The Top Drive is used to rotate the drill string during the drilling process, the Top Drive is a large motor that is suspended from the derrick of the rig. They can boast at least 1,000 horsepower that turn a shaft to which the drill string is screwed. Replacing the traditional rotary table, the Top Drive lessens the manual labor involved in drilling, as well as many associated risks.
The Top Drive is suspended from a hook below the traveling block, the Top Drive is able to move up and down the derrick.
Chosen both for increased safety and efficiency.
A Top Drive is capable of drilling with three joints stands, instead of just one pipe at a time. Here you can see the 3 joints of drill pipe already connected together on the right of the photo. The large yellow tool to the right of the Top Drive will grab the section of drill pipe top and bottom, the Top Drive will then be lifted to the top of the derrick, this tool will then swivel the drill pipe 180 degrees, the Top Drive will then connect to the top of the drill pipe and screw it into the section of pipe being held at the drill floor, then it all will be lowered to the seabed.
Reducing risk and increasing safety during the drilling process, Top Drives remove much of the manual labor that was previously required to drill wells. On some rigs Top Drives are completely automated, offering rotational control and maximum torque, as well as control over the weight on the bit.
On an offshore rig,(as opposed to a land based rig) the Top Drive travels up and down the vertical rails to avoid the mechanism from swaying with the waves of the ocean.