Because of their size, wind turbines are transported in individual parts to their offshore setup destination. Because individual components are manufactured at different sites, they are often put together for the first time at the mounting site on high seas. Therefore, it is necessary to verify and control the dimensions of the connections and mounting surfaces of the tower and foundation independently before mounting.
In this article we will look at how an optical coordinate measuring machine (CMM) was used to guarantee a smooth setup of offshore wind turbines by checking the dimensions of the mounting surfaces and bolts.
For this application a TRITOP CMM was used. This instrument, manufactured by Capture 3D, is a portable optical measuring system that precisely determines 3-D coordinates of specified object points (see figure 1). The gauging points are easily marked with self-adhesive or magnetic markers before the measuring process (as seen in figure. 2).
Figure 1: TRITOP CMM measuring system, photogrammetric camera with accessories
Figure 2: Consumables, such as self-adhesive and magnetic markers
The measuring object is then captured with a digital camera from different viewing angles. Based on all captured 2-D images the computer automatically calculates the 3-D coordinates of the gauge markers by means of bundle adjustment (see figures 3 and 4). Two certified scale bars guarantee the accuracy and process security of the measurement result.
Figure 3: Scheme of bundle adjustment of three camera positions
Figure 4: 3-D coordinates in the TRITOP CMM software measured from a 2-D image set
Primitives such as cylinders, holes, spheres, border lines, etc. can be measured by using corresponding adapters.
The entire system consists of a camera case, laptop, and scale bar case, and can easily be carried by one person. The measuring process as well requires only one operator. Because data acquisition is carried out with a camera and data evaluation takes place on a laptop, measurement projects can be recorded and inspected without an external power supply.
For the setup of offshore wind turbines, special concrete foundations are built on land. They are more than 20 meters high and have a diameter of approximately 10 meters on the upper platform. After completion, the foundations are tugged to their destination and anchored to the sea bed so that the upper edge of the platform rises two to four meters above the water surface (see figure 5). Because of the height of the wind turbine, the tower consists of several segments. These parts are also transported by boat to their offshore destination. The tower is then constructed on site using a special floating crane for heavy loads. For mounting the lowest tower segment, 120 steel bolts are recessed in the concrete foundation. These mounting bolts are positioned in two circles with a diameter of approximately four meters, as shown in figure 6.
Figure 5: Offshore anchored concrete foundation
Figure 6: Position of the mounting bolts in the concrete foundation and preparations for the measurement
Because foundations and tower parts are produced at different places, a test setup ashore is not always possible. If the position of the mounting bolts in the foundation does not fit the hole pattern in the lowest tower segment, time-consuming adjustments have to be made on site. In this case the tower mounting on high seas is extremely difficult, if not impossible. Because the floating crane is very expensive, such a situation causes extremely high additional costs and a large time lag.
To avoid such problems, the position of the mounting bolts in the concrete foundation has to be verified to the required accuracy of one-tenth of a millimeter. The measuring system enables the verification of the bolts directly after the completion of the foundations ashore as well as after the anchoring on high seas as seen in figure 7.
Figure 7: Measurement and inspection of 120 mounting bolts with the TRITOP CMM system on high seas
Harsh conditions such as the limited space and measuring distance on the concrete foundations, strong wind, or the lack of power supply are no obstacles at all for reliable measuring using the optical CMM.
Improperly positioned or tilted mounting bolts in the foundation can be identified quickly and clearly with the required accuracy. Thus, it is possible to eliminate problems before starting the mounting process.
For the measuring process, gauging markers and scale bars are placed on the foundation. Customized adapters, which were made from angle profiles, are used for inspection of the 120 bolts (see figure 8).
Figure 8: User-defined TRITOP CMM adapters for measuring the mounting bolts
The measuring area is then captured by taking several pictures from different directions and viewing angles, as shown in figure 9. During the measuring process the images are automatically transferred to the laptop. Immediately after the image transfer, the 3-D coordinates of all 120 mounting bolts are calculated within a few minutes on the basis of the unique dot pattern on the adapters. As the project is already evaluated on site, it can be immediately checked for missing data.
Figure 9: Measuring bolt placement
Depending on the definition of the adapters, single points or the axis of a mounting bolt can be calculated to determine the position or the direction of each single mounting bolt. The software allows the creation of measuring reports as well as the export of measuring data into tables (see figures 10 and 11).
Figure 10: Measuring report with position of mounting bolts and detail of geometric dimensioning and tolerancing (GD&T)
Figure 11: Report as tables (Excel, HTML)
The preparations for the measurement take less than 30 minutes and measurement itself about 10 minutes. Thus, inclusive of the transport by boat to the next platform, about one hour is required for each foundation.
The portability of the optical TRITOP CMM measuring system allows precise measurement and verification of mounting bolts and surfaces using CMM inspection. By this method, possible problems caused by incorrectly positioned or tilted mounting bolts can be identified and eliminated before assembly.
Due to the simple measuring process, the inspection of more than 120 mounting bolts per platform can be accomplished by a single person. The data evaluation directly on site guarantees that all features are captured completely and correct.
Using optical measuring technology for inspecting mounting surfaces ensures a smooth setup of offshore wind turbines, preventing high additional costs, and saving time.
Capture 3D would like to thank Zebicon, Denmark, and ITA, Poland, for the trust in its measurement technology and the professional realization of this project.
Capture 3D provides 3-D digitizer, surfacing, laser scanning, and dimensional inspection services. It also designs, manufactures, and supports 3-D scanners, and automated robotic and motion control systems. Capture 3D’s high-resolution, optical scanner, ATOS, delivers 3-D measurement data for industrial components. Instead of measuring single points, full part geometry is captured in a dense point cloud or polygon mesh describing the object’s surface and primitives precisely. Capture 3D is headquarted in Costa Mesa, California.
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