Kelly Drilling Rigs
The main parts that make up a kelly drilling system are: the swivel, kelly, kelly bushing, master bushing, and the rotary table. The swivel is attached to the hook of the traveling block and enables the drill string to rotate. The kelly is square or hexagonal portion of pipe, which fits snugly inside the kelly bushing, which in turn fits snugly into the master bushing. The master bushing is located on the rotary table and rotates with the rotary table as it turns.
The kelly bushing contains four studs that stick out on each corner of the bushing, enabling it fit snuggly and securely inside the master bushing. Thus, as the rotary table turns, the master bushing rotates, which in turn rotates the kelly bushing, kelly, as well as the rest of the drill string.
After a single joint of drill pipe has passed through the rotary table, a connection of another joint of drill pipe must be made in order for more pipe to be lowered into the hole. The kelly rig’s inability to handle more than one joint of pipe at a time is the main disadvantage when compared to the more innovate rigs equipped with top drives which are discussed below.
Top Drive Rigs
A top drive rig utilizes a heavy duty powered swivel which contains an internal motor enabling the top drive to grip onto and rotate the drill pipe on its own, without the need for a kelly to provide rotation. The top drive motor turns a threaded drive shaft which connects directly onto the top of the drill string to rotate it. Although a rig equipped with a top drive still needs a rotary table and slips to keep the drill string from falling into the hole when making a connection, a top drive eliminates the need for traditional rotary equipment such as a kelly and kelly bushing.
Unlike traditional kelly rigs, many rigs equipped with a top drive are able to handle stands (2-3 joints) of drill pipe, up to 90 feet in length at a time. This ability to handle multiple joints of drill pipe at a time increases the drilling rate and increases efficiency significantly. When a connection must be made, the driller simply pulls up the drill string until the bit is off the bottom and turns off the mud pumps. The crew then sets the slips in the rotary table to hold the drill string in place while the top drive is hoisted up to the top of the derrick to grasp the next stand of drill pipe to be added. Once the new stand of pipe is connected, the driller turns the pumps on, the crew removes the slips, and drilling can continue.
Pros and cons
The advantages of top drive systems are that they are safer, faster, and more efficient due to their ability to handle 2-3 joints of drill pipe at a time. In addition, top drive rigs are able to rotate the pipe while tripping pipe into/out of the hole, which helps reduce problems associated with stuck pipe, which is one of the most significant causes of loss of rig time. This process of rotating pipe while tripping is known as reaming, and is especially important in directional drilling, where larger portions of the pipe can come into contact with the wellbore, thus increasing the chances of encountering stuck pipe. By reaming while tripping into/out of the hole, the pipe is constantly moving, and never has a chance to become stuck once it contact the wellbore wall.
On the other hand, a significant con of top drive rigs is that they are more expensive and thus are not as common on smaller rigs. In addition, the ability of the top drive to handle 2-3 joints of drill pipe at a time requires a derrick that is significantly larger/taller than most derricks on traditional kelly drive rigs.