Sensor Innovation Drives Breakthrough in Mineshaft Safety Inspection
With regular mineshaft inspection mandatory in every major mining nation, an automated system based on cameras, laser scanning and inertial sensors promises substantial savings and improvements in safety.
Mineshafts are the lifeline to underground mines. Shafts and hoisting systems provide access to a network of openings used to access the underground resource, typically base and precious metal ore deposits, diamonds or coal, to carry out vertical transportation of miners and materials required for operations, and serve as a conduit for ventilation and cooling. They also provide an emergency exit.
Mineshaft inspections are therefore not only vital to ensuring the safe and reliable operation of the mine and its shafts, but are a legal requirement in many countries including: US, Ukraine, Russia, South Africa, Germany, Canada and Australia. Every mineshaft must be closed for a thorough visual inspection, typically once a week, (although the frequency varies according to local legislation), with each visual inspection taking between 4–12 hours to complete, depending on the depth of the shaft. This results in large losses in revenue for the operator of the mine, due to production being interrupted.
Moreover, visual inspections are often performed by engineers equipped with headlamps and torches in what would otherwise be complete darkness. The potential for human error is high, not just due to the lack of light, but because performing the inspection is arduous and fatiguing. Unfortunately, serious mine shaft accidents including fatalities still occur, some of which are caused by human error exacerbated by the fact that operations are conducted in a potentially dangerous and high risk environment.
Aiming to provide a much safer and more efficient alternative, Canada-based Sight Power Inc.. is working with Shaft Sinkers Holdings and Parsec in South Africa to develop an innovative automated mineshaft inspection technology. Based on laser scanning and high-resolution photography, the Mobile Shaft Scanning System (MS3) reduces the time it takes to perform an inspection from hours to minutes, providing far better accuracy than the human eye and enabling earlier detection of defects in shaft infrastructure.
Shaft Complexity
The mineshaft is a complex engineering structure. From a geometrical and topological point of view, it can be described as a cylinder with a diameter of between 5–20 meters and a depth of anywhere between 100–3,000 meters. There are a variety of objects to be found inside the shaft: vertical guides (rails for conveyances), pipes of various diameters, horizontal steel beams (buntons), fixing brackets, cables, and other constructional elements. Meanwhile, the mineshaft’s lining is normally constructed from reinforced concrete or cast-iron ‘tubbing’ rings. In addition, each shaft will have one or more stations, which are the intersections with the various mining levels.
Figure 1. MS3 unit in lab sensor located on scanning head.
Figure 2. Increase in mine’s revenue (based on only 50 % substitution of human visual inspection).
To ensure safe and uninterrupted mining operations, all of the shaft construction elements must be kept in good working condition. For instance, the guides must be perfectly aligned. Given that the vertical speed of conveyances such as elevators and 50-tonne skips can reach a velocity of up to 20 meters per second (m/sec), it is often the case that the bolts securing the guides to the steelworks can work loose. This can cause deviations in the guides that may result in conveyances becoming jammed or substantial damage to the shaft or its construction elements.
Misalignment between the guides and the conveyance may also occur due to movement in the shaft lining. The latter can be caused by rock pressure or failure of the supports, corrosion of buntons or guides, mechanical damage to steelworks due to rock falls, or simply due to dynamic interaction between the conveyances and the guiding system.
In order to check for any of the above, the conventional procedure is to lower engineers down the shaft in the elevator at a speed below 1 m/sec. Frequent stops are necessary to allow them to visually examine sections of the shaft in more detail, take measurements, and record any defects they find. With gold mines now having shafts deeper than 2000 metres in a single lift, it is not uncommon for an inspection to take up to 12 hours to complete – which means a substantial disruption to the mine’s production.
LiDAR Innovation
By automating mineshaft inspections, the use of Sight Power’s MS3 will mean a dramatic reduction in the time taken to complete them, as well as making them more accurate – improving safety substantially. MS3 is based on LiDAR (Light Detection and Ranging) technology, and combines three core elements – Lasers, Global Positioning System (GPS), and Inertial Navigation Systems (INS) – into a single platform. This combination allows the positioning of the footprint of a laser beam as it illuminates an object to a high degree of accuracy and is capable of acquiring spatial data to produce accurate Digital Elevation Models (DEMs).
DEMs provide a 3D representation of a terrain’s surface and are created from the billions of data points acquired by a Li- DAR scanning device. Typically, these devices are mounted on a mobile platform such as a satellite, aeroplane, or automobile, with GPS providing accurate geographical information and an INS recording the precise orientation (pitch, roll and yaw). Li- DAR systems are widely used to collect spatial data for DEM building, with applications ranging from construction and road maintenance to city management and planning. Street mapping is one well-known example.
The principle behind LiDAR is simple: shine a small light at a surface and measure the time it takes to return to its source. With distance and location information accurately determined, the laser pulses from a scanning device yield direct 3D measurements, creating a ‘point cloud’ of data that is then interpreted by sophisticated software in order to build a DEM or, in the case of sub-surface structures, a Digital Spatial 3D model. Sight Power’s integrated software models for example, allow the visualisation of complex spatial geological structures and components while enabling the discovery of new structural patterns.
LiDAR systems are ideal for mineshaft inspection, because the typical distance from the scanning device to objects within the shaft barrel is only 3–4 meters, meaning that an extremely dense point cloud can be created and much higher accuracy and lower variance achieved. However, the fact that GPS signals are not available underground presented the MS3 development team with a significant challenge in terms of obtaining the highly accurate geo-reference position data required.
Inertial Sensors Solve Underground Challenge
When the MS3 system was originally conceived, it was assumed that the technology would be fairly similar to conventional terrestrial scanning units. But without GPS, a variety of positional aids are required to assist the INS, including accelerometers and inclinometers. The development team also had to design a much more sophisticated system capable of achieving measurements to within 5 mm accuracy, compared with 20– 40 mm for any terrestrial system.
To reduce the risk of failure while proving the concept and testing the first prototype unit in a real mining environment, the MS3 hardware designers specified only the most reliable and precise instruments and system components, opting for Sherborne Sensors’ inclinometers and accelerometers. These inertial sensors are currently being employed to characterise and understand the mineshaft environment so that the Sight Power team can refine the MS3 system and its design for optimal accuracy and reliability.
During the laboratory test phase the sensors assisted in mapping and characterising the horizontal test setup performance, as well as indicating any flex, twist and tilt in the MS3 mechanical housing system. The movement of the MS3 system carriers – such as mine cages, skips and counterweights – will vary from normal speed to very slow during the inspection cycle. The accelerometers will measure the performance of the carriers with respect to vibration and acceleration during normal operation, as well as during inspection cycles.
Meanwhile, the high-precision inclinometers will measure the amount of roll and pitch of the carriers to give an indication of whether or not the movement down the shaft is well controlled using the shaft guide tracks. In the future, Sight Power aims to determine whether using clusters of accurate but lower cost sensors such as inclinometers and accelerometers can eliminate the requirement for the INS, which would lower the overall cost of the MS3 system.
Preventing Production Disruption
Ultimately, the objective is to use MS3 to enable quick and unobtrusive shaft scanning, with the device attached to the conveyance and moving up and down the shaft with an average speed significantly higher than that possible during manual visual inspections. A realistic 3D image of the entire shaft with all internal infrastructures (construction elements) can then be rendered by software.
Shaft engineers will be able to import the 3D image into a software package such as AutoCAD, or use Sight Power’s software to analyse any horizontal or vertical crosssection, measure sizes, distances, diameters and angles and compare them to the original construction measurements. The initial 3D model of a mineshaft will provide an accurate view of current shaft conditions, with subsequent scans then compared against it to determine all changes within the shaft and their severity and to identify any elements of the shaft infrastructure that have been affected. All change detection algorithms are part of Sight Power’s Shaft Inspector™ software.
MS3 will also have the ability to take digital photos by means of a stroboscopic, highresolution digital photographic module and then combine them with the LIDAR data to create more realistic 3D images. When analysing these 3D images, engineers will be able to immediately determine and highlight areas of concern – such as ground movements, cracks in lining, misalignment and deflections of pipes, buntons and guides.
With miners depending on the uninterrupted and efficient flow of materials, there is a compelling business case for Sight Power’s MS3. Assuming that it is accepted by the mining industry and regulatory authorities, it has the potential to reduce production stoppage times by at least 50 percent annually. It would also mean less man hours are spent underground in hostile environments.
Furthermore, by halving the number of visual inspections, MS3 would enable more available hoisting time and consequently significantly increase the mine’s revenue and profitability (Figure 1).
Crucially, engineers will be able to detect defects at a much earlier stage, cutting shaft maintenance repair costs and downtime and enabling engineers to address any issues before they become a major safety hazard and operational issue.
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