NORTHERN NATURAL GAS CO

Incident Narrative

AT APPROXIMATELY, 20:00 ON 1/20/19 A RUPTURE OCCURRED ON THE AUBURN (NEB46001) BRANCH LINE. AT 20:48, THE RUPTURE AT MILEPOST 18.45 WAS REPORTED BY FIRST RESPONDERS TO THE OPERATIONS COMMUNICATION CENTER (OCC). THE RUPTURE IS LOCATED ON THE WEST SIDE OF 619TH AVENUE, NORTH OF HIGHWAY 41 IN SECTION 6, TOWNSHIP 5N, AND RANGE 11E IN JOHNSON COUNTY, NEBRASKA. AT THE INCIDENT LOCATION, INVESTIGATORS OBSERVED A HOLE 15 FEET IN LENGTH AND 10 FEET IN WIDTH EXPOSING THE PIPELINE RUPTURE AT A DEPTH 48"" BELOW GRADE. A FAILURE MEASURING 113"" IN LENGTH ALONG THE 5 O'CLOCK POSITION OF THE PIPELINE OCCURRED ON THE 6-INCH-DIAMETER SEGMENT OF PIPELINE WITH A WALL THICKNESS OF 0.250"". THE PIPELINE IS VINTAGE 1932 DRESSER COUPLED PIPE WITH A SPECIFIED MINIMUM YIELD STRENGTH (SMYS) OF 24,000 PSI. THE PRESSURE AT THE POINT OF THE FAILURE WAS ESTIMATED 535 PSIG, PRODUCING A HOOP STRESS OF 29% SMYS. A DRESSER COUPLING WAS PRESENT, BUT REMAINED INTACT. NO EXTERNAL STRESSES WERE PRESENT THAT WOULD BE INDICATIVE OF THIRD PARTY DAMAGE. CALIPER MEASUREMENT OF THE PIPELINE WALL THICKNESS MEASURED CONSISTENTLY THROUGHOUT THE LENGTH OF THE FAILURE SO CORROSION IS NOT A FACTOR. THERE IS EVIDENCE OF A SEAM FOUND INTACT AT THE 10 O'CLOCK POSITION INDICATING THAT COMPOSITION OF CARBON, MAGNESIUM AND SULFUR ARE NOT EXCESSIVE AS CRACKING ALONG THE SEAM IS MORE COMMON WHEN COMPOSITION ISSUES EXIST. INITIAL INVESTIGATION REVEALED THAT A QUICK-CONNECT ON THE VALVE VRG CONTROLLER AT THE AUBURN TAKE-OFF SETTING SENSE LINE WAS LEAKING GAS. THIS SMALL LEAK CAUSED THE AUBURN TAKEOFF REGULATOR TO MALFUNCTION, RESULTING IN THE AUBURN BRANCH LINE REACHING A PRESSURE OF 535 PSIG PRIOR TO RUPTURE, WHEREAS THE TYPICAL OBSERVED LINE PRESSURE IS APPROXIMATELY 425 PSIG. LOCAL OPERATIONS EMPLOYEES WERE WORKING ON THE CONTROLLER WHEN THE RUPTURE OCCURRED ALLOWING A QUICK RESPONSE. THIS INCREASED PRESSURE IS CERTAINLY A CONTRIBUTING FACTOR TO THE PIPELINE FAILURE, BUT THE PIPELINE SHOULD NOT HAVE FAILED AT 525 PSIG, AS THIS IS LESS THAN THE MAOP PLUS ALLOWABLE BUILDUP. LABORATORY ANALYSIS OF THE PIPELINE WILL BE REQUIRED TO DETERMINE THE ROOT CAUSE OF THE PIPELINE FAILURE.