NORTHERN NATURAL GAS CO

Incident Narrative

ON 11/4/20, A CONTRACTOR WAS EVALUATING THE RIGHT-OF-WAY AND DISCOVERED BUBBLES. LEAK DETECTION FOUND VERY LIMITED AMOUNT OF GAS DETECTED. OPERATIONS CONFIRMED THIS LOCATION HAD A LEAK IN 2017 THAT WAS REMEDIATED WITH A PIPE REPLACEMENT AND ANTICIPATED THE EXISTING STOPPLE MAY BE LEAKING. A STOPPLE TECHNICIAN REPLACED THE BLEED VALVE ON THE STOPPLE AND THE LEAK WAS NO LONGER DETECTED. IT WAS CONFIRMED 11/5/20 AT 0800 HOURS THE LEAK HAD BEEN REMEDIATED. ON 11/6/20, A CONTRACTOR HEARD BUBBLES NEAR THE EDGE OF THE EXCAVATION THAT APPEARED TO BE MIGRATING FROM A SEPARATE LOCATION. HAND EXCAVATION FOUND A GIRTH WELD WAS THE SOURCE OF THE LEAK NOW DETECTED. A TECHNICIAN ARRIVED ON 11/7/20 TO INSTALL A CLAMP TO REMEDIATE THE LEAK. IT WAS DETERMINED THAT CLEARING OFF THE COATING AND PREPARING THE PIPE FOR A CLAMP THAT IT WOULD BE SAFER TO PERFORM A STOPPLE BYPASS. ONCE THIS WAS DETERMINED, THE NRC WAS NOTIFIED AS THE REPAIR WOULD EXCEED THE REPORTING THRESHOLD. A STOPPLE CREW ARRIVED ON SITE 11/9/20. THE PIPE WAS ON BYPASS AT 1103 ON 11/10/20. THE REPAIRS WHERE MADE AND THE PIPE WAS RETURNED TO SERVICE AT 1213 ON 11/11/20. METALLURGICAL EVALUATION DETERMINED THAT THE AMOUNT OF CORROSION PRESENT ON THE SURFACE AS WELL AS THE 80% OF THE WALL THICKNESS, TRANS GRANULAR BRITTLE FRACTURING THE CRACK, POSSIBLY OCCURRED DURING OR SHORTLY AFTER WELDING THE TWO JOINTS OF PIPE, WHICH PRESENTED THE LOCATION OF THE FAILURE POINT.