NORTHERN NATURAL GAS CO

Incident Narrative

NORTHERN IS IN THE PROCESS OF ABANDONING THE M520A MAINLINE BETWEEN OGDEN, IOWA AND VENTURA, IOWA. IN PREPARATION FOR ABANDONMENT, TOWNS SERVED OFF OF THE A-LINE ARE BEING MOVED TO A PARALLEL MAINLINE. ON JULY 23, 2020, MANHATTAN PIPELINE, THE CONTRACTOR WORKING FOR NORTHERN, WAS CAREFULLY EXCAVATING THE TAKE-OFF VALVE ASSEMBLY THAT SERVES THE TOWN OF DUCOMBE, IOWA. UNKNOWN TO THE CONTRACTOR, THERE WAS AN ABANDONED 2-INCH TAKE-OFF VALVE JUST 4 FEET DOWNSTREAM (NORTH) OF THE IN-SERVICE TAKE-OFF VALVE. THE CONTRACTOR DID NOT HIT OR STRIKE THE UNKNOWN, ABANDONED VALVE. ABOUT 3:30 PM, DURING THE EXCAVATION PROCESS, THE COMBINATION OF FORCES FROM THE HAND-DIGGING EFFORTS AND RELEASE OF OVERBURDEN CAUSED THE ABANDONED, 2-INCH-DIAMETER, 1931 VINTAGE, BRANCH CONNECTION AND VALVE ASSEMBLY TO FAIL, RESULTING IN BLOWING GAS. THE FAILURE APPEARS TO BE IN THE THREADED CONNECTION BETWEEN THE NIPPLE WELDED TO THE A-LINE AND THE VALVE. THE ABANDONED VALVE AND NIPPLE WERE SENT TO A LABORATORY FOR FURTHER ANALYSIS. THE M520A LINE WAS OPERATING AT APPROXIMATELY 238 PSIG AT THE TIME OF THE EVENT, AND ULTIMATELY VENTED 4.8 MMSCF OF NATURAL GAS. THE COMPANY HAS INSTITUTED AN EXPANDED PROGRAM OF RECORDS RESEARCH IN AN ATTEMPT TO LOCATE THESE ABANDONED FACILITIES TO PREVENT REOCCURRENCE. ONE CONTRACT EMPLOYEE WAS INJURED WHILE ESCAPING THE EXCAVATION. HE SUSTAINED A BRUISE TO HIS RIGHT WRIST AND ELBOW. HE WAS EXAMINED AND RELEASED FROM THE LOCAL HOSPITAL IN FORT DODGE, IOWA. UPDATE 9/29/20: METALLURGICAL ANALYSIS INDICATED THE ROOT CAUSE WAS THE RESULT OF DEFECTIVE WELDING ,EXTERNAL FORCES BENDING AND FLATTENING THE DAMAGED NIPPLE, AND POSSIBLY DEFECTS IN THE VALVE BODY. PHOTO OBSERVATIONS IN THE REPORT ALSO NOTE SIGNIFICANT CORROSION OF THE THREADED SECTION OF THE VALVE. WHILE IT APPEARS THERE WAS SOME EXTERNAL FORCE TO HAVE CAUSED THE BENDING AND FLATTENING, THE CORROSION OF THE THREADED PIPE NIPPLE IN THE ""AS RECEIVED"" CONDITION WOULD INDICATE THAT THE EXTERNAL FORCE OCCURRED SOMETIME IN THE PAST AND ONCE THE THREAD WAS DEFORMED AND EXPOSED TO THE ENVIRONMENT, CORROSION INITIATED. WITH THE POOR QUALITY OF THE FORGED VALVE FITTING THE CORROSION OF THE THREADS COULD HAVE BEEN RAPID.