The dragline, one of the largest pieces of mobile machinery in the world, is a key piece to modern surface coal mining operations. To put the size of these machines into perspective, consider BNI Coal’s “Liberty” dragline. Liberty, which became operational in 2004, consists of a 355-foot boom, is driven by not one – but 14 – motors that can use 12,600 horsepower at one time, and has a 77 cubic yard bucket that removes overburden to reach lignite coal. The massive size and scale of these machines is almost unimaginable. When considering these enormous dimensions – one may ask how a dragline is able to be moved substantial distances within a mine?
In 2015, BNI Coal’s Center Mine doubled the amount of land under permit available to mine from 9,000 acres to 18,000 acres. This move prompted BNI Coal to relocate Liberty and its smaller 736 dragline to new areas of the mine. The additional land was located about four miles south of Milton R. Young Generation Station. It required six years of planning, permitting, and infrastructure development that took place from 2009-2015 for this revamp of the Center Mine Operation. BNI Coal also extended its coal supply agreements with its customers through 2037 to ensure this move would allow the mine to sever enough coal to make the move cost-effective.
Transporting pieces of equipment like these was an unprecedented situation. Typically, the draglines are able to “move” at a very slow pace – seven feet per minute to be exact. This is done with a “walking” apparatus that uses camshafts and gears which are connected to two mechanical “shoes.” After deciding that this process was an extremely sluggish and inefficient option for an eight mile move – BNI partnered with Dutch-company Mammoet, who specializes in the heavy lifting and transport of large machinery.
Moving the draglines on “roller-skates” also was a viable option versus “walking” the machine due to the amount of wear and tear it saved – which proved to be large. Brac Mettler, Dragline Maintenance Planner at BNI provided many examples of why this served to be true: “It would have been comparable to several years of walking all at once. That would have meant rebuilding both propel cases and replacing large bushings and other pieces at a significant cost, well into the seven-figure range. Also, the wear on the bottom of the tub from the friction against the ground would have taken years off the metal and life of the tub – and we would have had to walk the machines down our main haul road.” Doing this would have been a very complicated process as Mettler further explained, “Walking these machines meant we would have had to remove the gravel from the road along with the crown on the road to make it flat enough for the dragline. To do that on such a long stretch of road would have been very costly. And again, if the dragline had broken down on the haul road it would have impeded our ability to deliver coal to the plant.”
With the help of Mammoet, a strategy was developed where both of the draglines would be transported by a wheeled vehicle. The vehicles that were used for this transport are known as “self-propelled modular transporters” (SPMT), with this particular one being a 36-axle trailer, and consisting of 708 tires.
Many difficulties arose throughout the whole moving process – as the move took place during a time with frigid temperatures, transmission lines had to lowered, and the Dragline had to cross Hagel Creek. Through all of these hardships though, Mettler found jacking/raising the machine to be the most difficult. “Since we hauled the machine with the boom in the air, the most difficult part was making sure that we had the correct center of gravity for the machine to jack/raise it safely without causing any damage to the machine.”
Once the dragline was on the “roller-skates” it took eight hours (over the span of four days) to fully complete the move from the time it was initially loaded on the trailer. During the move, the process was subjected to waiting a day with no movement, due to the lowering of the Bison Transmission Line. When asked if the draglines had moved a substantial distance like this since then, Mettler explained that 736 Page, the small dragline, has been “walked” short distances to different pits and one different area – but nothing compares to the original transport.
When reflecting on the success of the project, Mettler said he was just honored being there and being involved in the project itself. “To complete the entire project of jacking, hauling, and lowering the two machines with no significant injuries to anyone or damage to equipment was a big accomplishment considering the weather, different vendors and employees involved and all of the moving pieces. It’s not something that is done every day, and certainly not at BNI. It was amazing to see the machine in the air and to be able to walk under the tub of a machine that weighs around 10 million pounds.”