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thumb|A cleaning pig for a oil pipeline. The blue plastic disks seal against the inside of the pipe to propel the device and to remove loose sedimentation or scale buildup. The black rectangles at the top and the circular disks in the center are magnets to attract and remove any loose metal objects in the pipe. thumb|An Ultrasonic testing|ultrasonic leak-detection pig. thumb|A cleaning pig for a oil pipeline. The wire brush encircles the shaft and scours the interior of the pipeline. thumb|A scraper pig shown at the Trans-Alaska Pipeline System Visitors Center