NORTHERN NATURAL GAS CO

Incident Narrative

AT APPROXIMATELY 6:30 PM ON MARCH 19, 2014, RESIDENTS AND LOCAL EMERGENCY RESPONDERS REPORTED A STRONG GAS ODOR NEAR THE INTERSECTION OF ASHBURY AND TERRA GLENN COURT IN EAGAN, MN. FURTHER INVESTIGATON BY OPERATOR PERSONNEL DETECTED NATURAL GAS ALONG CURB LINES AND IN MANHOLES. NORTHERN NATURAL GAS WORKED WITH LOCAL DISTRIBUTION COMPANIES TO RE-ROUTE GAS AROUND THE 12-INCH PIPELINE WHILE REPAIRS WERE COMPLETED. NINE HOMES AND ABOUT 18 PEOPLE IN THE NEIGHBORHOOD WERE EVACUATED AND 5 OF THOSE HOMES HAD THE ELECTRICITY SHUT OFF. THE HOMEOWNERS WERE AWAY FROM THEIR HOMES OVERNIGHT WHILE PIPELINE CREWS RE-ROUTED GAS. THE PIPELINE WAS REPAIRED WITH A CYLINDER OF PIPE AND RETURNED TO SERVICE WITHOUT INCIDENT ON MARCH 23, 2014. THE PIPELINE HAD AN 8-INCH LONG CIRCUMFERENTIAL CRACK NEAR A GIRTH WELD. THE CRACK RESEMBLED A TEAR IN THE PIPE BODY. THE SECTION WAS SENT TO A LAB FOR ANALYSIS. THE PIPE JUMPED WHEN CUT TO BEGIN REPAIRS SO WE BELIEVE THE PIPE WAS IN A BIND. THAT IS, THE PIPE WAS BEING FORCED TO CHANGE DIRECTION (SLIGHTLY) WITHOUT A FITTING OR BEND. UPDATE: LABORATORY ANALYSIS: THE CRACK APPEARS TO BE CAUSED BY BENDING FORCES ON THE PIPELINE. THE LAB REPORT INDICATED THAT THE FAILURE OCCURRED AT DORMANT CIRCUMFERENTIAL SCC CRACKS. IN THIS CASE, IT WAS NOT THE GROWTH OF CIRCUMFERENTIAL SCC THAT RESULTED IN FAILURE. IT WAS HIGH BENDING/TENSILE STRESSES THAT CAUSED OVERLAOD WHERE THE CRACKS WERE PRESENT. THE LAB SITED LOWER THAN NORMAL TEMPERATURES (LOWER PIPE TOUGHNESS DUE TO LOWER TEMPS AND HIGH TENSILE STRESS DUE TO PIPE CONTRACTION) CONTRIBUTING TO THE FAILURE. SCC WAS NOT THE ROOT CAUSE OF FAILURE. IT WAS HIGH BENDING/TENSILE STRESS COUPLED WITH LOWER THAN NORMAL TEMPERATURES.