TRANSCOLORADO GAS TRANSMISSION COMPANY LLC

Incident Narrative

ON 6/25 AT 9:12AM, TRANSCOLORADO GAS TRANSMISSION COMPANY LLC'S OLATHE COMPRESSOR STATION LOCATED IN MONTROSE COUNTY, COLORADO EXPERIENCED AN ABNORMAL EVENT WHILE IN STANDBY MODE WITH NO COMPRESSION IN OPERATION AT THE TIME. THE STATION EXPERIENCED A COMMUNICATION FAILURE BETWEEN THE STATION PLC, ESD PLC AND THE I/O MODULE CONTROLLING THE ESD VALVES. THE I/O MODULE CONTROLLING THE ESD VALVES ALSO FAILED TO ""HOLD LAST STATE"" UPON LOSS OF COMMUNICATION. THIS RESULTED IN THE STATION VENT VALVE OPENING (INSTEAD OF REMAINING CLOSED) AND AN UNPLANNED GAS RELEASE EVENT FOR 112 MINUTES. THE TOTAL VOLUME RELEASED WAS 24,659.50 MCF. THE IMMEDIATE CAUSE OF THE EVENT WAS DETERMINED TO BE THE FAILURE OF THE I/O CONTROLLER DESIGNED TO PROVIDE A ""HOLD LAST STATE"" COMMAND THAT KEEPS THE STATION VALVES IN THE CURRENT STATE WHEN COMMUNICATIONS ARE LOST. INVESTIGATION REVEALED THAT THERE WERE SHORT LOSSES OF COMMUNICATION PRIOR TO THIS EVENT, BUT THE I/O MODULE ""HOLD LAST STATE"" SETTING ALWAYS FUNCTIONED AS DESIGNED. ANOTHER CONTRIBUTING CAUSE WAS IDENTIFIED TO BE LOSS OF COMMUNICATION BETWEEN THE STATION & ESD PLCS AND THE I/O MODULES. THE INTERMITTENT LOSS OF COMMUNICATION WAS CAUSED BY POOR CONNECTIONS ON THE COMMUNICATION WIRES AT THE PLCS, I/O MODULES, AND SURGE-SUPPRESSION EQUIPMENT. ALL CONNECTIONS WITH THE COMMUNICATIONS WIRE HAVE BEEN CHECKED AND CORRECTED AS REQUIRED. THERE HAS BEEN NO COMMUNICATIONS FAILURES SINCE. THE I/O MODULE WAS ALSO REPLACED.