API RBI - Methanation Unit North Dakota

Background

• Methanation Unit in North Dakota.
• Initial RBI assessment completed in 2010.
• Cracking found in stainless steel piping downstream of final reactor multiple times since 1999.
• Metallography of the piping showed transgranual-branched cracking similar to Chloride Stress Corrosion Cracking, but no chlorides were noted to be in the process.

RBI Analysis

• Literature research showed similarity to Process Water Cracking found in the nuclear industry.
• Due to the similarity in the morphology of the damage, Chloride cracking was assigned to the equipment upstream and downstream of the piping that had failed.
• No previous cracking inspections had been completed on these items.

Results

• The combination of the consequence of failure and high probability, since no previous inspections were complete, showed the items equipment items to be high risk.
• The API RBI software recommended an ‘A’-level cracking inspection.
• The inspections were completed in during an outage in 2011 and cracking that required repairs was found in one of the vessels.
• If the vessel would have failed during normal operating, the facility would have lost an approximate $3.7 million in lost production costs and a potential serious safety event.