ENOGEX LLC

Incident Narrative

PRIOR TO THE INCIDENT ON 11/19/2011 A SEGMENT OF L-4 BETWEEN N/E/ TAP AND KEETONVILLE YARD HAD BEEN REMOVED FROM SERVICE FOR ILI REMEDIATION AND REPAIRS. THIS OUTAGE REQUIRED GAS TO BE RE-ROUTED FROM NEPS AT OOLAHGA, OK DOWN L-4 TO THE L-4 TO L-4A JUNCTION AT KEETONVILLE, OK TO CONTINUE THE SUPPLY OF GAS THROUGH L-4A & L-4B TO CUSTOMERS CONNECTED TO L-4C INSIDE THE PRYOR INDUSTRIAL PARK. ON 11/18/2011, THE CLAREMORE COMPRESSOR STATION WAS REMOVED FROM SERVICE FOR MAINTENANCE OF MEASUREMENT EQUIPMENT. THIS OUTAGE ELIMINATED THE USE OF L-4A AND L-4B TO SUPPLY GAS TO L-4C; THEREBY REQUIRING THE USE OF AN ALTERNATE VALVE INSIDE THE AECI GAS YARD AT CHOTEAU, OK TO ROUTE GAS FROM L-281 INTO L-4E TO SUPPLY GAS TO CUSTOMERS CONNECTED TO L-4C INSIDE THE PRYOR INDUSTRIAL PARK. THE FLOW OF GAS FROM L-281 INTO L-4E TO SUPPLY GAS TO L-4C HAD NOT BEEN ATTEMPTED PRIOR TO THIS DATE. DUE TO THE GAS LOAD AT THE TWO POWER PLANTS AT AECI AND THE DEMAND REQUIRED BY CUSTOMERS IN THE PRYOR INDUSTRIAL PARK, IT WAS DETERMINED THAT PCV-281 IN THE AECI GAS YARD COULD NOT SUPPLY THE AMOUNT OF GAS NECESSARY. XV-102, A 16' VALVE WAS OPENED BY GAS CONTROL TO BYPASS GAS FLOW AROUND PCV-281 AND SUPPLEMENT THE FLOW THROUGH PCV-281. AT AN UNKNOWN TIME ON THE EVENING OF 11/18/2011, THE AECI POWER PLANT REDUCED THE DEMAND LOAD INTO THE POWER PLANTS. GAS FLOW THEN TOOK THE PATH OF LEAST RESISTANCE, WHICH WAS INTO L-4E AND L-4C. GAS FLOW CONTINUED TO FLOW BACKWARD INTO L-4B TOWARD THE CLAREMORE COMPRESSOR STATION. PC-6, A RECYCLE/SURGE CONTROL VALVE INSIDE THE CLAREMORE STATION, DETECTED THE INCREASING PRESSURE IN L-4B AND OPENED AT A SETPOINT OF 750 PSIG, THEREBY ROUTING GAS AROUND THE CLAREMORE STATION INTO L-4A. THE COMPRESSOR STATION WAS NOT NEEDED AND THEREFORE NOT OPERATING AT THIS TIME. THIS GAS FLOW CONTINUED TO FLOW INTO L-4, THUS STEADILY INCREASING THE PRESSURE IN L-4 UNTIL THE SETPOINT ON AVAILABLE OVERPRESSURE WAS SATISFIED AND THE PSV AT THE NEPS GAS YARD ACTIVATED. THE NEPS CONTROL ROOM OPERATOR INITIALLY NOTIFIED AN ONG REPRESENTATIVE TO RESPOND TO GAS VENTING IN THE GAS YARD. UPON ARRIVAL, THE ONG INDICATED TO THE NEPS CONTROL ROOM THAT THE GAS RELEASE WAS FROM A ENOGEX PSV. THE NEPS CONTROL ROOM OPERATOR THEN IMMEDIATELY NOTIFIED ENOGEX GAS CONTROL AT 1:40 AM. THE ENOGEX ON-CALL PERSONNEL ARRIVED AT NEPS AT 3:00 AM AND IMMEDIATELY CONFIRMED WITH GAS CONTROL THAT THE L-4 OPERATING PRESSURE WAS 369 PSIG OR 236 PSIG BELOW THE MAOP OF 605. THE PSV WAS THEN ISOLATED TO END THE RELEASE OF GAS AND GAS CONTROL WAS INSTRUCTED TO CLOSE XV-102 IN THE AECI GAS YARD AT CHOTEAU. THE ENOGEX ON-CALL PERSONNEL THEN DROVE TO THE CLAREMORE COMPRESSOR STATION TO ISOLATE PV-6 TO STOP THE FLOW OF GAS INTO L-4A. THE ENOGEX ON-CALL PERSONNEL THEN RETURNED TO NEPS TO PLACE THE PSV BACK INTO SERVICE. AN INSPECTION OF THE PSV AT NEPS GAS YARD REVEALED MINOR SCARRING OF THE SEAT MATERIAL THAT PREVENTED THE PSV FROM AUTOMATICALLY RESEATING ONCE THE L-4 PRESSURE FELL BELOW THE PSV SETPOINT OF 605. AN INVESTIGATION OF THIS INCIDENT REVEALED THAT PV-6 LOCATED IN THE CLAREMORE COMPRESSOR STATION WAS PROGRAMMED TO OPEN WHEN THE COMPRESSOR DISCHARGE PRESSURE IS EQUAL TO OR GREATER THAN 750 PSIG. IT WAS ALSO DETERMINED THAT THE PV-6 SURGE VALVE IS NOT NEEDED WHEN THE COMPRESSORS ARE NOT RUNNING. A CHANGE IN THE CONTROL LOGIC PROGRAM AT THE CLAREMORE COMPRESSOR STATION IS SCHEDULED FOR DECEMBER 22, 2011 TO INTERLOCK THE OPERATION OF PV-6 WHEN THE COMPRESSORS ARE NOT RUNNING OR DURING AN ESD CONDITION. THIS WILL PREVENT PV-6 FROM ROUTING GAS FROM L-4B INTO L-4A AND INTO L-4. UPDATE MARCH 5, 2013: COMPLETED SECTION E.5.F.