NORTHERN NATURAL GAS CO

Incident Narrative

WE BELIEVE THAT ON APRIL 9, 2014 AT ABOUT 10:00 A 3-INCH RELIEF VALVE ON THE TXM15501-M880B PLYMOUTH TO SPRABERRY MAINLINE BEGAN VENTING GAS. THE RELIEF VALVE IS LOCATED ON MAINLINE BLOCK VALVE IDENTIFIED AS BBB01 BETWEEN TWO MAINLINES; M880B AND M883B. THE PIPELINES ARE ALSO PROTECTED BY A CONTROL VALVE WITH PRESSURE OVERRIDE AND SLAM VALVES THAT THE ATLAS PIPELINE MIDKIFF AND WEST TEXAS GAS (WTG) SONORA / BENEDUM RECEIPT POINTS. THIS IS A VERY REMOTE AND UNATTENDED LOCATION. ON APRIL 11, 2014 AT 05:30 THE RELIEF VALVE VENTING WAS IDENTIFIED BY AN ATLAS EMPLOYEE WHILE PERFORMING ROUTINE MAINTENANCE ON THEIR EQUIPMENT NEARBY. THE VENTING WAS SO SMALL THAT SCADA DID NOT DETECT ANY NOTICABLE DROP IN PRESSURES. THE NATURAL GAS VENTING OVER A 3 DAY PERIOD MADE THIS EVENT A REPORTABLE INCIDENT. THE PIPELINE HAS A 550 MAOP AND THE RELIEF VENTED AT 545 AND NEVER COMPLETELY RESET. THE CAUSE OF THE RELIEF VALVE VENTING HAS NOT BEEN DETERMINED. **UPDATE** THE DOWNSTREAM COMPRESSOR STATION (SPRABERRY) HAD CONDUCTED ITS' ANNUAL ESD TESTING AND WAS NOT COMPRESSING. DELIVERIES WERE STILL BEING MADE TO THE PIPELINE SYSTEM AND THE PRESSURE BEGAN TO RISE. FORTUNATELY, THE 30-INCH PIPELINE IS NEARLY 100 MILES LONG SO PRESSURES INCREASED SLOWLY. AS MENTIONED ABOVE, THE RELIEF WAS SET BELOW THE PIPELINE MAOP AND BEGAN TO WEEP. FURTHERMORE, THE RELIEF VALVE IS AN OLD STYLE AND DID NOT RESET IMMEDIATELY, THEREBY CAUSING GAS TO EXPEL TO ATMOSPHERE FOR SOME TIME. ***ADDITIONAL INFO*** AFTER DISCOVERY, THE RELIEF VALVE WAS TESTED AND RESET. IT IS STILL SET BELOW THE ACTUAL MAOP OF THE PIPELINE. THE MAOP IS ""GRANDFATHERED"". THE PIPELINE IS UNDER A SELF-IMPOSED PRESSURE RESTRICTION CONSIDERING OPERATING HISTORY AND KNOWN CORROSION. THE RELIEF WAS LAST INSPECTED AND TESTED BY MORRIS FIELD SERVICES, LLC ON MARCH 25, 2014. NORTHERN PERSONNEL WILL VERIFY SCHEDULED COMPRESSOR STATION OUTAGES AND CUSTOMER SHUT-INS DO NOT ADVERSELY AFFECT PIPELINE FLOWS. ***UPDATE***THE PIPELINE WAS NOT OPERATING UNDER A SELF-IMPOSED PRESSURE RESTRICTION. THE MAOP HAS BEEN 550 SINCE 2004. UPDATED PART E. ***MORE INFO***THE PRESSURE OVERRIDE SET POINTS FOR FLOW AND PRESSURE CONTROL VALVES WERE RESET TO SLIGHTLY UNDER MAOP. THE SLAM VALVES WERE ADJUSTED TOO. NORTHERN HAS NOT EXPERIENCED ANY FURTHER PROBLEMS OR ISSUES AT THIS RECEIPT POINT.