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.
An advantage of a top drive is that it allows the drilling rig to drill longer sections of a stand of drill pipe. A rotary table type rig can only drill 30'-60' (double-triple respectively) 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.
A top drive is a hydraulic or electric motor suspended in the derrick (mast) of a drilling rig, which rotates the drill string and bit and is used in the actual process of drilling the well. Using a top drive eliminates the need for the traditional kelly and rotary table and reduces the amount of manual labour and associated hazards that have traditionally accompanied this task. For drilling contractors and producers, utilizing top drive technology translates into increased efficiency in its operations. Top drive ratings are based on the safe static hook load, this rating relates to the main thrust bearing (usually the weakest link). Top drive ratings of 500 short tons typically have an API rating lower than 500T, due to required calculations of rotation with a percentage of rated load.