Evolution of Drilling Technology: From Square to Round
The evolution of drilling technology may be best characterized as a shift away from typical rotary tables and Kelly drives to top drives rather than a simple shift from square to round Kelly designs.
Normally the rotary table is the source of rotational power. The Kelly bushing takes that rotation and puts it into the Square Kelly on the drillstring. This became a well-established technique for rotary drilling.
With the growing demands of drilling operations, notably for directional, horizontal and extended reach wells, the top drive technology found more and more use among operators. A top drive provides rotation to the drillstring from the top of the derrick and may also move vertically with the drillstring. It enables for the use of pipe stands instead of being limited to the length of a single joint between connections.
The Rise of Square Kelly Systems
The Square Kelly, in traditional rotary drilling, offered a direct mechanical connection between the rotary table and the drillstring. Its square shape was designed to match the internal profile of the Kelly bushing, which allowed the torque to be transferred via the mating flat surfaces while the Kelly could move vertically during drilling.
This architecture was convenient in that the key parts of the system performed well defined functions. The rotary table provided rotation which was passed to the Kelly via the Kelly bushing and then by the Kelly to the drillstring.
The mechanism also provided for the circulation of drilling fluid. As the Kelly was rotated and lowered, drilling fluid could be pushed through the hollow Kelly and into the drillstring.
Transition to Round Kelly Designs
The evolution of contemporary drilling techniques cannot be characterized as a simple shift from Square Kelly to circular Kelly. The most notable modification was the shift from traditional Kelly drive systems to top drive systems.
Top drives changed the way the drillstring was rotated. Rather of sending spin from the rig floor via a rotary table and Kelly, the top drive delivers rotation from above the drillstring. Because the top drive moves up and down the derrick , it may stay attached to the drillstring while drilling . It can also support the drilling of many linked joints , or stands .
This operating concept was especially useful when drilling plans evolved to include directional, horizontal and extended-reach wells.
Advantages of Round Kelly Systems
The first description of "Round Kelly Systems" did not correctly describe the major technical change in rotary drilling. Therefore, a round Kelly should not be portrayed as the straight successor to the Square Kelly in the absence of a particular technical context.
The Kelly driven vs. top drive contrast is more important. The top drive technique enables the drillstring to be rotated continuously and to drill with stands of pipe, which may alter connecting and tripping procedures. These features are particularly valuable for more complicated well profiles.
Advantages and Limitations of Square Kelly Systems
The Square Kelly worked because it fulfilled the fundamental needs of traditional rotary drilling with a reasonably simple mechanical design. At the same time, the technique of operation created limits which grew more relevant as drilling procedures developed.
Advantages of Square Kelly Systems
- Direct torque transmission: The square profile works with the matching Kelly bushing to transfer rotary motion from the rotary table to the Kelly and drillstring.
- Established operating principle: Kelly-driven rotary systems have a long history in conventional drilling operations, with well-defined components and procedures.
- Straightforward system architecture: The rotary table, Kelly bushing, Kelly, and drillstring each have a clear role in the rotation system.
- Integrated fluid passage: The hollow Kelly provides a path for drilling fluid to enter the drillstring during drilling.
Limitations of Square Kelly Systems
The limitations of the Square Kelly are better understood as limitations of the conventional Kelly-driven drilling workflow, rather than as defects caused only by its square cross-section.
- Connection-related interruptions: Conventional Kelly drilling requires the Kelly to be disconnected and reconnected as additional drillpipe is added.
- Limited drilling length per connection: The Kelly itself has a finite working length, so drilling must stop when the Kelly reaches the bottom of its travel and another joint must be added.
- Less efficient for some complex well profiles: The conventional system is less suited to certain directional and extended-reach drilling workflows than modern top drive systems.
- More manual handling steps: Conventional Kelly operations involve additional connection and pipe-handling procedures compared with systems that can drill with stands of pipe.
These limitations do not mean that every Kelly-driven rig is unsuitable for demanding drilling. Rather, they explain why top drive systems became attractive as drilling programs required greater operational flexibility.
Impact on Drilling Operations
The Kelly system provided a dependable way to rotate a drillstring from the rig floor. The operating procedure, however, required the drilling team to cease rotation and establish a connection when the available Kelly travel was consumed. The connection method included pausing the rotary and pumps, disconnecting the Kelly, inserting drillpipe, reconnecting the system, and continuing drilling.
Top drive technology, which enabled the drillstring to be turned from above and drilling with stands of pipe, altered this process. This may eliminate the pauses involved with establishing connections and is one of the reasons why top drives became significant in directional and extended-reach drilling.
Modern Alternatives: Top Drive vs. Kelly Drive Rigs
The most relevant parallel in current terms is between Top Drive and Kelly Drive systems. Both can spin the drillstring, but they are set up mechanically different and enable various drilling procedures.
Top Drive Systems: The New Standard
Top drive systems are widely used in modern drilling because the drive mechanism is suspended from the derrick and rotates the drillstring from above. The top drive can travel vertically along the derrick, allowing the drilling crew to work with stands of connected drillpipe rather than relying only on individual joints.
This design provides several operational advantages:
- Continuous rotation during appropriate drilling operations
- Drilling with stands of pipe
- Improved suitability for directional and horizontal drilling
- More flexible pipe-handling and connection procedures
- Better applicability to extended-reach drilling workflows
SLB notes that top drives are a major improvement in drilling-rig technology and contribute to the ability to drill more difficult extended-reach wellbores.
Comparing Top Drive and Kelly Drive Systems
While top drives are widely used, Kelly drive systems remain an important part of drilling history and may still be relevant where existing rigs, operating requirements, or specific applications support their use.
Top Drive Advantages:
- Rotation is applied from the top of the drillstring
- Drilling can be performed with stands of connected pipe
- Well suited to directional, horizontal, and extended-reach applications
- Can improve the efficiency of connection and tripping workflows
Kelly Drive Advantages:
- Established conventional drilling method
- Clear and relatively straightforward mechanical arrangement
- Compatible with rotary-table drilling rigs designed around Kelly systems
- Proven operating principle for conventional drilling applications
The right choice depends on the drilling rig, well profile, operating requirements, and existing equipment configuration. It is therefore more useful to evaluate the complete drilling system than to judge the Square Kelly by its geometry alone.
The Future of Drilling Technology
Modern drilling technology is constantly evolving, based on better automation, well control, directional precision, data collection and operational efficiency. Top drives are one element of this progress, but other technologies, such as downhole motors and rotary steerable systems, also have a role in today’s directional drilling.
The Square Kelly continues to be significant to the development of conventional rotary drilling because it shows the integration of rotation, fluid circulation and drillstring motion into a proven drilling system.
Today, engineers and buying teams looking for Kelly-related equipment have to look at more than just whether the Kelly is square. The whole design should be examined and the Kelly dimensions, matching Kelly bushing, drillstring connection, rig arrangement and planned drilling application considered.
Conclusion
The Square Kelly was popular because it was a practical, well-established means of communicating rotation from the rotary table to the drill string. Its square design cooperated with the Kelly bushing to transmit torque, but yet let the Kelly to move vertically, while drilling. Its hollow center offered a route for drilling fluid.
The eventual reduction in its employment in many contemporary drilling applications was not only because the square form was replaced with a round design. The most crucial advance was the advent of top drive systems which altered the methods by which the drillstring was spun and enabled drilling operations to function more effectively with stands of pipe.
If you are evaluating a Square Kelly for a conventional drilling rig or need to confirm the appropriate configuration for a specific application, Welong can be contacted for product and technical discussions at oiltools15@welongpost.com.
References
1. Raymond, L. R. Drilling Technology in Nontechnical Language. PennWell Books, 2010.
2. Smith, R. C., and Ravi, K. Advanced Drilling Engineering: Principles and Applications. Gulf Publishing, 2018.
3. Lyons, W. C., Plisga, G. J., and Lorenz, M. D. Standard Handbook of Petroleum and Natural Gas Engineering. Elsevier, 2016.
4. Mitchell, R. F., and Miska, S. Z. Fundamentals of Drilling Engineering. Society of Petroleum Engineers, 2011.
5. Black, A. D., and Wu, B. Applied Well Cementing Engineering and Drilling Practices. Springer, 2019.
6. Economides, M. J., and Nolte, K. G. Reservoir Stimulation. Wiley, 2000.
