A top drive is a big motor system which is hoisted in a derrick or mast of a drilling rig. A top drive is a modern rotating system which has been popular for many drilling contractors and oil operators. Top drives can be used on all types of rigs, from truck-mounted rigs to offshore rigs.
Rotation provided to a drill stem is accomplished by a top drive. Therefore, a Kelly and a Kelly bushing are not required for a top drive system. Moreover, a master bushing and a rotary table serves as support for slip and weight of a drill stem and as a conduit for a drill stem to be raised or lowed into a wellbore.
Since a Kelly is not required, the length of each stand is more than a single joint. Typically, drilling with a top drive can be drilled with a stand of drill pipe which consists of 3 joints of drill pipes. A top drive can drill about 90 ft before making a connection, as opposed to 30 ft like a Kelly. A top drive system allows rotation and circulation while pulling out of a hole (back reaming). However, this operation cannot be performed with a Kelly system.
A top drive is attached to a dolly track acting like a guide rail in a derrick. This allows straight movement up and down while drilling and tripping.
This video below briefly demonstrates how a top drive is operated.
Figure 1 – Basic Configuration of Top Drive
A – Elevator
B – Bail or Link
C – IBOP (both manual and pneumatic operated)
D – Rotating Head
E – Top drive motor
F – Dolly Track
G – Hook
H – Travelling Block
