JOHANNESBURG (miningweekly.com) − Gold Fields has completed the world’s tallest steel headgear, standing at 87 m, at its South Deep Twin Shafts complex, near Westonaria, in South Africa.
The 1 900 t structure, which used 100% South African made materials, has steelwork value of about R40-million and was installed by South African steel provider Steel Services and Allied Industries.
Gold Fields contracted Steel Services to supply, fabricate, corrosion protect, deliver and erect a new A-Frame steel headgear over the existing concrete vent shaft headgear. Gold Fields believes that the new headgear will contribute towards extending the life of the mine by well over 50 years.
The design and engineering of the new headgear was done by Hatch. Overall project management of the project was the responsibility of the South Deep vent shaft deepening team.
The new headgear is part of the conversion of the South Deep existing vent shaft into a production shaft that will add another 195 000 t/m rock hoisting capacity for the mine. This, together with the existing main shaft rock winder, will contribute to achieving the mine’s target of gold production of 750 000 oz/y by end 2014.
Steel Services and Allied Industries group MD Lawrence Bartlett states that, owing to the expected heavy loading on the steelwork, a decision was taken to construct the structure in 1,2 m x 1,2 m x 45 mm box sections.
The fabrication and trial assembly of the box section headgear was completed at the Steel Services workshops in Carletonville, and transported the short distance to the South Deep Mine south of Westonaria, where final assembly was carried out in preparation for the headgear erection
The majority of the fabrication was completed in 2010 and the site erection was completed in two stages over the December 2010 and April 2011 shutdowns.
Automatic submerged arc welding procedures was used in the fabrication of the box girders and knuckle joints. All steel was preheated prior to welding to ensure the highest standards in quality and strength were met. All welds were also fully tested. Extensive use of jigs in the fabrication process was used to ensure that the required assembly tolerances were maintained.
“The existing headgear is still in operation and is still required in the daily operations of the mine, which means that destroying it and rebuilding the headgear was also not an option. The only other viable option was to construct a new headgear from steel and erect it over the top of the existing headgear,” Bartlett adds.
The Gold Fields project team and contractors faced a number of challenges, such as erecting the entire structure from the west side as access from the north, south or east was not possible. However, with the assistance of the largest mobile cranes in South Africa, the engineers were able to overcome this challenge.
The project also included special considerations. The size of the knuckles used on the structure meant these had to be rotated into special positions on abnormal load trucks to fit under road bridges between Carletonville and the site.
The legs of the structure, up to the 44 m level, had to be propped up against the concrete headgear by made-for-purpose structures, which remained until the first level could be installed. The propping steel was anchored against the existing headgear and suspended from cables tethered to the top of the headgear.
The company used automatic submerged arc welding procedures in the fabrication of the box girders to ensure uniform and sustainable quality standards. All welds were also fully tested.
Steel Services appointed design, draughting and detailing company Cadhouse Detailers for the workshop detailing, Omnistrut for additional fabrication and Sarens South Africa for the heavy lifting and rigging portion of the project.
Bartlett states this was Steel Service’s largest project to date, with over 5 000 bolts being installed to erect the structure without needing the use of a cutting torch. “We are proud to be a critical part of the project team that successfully supplied, fabricated and installed the tallest, if not largest, steel headgear in the world,” he concludes.
The company has now nominated the project for the Southern African Institute of Steel Construction Steel Awards, which will take place in October, this year. It will compete in the category for excellence in the use of structural steel.
The Gold Fields headgear structure replaces the world’s former tallest steel headgear structure, which is 81 m tall, at mining and metals company Konkola Copper Mines’ Konkola Deep mine, in Chiliabombe, Zambia.
The third tallest structure in the world also belongs to South Africa. Shaft 17 headgear of Impala Platinum near Rustenburg is 81 m tall.