of drill pipe refers to the grade of drill pipe steel. Which is determining by the minimum yield strength of drill pipe steel.

According to API, there are five steel grades of drill pipe:

D, E, 95(X), 105(G) and 135(S), among which X, G and S are high strength drill pipe.

The higher the steel grade of the drill pipe, the greater the yield strength of the pipe. and the greater the various strengths of the drill pipe (tensile, torsional, extruding, etc.). In the strength design of drill string, it is recommended to improve the strength of drill string by increasing the steel grade rather than increasing the wall thickness.

(3) Drill pipe joint and wire buckle

Drill pipe joint is a component of drill pipe, male and female joints, connected at both ends of the drill pipe body. The connector is provided with a wire link (thick link) to connect each single drill pipe. During the drilling process, the joints are often removing and the joint surface is subjecting to considerable clamp force, so the drill pipe joint has a thicker wall, the joint OD is larger than the pipe OD, and the stronger alloy steel is used. 35CrMo alloy steel is generally using in domestic drill pipe joints.

The connection of the buckles must meet three conditions. that is, the same size, the same type of buckles, male and female buckles matching. Different sizes of drill pipe have different joint sizes. The same size drill pipe wire type is not the same. The joint types used by drill pipe manufacturers are also difficult to be identical. Therefore, in order to facilitate the distinction between drill pipe joints and engineering applications. API has made uniform regulations on the types of drill pipe joints, resulting in the API drill pipe joints commonly used in the petroleum industry.

API drill pipe joints are available in both old and new standards. The old API drillpipe joints were developing for the early use of drill pipe with fine connections and were classifiing into three types: inline (IF), perforating (FH), and regular (REG).

drill string
drill string

The inner flat joint is mainly using for the outer thickening 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 the joint. The flow resistance of drilling fluid is small, which is conducive to improving the water power of drill bit.·

The perforated joint is suitable for the internal thickening drill pipe. It is characterizing by the fact that the drill pipe has two inner diamediamets. The joint inner diameter is equal to the inner diameter at the thickening of the pipe body. but less than the inner diameter of the pipe body part. The resistance of drilling fluid flowing through the joint is greater than that of the inner flat joint, but the outer diameter is smaller than that of the inner flat joint.

Normal joints are suitable for internal thickening of drill pipe. This joint has a smaller bore than the bore where the drill pipe is thickening. So there are three different bore sizes of drill pipe with normal joints. Drilling fluid flows through this joint with the greatest resistance, but it has the smallest outside diameter and is stronger. 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” shape thread, but the buckle type (expressed by the top cutting width of the thread) buckle distance, taper and size are very different.

drill string
drill string

With the rapid development of butt welded drill pipe, the fine button drill pipe is gradually replacing by butt welding drill pipe. The old API drill pipe joint is very inconvenient to use due to various specifications. Therefore. The American Petroleum Institute also proposed a new series of NC joint (some people call it the digital joint). And the NC joint is representing by the letter NC and two digits, such as NC50, NC26, NC31, etc.

NC(National Coarse Thread) means national standard coarse threads. Two digits indicate the diameter of the coarse thread on the base surface of the silk thread. (the first two digits of the diameter of the coarse thread are using). For example, NC26 means the joint is NC type, the base surface of the screw joint circle diameter is 2.668in.

The NC thread is also “V” shape thread. with 0.065in flat thread top and 0.038 round thread bottom. with V-0.038R for the buckle type, can be connecting with V-0.065 thread.

All inner flat (IF) and perforation (FH) in the old API standard are V-0.065 thread. So it is considering to abolish the old standard and adopt the NC joint.

NC type joints are more and more widely used in the petroleum industry. But some old API standard joints (inner flat, perforated, normal) are still using in the field.

NC type joints and old API standard joints have the same pitch diameter, taper, pitch and thread length, and can be using interchangeably. In drill string, in addition to drill pipe joints.

there are a variety of fit joints (used to connect different sizes or different connection types of pipe). guard joints (protection of the pipe string is often removing from the wire), etc. In addition, the kelly pipe, drill collars, drill bits, and other downhole tools are also connecting by wire connections. (Note that the type of wire connections used for each of these connections and tools is consistent with the standard for drill pipe connections.)

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