Abstract: 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.