The purpose of the rotation system is to rotate the drill string and, subsequently, the drill bit, hence the name rotary drilling. Even when a downhole motor, called a “mud motor,” is used to spin the drill bit, the rotation system still turns the drill string (although usually at a slower rate). This prevents the drill string from getting stuck and reduces friction with the borehole walls. During drilling operations, the drill string normally turns anywhere from 60 to 120 rotations per minute (rpm), but the rotation usually is not any faster than 200 rpm. When a mud motor is used, string rotation is usually slowed to 25 to30 rpm. There are occasions, such as when the bit encounters very hard rock, that the string rotations may start oscillating between high and low rpm, from high vibrations (whirling) down to just a few rpm (stick-slip). During directional drilling operations, the rotation of the drill string will stop completely and rely entirely on the down-hole drilling motor to turn the drill bit in the proper direction.
These days, the drill string is turned either by a rotary table or a top drive system.
Top Drives
The primary means of imparting drill string rotation in modern drilling rigs is the top drive. This, in essence, is a 500 to 1000 hp motor that rides up and down with the traveling block. The motor can be hydraulically driven or powered with electric motors. The top drive has the capability of screwing directly into the drill string. They are very versatile; hence, their popularity today. You can tell if a rig has a top drive by first, noting the motor attached to the traveling block. But if you are far away, note if the mast has a bar or other anti-torque devices. As one might surmise, the top drive being hung on a set of wire ropes will impart a reaction torque that must be opposed, or otherwise, the wirerope will wrap up into what is euphemistically called a “birds nest.”
Rotary Tables
Although rotary tables are declining in popularity as a primary rotation mechanism, they are still found on many rigs.
The operating concept of the rotary table system is simple: a rotating turntable is fitted with a series of adapters called bushings that grip a square or hexagonal pipe called the kelly. The kelly is screwed into the top of the drill string and transmits the rotation of the turntable to the string.
The key to the whole system is that the kelly can slide up and down within the bushing even as it is being rotated. This allows the pipe to move downward while rotating as drilling progresses.
The master bushing sits right inside the turntable. It often contains a bowl shaped depression that allows drillers to insert wedges, which hold up the drill string while it is unscrewed from the kelly.
The kelly bushing is an adapter that fits inside the master bushing and transmits rotation to the kelly, which will likely vary in size as drilling progresses and the diameter of the hole decreases.
Top-drive Systems
Top drive systems do away with the kelly and kelly bushings by suspending a drive system from the traveling block. Instead of rotating the drill string with a kelly and turntable, top drive systems use an electrically powered drive shaft that screws directly into the top of the drill string (kind of like a really big electric drill). As drilling progresses, the top drive system is lowered towards the hole.
Because of the rotational forces exerted on the top drive system, a system of tracks is installed on the rig to allow the top drive mechanism to be raised and lowered without itself rotating.
One last note: the drill string always rotates clockwise. Since the drill pipe is screwed together from many sections, reverse rotation would cause the drill string to come apart.
Images: “Top Drive” by Michael Black ; “Drilling rig detail” by wpohldesign via iStock; “Top Drive Tracks” by Michael Black