Kelly Drive vs. Top Drive: How the Drill String Is Rotated
A clear explanation of how Kelly drive rigs and top drive rigs rotate the drill string, and why the bottom‑hole assembly (BHA) remains the same in both systems. The video walks through the drill string components, the role of the rotary table, and how modern top drives replaced the older Kelly‑driven method.
Concept Summary
Kelly drive rigs use a rotary table to turn a square or hexagonal Kelly, transmitting rotation to the drill string. Top drive rigs replace this with a motorized drive unit suspended from the traveling block, allowing continuous rotation, improved directional control, and safer pipe handling. Despite the difference in surface equipment, the BHA architecture remains identical: drill pipe on top, then HWDP, then drill collars, and finally the drill bit cutting the formation.
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