Feature: Round Numbers
Professional Surveyor Magazine - June 2012
An energy company measures a large hydrocarbon storage tank using both 3D scanning and total stations. The resulting data reveals interesting facets of the tank’s shape and behavior, as well as 3D scanning’s benefits.
By Jean-Paul Rey
The petroleum industry subjects its facilities to strict controls and measures for safety reasons. This is certainly the case with the large, circular hydrocarbon storage tanks situated near refineries or terminals belonging to the group Total, one of the largest integrated petroleum and gas companies in the world.
As part of their inspection process, Total
conducts measurements to determine deformation of the tanks, which can be as large as 100 m (328 ft) in diameter with heights ranging between 15 and 25 m (50 and 82 ft). The work is conducted by Total’s TEC/GEO department, which is responsible for geometric integrity of petroleum infrastructure at the core of Total’s exploration/production sector.
“To measure a tank, we first perform a series of measurements on the empty tank. We then repeat the measurements at different stages of filling—at the quarter, half, and three-quarter levels as well as filled—and conclude with a new series of measurements of an empty tank,” explains Arnaud Vidal, engineer-topographer at TEC/GEO. “We then analyze the collected measurements and compare them with the admissible tolerances for the different codes of construction.” The operation is repeated on average once every three years.
“Besides the verticality of its walls, it was important to know the roundness of the tank,” Vidal said. “The tank has a floating roof, which rises and falls according to the amount of hydrocarbon in the tank. So we must ensure that it cannot be blocked at any point—that is, the tank must be truly round and not oval.”
Speed and Precision
Because of the size of the tanks and the need for precise measurements, TEC/GEO initiated a test project to develop techniques for measuring and analyzing the shape of the tanks using the Trimble
FX 3D phase-shift scanner. The results of the measurements were not intended to substitute for the work of a team of surveyors, nor were they used to verify the integrity of the tank. The primary goal was to evaluate the precision of the measurements obtained by the 3D scanner and compare them with traditional measurements made using a total station.
The test project was carried out on storage tank T7707 at the plant in Lacq, France in April 2011. T7707 is a steel tank fed by a pipeline connected to a terminal on the Atlantic coast. The tank is relatively small, with a diameter of 20 m (66 ft) and a height of about 15 m (50 ft), and during the test the tank was half full. Vidal was aided by an intern, Jean-Baptiste Geldof, a topography student at The National Institute of Applied Science of Strasbourg in France.
Scanning the Tank
It took Vidal and Geldof half a day to collect the data. They set up the scanner at ground level only a few meters away from the tank. From each location, they scanned the visible portion of the tank, making full-height scans to capture the entire structure. This required a total of eight successive positions for the scanner, with a sufficient overlap of points between the scans. Some of the targets had unknown coordinates, while others carried georeferenced coordinates. Spherical targets and flat targets printed on paper were placed on the tank and its supporting walls. To prevent oblique views and exceptional reflections, a scanning overlap of 50% between the different stations was necessary.The scanning was conducted to obtain a resolution of approximately one point every 2 cm (0.8 in) on the walls of the tank. Because of the tank’s close range and curved shape, the actual spacing ranged between 1 to 3 cm (0.39 to 1.18 in). This resolution was sufficient for the project and avoided the need to scan each spherical target individually. Each scan required about 15 minutes, including one or two denser scans on the flat targets.
Complementary scans of higher density were made in the zone at the foot of the tank where larger deformations were suspected. The spherical targets were used primarily for the consolidation of the point cloud. The flat targets were used to realign the completed point cloud in a three-dimensional system and for georeferencing of all the measured points.
Analyzing the Results
In the office, Vidal and Geldof used Trimble RealWorks software to process the data obtained by the scanner. The software enables the user to register and manipulate an as-built scene of point-cloud data and provides tools for visualization and analysis. Prior to consolidating the eight scans, the target positions and scanner stations were calculated using the method of least squares. The residuals of adjustment on the target and station positions were less than 2 mm (0.08 in).
When the adjustment and point cloud were complete, Vidal carried out several analyses. He took several primary approaches, including
- analyzing the data/georeferencing of the several-million cloud points;
- analyzing the verticality and roundness of the walls of the tank; and
- studying the correlation between the differential decline of the annular bottom plate and the deformations of the walls.
Vidal also compared the scanning results with data collected using a Trimble VX Spatial Station. The team used the VX as a conventional total station, collecting points and vertical profiles at discrete locations around the tank.
The scanning method of measurement enabled Vidal and Geldof to georeference the measured points in accordance with recommended procedures and to attain the desired precision of measurement. Using the processing software, the team could create horizontal cross sections and compare them against true circles at the same height. They could also create vertical lines at any location and compare measured points against the true vertical line.
Their analysis detected an ovalization of the walls at a height of 6 m (20 ft) above the foot of the tank. Those deformation values were within the admissible tolerance range. The scanner measurements also revealed a correlation between the nominal roundness of the walls and the leveling of the tank base. The analysis concluded that the ovalization of the tank was caused by controlled subsidence of the base, still within acceptable tolerances.
It became apparent that high-precision measurement is needed for determining the differences in height that define the base tilt. The precise data is needed to estimate the roundness that can result from subsidence or slight tilts of the base. Another correlation was found between the differential declines of the annular bottom plate and deformations of the walls on the same vertical line.
The measurements taken by the scanner provided millimeter precision. The detected deformations were only several centimeters, the majority being completely logical and the others in compliance with the set tolerances. The scanner and resulting point cloud allowed cross sections and vertical profiles to be taken at any location on the tank. By contrast, measurements using a conventional total station can collect vertical profiles at only a few locations around the tank.
“Possible important deformations may exist between the vertical profiles measured using the total station, and we would not detect them,” Vidal explained. “We are very interested in prioritizing use of the 3D scanner, whose advantage is the great density of the points that can be obtained from it. This enables us to measure elements between the vertical profiles that are not covered by the total station.”
Vidal believes that TEC/GEO can use the 3D scanner to measure during the construction of new storage tanks. The data will create an internal repository of the original dimensions of the tanks. He recommends the use of scanning on existing storage facilities, as well.
Vidal sees scanning and total stations as complementary. “At some of our facilities, laser scanning technology is not yet systematically present,” he said. “In these locations where high accuracy is required with no need for high point density, we use a total station and focus on potential deformations that foreseeably would have been observable with a scanner during construction of the tank.” In this case, more detailed and systematic measurements will be necessary, providing full proof that total stations and 3D laser scanning each has its place in today’s industry.
Jean-Paul Rey is a freelance journalist and translator living in France. Prior to doing freelance, Rey was chief editor for 10 years at the two largest communications consulting agencies in Paris.
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