Drill pipe is classified into three categories: square drill pipe (Kelly), standard drill pipe, and heavy-weight drill pipe. The connection sequence is as follows: square drill pipe (1 joint) + standard drill pipe (*n* joints, determined by well depth) + heavy-weight drill pipe (*n* joints, determined by the drill string assembly design).
Square Drill Pipe (Kelly)
The square drill pipe is positioned at the uppermost end of the drill string; it is available in two configurations: square (four-sided) and hexagonal (six-sided).
Functions
① During drilling operations, the square drill pipe engages with the master bushing and rotary table insert to transmit rotational torque from the surface to the drill pipe, thereby driving the entire drill string and drill bit to rotate;
② It supports the entire weight of the drill string;
③ It serves as the conduit for the circulation of drilling fluid.
Structure
The square drill pipe consists of upper and lower tool joints and a pipe body section. The pipe body features either a square or hexagonal cross-section (square configurations are predominant in oil drilling operations). The upper tool joint features a left-hand internal (female) thread, which connects to the swivel; this left-hand thread prevents accidental unscrewing (back-off) during rotation. The lower tool joint features a right-hand external (male) thread, which connects to the standard drill pipe. During operation, the upper end of the square drill pipe remains above the rotary table level, while the lower section remains below it.
Standard Drill Pipe
Functions
Standard drill pipe constitutes the fundamental component of the drill string. Its primary functions are to transmit rotational torque and convey drilling fluid, and-through the gradual addition of pipe joints-to continuously deepen the wellbore. Consequently, standard drill pipe occupies a position of critical importance in oil drilling operations.
Structure
Standard drill pipe is manufactured from seamless steel tubing, typically featuring a wall thickness of 9 to 11 mm. It consists of two main parts: the pipe body and the tool joints; the pipe body and tool joints are joined together using friction welding.
Heavy-Weight Drill Pipe (HWDP)
Heavy-weight drill pipe is similar in appearance to standard drill pipe, consisting of a hollow steel tube approximately 10 meters in length. However, the weight of a single joint is significantly greater than that of standard drill pipe, and its wall thickness is two to three times that of standard pipe. Heavy-weight drill pipe is positioned between the standard drill pipe and the drill collars; its purpose is to prevent fatigue failure caused by abrupt changes in the cross-sectional geometry of the drill string assembly. Additionally, it can serve as a partial substitute for drill collars; however, it offers the advantages of simpler handling and greater ease during tripping operations (running in and pulling out of the hole), thereby helping to reduce overall tripping time.