A kelly bushing, also known as a rotary table bushing, is a component used in the rotary table of an oil rig for onshore and offshore drilling operations. It is an essential part of the drilling process, as it allows the drill string to rotate and transmit torque to the drill bit.
Construction of a Kelly Bushing for Offshore Drilling:
- Material: Kelly bushings are typically made from high-strength steel or alloy materials to withstand the mechanical stresses and harsh conditions encountered during drilling.
- Design: The kelly bushing is designed to fit into the rotary table of the drilling rig. It has an inner bore with a square or hexagonal cross-section, which matches the corresponding shape of the kelly (a square or hexagonal pipe that connects to the top of the drill string).
- Bearings: The kelly bushing has roller bearings or other suitable bearings on its inner surface. These bearings allow the kelly to rotate smoothly inside the bushing.
- Tongs and Clamps: The kelly bushing may have tongs or clamps to secure the kelly in place during drilling operations. This prevents the kelly from slipping or rotating uncontrollably when torque is applied.
- Drive System: The kelly bushing is connected to the rotary table’s drive system, which provides the power and rotation necessary to turn the drill string.
Function in Offshore Drilling:
During offshore drilling, the kelly bushing serves as a connection point between the drill string and the rotary table. When drilling begins, the kelly is inserted into the kelly bushing, and the drill string is suspended from the derrick. As the rotary table turns, it transmits torque to the kelly through the kelly bushing. The kelly, in turn, rotates the entire drill string, including the drill bit at the bottom of the wellbore.
As the drilling progresses, sections of drill pipe are added to the drill string, and the kelly is lifted to compensate for the increased length. This process, known as “making a connection,” allows continuous drilling while maintaining the rotation of the drill bit.
In summary, the kelly bushing is a critical component in offshore drilling rigs, as it enables the rotation of the drill string and, ultimately, the drill bit during drilling operations. Its design and construction ensure smooth rotation, secure attachment, and effective drilling performance in challenging offshore environments.
