A majority of pipeline failures are caused by external corrosion. Failures in pipelines that carry oil result in oil spills that cause widespread environmental damage and economic losses for the oil companies, pipeline operators, and communities through which pipelines pass. Failures in pipelines that carry natural gas can cause far more damage. In extreme cases, thinning of the pipeline wall from corrosion can cause the pipeline to explode. A critical part of keeping our pipeline infrastructure safe is frequent and accurate inspection.
The images below were taken with the Seikowave eVox LCG 3D scanner. The image on the left is a 2D image of a section of pipeline that was inspected. The image on the right is a 3D image. The areas of corrosion are apparent in both images. A critical part of determining the severity of the corrosion is the calculation of the metal loss.
In order to calculate the metal loss, the nominal surface of the pipeline must be determined. To do this, the 3D data is fit to a cylinder; the 3D data is used to determine the best fit cylinder. The nominal surface of the pipeline is the surface of the cylinder. The metal loss is calculated by determining the volumetric different between the location of the surface of the cylinder and the remaining metal, as determined from the 3D data of the pipeline scan. The pipeline inspection industry has identified three approaches for calculating the metal loss. These approaches are referred to as Level One, Level Two, and Level Three. The approach described above is the Level Three calculation and is generally considered to be the most accurate. The Level One and Level Two calculations can be derived from the data used in the Level Three calculation. The Pipeline Research Council International, or PRCI, is an excellent source for detailed material on this topic. Please refer to the PRCI web site.
In the above image, the section of 3D data taken on the pipeline is shown fit to a cylinder. The cylinder serves as the nominal pipeline. From the 3D data, the radius of the pipeline as well as the metal loss can be determined. For this pipeline, the deepest corrosion pit was 1.6mm and the metal loss was 3,350 cubic millimeters.