GREAT LAKES GAS TRANSMISSION CO

Incident Narrative

THE INCIDENT INVOLVED A MINOR LEAK ORIGINALLY DISCOVERED ON 2/12/18 (THEN ISOLATED AND BLOWN DOWN THE SAME DAY) AT THE GLGT ST. VINCENT COMPRESSOR STATION IN MINNESOTA. THE LEAK LOCATION WAS EXCAVATED ON 4/10/18 AND VISUALLY DETERMINED TO BE ORIGINATING FROM A BURIED 24 INCH ELL FITTING. AN UNDERGROUND 24"" ELL DEVELOPED A PIN HOLE LEAK. THE 24"" ELL CONNECTED TO A 1"" WELD-O-LET, THEN TO 1"" PIPING LEADING TO AN ABOVE GROUND PRESSURE TRANSMITTER, USED FOR FLOW MEASUREMENT PURPOSES. THE 24"" ELL WAS CUT OUT AN REPLACED. THE CUT OUT SECTION WAS SENT TO METALLURGICAL ANALYSIS COMPANY FOR IMMEDIATE CAUSE DETERMINATION. THE METALLURGICAL ANALYSIS SUMMARY CONCLUDED THERE WERE TWO LEAK LOCATIONS PRESENT IN THE ELBOW SECTION; ONE IN THE ELBOW AND ONE IN THE ATTACHED PIPING. THE LEAK IN THE PIPING WAS LOCATED AT A THROUGH-WALL CRACK AT THE TOE OF THE WELD BETWEEN THE PIPING AND WELD-O-LET. THE CRACK WAS MOST LIKELY CAUSED BY OVERLOAD, ALTHOUGH THIS CAUSE COULD NOT BE DEFINITIVELY DETERMINED BECAUSE OF EROSION DAMAGE TO THE FRACTURE SURFACES. THE CAUSE OF THE LEAK IN THE ELBOW WAS EROSION FROM THE GAS ESCAPING FROM THE CRACK IN THE PIPING. TRANSCANADA SME EVALUATION CONCLUDED, THE LIKELY SOURCE OF THE OVERLOAD REFERENCED IN THE METALLURGICAL REPORT WAS THE 1"" RISER BEING PULLED UPWARD BY THE SOIL AROUND IT NEAR THE GROUND SURFACE IN A PROCESS CALLED ""FROST JACKING"" DURING THE COLD MONTHS. THE REPEATED CYCLES OF FREEZING AND THAWING MAY HAVE INDUCED STRESSES ON THE ELBOW-PIPE JOINT, THEREBY, CAUSING THE CRACK. REGARDING THE EXISTENCE OF SIMILAR DESIGNS IN THE SYSTEM, IT HAS BEEN CONFIRMED ST. VINCENT WAS THE ONLY STATION THAT HAD THE ELBOW FLOW METER OUTSIDE A UNIT BUILDING. THE OTHER GLGT STATIONS HAVE THE ELBOW FLOW METERS INSIDE THE BUILDINGS AND NOT SUBJECTED TO SIMILAR CONDITIONS. TRANSCANADA WILL CONTINUE TO EVALUATE PIPING ARRANGEMENTS WHERE FROST-SUSCEPTIBLE SOIL CONDITIONS MAY EXIST WITHIN THE GLGT SYSTEM UNDER THE THREAT MANAGEMENT PROGRAM.