The big pull

A massive salvage operation lasting 17 months finally saw a 2,000t walking dragline restored to like-new condition and put back to work, after it fell more than 15m into the pit it was excavating. Landing upside down, the machine looked like a total write-off until an ingenious and unique procedure, devised and employed by engineers of Minserco Inc, the field erection division of Bucyrus International Inc, achieved the seemingly impossible, and rolled it back on its feet.

The dragline, a Marion model 7820 with 93m of boom and a 32m3 bucket, started work in mid 1994 at the limestone quarry of Southdown Cement Company (now Cemex) at Wampum, West Pittsburg, Pennsylvania USA, after an erection period lasting eight months. It formerly worked at a coal mining operation in Kentucky. The dragline operates in strips, removing approximately 25m of overburden to uncover valuable limestone. The overburden is cast into the previously mined-out pit alongside.

The mishap occurred at 5am on 24 September 1999, when the bench supporting the dragline suddenly gave way while the machine was swinging a loaded bucket toward the dumping point over the pit. The machine rolled completely upside down as it tumbled, injuring two crew members who later recovered in hospital. When the dust settled, the once proud giant excavator looked like a monumental scrapping job, but after costing out all alternatives for resuming stripping operations and carefully analysing the recovery plan, management made the brave decision to salvage the machine and put it back to work. The ingenious recovery operation hinged on a 'simple' roll-back idea using the pull from a bevy of motor scrapers tied to a multiple sheave block arrangement attached to the upturned machine.

After excavating around the machine and removing the boom, the next step was to fabricate seven sheave support assemblies and attach them to the dragline. Welded to the dragline's deck and tub, they tied both together to form a rigid unit. They not only provided a secure anchor for the sheaves, but also maintained alignment of the tub, swing rollers, and centre pintle during the big pull.

Meanwhile, at the edge of the pit some 122m from the upturned dragline, two multiple sheave block assemblies, mounted on heavy duty fabricated pedestals, were erected. They were each threaded with a 2,130m long, 50mm diameter wire rope to a similar number of sheaves on the dragline. To ensure everything stayed in place, steel beams were welded to the sheave assemblies and tied back to two old dragline buckets. These were positioned approximately 8m below ground level and then buried with compacted earth. This arrangement provided a secure anchor for the sheave blocks during the tremendous pull required to right the toppled dragline.

Eight twin-engined scrapers, comprising Caterpillar 627 and 637 models, provided the pulling power. Their 16 engines provided traction to the 32 driving wheels. Grouped in two teams of four, the scrapers were all hooked to the long cables and, when everything was ready, signalled to pull in unison. The crumpled dragline gradually rolled upright as it yielded to the tremendous pull from the multiple-part ropes and the teams of scrapers. Further excavation and more pulling levelled the machine ready for its rebuild. But first it had to be moved to a site offering better access for repair crews. Before the move, the propelling unit on one side of the machine had to be replaced as it was broken off during the fall. Then, using power from a portable generator mounted on a trailer, and the dragline's own motors, the crumpled remains of the dragline minus house and boom, walked 2.4km to the chosen repair site. Some extra ballast was required at the front of the machine to achieve the correct balance during walking.

The operator's cab, walking shoes and tub had survived relatively unscathed. The only damage to the cab was a window broken by the operator to escape after the accident but the rest of the machine required major rebuilding. The machinery house and mast were totally destroyed, and some of the rotating machinery, including the motor-generator sets, needed replacing. Components were salvaged as far as possible, including boom foot and point assemblies, but much of the original machine had to be scrapped. Lifting equipment to assist in the re-erection of the Marion 7820 dragline included two Manitowoc cranes, models 4100 and 888, both with a rated lifting capacity of 208t. After several months of hard work, and a newly-fabricated machinery house, the machine gradually took on the appearance of a dragline again.

While the big machine was down, a much smaller 14-yard Marion 7400 that the quarry had previously used, and in fact almost sold, was fired up and worked to maintain some production. Says quarry manager Jim Houk: 'We won't be selling that machine for some time yet. It really saved the day during our ordeal.'

On 26 February 2001, some 17 months after the Marion 7820 dragline was almost totally destroyed, it went back to work looking like a new machine. With shiny new paint, rebuilt motors, and many new components, the machine has been brought back to life. During the period the dragline was out of service, management also took the opportunity to rectify a few minor problems, and make some modifications to the machine that would not have been attempted under normal circumstances.

Says Houk: 'We now have a dragline that's better than the one we had before. It's digging well and doing a good job for us. We were very impressed with the work done by Minserco. The resurrection plan and salvage operation were entirely in their hands. We've also tightened up our working procedure to everyone's mutual benefit!'