NORTHERN NATURAL GAS CO

Incident Narrative

ON JULY 1, 2016, A NORTHERN EMPLOYEE INADVERTENTLY OPENED A MAINLINE BLOW DOWN VALVE RELEASING 9.7 MMCF OF NATURAL GAS TO ATMOSPHERE. THE EMPLOYEE WAS ASSIGNED TO CALIBRATE THE FIRE AND GAS DETECTION SYSTEMS ON THE TWO TURBINE UNITS IN BUILDING 1 SO THE BUILDING HAD BEEN PLACED IN BYPASS MODE. BYPASS MODE PREVENTS THE EMERGENCY SYSTEM FROM PERFORMING A BUILDING SHUTDOWN OR AN EMERGENCY SHUTDOWN WHILE THE TECHNICIANS WORK ON THE GAS DETECTION SYSTEMS. WHILE THE EMPLOYEE WAS TESTING THE FIRE AND GAS DETECTION SYSTEM ON UNIT 2, THE DISPLAY SHOWED ALARMS FOR GAS DETECTION ON UNIT 1 WHICH WAS UNEXPECTED. SHORTLY AFTER SEEING THE VISUAL ALARM, THE EMPLOYEE HEARD THE VENTING OF GAS OCCURRING OUTSIDE THE BUILDING, WHICH WAS NOT EXPECTED, AND ASSUMED AN EMERGENCY SHUTDOWN HAS COMMENCED. THIS INITIAL VENTING WAS CAUSED BY AN ERROR IN THE BYPASS MODES CODE, WHICH FAILED TO PREVENT THE VENTING OF GAS WHEN THE GAS DETECTION HEAD ON THE UNITS DETECTED GAS. IN RESPONSE TO HEARING THE VENTING OF GAS, THE EMPLOYEE LOOKED ON ANOTHER VISUAL DISPLAY WITHIN THE BUILDING TO CONFIRM THE ALARMS SIGNALED AN EMERGENCY SHUTDOWN, THEN PROCEEDED OUTSIDE TO ATTEMPT TO MANUALLY CLOSE THE CLOSEST OF THE THREE SHUTDOWN VALVES. THE VALVE, HOWEVER, WAS CLOSED AND THE EMPLOYEES ACTIONS OPENED THE VALVE CAUSING ADDITIONAL GAS TO VENT. 12/3/20 UPDATE - CORRECTED PART G APPARENT CAUSE TO ERROR IN THE PROGRAM VERSUS VALVE POSITION.