A Bit of Good Sense
A visit with a geothermal drilling firm, Mid-State Construction Co. in Livingston, Tenn.
Mid-State Construction Co. in Livingston, Tenn., is a geothermal pioneer, having entered the market back in the early 1990s. Mid-State CEO and founder Johnny Coleman had grasped geothermal’s significance early on to become an outspoken proponent of the technology. However, Coleman says geothermal today no longer needs a spokesperson. This energy-saving strategy for cooling and heating now is sought after both in new construction and in retrofitting.
Mid-State’s geothermal reputation keeps its drilling division almost continuously in the field on large-scale installation projects, often on multiple sites simultaneously throughout its five-state area of operation. The jobs include high schools and colleges, federal, state and municipal buildings, and even Fort Knox, where the company has completed several projects.
Mid-State has exactly the right equipment to maintain uninterrupted production at each of its concurrent projects. Of its 125-piece fleet, 30 units are drilling rigs. Among the larger machines are 12 Atlas Copco T4W rigs and a T3W. As powerful as these rigs are, their production capabilities rely heavily on the bit at the bottom of the drill string. In the difficult conditions at the South-eastern Tennessee State Regional Correctional Facility near Pikeville, Tenn., one bit proved cost-effective – an Atlas Copco 53⁄4-inch bit with conical carbide buttons on an Atlas Copco Secoroc QL 50 DTH Hammer.
The correctional facility is capable of housing 971 inmates. Operating since 1980, the prison is undergoing an expansion that will allow it to house another 1,444 inmates. Mid-State was contracted to create a geothermal loop field consisting of 580, 53⁄4-inch vertical holes to 500 feet.
At this site, the crews encountered extremely abrasive sandstone within only a few feet of the surface, continuing to 480 feet. Jason Gentry, project manager for Mid-State, says the complex formation was “a mix of every color of sandstone in the book – red, pink, white – with three water seams.” The water seams were found to run consistently throughout the field. The sandstone ate up most bits so quickly that some could not complete the holes they started.
Gary Brown, project superintendent, says the company did have success with diamond bits. While the diamond bits could each manage two holes before they were exhausted, they were not cost-effective compared to the Atlas Copco carbides in these conditions. Mid-State purchased 600 of the bits for this project.
Bit by Bit
Atlas Copco T4 rigs started each hole with a new bit, drilling to total depth. Then drillers retrieved the bit, sharpened it, and set it aside for later use in projects with less difficult conditions.
Up to five T4 rigs were in use at a time during the year-long contract. As the T4 rigs were drilling, their electronics simultaneously logged the seams and water voids found in each bore. This precise information allowed grouter Jacob Collins to stop exactly at the voids to pack them with gravel. Otherwise, Collins explains, he’d lose his grout. This would waste untold amounts of the thermally-enhanced mix used for this application, since the seams ran throughout the breadth and width of the loop field.
The loops were connected by eight-foot trenches, and then run to an exchange house. Two large pipes exiting the rear of the exchange house will permit auxiliary air conditioning or heating if ever the geothermal loops need to be turned off.
To give an idea of how much energy relief the geothermal system can provide, an auxiliary unit would need up to six tons of refrigerant to match this system’s air-conditioning capacity.
A Bit Better
Mid-State was engaged in several large geothermal projects during the prison expansion work. On-site at Belmont University in Nashville, Tenn., the company was using the same 53⁄4-inch carbide bits. This project was for the College of Law’s Randall and Sadie Baskin Center, a 71,000-square-foot facility with a five-level, 500-space underground parking garage. Brown says the project plans called for 100 bores. These also were drilled to 500 feet. These conditions provided a strong contrast for comparison. “This was better drilling than the prison job,” Brown notes. “All limestone, no sandstone. Very little water, and no voids. We could get 9,000 feet to 11,000 feet from each bit.”
Though the bits at the correctional facility have only 500 feet on them so far, Brown says the resharpened bits will see additional duty in Mid-State’s quarry projects.
At first glance, such frugality over a bit may seem excessive, considering the size of the projects and the expense of the rig and hammer. In the increasingly competitive geothermal market, however, these two projects prove Mid-State’s tooling choice to be … a bit of good sense. ND