Collaboration and innovation key to managing subsidence and keeping coal flowing
Anglo American’s Capcoal team has achieved substantial cost saving and put real innovation into action through a new way to manage subsidence by a longwall panel of the Grasstree mine.
As part of the normal mining process that occurs more than 100 metres below ground level, longwall mining can gradually cause the surface above to subside as the mine progresses and extracts the coal seam, which can be three to four metres in height.
Six panels of the Grasstree longwall mine were located beneath Anglo American’s Lake Lindsay Overland Conveyor, which transports coal from Lake Lindsay to the Coal Handling Processing Plant (CHPP) at Capcoal, as well as the main 66KV power-line servicing the town of Middlemount. Usual practice to manage longwall subsidence of this nature is to relocate the above ground asset, resulting in time delays, potentially up to 12 weeks, and substantial additional cost through the alternate long-haul transport of coal to the CHPP while the conveyor is stood down.
Anglo American adopted a new approach to manage the first of six longwall panels to avoid the conventional extensive delays and additional costs. After an extensive risk management process, it was decided to keep the conveyor in operation and to make real time adjustments as underground mining progressed and subsidence occurred. To bring the conveyor back into alignment once predetermined tolerances were exceeded, 55 bespoke Line-stand Adjustment Frames (LAFs) were designed. The LAFs were modular in construction due to the four different foundation types that existed, including both concrete piers and sleepers.
Project Manager Chris Ryan said Anglo American’s technical group led by John Soar with the assistance and support of site colleagues developed and engineered innovative plans that considered all the risks while giving stakeholders confidence that all surface infrastructure could be left in-situ while mining was carried out underground.
“The process involved being able to engineer and install adjustable supports to the conveyor to ensure it remained within its operating structural integrity tolerances so that the belt did not dislodge.
“The system was based on a subsidence event in the Hunter Valley some 30 years ago with a similar rope conveyor system and to our knowledge subsiding above ground infrastructure had never been undertaken in Queensland in this way before,” Chris said.
A total of 690 metres of overland conveyor was impacted with a maximum of 1800mm vertical movement measured during longwall mining. The conveyor was stood down 29 September to 15 October in preparation for the subsidence and was handed back to Capcoal CHPP operations on 28 October.
“These innovations significantly reduced our cost exposure while turning potential major issues into well risk assessed and managed solutions,” Chris said.
The exercise has strengthened Anglo American’s relationship with contractors and asset owners, particularly Ergon Energy, who owns the 66KV powerline, which not only crossed the underground panel, but also crossed the overland conveyor.
Working collaboratively with Ergon Energy by involving them early in the planning and ensuring they were part of the process allowed Anglo American to avoid relocating the powerline.
Anglo American’s Manager Infrastructure, Energy and Carbon Scott McEwan said an agreement between the two parties resulted in Anglo American contributing a fraction of the amount it would have if the line was relocated to cover costs to stabilise five power poles during longwall mining of the first panel.
Scott confirmed that Anglo American approached Ergon earlier this year to discuss their mining activity plans which were set to start in September.
“We presented Ergon with a plan to manage the issues related to the longwall project,” Scott said
“A group of managers and engineers from Ergon’s Major Customer group reviewed the subsidence plan and reports and agreed that the risk to their network could be managed while assisting us as their customer to reduce our costs,” he said.
“Anglo American and Ergon have worked incredibly close throughout the process to date and as part of the agreement we will continue to do so.”
The key to the innovation of this project will be in the repetition of the process during the remaining five panels which will be mined between 2014 and 2019. Although each panel will present its own challenges, each time the process is repeated with the additional learnings and knowledge, the time and cost savings are likely to increase, resulting in an effective and efficient system resulting from innovative problem-solving by a multi-disciplinary Anglo American team.
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