Top drive systems primarily function like a power swivel, applying rotary torque to the drill pipe at the top of the drill string. Some early top drives are suspended below a separate rotary swivel, while some have an integrated swivel. The prime difference from a power swivel is the power, load rating, and ability to connect the stand or drill string to the powered main shaft at well center without human intervention through the use of a “pipe handler” under the top drive. This facilitates drilling ahead with stands of drillpipe where prior to this all connections had to be made at the drill floor which meant only one single from the rathole could be added at a time. Top Drives are an alternative to Kelly drive systems powered by a rotary table. Top drives also provide improved back reaming capability to aid in pulling out of the well while rotating. Top Drives allow both pumping and rotating during hoisting, making it possible to pull out of a wellbore with mechanical obstructions. Additionally, a remote kelly cock, or integrated BOP valve, is connected to the top drive main shaft and can be remotely actuated from the driller’s console. Top drives are generally rated for hoisting loads at 150 ton, 250 ton, 350 ton, 500 ton, 750 ton, 1,000 ton, and 1,250 ton. A rotary table is occasionally still used for a back-up rotation system, or as a method to turn casing in the well while adding a joint of casing to the string to avoid potential sticking problems. In a few cases no rotary exists and a simple device to handle the slips is put in its place at well center on the rig floor.