COLORADO INTERSTATE GAS CO

Incident Narrative

ON AUGUST 31, 2020, THE ROBERSON CREEK COMPRESSOR STATION WAS ON LINE AND ONE OF THE THREE UNITS WAS OPERATING. DURING THE DAY, OPERATIONS WAS WORKING WITH THE ORIGINAL EQUIPMENT MANUFACTURER TO INSPECT UNIT #2 VARIABLE FREQUENCY DRIVE (VFD) FOR THE PRIMARY DRIVER/ELECTRIC MOTOR. DURING THE INSPECTION, ONLY THE POWER TO THE VFD HAD BEEN LOCKED OUT AND NOTHING WAS TOUCHED ON THE GAS PIPING. AT APPROXIMATELY 2:10 PM MT, OPERATIONS STARTED UNIT #2 AT THE ROBERSON CREEK COMPRESSOR STATION TO CHECK THE OPERATION OF THE VFD. AFTER OPERATING THE UNIT FOR APPROXIMATELY 10 MINUTES THE UNIT WAS SHUT DOWN AT APPROXIMATELY 2:20 PM MT BY LOCAL OPERATIONS. EACH TIME A UNIT SHUTS DOWN, A TWO HOUR TIMER STARTS TO BLOWDOWN THE COMPRESSOR CASE. AT APPROXIMATELY 4:00 PM MT, KM PERSONNEL LEFT FOR THE DAY. AT 4:26 PM MT, THE UNIT VENT VALVE OPENED TO VENT THE COMPRESSOR CASE. NORMALLY IN THIS SITUATION, THE UNIT DISCHARGE VALVE WOULD BE IN THE CLOSED POSITION; HOWEVER, THE DISCHARGE VALVE WAS IN THE OPEN POSITION. THIS ALLOWED GAS TO CONTINUE TO VENT FROM THE STATION DISCHARGE. ON SEPTEMBER 1, 2020, AT APPROXIMATELY 6:50 AM MT, STATION PERSONNEL ARRIVED AT THE FACILITY, HEARD THE GAS BLOWING AND CLOSED THE VENT VALVE THAT THEY FOUND IN THE OPEN POSITION. GAS LOSS WAS CALCULATED TO BE APPROXIMATELY 150 MMCF. THE INVESTIGATION REVEALED THAT THE UNIT DISCHARGE VALVE WAS COMMANDED OPEN BY THE UNIT PLC AT 2:25PM. IT COULD NOT BE DETERMINED WHY THE VALVE WAS COMMANDED OPEN OR WHERE THE COMMAND CAME FROM. THE INVESTIGATION TEAM WAS UNABLE TO REPLICATE THE CAUSE OF THE EVENT DURING THEIR INVESTIGATION AND SUSPECTS THAT THE PLC EXPERIENCED A MALFUNCTION THAT CAUSED IT TO SEND THE UNINTENDED SIGNAL TO OPEN. THE INVESTIGATION ALSO REVEALED THAT THERE WAS A FLAW IN THE PLC PROGRAMMING. THERE WAS A LOGIC PATH THAT ALLOWED THE CONFIRMATION OF THE UNIT VALVE CLOSURES TO BE BYPASSED WHEN BLOWING DOWN THE COMPRESSOR UNIT. IN ORDER TO PREVENT A REOCCURRENCE, THE PLC LOGIC WAS MODIFIED TO ENSURE THAT THE UNIT SUCTION AND DISCHARGE VALVES ARE CLOSED BEFORE THE UNIT VENT VALVE IS OPENED. ADDITIONALLY, CIG ASSESSED THE PLC LOGIC OF THE OTHER TWO COMPRESSOR UNITS AND MADE CHANGES AS APPROPRIATE. IN ADDITION, A CALLOUT ALARM WAS ADDED IN THE EVENT THAT UNIT VENT VALVE IS OPEN AT THE SAME TIME AS A UNIT SUCTION OR DISCHARGE VALVE. FINALLY, A VENT STACK FLOW SWITCH WAS INSTALLED TO HELP DETECT ANY FUTURE UNINTENDED RELEASES.