NORTHWESTERN CORPORATION

Incident Narrative

"AT APPROXIMATELY 08:57 ON DECEMBER 3, 2025, SOUTH OF ABERDEEN SD, A VEHICLE SLID OFF AN ICY, GRAVEL ROAD STRIKING A 1"" STEEL FARM TAP WITH A 4"" STEEL BOLLARD BARRICADE IN THE RIGHT OF WAY. DRIVER WAS NOT INJURED. THE NIGHT BEFORE THE INCIDENT THERE WAS SNOW AND FREEZING RAIN WHICH AFFECTED ROAD CONDITIONS. AT 09:11, BROWN COUNTY 911 DISPATCH CALLED NORTHWESTERN ENERGY TO REPORT HIT LINE AND FIRST RESPONDER WAS ON SCENE AT APPROXIMATELY 09:26. NORTHWESTERN ENERGY FIRST RESPONDER IDENTIFIED THIS WAS THE MAIN PIPELINE INTO ABERDEEN, SD AND COMPANY LEADERSHIP WAS NOTIFIED IMMEDIATELY. MULTIPLE TEAMS COORDINATED TO ASSESS SYSTEM INTEGRITY, MANAGE GAS LOAD TO THE CITY OF ABERDEEN, AND ENSURE PUBLIC SAFETY. ABERDEEN RURAL FIRE DEPARTMENT (ARFD) TURNED OFF THE ABOVE GROUND VALVE AT THE FARM TAP AT APPROXIMATELY 09:30 WHICH MINIMIZED THE BLOWING GAS; HOWEVER, AN UNDERGROUND LEAK WAS STILL PRESENT AND RELEASING GAS (INITIALLY ~610PSIG). THE BROWN COUNTY SHERIFF DEPARTMENT BLOCKED THE ROAD AT THE INTERSECTION NORTH AND SOUTH OF THE ACCIDENT. THE ARFD VERIFIED NO ONE WAS HOME AT THE WEST RESIDENCE AND EVACUATED THE TWO RESIDENTS AT THE EAST RESIDENCE. MITIGATION STEPS INCLUDED MONITORING AMBIENT AIR, BARHOLING FOR GAS MIGRATION, AND PLANNING SEGMENT ISOLATION TO CONTROL THE PLUME OF GAS FOR SAFE APPROACH TO THE FARM TAP CURB VALVE. TO SUSTAIN THE GAS LOAD FOR THE CITY OF ABERDEEN DURING FRIGID WINTER WEATHER CONDITIONS, FOUR LARGE INDUSTRIAL CUSTOMERS WERE CURTAILED AND ALTERNATE SUPPLY INTERCONNECT SUPPORT FROM NORTHERN NATURAL GAS WAS ARRANGED. SEVERAL ATTEMPTS OF ISOLATION WERE ORGANIZED AND COMPLETED; HOWEVER, GAS LOAD DEMANDS WERE A FACTOR IN EXECUTION DUE TO FRIGID WINTER WEATHER CONDITIONS. AT 18:33, WITH DIMINISHED GAS LOAD, THE DAMAGED FARM TAP WAS ISOLATED, GAS FLOW STOPPED, AND SYSTEM PRESSURE RESTORED. CONTINGENCY PLANS FOR HEATING TO THE TWO CUSTOMERS AFFECTED BY THE FARM TAP SHUT OFF AND REPAIR LOGISTICS WERE IMPLEMENTED. FINAL REPAIR WAS DEFERRED TO NEXT DAY WITH OVERNIGHT MONITORING IN PLACE."