NORTHERN NATURAL GAS CO

Incident Narrative

AT APPROXIMATELY 22:45 ON 29 DECEMBER 2014, AN 8-INCH ANDERSON-GREENWOOD RELIEF VALVE OWNED BY NORTHERN NATURAL GAS COMPANY OPENED ON NORTHERN'S 12-INCH-DIAMETER NBPL/WELCOME INTERCONNECT PIPING. APPARENTLY, NBPL EXPERIENCED SOME REGULATOR ISSUES THAT CAUSED FLOWS TO INCREASE AND THE PRESSURE TO EXCEED THE 830 PSIG SET POINT. NORTHERN BELIEVES NBPL EXPERIENCED TWO REGULATOR FAILURES AS THE RELIEF HAS SUFFICIENT CAPACITY TO PROTECT THE 800 PSIG MAOP IN A SINGLE REGULATOR FAILURE MODE; THE PRESSURE IN THE 800 PSIG MAOP MNB91501 LINE REACHED 894 PSIG FOR A BRIEF PERIOD EXCEEDING THE MAOP PLUS ALLOWABLE BUILDUP BY 14 PSIG. CALCULATIONS INDICATE IT WOULD TAKE TWO REGULATORS TO FAIL TO REACH THIS PRESSURE WITH THE RELIEF WIDE OPEN. MOREOVER, THE PIPELINE PRODUCES A HOOP STRESS OF 39% SMYS AT ITS 800 MAOP. THE HOOP STRESS WAS CALCULATED AS 42% AT THE OVERPRESSURE CONDITION, WHICH IS WELL BELOW THE ALLOWABLE STRESS FOR PIPE IN THIS LOCATION. THIS EVENT RESULTED IN A REPORTABLE INCIDENT BECAUSE 13.9 MMCF OF NATURAL GAS WAS LOST. ACCORDING TO 191.23(B)(2), A SRC SHOULD NOT BE REQUIRED SINCE THE EVENT RESULTS IN AN INCIDENT BEFORE THE DEADLINE FOR FILING A SAFETY RELATED CONDITION REPORT. NORTHERN BORDER PIPELINE CURRENTLY IS OPERATING ITS REGULATORS AT A REDUCED PRESSURE AND VOLUME UNTIL THE PROBABLE CAUSE CAN BE DETERMINED. THE PROBABLE CAUSE WAS A FAILURE OF CONTROL VALVES OWNED AND OPERATED BY NOTHERN BORDER PIPELINE COMPANY (NBPL). IT IS DIFFICULT TO ESTABLISH THE EXACT SEQUENCE OF EVENTS DUE TO THE LACK OF VALVE POSITION FEEDBACK ON NBPL'S SYSTEM, AND SINCE NO DETERMINATION HAS BEEN MADE WHY THEIR PNEUMATIC BACK-UP CONTROLLERS DID NOT OVERRIDE THEIR DIGITAL CONTROL LOOP TO TAKE OVER CONTROL OF THE PRESSURE. IT IS CLEAR, HOWEVER, THAT NBPL FAILED TO CONTROL PRESSURE TO THE POINT WHERE PRESSURE INCREASED TO CAUSE NORTHERN'S RELIEF VALVE TO VENT. NBPL DID INDICATE BOTH OF THEIR CONTROL VALVES WERE AT 96% OPEN.