A top drive is a mechanical device on a drilling rig that provides clockwise rotational force to the drill string to facilitate the process of drilling a borehole. It is an alternative to rotary table. It is located at the swivel place and allows a vertical movement up and down the derrick.
A good advantage of a top drive is it allows the drilling rig to drill longer sections of a stand of drill pipe. A rotary table type rig can only drill 30' sections of drill pipe while a top drive can drill 60' to 90' stands, depending on the drilling rig type. Having longer sections of drill pipe allows the drilling rigs to drill deeper sections of the wellbore, thus making fewer connections of drill pipe.
Several different kinds of top drives exist, and are usually classified based on the "Safe Working Load" (SWL) of the tool, and the size and type of motor used to rotate the drillpipe. For offshore and heavy duty use, a 1000 short ton, top drive would be used, where a smaller land rig may only require a 500. Motors are available in all sizes, and come in Hydraulically powered, AC, or DC motors.
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