Drill pipes are categorized into three types: kelly pipes, standard drill pipes, and heavy-weight drill pipes. The connection sequence is: kelly pipe (1 joint) + standard drill pipes (n joints, determined by well depth) + heavy-weight drill pipes (n joints, determined by the bottom-hole assembly design).
Kelly Pipe
The kelly pipe is located at the top of the drill string and comes in two cross-sectional shapes: square and hexagonal.
Functions
① During drilling, it works with the master bushing and rotary table bushing to transmit surface rotational torque to the drill pipe, thereby driving the rotation of the drill string and bit;
② It bears the entire weight of the drill string;
③ It serves as a conduit for drilling fluid circulation.
Structure
The kelly pipe consists of upper and lower subs (connectors) and a pipe body. The body features either a square or hexagonal cross-section (square is most common in oil drilling). The upper sub has a left-hand female thread (reverse thread) to connect to the swivel; the left-hand threading prevents unscrewing during rotation. The lower sub has a right-hand male thread to connect to the standard drill pipe. During operation, the upper end of the kelly remains above the rotary table, while the lower section extends below it.
Standard Drill Pipe
Functions
The standard drill pipe is a fundamental component of the drill string. Its primary roles are transmitting torque and conveying drilling fluid, as well as allowing the wellbore to deepen through the progressive addition of pipe joints. Consequently, it plays a crucial role in oil drilling.
Structure
It is manufactured from seamless steel tubing, typically with a wall thickness of 9–11 mm. It consists of two parts-the pipe body and the tool joints-which are joined together via friction welding.
Heavy-Weight Drill Pipe
Heavy-weight drill pipes resemble standard drill pipes; they are hollow steel tubulars, typically around 10 meters in length. However, each joint is heavier than a standard drill pipe, with a wall thickness two to three times greater. Connected between the standard drill pipe and the drill collars, they serve to prevent fatigue failure caused by abrupt changes in the cross-sectional area of the drill string. They can also perform some of the functions of drill collars while offering simpler suspension and easier handling during tripping operations, thereby saving time.
