Top Drives Information
Top drives are suspended mechanical drives that provide clockwise torque to the drill string and serve as an alternative to the use of a Kelly drive and rotary table. They consist of one or more electric or hydraulic motors and are connected to the drill string by a short section of pipe that is referred to as the quill.
Top drives are suspended below the traveling block at the swivel joint and move up and down the derrick as controlled by the draw-works. They are rated by a safe working load (SWL) that dictates the length of drill string they are able to navigate through the borehole. In contrast to how a rotary table and Kelly drive provide torque to the drill string at the rig floor the top drive transmits torque through a short section of pipe referred to as a quill.
The absence of a Kelly drive which requires a make and break cycle for each section of pipe fed through the well bore, top drives allow for the advancement of longer sections of drill string referred to as doubles and triples. They also reduce manual labor and are deployed in automated pipe handling operations.
Types of top drives include the following:
- Hydraulic top drives are powered by one or more hydraulic motors. They are deployed on land rigs, mobile rigs, and work-over rigs.
- AC powered top drives are coupled to one or more AC motors with a gearbox used to increase torque and decrease speed as necessary to rotate the drill string. They are favored over hydraulic top drives in offshore well drilling operations as well as when dealing with larger loads.
- Torque capacity – Torque capacity expressed in newton meters (N·m) or foot-pound of force (ft·lbf) is the rated torsional load that the top drive is able to transmit to the drill string.
- Safe working load (SWL) – The SWL refers to the lifting capacity of the top drive and is typically expressed in short tons or metric tons. Short tons are equivalent to 2000 lbs while a metric ton is equivalent to 1000 kilograms or 2204 lbs. The long ton or British is yet another distinct measurement and is equivalent to 2240 lbs.
API Specification 8C — Standard requirements for the design, manufacture and testing of hoisting equipment suitable for use in drilling and production operations in the oil and gas industry.