Bit of good sense

March 7th, 2012 | Posted by in T4W

Geothermal pioneer finds the right bit for bad stone

Mid-State Construction Company in Livingston,Tennessee, 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 said geothermal today no longer needs a spokesperson. This energy-saving strategy for cooling and heating is now 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 evenFt.Knox, where they have completed several projects.

The company 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.

The geothermal loop field was installed alongside the 60-foot excavation for the College of Law's new facility.

Powerful as these rigs are, their production capability relies heavily on the bit at the bottom of the drill string. In the difficult conditions at the Southeastern Tennessee State Regional Correctional Facility near Pikeville, Tennessee, only one bit proved cost effective: an Atlas Copco 5 ¾-inch bit with conical carbide buttons on an Atlas Copco Secoroc QL 50 DTH Hammer.

Hard Time

The correctional facility is a close security prison 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, 5 ¾-inch vertical holes to 500 feet (152 meters).

At this site, the crews encountered extremely abrasive sandstone within only a few feet of the surface, continuing to 480 feet (146 meters). Jason Gentry, project manager for Mid-State, said 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 competitor brands could not complete the holes they started.

Gary Brown, project superintendent, said 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.

Atlas Copco 5 3/4-inch bit with BH66 hemispherical carbide buttons

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 said, 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 8-foot (2.4-meter) 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 6 tons of refrigerant to match this system’s air conditioning capacity BTU for BTU.

Bit better

Mid-State was engaged in several large geothermal projects during the size of the prison expansion work. On-site at Belmont University in Nashville, the company was using the same Atlas Copco 5 ¾-inch carbide bits. This project was for the College of Law’s Randall and Sadie Baskin Center, a 71,000 square foot (21,640 square meter) facility with a five-level, 500-space underground parking garage.

Brown said the project plans called for 100 bores. These were also drilled to 500 feet with T4 and T3 rigs. These conditions provided a strong contrast for comparison.

“This was better drilling than the prison job,” Brown said. “All limestone, no sandstone. Very little water and no voids. We could get 9,000 to 11,000 feet (2,743 to 3,352 meters) from each bit.”

Extended use

Though the bits at the correctional facility have only 500 feet on them so far, Brownsaid the re-sharpened 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.

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