Controlling drill pipe erosion keeps drill steel costs down
Regulations two years ago caused changes in blasting plans that took mild steel drill pipe from 400 hours of life to just 130. Operations such as this in eastern Kentucky have limited their blasthole diameter to 6 3/4 inches and brought their blast pattern in from 18 by 18 to 16 by 16 feet. Reducing bit size, however, completely reconfigured a highly productive drilling formula that had evolved over years of careful engineering and practical experience with driling in these ground conditions. In effect, maintaining regulatory compliance doubled and in some instances tripled drill steel cost per hour drilled.
Regulations two years ago caused changes in blasting plans that took mild steel drill pipe from 400 hours of life to just 130. Operations such as this in eastern Kentucky have limited their blasthole diameter to 6 3/4 inches and brought their blast pattern in from 18 by 18 to 16 by 16 feet. Reducing bit size, however, completely reconfigured a highly productive drilling formula that had evolved over years of careful engineering and practical experience with driling in these ground conditions. In effect, maintaining regulatory compliance doubled and in some instances tripled drill steel cost per hour drilled.
Working alongside their customer, Atlas Copco was asked to come up with a solution to get drill steel cost per foot drilled back down to reasonable levels. Because drill pipe can run so long between changes, it was best to calculate the "drill steel cost per hour" by dividing the total cost of the drill pipe by the total "air compressor hours" logged between installing the pipe and removing it. This provided a reliable and simple basis on which to make a comparison.
When the large blasthole crawlers were drilling with 7 7/8-inch rotary bits on 5 1/2-inch mild steel drill pipe, bailing velocity (BV) was almost ideal at 6,000 feet per minute (fpm).
Putting 6 3/4-inch bits on the same 5 1/2-inch pipe decreased the annulus so much that it double the BV to 12,000 fpm. the increased velocity mashed the sandstone into grit that scoured the mild steel drill pipes so aggressively they lasted only on-third of their normal life compared to when they used to make 7 7/8-inch holes. The tricone bits also suffered in these holes, their inserts shearing in half in the various sandstones. The rate at which drill steel wore down posed a new problem. The gap that quickly widened between the steel and the deck bushing gave dust an escape-way. There was a risk it would not be captured effectively by the rig's dust control vacuum. To operate a rig with uncontrolled dust would put the mine in jeopardy of violating dust level limits. So drill steel had to be switched out frequently to keep that gap tight. The result was that rigs were now going through drill steel three times faster than before. The sole cause was the physics involve in a change of bit size.
Together with an experienced driller, Ron Johnson, Atlas Copco's Regional Sales Manager for the 11-state eastern U.S. coal region, performed a complete drill audit, which included an air compressor capacity test. This audit showed that there was much more air than required for adequate bailing, due to the smaller annular area. One solution was to decrease the drill steel diameter, so they replaced the 5 1/2-inch steel with 5-inch. But this created its own problems. The 5-inch drill steel was not as rigid. It's flexing caused drill string vibration and chatter. Inefficient drilling will also tend to shorten tool life. To address the air issue, rather than "choke" the compressor intake to reduce the volume, they not only regulated it down but vented excess volume up the mast to decrease air volume in the hole. As finely tuned as they could get it, the sandstone still rapidly eroded their drill steel.
That's when Johnson suggested TEAMALLOY drill steel. If this test was successful, almost every coal company in the eastern U.S. would be keenly interested in TEAMALLOY. So far it has. While mild steel lasted only 130 hours, the TEAMALLOY drill steel was indistinguishable from new at 300 hours. Approaching 400 hours, the steel was starting to show only minimal wear. The wear was controlled and predictable, showing the embedded alloy wear strips slightly more prominently than when new. Johnson predicted they would likely see 500 hours before they change it out.
It would seem that Atlas Copco has found a successful formula. While drilling, TEAMALLOY pipe offered greater stabilization and perfect rotation. Dust is down. Drill steel cost per hour is down. Production is up. And Atlas Copco Secoroc's TEAMALLOY drill steel is living up to its billing; that it would beat mild steel in cost per hour in any application, anywhere, every single time.
