Surveyingtechnical

24 PositionIT – Jan/Feb 2011

Surveying on the Medupi Power Station project

Information from Optron Geomatics

Confirming that structures are built correctly is a key activity in any construction project. At a new power station in South Africa, Trimble technology was used to make quick work of a tall order.

The Medupi Power Station in Limpopo Province is owned and operated by Eskom and

will provide reliable electric power to customers in Limpopo Province.

The coal-fired plant uses supercritical boiler technology to operate at higher temperatures and pressures than conventional boilers. Medupi will have six generating units and overall capacity to deliver 4788 MW of power to the South African grid. To reduce demand for water, Medupi will utilise dry-cooling technology as part of its generation cycle. On completion in 2015, this plant will be the largest dry-cooled plant in the world and will provide greater efficiency and improved utilisation of coal and water compared to similar plants that utilise older, conventional technologies.

Covering 883 hectares, Medupi’s size and complexity called for expert surveyors. Trail Surveys in Pretoria was selected to serve on the project’s quality control team, and represent Eskom in all survey matters on the site. “We conduct verification surveys on work done by surveyors from the various construction subcontractors working on the project,” said Trail Surveys CEO Philip Schalekamp. “We also do contour and detail surveys, volume verification, height analysis on earth works and blasting, as-built surveys and other ad hoc surveys and reports.”

One of their tasks was to verify the construction of the lift shaft for the boiler on Medupi’s Unit 6. The boiler lift shafts, which are key structures in a generating unit, are erected early in the construction of each unit. Not only must each shaft be in the correct location, it must also be constructed to specified dimensional and vertical

Fig. 1: Scanning the north face of the lift shaft and the surrounding concrete piles.

Fig. 2: Checking the control network for the next day's scan. From left: Danie Roelvert, Francois Marias and Sean Dane.

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tolerances. The concrete shaft at Unit 6 is roughly 7,6 m x 7,6 m and 120 m tall. To verify that it has been built to plan, Trail Surveys combined precise survey and laser scanning technologies.

The first step was to establish control for the scanning. The survey crew used a Trimble S8 Total Station and Trimble CU Controller to establish four new instrument stations around the lift shaft at Unit 6. They made multiple measurements to each new station from Medupi’s network of geodetic control pillars. The fieldwork for the control took less than half a day. The team downloaded the measurements to Trimble Geomatics Office Software and completed the calculations to produce coordinates for the new stations. With the control in place, the scanning work began.

Using the survey workflow in the Trimble system, the surveyors set up their Trimble GX 3D Scanner on the new points and oriented the scanner into the project control network. From each point, they scanned an entire face of the lift shaft. Trimble PointScape Software “was an important part of the work,” said surveyor Danie Roelvert. “We were able to limit the scans to only the areas of interest, which speeds up the work in the office.” The Trimble SureScan technology allowed the crew to “achieve a more regular grid of points,” he said, “which was

Fig. 3: 3D point cloud with comparison surface overlay of the lift shaft.

a tremendous help in cleaning and processing the data.”

Each setup required roughly one hour, and all of the scanning work was completed in about four hours. To avoid the congestion (Medupi employs more than 8000 construction workers) as well as heat shimmer and dust, the team conducted the scanning work during early morning hours from 3h00 to 07h00.

In the office, it took about four hours to process the scans using Trimble RealWorks software. The registration of the scans went quickly, with residuals as small as 8 mm. The technicians filtered the point cloud to produce a 10 cm spatial grid on each face of the shaft.

With the scanning results in place, the surveyors used the Surface to Model and 3D Inspection tools of Trimble RealWorks to compare the measured data to the lift shaft’s design. Each face of the shaft was analysed independently as a complete 120 m surface, and also broken into 15 m sections. By colour coding the differences between the design and as-built shaft, it was easy to identify deviations from the design.

The Trail Surveys team delivered the data to Eskom in Trimble RealWorks format. By using Trimble RealWorks Viewer, Eskom engineers could inspect the results and access the data for further analysis. Trail Surveys also provided colour

maps of each face, plus cross sections and 3D views of the shaft. Schalekamp said that his clients were pleased with the deliverables. “The scan is very helpful to analyse each side of the shaft at any given point,” he said, “and also to visualise the shaft in general. The ‘heat plot’ made the data user-friendly so that any person can understand the report.”

According to Roelvert, the project could not have succeeded without the combined Trimble technologies. The Trimble S8 Total Station provided accurate control, and the Trimble GX took over from there. “If we had used only a total station, we would only be able to determine whether the top is within tolerance,” Roelvert said, “and that would have required a surveyor to access the top of the shaft to place a prism. The scanner allowed us to analyse all four sides of the structure, from bottom to top. There were no access issues as the shaft is remotely surveyed. In terms of positioning, you can see exactly what happened during construction.”

Contact Sean Dane, Optron Geomatics, Tel 031 566-6221, sdane@optron.com