NORTHERN NATURAL GAS CO — Gas transmission and gathering — incident on 2008-05-23

Gas transmission and gathering incident in BEAVER BAY, LAKE County, MN. Reported cause: 3. Reported consequences: 0 fatalities, 0 injuries, $530,000 reported property damage. Operator-reported incident data submitted to PHMSA, generally within 30 days. Records may be supplemented or corrected and do not represent final agency causal findings.

Report number: 20080058.

Operator ID: 13750.

Reported incident date: 5/23/2008.

Cause detail: EARTH MOVEMENT

Cause detail: ON MAY 23, AT APPROXIATELY 14:57 THE CARLTON LOCATION WAS NOTIFIED BY ANGEL SERVICE ITT CORP, THAT THEY HAD COMPLETED THEIR AERIAL LEAK SURVEYING. UPON REVIEW OF THE INFORMATION IT APPEARED THAT THERE WAS A DETECTION OF A SINGLE METHANE INDICATION IN LAKE COUNTY, MN ON THE 16-INCH RESERVE (MNB69801) BRANCH LINE ON THE PIPELINE RIGHT OF WAY APPROXIMATELY 1.1 MILES WEST OF LAX LAKE ROAD @ APPROXIMATE MILEPOST 59.5. NORTHERN PERSONNEL PROCEEDED TO REDUCE THE PRESSURE FROM 405 TO 300 PSIG AND SENT PERSONNEL TO THE SITE TO UTILIZE PORTABLE LEAK DETECTION EQUIPMENT TO VERIFY IF THERE WAS A LEAK. A LEAK WAS DETECTED @ THE APPROXIMATE LOCATION INDICATED. A STRONG HYDRO CARBON SMELL WAS DETECTED, A READING OF 5% LEL WAS INDICATED AND A PATCH OF DEAD VEGETATION APPROXIMATELY 15 FT IN DIAMETER WAS NOTED. ACTIONS WERE TAKEN TO ACQUIRE THE SERVICES OF A CONTRACTOR AND ARRANGE FOR NORTHERN'S STOPPLE CREW TO PROCEED TO THE SITE. AN 8-INCH STOPPLE AND BYPASS WAS INSTALLED AROUND THE LEAK AREA, THEN THE SECTION OF LINE WAS BLOWN DOWN AND EXCAVATED. A ROCK WAS FOUND TO HAVE IMPINGED ON THE PIPELINE AT THE 6 O'CLOCK POSITION. THE IMPINGMENT RESULTED IN A DENT MEASURING APPROXIMATELY 11 INCHES LONGITUDINALLY, SEVEN INCHES CIRCUFERENTIALLY AND 0.903 INCHES DEEP. THERE WAS A CRACK LOCATED AT THE BOTTOM OF THE DENT MEASURING APPROXIMATELY 1-INCH LONG. IT WAS DETERMINED THAT THE LEAK COULD BE REPAIRED BY INSTALLATION OF A 2-FOOT BY 16-INCH TYPE B PRESSURIZED CONTAINING SLEEVE. INSTALLATION OF THE SLEEVE WAS COMPLETED ON JUNE 3. ALL WELDS WERE INSPECTED USING MAGNETIC PARTICLE AND ULTRASONIC THICKNESS METHODS. THE PIPELINE BETWEEN THE STOPPLE FITTINGS WAS PURGED, PRESSURE TESTED AND RETURNED TO NORMAL 405 PSIG SERVICE. THERE WAS NO FIRE, NO INJURIES AND NO LOSS OF SERVICE TO ANY CUSTOMERS. THE ROOT CAUSE OF THE LEAK WAS A ROCK IMPINING ON THE LINE AND GROUND SUBSIDENCE. A FINAL REPORT WILL BE COMPLETED UPON DETERMINATION OF THE TOTAL COST OF REPAIRS. SUPPLEMENTAL REPORT: IN THE NARRATIVE IT WAS INDICATED THAT THE LINE NUMBER WAS MNB69801, WHICH IS INCORRECT AND SHOULD INSTEAD BE MNB72701. SUPPLEMENTAL/FINAL REPORT: TOTAL COST OF REPAIRS ESTIMATES, INCLUDING GAS LOSS HAVE BEEN UPDATED. THE ROOT CAUSE OF THE LEAK WAS DETERMINED TO BE THAT GROUND SUBSIDENCE RESULTING IN A ROCK IMPINGING ON THE LINE, WHICH RESULTED IN DAMAGE TO COATING AND A DENT IN THE LINE AND AN APPROXIMATE ONE INCH CRACK IN THE BOTTOM OF THE DENT WHICH ALLOWED GAS LEAKAGE TO THE ATMOSPHERE. THIS EVENT IS CONSIDERED CLOSED.

PHMSA trend classification: significant incident, not serious.

PHMSA standardized cause: NATURAL FORCE DAMAGE — EARTH MOVEMENT.

PHMSA indexed costs: $530,000 reported total cost, $293,481.168 in 1984 dollars, $714,358.736 in current-year dollars.

PHMSA trend flags, standardized causes, and indexed costs are analytical fields added to operator-reported incident data. They do not represent final agency causal findings.