NORTHERN NATURAL GAS CO

Incident Narrative

ON JUNE 23, 2010, A LANDOWNER, EUGENE MULLIGAN, CALLED NORTHERN NATURAL GAS COMPANY'S OPERATIONS COMMUNICATION CENTER TO REPORT BLOWING GAS TO ATMOSPHERE AT A VALVE SITE NEAR WHERE HE LIVED. NORTHERN EMPLOYEES IMMEDIATELY RESPONDED. IT WAS DETERMINED THAT IT WAS NATURAL GAS BLOWING FROM NORTHERN'S MNM80104-M500D MAINLINE AT A VALVE SITE AT APPROXIMATE MILEPOST 95.89. PIPELINE PRESSURE ON THE LINE WAS 739 PSIG. THE LINE HAS AN MAOP OF 1,000 PSIG. A PIR OF 563 FEET WAS CALCULATED. LOCAL POLICE ASSISTED WITH BARRICADING OFF 320TH STREET WHICH PASSES BY THE VALVE SITE. THERE WAS NO FIRE, NO EXPLOSION AND NO INJURIES. AS A PRECAUTION 5 PEOPLE WERE EVACUATED FROM RESIDENCES IN THE NEAR AREA. APPROXIMATELY 13 MILES OF C LINE AND 6 MILES OF D LINE WERE SHUT IN TO ENABLE ISOLATION OF THE LINE SEGMENT. AN ATTEMPT WAS MADE TO DRAW GAS PRESSURE DOWN WITH COMPRESSION HORSEPOWER @ THE FARMINGTON COMPRESSOR STATION. AT 490 PSIG COMPRESSION WAS SHUT DOWN DUE TO HIGH ROD LOAD ON UNITS. AN EMERGENCY 1-CALL WAS MADE AND CONTRACTOR PERSONNEL WERE CALLED TO ASSIST WITH REPAIRS. IT WAS DETERMINE THAT LINE WOULD BE BLOWN DOWN TO 50 PSIG FOR OVERNIGHT, ONE PERSON WOULD MAN SITE THRU THE NIGHT. ON JUNE 24 THE LINE WAS BLOWN DOWN TO ZERO PRESSURE. FOUR FARMTAP CUSTOMERS WERE IMPACTED. EXCAVATION WAS COMMENCED AND IT WAS FOUND THAT THERE WAS A SIGNIFICANT FROST BALL. A HOT WATER HYDRO VAC WAS UTILIZED TO REMOVE FROST. INVESTIGATION INDICATED THAT THERE WAS A PORTION OF THE FLANGE GASKET BLOWN OUT, WHICH ALLOWED GAS LEAKAGE. THE FLANGE WAS DISASSEMBLED AND A NEW LAMONS 30"" 600 ASME B16 47A, ASME B1620, 304IR GASKET WAS INSTALLED. THE FLANGE BOLTS WERE RE-INSTALLED AND RETORQUED. ADDITIONAL CONCRETE SUPPORT BLOCKS WERE INSTALLED AND ALLOWED TO CURE. ON JUNE 28TH THE LINE WAS PURGED AND SLOWLY RELOADED TO OPERATIONAL PRESSURE, LEAK CHECKED, COATED AND BACKFILLED. A SUPPLEMENTAL FINAL REPORT WILL BE FILED WHEN ALL COST OF REPAIRS IS CALCULATED. FINAL COST WERE CALCULATED AND INITIAL COSTS WERE CHANGED/UPDATED. ON APRIL18,2011 A SUPPLEMENTAL FINAL REPORT WAS FILED AS PHMSA REQUESTED TO PROVIDE ADDITIONAL INFORMATION ON PIPE SPEC'S, PART C, ITEM 3 OF THE REPORT AND COST OF GAS RELEASED UNINTENTIONALLY IN PART D, ITEM 7B. THIS INFORMATION HAS BEEN ADDED TO THIS SUPPLEMENTAL FINAL REPORT AS REQUESTED. SUPPLEMENTAL/FINAL REPORT - THIS REPORT IS BEING FILED AS A SUPPLEMENTAL/FINAL REPORT TO REPORT # 20100039-15215 DATED 08/31/2010 AS A FOLLOW UP TO DISCUSSION WITH PHMSA REPRESENTATIVES. PART A ITEM 7 WAS CHANGED TO MATCH TIME ON THE NRC FORM, THE CAUSE OF THE EVENT WAS CHANGED FROM EQUIPMENT FAILURE TO INCORRECT OPERATIONS. AN ANALYSIS OF THE EVENT INDICATES THAT THERE WAS ADEQUATE, PROPER SUPPORT FOR THE PIPING, NO INDICATION OF SUBSIDENCE AND ALTHOUGH THE GASKET MATERIAL WAS NOT ANALYZED DUE TO IT'S DAMAGED CONDITION, THERE ARE TWO OTHER SIMILAR 30-INCH VALVES WITH THE SAME TYPE OF GASKETS INSTALLED ON THIS PIPING AND NEITHER OF THOSE HAVE HAD ANY PROBLEMS OR LEAKS, THIS IS A VERY GOOD, HIGH QUALITY GASKET. THE LEAKAGE APPEARS TO HAVE BEEN BETWEEN THE 6 O'CLOCK AND 9 O'CLOCK POSITION ONLY. NORTHERN BELIEVES THAT THE ROOT CAUSE OF THIS EVENT IS MOST LIKELY THAT ALL OF THE BOLTS ON THE FLANGE WERE NOT CONSISTENTLY TORQUED WHICH LED TO LEAKAGE, WHICH ULTIMATELY LED TO THE GASKET FAILURE. NORTHERN IS ISSUING A TECHNICAL BULLETIN TO ALL EMPLOYEES, INSPECTORS AND CONSTRUCTION PERSONNEL RE-EMPHASIZING THE IMPORTANCE OF PROPER TORQUEING AND THE REQUIREMENTS OF ENGINEERING STANDARD 7570 WHEN JOINING OF PIPE & PIPING COMPONENTS OTHER THAN BY WELDING. SHOULD THERE BE AN EVENT OF THIS NATURE IN THE FUTURE EVERY EFFORT WILL BE MADE TO ENSURE THAT EVEN THOUGH THE GASKET MATERIAL MAY BE DAMAGED IT WILL BE SENT TO A LABORATORY FOR ANALYSIS TO DETERMINE IF THE QUALITY OF THE GASKET WAS OR IS A FACTOR IN THE EVENT.