NORTHWESTERN CORPORATION

Incident Narrative

"A7 - UNABLE TO DETERMINE AS TIME PIPE HAS BEEN LEAKING IS UNKNOWN 21C - NO ADDITIONAL GAS WAS RELEASED BEYOND UNDERGROUND PIPE LEAKING FOR UNKNOWN AMOUNT OF TIME PART C 4.A. - EDUCATED GUESS DUE TO AGE OF PIPE TIMELINE OF EVENTS PRIOR TO EXPLOSION: 10:35 AM: RECEIVED ODOR CALL FOR 1342 S. 5TH ST W 10:52 AM: ARRIVED AT THE SITE OF THE ODOR CALL 1342 S. 5TH ST W WORKED INSIDE, CHECKING APPLIANCES, VENTING, AMBIENT CHECKED OUTSIDE AMBIENT, METER SET, BAR HOLED ALONG SERVICE 11:30 AM: REQUESTED ADDITIONAL HELP WITH LEAK SURVEYING 11:36 AM: GAS SUPERVISOR INFORMING ABOUT ACTIVE LEAK, UNDER INVESTIGATION 11:42 - 11:48 AM: EMERGENCY LOCATES REQUESTED 11:50 AM: ADDITIONAL HELP ARRIVED ONSITE AND BEGAN TO MONITOR AMBIENT, SURVEY SURROUNDINGS, MAPPING 12:00 PM: NOTIFIED MISSOULA FIRE DEPARTMENT (MFD), GAS LEAK, REQUESTED FD TO BE DISPATCHED BEGIN EVACUATING HOMES 12:04 PM: DE-ENERGIZED POWER @ 1410 A/B S. 5TH ST W. MFD ASSISTANCE CHECKING AMBIENT INSIDE HOMES 12:36 PM: DE-ENERGIZED SECONDARY POWER TO 1410 A/B S. 5TH ST W. 12:36 - 12: 48 PM: BEGUN BARHOLING PRIOR 12:48 PM: REQUESTED BACKHOE TO ASSIST WITH BARHOLING 12:50 PM: HOUSE EXPLOSION POST EXPLOSION: 12:50PM: 1413 S. 4TH ST W HOUSE EXPLOSION 12:50 PM: DE-ENERGIZE PRIMARY FOR MULTIPLE BLOCKS FROM RUSSELL & 5TH ' EAST (FOUR BLOCKS EAST) MFD - INCREASED PERIMETER CONTINUED EVACUATIONS, ROAD CLOSURES, FIRE IC DAVE ANY INJURIES RESULTED: HIS RESPONSE NO ONE WAS HOME, NO INJURIES 1:05 PM: DE-ENERGIZED PRIMARY RUSSELL & 5TH ' EAST (FOUR BLOCKS EAST) 1:10 PM: ADDITIONAL SUPPORT ARRIVED ONSITE. INFORMED FIRE IC DAVE ' OF PLAN TO OPERATE ZV # 24, TO ELIMINATE GAS FLOW FROM EAST AND INSTALL PRESSURE CONTROL FITTING (PCF) TO ELIMINATE GAS FLOW FROM WEST 1:39 PM ZONE VALVE # 24 STOPPING FEED FROM THE EAST. 1:40 PM: BEGAN BELLHOLE EXCAVATION AT 1501 S. 4TH ST W ALLEY 2:00 PM: BEGAN SHUTTING OFF ALL LOCKWING VALVES 2:15 PM: MFD SHUT OFF LOCKWING VALVE TO 1413 S. 4TH ST W 4:00 PM: PCF # 1 PILOT BIT BROKE (WEST OF RUSSELL) 4:00 PM: BEGAN PCF # 2 (WEST OF RUSSELL) 5:10 PM: PCF # 2 STOPPERED, FEED FROM WEST WAS SHUT OFF. 5:15 PM: GAS SUCCESSFUL STOPPED.CONFIRMED FEED FROM WEST WAS SHUT OFF BY BREAKING DOWN METER SET AT 1501 S. 4TH ST W. 5:15 PM TO 1 AM: CONTINUED BARHOLING, AMBIENT MONITORING, TURNED OFF ALL THE LOCKWING VALVES EAST OF PRINCE. RESTORATION EFFORTS COMPLETED IN THE FOLLOW DAY. UPDATE 5-7-2025 ON JUNE 3, 2024 DESTRUCTIVE TESTING WAS CONDUCTED AT MATERIAL EVALUATION AND ENGINEERING. ENCLOSED IS THE TESTING PROTOCOL UTILIZED TO DESTRUCTIVELY TEST THE PIPE REMOVED FROM EVENT IN MISSOULA, MT ON JANUARY 17, 2024. THE FOLLOWING TESTS WERE CONDUCTED. 'CHEMISTRY IDENTIFICATION (ASTM E415-2021) ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDS) KEYENCE MICROHARDNESS ROCKWELL HARNESS SCANNING ELECTRON MICROSCOPY (SEM) XS RESULTS INDICATED A SINGULAR IMMEDIATE (NOT OVER TIME) FACTURE CONSISTENT WITH THE PIPE COMING UP. GIVEN THE DIRECTION OF THE FRACTURE SOMETHING BELOW THE PIPE CAUSED THE SURFACE TO CHANGE/RISE UP TO CREATE THE FRACTURE."