Retrofitting an aging power plant is far from easy, even in the best of circumstances. Before work can begin, existing conditions must be accurately documented. Using traditional tools such as clipboards, tape measures and 2D layouts to capture information is time consuming, fraught with risk, and prone to errors and missed information. High-speed 3D laser scanners with intuitive software have revolutionized this process over the last decade, but some projects require more versatility than a dedicated laser scanner can provide. In these applications, surveyors and engineers have sought innovations that will streamline their workflows and minimize risk while capturing the precise data they need to ensure client satisfaction.
Achim Hoffstiepel, technical director of engineering surveying for Heitkamp Ingenieur- und Kraftwerksbau GmbH, along with team members Marvin Diehl (M.Sc.) and Dennis Alius (B.Eng.), are well acquainted with these challenges. The Germany-based surveying and engineering firm specializes in building and modernizing power plants, cooling towers, sewage plants, high-rise buildings, industrial structures, locks and bridges, with an eye toward continued innovation. When Limak Holding, the owner of the Hamitabat gas power plant in Turkey, approached the firm with a project related to a performance upgrade of the 1,200 MW plant, Hoffstiepel and his team readily agreed.
The project involved determining the radii of the inner and outer shells of two existing cooling towers at an interval of every 2 meters (6.6 feet) and comparing them to the planned radii to identify whether the cooling towers had deformed over the years due to the static load, the long period of use and the influences of weather. For Heitkamp Ingenieur- und Kraftwerksbau, it was a relatively straightforward assignment except for one major obstacle: the height of the cooling towers required them to be scanned from a distance that the firm’s dedicated 3D laser scanners could not bridge. Hoffstiepel and his team would have to find a new approach.
Integrated Laser Scanning Goes the Distance
The two cooling towers stand nearly 450 feet tall (135 meters) with a diameter that varies between 220 feet (67 meters) at their narrowest point and more than 330 feet (101 meters at their base). Capturing enough detail on the towers would require scanning from a sighting distance of nearly 1,000 feet (300 meters) away. Hoffstiepel knew of only one solution capable of meeting these requirements—the Leica Nova MS50 MultiStation. “With the Leica Nova MS50, sighting distances like this are no problem at all,” he explained.
The team created a local network for both cooling towers with survey and scanning positions specified so that each cooling tower could be completely mapped. In all, four scanning survey points were required outside each cooling tower and three inside it. Within this network, the team was able to guarantee a stable orientation from every survey point with the Nova MS50 using a “free station” method. Since the Leica Nova MS50 is a total station with integrated scanning capabilities, the team was able to handle both the network measurements and laser scans with the same instrument.
“The advantage of the Nova MS50 is that it supports all the workflows of a total station,” said Hoffstiepel. “We were thus able to use the ‘free station’ method in the specified network and then scan right away from each defined position.”
The mapped point cloud was directly available in the local coordinate system, which eliminated the need to register the scans from the individual survey positions into a superordinate coordinate system. The surveying team selected a scan point grid with a medium spacing of 5 cm. Since the multistation scans up to approximately 1,000 points per second, the scanning time for each survey point was about two to three hours. This means that the team was able to capture about 46 million points for each of the two cooling towers—more than enough density to accurately calculate their radii.
Streamlined Workflows Save Time
The team exported the complete point clouds of both cooling towers from the Nova MS50 to Leica Cyclone 8.1 using an XML file. In Leica Cyclone, the point clouds were first cleaned up and then the inner shell of the cooling tower was digitally separated from the outer shell to enable the radii of the inner and outer shells to be calculated separately. The team then exported the cleaned and separated data from the cooling tower shells so that the 232 radii could be estimated using additional software.
As a result of the chosen scanning density, about 3,000 points were available to use for each radius estimate, which enabled the radii to be calculated with very high accuracy. Comparing the calculated actual radii to the planned radii showed the deviations between the plan and the current condition to be approximately 2 cm on average, which was within tolerance.
The innovative method provides a new solution for mapping and analyzing large engineering jobs.
“The Nova MS50 was the optimum solution for this job because its range exceeded that of conventional laser scanners,” said Hoffstiepel. “The direct linking of the individual scan positions saved valuable time when post-processing the scan data, as the scanned point cloud was already directly available in the defined coordinate system using the ‘free station’ approach.”