NORTHERN NATURAL GAS CO

Incident Narrative

ON AUGUST 16, 2023, NNG GAS CONTROL RECEIVED ALARMS OF LOW PRESSURE AND ZERO FLOW AT MULTIPLE STATIONS IN THE SEMINOLE SUBSYSTEM, UPSTREAM OF THE KERMIT, TX, COMPRESSOR STATION. THE GAS CONTROLLERS CONTACTED CUSTOMERS IN THE AREA CONFIRMING THE PRESSURE AND FLOW DEVIATIONS. NNG OPERATIONS COMMUNICATIONS CENTER CONTACTED THE FIELD OPERATIONS TEAM ON-CALL TECHNICIAN TO INVESTIGATE THE ALARMS. NNG WAS NOTIFIED BY LOCAL AUTHORITIES OF BLOWING GAS AND FIRE NEAR COYANOSA, TX. THIS CONFIRMED THE INCIDENT WITH NNG GAS CONTROL. NNG OPERATIONS COMMUNICATION CENTER INITIATED AN ALL-CALL TO THE KERMIT, TX, FIELD OPERATIONS TEAM TO RESPOND TO THE EMERGENCY. NNG GAS CONTROL COMPLETED SHUTTING-IN ALL RECEIPT AND DELIVERY POINTS. THE FIELD OPERATIONS TEAM CLOSED BLOCK VALVE 3 WHICH COMPLETED THE ISOLATION OF THE AFFECTED SEGMENT. FIRST RESPONDERS CLOSED TWO ROADS IN THE PROCESS. THE FIRE FROM THE FAILURE WAS REDUCED TO BURNING RESIDUAL GAS WITHIN 75 MINUTES OF THE SEGMENT BEING ISOLATED. BASED ON LAB RESULTS, THE PIPELINE RUPTURED INITIALLY IN APPROXIMATELY THE 6 O'CLOCK POSITION IN AN AREA, ABOUT A FOOT LONG, OF SEVERE, MOSTLY CIRCULAR, PITTING AND CORROSION. THIS PITTING BEARS ALL THE HALLMARKS OF MICROBIOLOGICALLY INFLUENCED CORROSION (MIC), A PROCESS WHERE ONE OR SEVERAL STRAINS OF BACTERIA CORRODE STEEL. THE MATERIAL HAD BEEN CORRODED TO APPROXIMATELY 0.041"" WALL THICKNESS AT THE RUPTURE ORIGIN, APPROXIMATELY 12% OF THE ORIGINAL MATERIAL THICKNESS. FURTHER EXAMINATIONS OF THE MATERIAL DID NOT LOCATE ANY OTHER FAILURE MECHANISM.