apparatus and methods of repairing an in - service pipeline , tank , and / or vessel are provided . generally , a metal arc welding under oil process employing an automated metal arc welding setup with a continuous wire feed is utilized . the process may be used in connection with a smart pig to perform in - situ internal repairs of in - service pipelines , tanks , and / or vessels . for example , a pipeline pig or other device is contemplated that employs an internal power supply , a navigation system , and a metal arc welding under oil system that is able to travel vast distances within a pipeline to reach pipeline segments that are either buried underground , under highways , or underwater making access very difficult . once at its desired location , the pipeline pig performs in - situ welding or other internal repairs to the in - service pipeline . the disclosed apparatus and methods provide great flexibility to the repair of pipelines , tanks , and / or vessels .