NORTHERN NATURAL GAS CO

Incident Narrative

A CONTRACTOR AND SUBCONTRACTOR WERE PREPARING TO INSPECT A CASING ON WELL 02-05 ON JULY 15, 2021. AFTER INSTALLING A REDUCING ASSEMBLY THAT ALLOWS A TOOL TO BE LOWERED IN THE CASING PROVIDED BY THE CONTRACTOR, BOTH PARTIES PROCEEDED TO TEST THE COMPLETED ASSEMBLY. THE WELLHEAD MASTER VALVE WAS PARTIALLY OPENED TO EQUALIZE PRESSURE. THE CONTRACTOR NOTED THAT THE MASTER VALVE WAS DIFFICULT TO OPERATE AS EXPECTED DUE TO RELATIVELY HIGH-PRESSURE DIFFERENTIAL ACROSS THE VALVE. THEY REPORTED THE PRESSURE WAS 1,140 PSIG AT 0900 HOURS. NO GAS COULD BE HEARD FLOWING THROUGH THE VALVE, BUT THE SUBCONTRACTOR INDICATED THERE WAS A SLIGHT LEAK BUBBLING BETWEEN THE COMPANION FLANGE AND THE REDUCING FITTING, THEREFORE, BOTH THE CONTRACTOR AND SUBCONTRACTOR CLOSED THE MASTER VALVE AND THEN OPERATED A BLEED-OFF VALVE, RELEASING THE PRESSURE BUILDUP. THE LEAK WAS REMEDIATED BY TIGHTENING THE REDUCING FITTING WITH A PIPE WRENCH BY APPROXIMATELY A HALF-TURN ON THE FITTING. WITH THE LEAK REMEDIATED, THE MASTER VALVE WAS SLOWLY OPENED AGAIN WITH DIFFICULTY. IT TYPICALLY TAKES 2-4 TURNS OF THE VALVE TO ACHIEVE FULL-OPEN STATUS, AFTER APPROXIMATELY 1 1/2 TO 2 TURNS OF THE MASTER VALVE, THE ASSEMBLY FAILED. THE SIX PERSONNEL ON-SITE VACATED THE WORK AREA, THREE OF WHICH RECEIVED MEDICAL ATTENTION DUE TO FACIAL ABRASIONS AND POTENTIAL HEARING LOSS. ONCE THE AREA WAS SECURED AND NORTHERN NATURAL GAS EVALUATED THE SCENARIO, A WELL CONTROL CONTRACTOR WAS TO MOBILIZE TO REMEDIATE THE VENTING GAS. THE WELL CONTROL CONTRACTOR ARRIVED ON-SITE AT AND CONFERRED WITH EMERGENCY RESPONDERS AT 2345 HOURS, AND DUE TO LACK OF DAYLIGHT AND PRESENCE OF NEARBY EQUIPMENT AND VEHICLES, IT WAS DECIDED TO RE-EVALUATE THE NEXT MORNING. AROUND 0700 ON JULY 16, 2021, THE WELL CONTROL CONTRACTOR DELAYED THE RESPONSE EFFORT DUE TO POOR LIGHTING AND LIMITED VISIBILITY CAUSED BY EARLY MORNING FOG. ONCE THE FOG LIFTED, THE WELL CONTROL CONTRACTOR ASSESSED THE SITE AND CLOSED THE MASTER VALVE AT 1001 HOURS ON JULY 16, 2021. IT WAS NOTED BY THE WELL CONTROL CONTRACTOR THAT THE MASTER VALVE HAD A SLIGHT LEAK AND WAS REMEDIATED BY INSTALLING A SECOND MASTER VALVE ABOVE THE ORIGINAL CONTAINING THE LEAK FULLY AT 1125 HOURS. THE PRELIMINARY EVALUATION DETERMINED THE THREADED CONNECTION BETWEEN THE COMPANION FLANGE AND REDUCING FITTING DID NOT HAVE SUFFICIENT THREAD ENGAGEMENT. THE INADEQUATE CONNECTION COULD NOT PROVIDE SUFFICIENT HYDRAULIC STRENGTH TO WITHSTAND THE PRESSURE NOR THE MECHANICAL STRENGTH TO RESIST THE BENDING MOMENT IMPARTED BY THE SUSPENDED WIRELINE ASSEMBLY. PRELIMINARY LAB ANALYSIS CONDUCTED BETWEEN AUGUST 30, 2021 ' SEPTEMBER 1, 2021 FOUND THE COMPANION FLANGE, SWAGE PIN, WAS UNDERSIZE BY 0.0325 INCHES AND THE REDUCING FITTING WAS OVER SIZED BY 0.0305 INCHES. THE THREADS ON THE COMPANION FLANGE WERE ALSO SHORT LENGTH 2-INCHES INSTEAD OF THE FULL LENGTH OF 4-INCHES, WHICH LIMITED THE NUMBER OF THREADS AVAILABLE FOR FULL ENGAGEMENT ACROSS 4-INCHES. THIS LACK OF AVAILABLE ENGAGEMENT AND COMPONENTS OUTSIDE ACCEPTABLE TOLERANCE, PREVENTED THE ASSEMBLY FROM PROPERLY HOLDING TOGETHER UNDER THE DESIGNED PRESSURE.