GREAT LAKES GAS TRANSMISSION CO

Incident Narrative

ON SEPTEMBER 5, 2015, GREAT LAKES GAS TRANSMISSION COMPANY (GLGT) EXPERIENCED AN INCIDENT ON ITS 200 LINE PIPELINE IMMEDIATELY DOWNSTREAM OF A PIG LAUNCHER ON PROPERTY CONTROLLED BY GLGT AT ITS ST. VINCENT COMPRESSOR STATION IN KITTSON COUNTY, MINNESOTA. THE INCIDENT, WHICH RESULTED IN A SHUTDOWN OF THE PIPELINE VALVE SECTION BETWEEN THE COMPRESSOR STATION AND MLV 1-1-2, WAS THE RESULT OF A CONNECTION FAILURE OF A TWO-INCH DIAMETER PIG SIGNAL COMPONENT AT THE ABOVEMENTIONED LOCATION. GAS WAS RELEASED AND SUBSEQUENTLY IGNITED. FOLLOWING CONTROL OF THE RELEASE AND CONTROLLED BLOWDOWN OF THE PIPELINE SECTION, THE AREA WAS EXCAVATED AND THE SOURCE OF THE GAS LEAK CONFIRMED AS A FAILED CONNECTION BETWEEN THE BASE OF A PIG SIGNAL OFFTAKE TUBE AND A BOLTED MOUNTING PLATE ATTACHED TO THE GLGT 200 LINE. THE DAMAGED PIG SIGNAL COMPONENTS, INCLUDING THE PIG SIGNAL OFFTAKE TUBE WITH ASSOCIATED BASE (WHICH WAS ALREADY DISCONNECTED FROM THE PIPELINE) AND THE LOWER MOUNTING PLATE, WERE REMOVED FROM THE PIPELINE AND SENT TO A THIRD-PARTY LABORATORY FOR FAILURE ANALYSIS. THE PIG SIGNAL WAS NOT REPLACED. THE PIPELINE WAS REPAIRED BY REMOVAL OF THE WELD-O-LET THAT CONNECTED THE BASE MOUNTING PLATE TO THE PIPELINE AND THEN INSTALLING A CONTOURED INSERT FITTING (VESSELET) AND CAP. FOLLOWING COMPLETION OF THE REPAIR, THE PIPELINE WAS PLACED BACK INTO SERVICE THE AFTERNOON OF SEPTEMBER 22, 2015. FAILURE ANALYSIS OF THE PIG SIGNAL COMPONENTS INDICATED THAT THREE (3) OF THE FOUR (4) FASTENERS, SOCKET HEAD CAP SCREWS (SHCS), OF THE BOLTED MOUNTING PLATE CONNECTION FAILED BY A TIME-DEPENDENT, STRESS CORROSION CRACKING (SCC) MECHANISM. THE WEST SHCS FAILURE MAY HAVE BEEN EXACERBATED BY ADDITIONAL FACTORS DUE TO ITS VERY HIGH HARDNESS AND POSSIBLE UN-TEMPERED MARTENSITIC CONDITION. THE SOUTH SHCS FAILED LAST AND EXHIBITED WITH MIXED SCC AND DUCTILE RUPTURE MODES. ITS REMAINING LIGAMENT FAILED SUDDENLY WHEN ITS LOAD BEARING CAPACITY WAS EXCEEDED.