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Pony-Sub Walking Box Rig

Higher efficiency with lower ground-bearing pressure.

In 2011 Shawn Smith and Harlan Smith traveled to Houston, Texas to discuss a new project with a major client. They were in the early stages of developing a new box-on-box rig with a new side saddle design that was intended to walk well-to-well from the start. By orienting the rig boxes to be in line with the well row, and orienting the pipe handling perpendicular to the well row, the rig would be much more efficient moving well-to-well.

Additional goals included integrating the walkers into the lower rig boxes, keeping the controls extremely simple, and making the system robust. The brand new Entro King-Pin Walking System would fit the bill perfectly. The client needed a system that was fast, but they also needed the ground bearing pressure to be carefully managed.

The Ground Bearing Problem

Entro Industries conducted an engineering study on how ground-bearing loads are transmitted through rig mats and into the ground. The study showed that the ground-bearing pressure depends on several factors, including the shape of the walking foot and the orientation of the rig mats. Entro found that by rotating their rig mats perpendicular to the rig and by using Entro's Kingpin Walking Feet, the customer would be able to reduce the ground bearing pressure and stress in their rig mats to less than 50% of competing designs.


By 2013 the project was in full swing, and Entro got the call to begin fabrication of not only the walking machinery, but also the level 1 pony-sub that supported the rig. The project required delivery of one complete walking system per week. By optimizing their production processes, Entro Industries was able to deliver every system on time, helping keep their customer on schedule.


The customer has been using their Entro walking systems throughout the U.S. with no issues. The typical startup involves 1/2 days of commissioning checks and training, and the pony sub design keeps the installation simple.