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28 Spring 2019 By using light detection and ranging (lidar) technology to save time during the inspection process, TechCorr can increase the value it provides to tank operators. Each inspection begins by identifying at least eight stations around the circular tank shell. The API 653 standard calls for a maximum distance of 32 ft (9.7 m) between stations. Large tanks with diameters of more than 300 ft (91 m) may require more than 30 stations. The stations are defi ned in diametrically opposed pairs and serve as the basis for measurements on the tank fl oor and shell. The traditional approach to the measurement work begins by using a rotating laser level to measure the fl oor. At each station, an inspector walks along a radial line from the shell to the centre of the tank, measuring the fl oor elevation at 1 ft intervals. Each reading is recorded by hand. To check the shell, they use a surveying total station to measure each station, where a series of points is captured on a vertical line from the bottom of the shell up to the roof. The inspector may need to set up the total station in several different locations to gather the required information. In the offi ce, the handwritten measurements are entered into spreadsheets to produce fi nished elevations for the points on the tank fl oor. Technicians review the numbers to check fl atness and look for any settlement or depressions. A similar process handles the total station data, which is analysed to detect any bulges or departure from vertical. The results are plotted in pairs of opposing stations. As well as being time-consuming and tedious, manually recording data introduces the potential for error both in the fi eld and offi ce, so crews must check their work thoroughly. In the offi ce, even using customised spreadsheets, data processing usually takes a few hours before the results can go to CAD operators to prepare fi nished drawings. Shifting gears on tank measurement Things change when lidar, also known as laser scanning, is used for tank inspections. Before starting the scanning, an inspector walks the area to identify optimal locations for the scanners. Generally, four scanner setups are required to capture a tank interior, although larger tanks may require additional setups. TechCorr uses Trimble TX5 or TX8 3D scanners for the work. The instruments are fully automatic and can measure over 1 million points per second. Each scan takes about 12 min. and covers a 360˚ area around the scanner. The measurements capture the tank fl oor, shell, roof and any piping or structures. The result is a point cloud of tightly spaced 3D points; each point is measured to an accuracy of a few millimetres. Compared to manual measurements, the scanner cuts the time in the fi eld by 75%. While the savings in the fi eld are signifi cant, the larger payoff comes in the offi ce. Data from the scanner can be downloaded directly to Trimble RealWorks software. The software then combines (or 'registers') the multiple scans into a single large point cloud. To make viewing easier, the cloud can be colourised using photographs taken by the scanner or colour information captured with each point. After cleanup to remove extraneous points – the scanner's rapid measurement often captures other technicians and their equipment in the area – the fi nished point cloud is ready for analysis. Figure 1. A finished point cloud provides a detailed view of a storage tank. Laser scanning is used for both interior and exterior scans. Figure 2. Storage tank exterior with comparison to a vertical cylinder reveals deformation.

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