Rotary table (drilling rig)
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A Rotary table is a mechanical device on a drilling rig that provides clockwise (as viewed from above) rotational force to the drill string to facilitate the process of drilling a borehole.
In the diagram, #20 (in blue) is the rotary table. The kelly #19, is inserted through the center of the rotary table and kelly bushings, and has free vertical (up & down) movement to allow downword force to be applied to the drill string, while the rotary table rotates it. (Note: Force is not actually applied from the top (as to push) but rather the weight is at the bottom of the drill string like a pendulum on a string.)
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[edit] Components
[edit] Chain Drive
Most rotary tables are chain driven. These chains resember very large bicycle chains. The chains require constant oiling to prevent burning and seizing.
[edit] Rotary Locks
Virtually all rotary tables are equiped with a rotary lock. Engaging the lock can either prevent the rotary from turning in one particular direction, or from turning at all. This is commonly used by crews in lieu of using a second pair of tongs to makeup or break out pipe.
[edit] Bushings
The rotary bushings are located at the center of the rotary table. These are generally able to be removed in two sepearate pieces to facilitate large items, i.e. drill bits, to pass through the rotary table. The large gap in the center of the rotary bushings is referred to as the "bowl" due to its apperance. The bowl is where the slips are set to hold up the drill string during connections and pipe trips as well as the point the drill string passes through the floor into the wellbore. The rotary bushings connect to the kelly bushings to acually induce the spin required for drilling.
[edit] Alternatives
Most recently manufactured rigs no longer feature rotary drives. These newer rigs have opted for top drive technology. In top drive, the drill string is turned by mechanisms located at the kelly as opposed to being located in the floor.
[edit] Terminology
Rotary speed is the number of times the rotary table makes one full revolution in one minute (rpm).
See Drilling rig (petroleum) for a detailed description of the diagram.
