{"operation":"document","citation":"DCA02MP002","title":"Rupture of Enbridge Pipeline and Release of Crude Oil","source_type":"incident","agency":"National Transportation Safety Board","status":"current","official":true,"published_on":"2026-01-30","effective_on":"2002-07-04","summary":"Accident. in Cohasset, MN, USA. on 2002-07-04. Enbridge Energy Partners L.P.. Rupture","machine_formats":{"json":"https://regulus.evalyn.ai/document/ntsb-case-dca02mp002.json","markdown":"https://regulus.evalyn.ai/document/ntsb-case-dca02mp002.md"},"app_url":"https://regulus.evalyn.ai/document/ntsb-case-dca02mp002","source_url":"https://www.ntsb.gov/investigations/Pages/DCA02MP002.aspx","body":"NTSB investigation DCA02MP002.\n\nEvent Type: Accident\n\nEvent Date: 2002-07-04\n\nEvent City: Cohasset\n\nEvent State Or Region: MN\n\nEvent Country: USA\n\nPipeline Operator: Enbridge Energy Partners L.P.\n\nPipeline Type: Hazardous Liquid - Regulated\n\nAccident Type: Rupture\n\nCompletion Status: Completed\n\nReport Number: PAR-04-01\n\nProbable cause: The National Transportation Safety Board determines that the probable cause of the July 4, 2002, pipeline rupture near Cohasset, Minnesota, was inadequate loading of the pipe for transportation that allowed a fatigue crack to initiate along the seam of the longitudinal weld during transit. After the pipe was installed, the fatigue crack grew with pressure cycle stresses until the crack reached a critical size and the pipe ruptured.\n\nTier1Name: System operating\n\nTier2Name: Product leak/release\n\nTier1Name: System operating\n\nTier2Name: Pipe structural malfunction/failure\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Support/oversight/monitoring\n\nFinding Tier3Name: Oversight\n\nFinding Modifier Name: Federal agency\n\nFinding Report Text: Organizational - Support/oversight/monitoring - Oversight - Federal agency\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Development\n\nFinding Tier3Name: Design\n\nFinding Modifier Name: Other institution/organization\n\nFinding Report Text: Organizational - Development - Design - Other institution/organization\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipeline structure\n\nFinding Tier3Name: Joint/weld/seam\n\nFinding Modifier Name: Fatigue/wear/corrosion\n\nFinding Report Text: Pipeline - Pipeline structure - Joint/weld/seam - Fatigue/wear/corrosion\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipeline handling/service\n\nFinding Tier3Name: (general)\n\nFinding Tier4Name: (general)\n\nFinding Modifier Name: Damaged/degraded\n\nFinding Report Text: Pipeline - Pipeline handling/service - (general) - Damaged/degraded\n\nOfficial NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations.\n\nWhat Happened\nAbout 2:12 a.m., central daylight time, on July 4, 2002, a 34-inch-diameter steel pipeline owned and operated by Enbridge Pipelines, LLC ruptured in a marsh west of Cohasset, Minnesota. Approximately 6,000 barrels (252,000 gallons) of crude oil were released from the pipeline as a result of the rupture. The cost of the accident was reported to the Research and Special Programs Administration Office of Pipeline Safety to be approximately $5.6 million. No deaths or injuries resulted from the release.\n\nWhat We Found\nWe determined that the probable cause of the July 4, 2002, pipeline rupture near Cohasset, Minnesota, was inadequate loading of the pipe for transportation that allowed a fatigue crack to initiate along the seam of the longitudinal weld during transit. After the pipe was installed, the fatigue crack grew with pressure cycle stresses until the crack reached a critical size and the pipe ruptured.\nThe following safety issues were identified during this investigation:\n\nThe effectiveness and application of line pipe transportation standards.\nThe adequacy of Federal requirements for pipeline integrity management programs.\n\nWhat We Recommended\nAs a result of this investigation, we made the following new safety recommendations.\nTo the Research and Special Programs Administration:\n\nRemove the exemption in 49 Code of Federal Regulations 192.65 (b) that permits pipe to be placed in natural gas service after pressure testing when the pipe cannot be verified to have been transported in accordance with the American Petroleum Institute recommended practice 5L1. (P-04-01)\nAmend 49 Code of Federal Regulations to require that natural gas pipeline operators (Part 192) and hazardous liquid pipeline operators (Part 195) follow the American Petroleum Institute recommended practice 5LW for transportation of pipe on marine vessels. (P-04-02)\nEvaluate the need for a truck transportation standard to prevent damage to pipe, and, if needed, develop the standard and incorporate it in 49 Code of Federal Regulations Parts 192 and 195 for both natural gas and hazardous liquid line pipe. (P-04-03)\nTo the American Society of Mechanical Engineers:\n\nAmend American Society of Mechanical Engineers B31.8, Gas Transmission and Distribution Piping Systems, section 816, to remove the provision that pressure testing may be used to verify the integrity of pipe that may not have been transported in accordance with the American Petroleum Institute recommended practices for transportation of pipe by railroad or marine vessels. (P-04-04)\nAmend American Society of Mechanical Engineers B31.4, Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids, section 434.4, to require the use of the American Petroleum Institute recommended practice 5LW for marine transport of pipe. (P-04-05)\nTo the American Petroleum Institute: Review the equations in American Petroleum Institute recommended practice 5L1, Recommended Practice for Railroad Transportation of Line Pipe, and American Petroleum Institute recommended practice 5LW, Recommended Practice for Transportation of Line Pipe on Barges and Marine Vessels, for calculating the static load stresses at the bearing or separator strips and revise the recommended practices based on that review. (P-04-06)\n\nPAR-04-01\n<<<PAGE 1>>>\n\nRupture of Enbridge Pipeline and Release of Crude Oil\nnear Cohasset, Minnesota\nJuly 4, 2002\nPipeline Accident Report\nNTSB/PAR-04/01\nPB2004-916501\nNotation 7514A\nNational\nTransportation\nSafety Board\nWashington, D.C.\n\n<<<PAGE 2>>>\n\nthis page intentionally left blank\n\n<<<PAGE 3>>>\n\nPipeline Accident Report\nRupture of Enbridge Pipeline and Release of\nCrude Oil near Cohasset, Minnesota\nJuly 4, 2002\nNTSB/PAR-04/01\nPB2004-916501 National Transportation Safety Board\nNotation 7514A 490 LíEnfant Plaza, S.W.\nAdopted June 23, 2004 Washington, D.C. 20594\n\n<<<PAGE 4>>>\n\nNational Transportation Safety Board. 2004. Rupture of Enbridge Pipeline and Release of Crude Oil\nnear Cohasset, Minnesota, July 4, 2002. Pipeline Accident Report NTSB/PAR-04/01. Washington, DC.\nAbstract: About 2:12 a.m., central daylight time, on July 4, 2002, a 34-inch-diameter steel pipeline owned\nand operated by Enbridge Pipelines, LLC ruptured in a marsh west of Cohasset, Minnesota.\nApproximately 6,000 barrels (252,000 gallons) of crude oil were released from the pipeline as a result of\nthe rupture. The cost of the accident was reported to the Research and Special Programs Administration\nOffice of Pipeline Safety to be approximately $5.6 million. No deaths or injuries resulted from the release.\nThe safety issues identified in this accident are the effectiveness and application of line pipe transportation\nstandards and the adequacy of Federal requirements for pipeline integrity management programs.\nAs a result of its investigation of this accident, the Safety Board issues safety recommendations to the\nResearch and Special Programs Administration, the American Society of Mechanical Engineers, and the\nAmerican Petroleum Institute.\nThe National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine,\npipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board\nAct of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study\ntransportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board\nmakes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and\nstatistical reviews.\nRecent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also\nmay be obtained from the Web site or by contacting:\nNational Transportation Safety Board\nPublic Inquiries Section, RE-51\n490 LíEnfant Plaza, S.W.\nWashington, D.C. 20594\n(800) 877-6799 or (202) 314-6551\nSafety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To\npurchase this publication, order report number PB2004-916501 from:\nNational Technical Information Service\n5285 Port Royal Road\nSpringfield, Virginia 22161\n(800) 553-6847 or (703) 605-6000\nThe Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports\nrelated to an incident or accident in a civil action for damages resulting from a matter mentioned in the report.\n\n<<<PAGE 5>>>\n\niii Pipeline Accident Report\nContents\nExecutive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv\nFactual Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1\nAccident Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1\nAccident Narrative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1\nEmergency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3\nDamage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5\nPostaccident Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5\nTests and Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6\nPreaccident Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8\nFatigue Cracking in Enbridge Pipe Manufactured by U.S. Steel . . . . . . . . . . . . . . . . . . 8\nOperational Reliability Assessments of the Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8\nElastic Wave In-Line Inspection at Rupture Location . . . . . . . . . . . . . . . . . . . . . . . . . . 11\nPipe Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12\nRailroad Transportation of Thin-Walled Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12\nRailroad Transportation of Accident Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16\nSafety Board Materials Laboratory Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18\nRSPA Postaccident Corrective Action Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20\nEnbridge Postaccident Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21\nAmerican Society of Mechanical Engineers Pipeline Codes . . . . . . . . . . . . . . . . . . . . . . . . 23\nAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25\nThe Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25\nTransportation of Accident Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25\nTransportation Fatigue Cracking in Line Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27\nNatural Gas Pipeline Safety Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28\nLiquid Pipeline Safety Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28\nMarine Transportation of Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29\nTruck Transportation of Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29\nASME Pipeline Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29\nPipeline Integrity Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30\nConclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33\nFindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33\nProbable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33\nRecommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34\nAppendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37\nInvestigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37\n\n<<<PAGE 6>>>\n\niv Pipeline Accident Report\nExecutive Summary\nAbout 2:12 a.m., central daylight time, on July 4, 2002, a 34-inch-diameter steel\npipeline owned and operated by Enbridge Pipelines, LLC ruptured in a marsh west of\nCohasset, Minnesota. Approximately 6,000 barrels (252,000 gallons) of crude oil were\nreleased from the pipeline as a result of the rupture. The cost of the accident was reported\nto the Research and Special Programs Administration Office of Pipeline Safety to be\napproximately $5.6 million. No deaths or injuries resulted from the release.\nThe National Transportation Safety Board determines that the probable cause of\nthe July 4, 2002, pipeline rupture near Cohasset, Minnesota, was inadequate loading of the\npipe for transportation that allowed a fatigue crack to initiate along the seam of the\nlongitudinal weld during transit. After the pipe was installed, the fatigue crack grew with\npressure cycle stresses until the crack reached a critical size and the pipe ruptured.\nThe following safety issues were identified during this investigation:\nï The effectiveness and application of line pipe transportation standards.\nï The adequacy of Federal requirements for pipeline integrity management\nprograms.\nAs a result of its investigation of this accident, the Safety Board issues safety\nrecommendations to the Research and Special Programs Administration, the American\nSociety of Mechanical Engineers, and the American Petroleum Institute.\n\n<<<PAGE 7>>>\n\n1 Pipeline Accident Report\nFactual Information\nAccident Synopsis\nAbout 2:12 a.m., central daylight time, on July 4, 2002, a 34-inch-diameter steel\npipeline owned and operated by Enbridge Pipelines (Lakehead), LLC1 ruptured in a marsh\nwest of Cohasset, Minnesota. (See figure 1.) Approximately 6,000 barrels (252,000\ngallons) of crude oil were released from the pipeline as a result of the rupture. No deaths\nor injuries resulted from the release.\nFigure 1. Enbridge pipeline system.\nAccident Narrative\nThe crude oil pipeline involved in the accident originated at Edmonton, Alberta,\nCanada, and terminated at Superior Terminal in Superior, Wisconsin. The 34-inch-\ndiameter pipeline, designated line no. 4 at the time of the accident, was operated by\npipeline controllers in the Enbridge control center in Edmonton using a supervisory\ncontrol and data acquisition (SCADA) system.2 About 2:12 a.m. on July 4, 2002, the\n1 Enbridge Pipelines (Lakehead), LLC is the operator of the pipeline system formerly named Lakehead\nPipe Line Company.\n2 Pipeline controllers use a computer-based SCADA system to remotely monitor and control\nmovement of oil through pipelines. The system makes it possible to monitor operating parameters critical to\npipeline operations, such as flow rates, pressures, equipment status, control valve positions, and alarms\nindicating abnormal conditions.\n\n<<<PAGE 8>>>\n\nFactual Information 2 Pipeline Accident Report\ncontroller operating the line observed a SCADA system indication of a loss of suction and\ndischarge pressure at the Deer River pump station. (See figure 2.) At 2:13 a.m., the\nFloodwood pump station suction pressures began dropping, and then audible and visual\nalarms were received for an invalid suction pressure. The controller initially suspected an\ninaccurate pressure transmitter at Floodwood, because the suction pressure had gone to\nzero. Subsequently, he noticed that the discharge pressure for Floodwood was also\ndropping and realized that he had an abnormal condition. The controller showed the shift\ncoordinator the situation, and, suspecting a possible leak, they agreed at 2:14 a.m. to shut\nthe pipeline down. At 2:15: a.m., the controller initiated closure of the pipeline injection\nvalve at the Clearbrook Terminal and began shutting down pumps and remotely closed\nvalves to isolate the suspected leak. The upstream valve at Deer River and the downstream\nsectionalizing valve at milepost (MP) 1017.9 were remotely closed by 2:21 a.m., which\nisolated the ruptured section. All remotely controlled valves on the pipeline from\nClearbrook to Superior Terminal were closed by 2:32 a.m.\nFigure 2. Enbridge pipeline facilities and rupture site.\nAbout 2:25 a.m., the Enbridge control center notified the Deer River and\nFloodwood police departments of the suspected leak, and about 2:30 a.m., Enbridge field\npersonnel were notified. About 5:20 a.m., Enbridge field personnel dispatched to\ninvestigate along the pipeline right-of-way detected the odor of crude oil in a marshy area\nnear Blackwater Creek and manually closed the closest valve to the failure. This valve was\nnear MP 1007.32, about 4 1/2 miles downstream (east) of the rupture.\n\n<<<PAGE 9>>>\n\nFactual Information 3 Pipeline Accident Report\nAt 7:00 a.m., after Enbridge field employees verified the release, Enbridge notified\nthe National Response Center of a crude oil leak in the companyís 34-inch pipeline. This\nnotification indicated that an unknown amount of crude oil had been released. The pipe\nwas found to have ruptured at MP 1002.73, about 7 miles downstream of the Deer River\npump station. The company then contacted local, State, and Federal officials, as well as\nEnbridge spill response contractors, who proceeded to the spill site. Enbridge also had\nright-of-way representatives contact landowners in the vicinity of the spill. At 12:09 p.m.,\nEnbridge called the National Response Center again and updated the spill volume to 6,000\nbarrels of crude oil. At the time of the accident, Enbridge had not designated the area\nwhere the rupture occurred as a high-consequence area3 based on the criteria defined in 49\nCode of Federal Regulations (CFR) Part 195, ìTransportation of Hazardous Liquids by\nPipelines.î\nEmergency Response\nBooms were placed in Blackwater Creek as a precaution to prevent crude oil from\nmoving away from the spill site toward nearby waterways, including the Mississippi\nRiver. Enbridge started building a 1/4-mile-long road along the right-of-way to the spill\nsite using wood mats. With heavy rain forecast, responders were concerned that the crude\noil might spread farther and contaminate the Mississippi River. The unified command for\nthe accident response was established and included the Cohasset Fire Department,\nEnbridge, the Minnesota Pollution Control Agency, the Minnesota Department of\nEmergency Management, and the Forestry Division of the Minnesota Department of\nNatural Resources.\nThe unified command decided that the best way to prevent the crude from entering\nnearby waterways was to perform a controlled burn. As a precaution, the command\ndesignated 12 homes in the local area to be evacuated, and seven residents were\nevacuated. Later in the afternoon, the Minnesota Department of Natural Resources coated\nthe spillís perimeter with chemical fire retardant from tanker planes. After the chemical\nwas placed, flares were shot into the crude oil to ignite the oil.\nThe controlled burn was ignited about 4:45 p.m. (See figure 3.) The burn created a\nsmoke plume about 1 mile high and 5 miles long. (See figure 4.) The controlled burn\nlasted until about 5:00 p.m. the next day, July 5. While they monitored the fire, Enbridge\npersonnel, firefighters, and environment authorities also monitored the spill perimeter to\nensure that no crude was getting into area waterways. Reportedly, no free-flowing product\nreached any of the boomed areas.\n3 High-consequence area refers to commercially navigable waterways, high population areas,\nconcentrated population areas, or unusually sensitive areas that might be affected by an accident involving\nthe pipeline in that area. Title 49 CFR 195.450, 195.452, and 195.6 contain the criteria for designating an\narea a high-consequence area for hazardous liquid pipelines.\n\n<<<PAGE 10>>>\n\nFactual Information 4 Pipeline Accident Report\nFigure 3. Controlled burn surrounded by white fire retardant.\nFigure 4. Smoke plume 1 mile high and 5 miles long.\n\n<<<PAGE 11>>>\n\nFactual Information 5 Pipeline Accident Report\nDamage\nThe cost of the accident was reported to the Research and Special Programs\nAdministration (RSPA) Office of Pipeline Safety to be approximately $5.6 million.4\nEnbridge recovered 2,574 barrels of oil and estimated that the in situ burn consumed\napproximately 3,000 barrels, with the remainder being lost to evaporation or entrapment\nin the soil.\nPostaccident Inspection\nOn July 6, after vacuum trucks had removed the remaining oil and water, the\nruptured pipe was exposed. The pipe was fractured along the edge of a longitudinal weld.\nWhen the pipe that failed was installed, the longitudinal weld was at the 5:30 clock\nposition when viewed facing downstream (eastward). The rupture was about 69 inches\nlong and gapped open about 6 1/4 inches at the center. (See figure 5.) At the rupture\nlocation, the pipeline was rated for a regulatory maximum operating pressure of 687\npounds per square inch, gauge (psig). The pressure at this location at the time of failure\nwas calculated to be 526 psig. The United States Steel Corporation (U.S. Steel)\nmanufactured the pipe at its National Tube Works in McKeesport, Pennsylvania.\nFigure 5. Rupture in accident pipe.\n4 This total includes estimated property damage, including cost of cleanup and recovery, value of lost\nproduct, and damage to the property of the pipeline operator and others.\n\n<<<PAGE 12>>>\n\nFactual Information 6 Pipeline Accident Report\nTests and Research\nTwo sections of pipe, one containing the rupture and one from the same length of\npipe, were removed and sent to the Safety Boardís Materials Laboratory for metallurgical\nexamination. The pipe that ruptured was manufactured in accordance with American\nPetroleum Institute (API) standard 5L, grade X52, indicating that the steel had a specified\nminimum yield strength5 of 52,000 pounds per square inch (psi). The 34-inch outside\ndiameter pipe was specified as 0.312-inch nominal wall thickness with a double\nsubmerged arc weld (DSAW) longitudinal seam weld. The pipe had a diameter-to-wall\nthickness (D/t) ratio of 109:1. The pipe was coated with a spiral wrap tape that was applied\nin the field during construction in 1967.\nSurface corrosion was visible on the outer surface of the pipe adjacent to the\nrupture, but no dents, scratches, or gouges were present at any location on the pipe\nsections examined. The corrosion was assessed as light, with no apparent pitting and little\napparent loss of wall thickness. Both pipe sections were ultrasonically inspected for cracks\nalong the longitudinal seam weld, and, other than the rupture that caused the accident, no\nadditional cracks or discontinuities were uncovered. Fatigue cracking6 has been shown to\ninitiate at seam welds because of changes in geometry, residual stress, and material\nproperties associated with the weld. Metallurgical testing and examination of the ruptured\narea found no material or manufacturing defect in the steel or the welded seam of the pipe.\nInitial examination of the rupture revealed a preexisting fatigue region at the center\nof the rupture. The fatigue region was 13 inches long adjacent to the inside surface of the\npipe and did not extend all the way through the pipe wall. (See figure 6.) More detailed\nexamination showed that the fatigue cracking initiated at multiple locations along the\ninside surface (see figure 7) at the toe of the longitudinal weld bead. (See figure 8.)\nExamination of the cleaned fracture surface revealed a darker, more heavily oxidized band\nadjacent to the inside surface of the pipe that extended the entire length of the fatigue area.\nThe more heavily oxidized portion of the fatigue area penetrated a maximum of about 0.04\ninch deep at the center of the rupture. The oxidized band was visible for almost the entire\nlength of the fatigue area. Near its ends, the oxidized portion of the fatigue crack extended\nabout 0.010 inch into the pipe wall. The remainder of the fatigue crack was less oxidized\nand extended more deeply into the pipe wall over the central 6 inches of the fatigue\nregion. Along approximately 2.5 inches in the central region, the fatigue crack almost\npenetrated the pipe wall. At its maximum depth, the fatigue crack penetrated through 0.270\n5 Yield strength is a measure of the pipeís material strength and is the stress level, expressed in pounds\nper square inch, at which the material starts to exhibit permanent deformation. Although yield strength is\nexpressed in pounds per square inch, this value is an expression of a pipe materialís strength, which is not\nequivalent to a pipeís internal pressure.\n6 The term fatigue cracking is used to describe a progressive cracking of structural material that occurs\nunder repeated loading and may eventually lead to failure. The fatigue crack grows with cyclic loading until\nthe crack reaches a critical length at which the stresses cause it to grow unstably leading to structural failure.\nFatigue cracks can initiate at microscopic flaws or weak spots in the material. Once initiated, cracks can\ngrow at stress levels that are quite low in comparison to the materialís yield strength.\n\n<<<PAGE 13>>>\n\nFactual Information 7 Pipeline Accident Report\ninch of the 0.297-inch measured wall thickness.7 Measurement and testing of the pipe\nshowed that it met thickness and strength requirements. The pipe fracture beyond the\nfatigue crack contained features typical of overstress fracture.\nFigure 6. View of top fracture surface of 13-inch-long crack, showing penetration nearly\nthrough pipe wall in center.\nFigure 7. Face of fracture in accident pipe.\n7 The 0.297-inch measured wall thickness is within the allowable range for a pipe with 0.312-inch\nspecified nominal wall thickness.\n\n<<<PAGE 14>>>\n\nFactual Information 8 Pipeline Accident Report\nFigure 8. Fatigue initiating at toe of weld on interior surface of pipe.\nPreaccident Events\nFatigue Cracking in Enbridge Pipe Manufactured by U.S. Steel\nEnbridgeís 34-inch U.S. Steel DSAW pipe had a documented history of\nlongitudinal seam weld failures due to fatigue cracks. Metallurgical analysis reports of\nlongitudinal seam weld failures in Enbridgeís U.S. Steel pipe in 1974, 1979, 1982, 1986,\n1989, and 1991 identified the causes as fatigue cracking at the toe of the weld. Enbridgeís\n34-inch pipeline system also used A.O. Smith flash-welded pipe, Canadian Phoenix\nelectric resistance welded pipe, and Kaiser Steel submerged arc welded (SAW) pipe. All\nof the longitudinal seam weld failures caused by fatigue cracks in this pipeline have\noccurred in pipe manufactured by U.S. Steel.\nOperational Reliability Assessments of the Pipeline\nAfter the 1991 pipe rupture at the toe of the weld in the 34-inch pipeline resulted in\nthe release of 40,500 barrels (1,701,000 gallons) of crude oil, Enbridge signed a consent\norder with RSPAís Office of Pipeline Safety to conduct an operational reliability\nassessment of the 34-inch pipeline from Gretna, Manitoba, Canada, to Superior,\nWisconsin. The assessment was to include a review of pipeline operating conditions and\nan analysis of the previous pipe failures. The operator was also required to restrict\n\n<<<PAGE 15>>>\n\nFactual Information 9 Pipeline Accident Report\nallowable operating pressures, to hydrostatically pressure test8 the pipeline to establish\nthat the line was safe to operate, and to develop a program to ensure that the line would\ncontinue to be safe in the future.\nIn December 1992, Enbridge performed an operational reliability assessment9 of\nthe 34-inch pipeline in the United States. As a result of the study, changes were made in\npipeline operations that reduced the number of pressure cycles10 and their associated\npressure ranges. Among other actions it took as a result of the 1991 rupture, Enbridge\nfinancially and technically supported British Gasís development of the Elastic Wave in-\nline inspection tool to identify pipe cracks before they precipitate a failure. British Gas did\nthe inspections in 1995 and 1996. PII North American, Inc. (PII), the successor to British\nGas, currently provides the inspection tool data report of the Elastic Wave inspection tool\nin the United States.\nThe pipeline section in which the 2002 rupture occurred was pressure tested to 835\npsig after its construction in 1967. Enbridgeís first longitudinal seam weld in-service\nfailure of U.S. Steel pipe from a fatigue crack occurred in July 1974. The entire pipeline,\nincluding the pipe joint11 containing the failure, was pressure tested between 1974 and\n1976 at a test pressure of 764 psig. The entire 34-inch pipeline was pressure tested in 1991\nand 1992 at higher stress levels than had been used before. Because of variations in pipe\nwall thickness and changes in elevation in each section of the pipeline, the test pressure\nrange was from 85 percent to 105 percent of the specified minimum yield strength of the\npipe, or up to 1,002 psig.12 The 1991 test pressure at the point of the July 4, 2002, rupture\nwas 937 psig. The operator agreed in 1991 to pressure test the pipeline again in 5 years\nunless an in-line inspection tool capable of identifying cracks in the longitudinal seam of\nthe pipe was developed. RSPA did not allow the operator to raise the pressures above\nthose in effect at the time of the 1991 accident while the consent order was in effect.\nDuring the 1991 and 1992 pressure testing program, Enbridge found four crack-\nlike/manufacturing defects, four corrosion defects, and one blister. Two subsequent leaks\noccurred that resulted from pressure-cycle-induced growth of fatigue cracks in U.S. Steel\npipe. The two in-service leaks occurred in the first 6 months of 1994 at the site of fatigue\ncracks that had survived the pressure test levels of the 1991ñ1992 program. A reassessment\nreport was completed in December 1994 following those two failures. Enbridgeís\nmetallurgical report indicated that the initiating fatigue cracks were readily apparent\nadjacent to the inside pipe wall and had been introduced during the transportation of the\npipe, as they were smoother and darker than subsequent fatigue crack growth. The report\n8 A hydrostatic test of a pipeline involves filling the pipeline with water or similar liquid, gradually\nincreasing the pressure of the liquid to a predetermined maximum, and examining the line and/or test\nrecords for indications of a leak.\n9 The 1992 assessment was updated in 1994, 1995, and 1998.\n10 One pipeline pressure cycle is the pressure variation from a minimum to a maximum pressure and to\nthe minimum again.\n11 A joint is a single length of pipe, nominally 40 feet long.\n12 Using the internal design strength formula in 49 CFR Part 195, a test pressure of 954 psig is calculated\nat 100 percent of specified minimum yield strength for line pipe with the specification of the pipe that ruptured.\n\n<<<PAGE 16>>>\n\nFactual Information 10 Pipeline Accident Report\nnoted that both defects at the point of failure showed evidence of having grown during the\n1991ñ1992 pressure tests and concluded that ductile tearing of the metal caused the growth\nof these existing defects. Another Enbridge conclusion was that the operating histories of\nthe upstream operating stations showed that pressure cycles also contributed to the failures.\nAfter Enbridge ran tests with the Elastic Wave inspection tool, the results were\nreviewed and recommendations were included in Enbridgeís 1995 integrity assessment\nreport. As a result of the recommendations, Enbridge proposed to RSPA an in-line crack\ninspection program as the most appropriate means of reducing or eliminating the risk of\npipeline failures. The detection level specification for the Elastic Wave tool stated that the\ntool would find a defect equal to or greater than 2.5 inches long with an accuracy of ±0.4\ninch at 4.5 mph. The detection level specification for crack depth was 25 percent of the\npipe wall thickness with a sizing accuracy of ±25 percent of the wall thickness. For an\nindication to be reported to the operator as a defect, both the crack length and the crack\ndepth threshold requirements had to be met.\nRSPA agreed in 1995 to the use of the in-line crack inspection program in lieu of\nhydrostatic pressure testing. As a condition for accepting the proposal for 1996, RSPA\nstipulated that it would review the inspection program before deciding on future pressure\ntesting. One of the reasons for conditional approval in RSPAís stipulations was that RSPA\nwanted to know whether the Elastic Wave inspection tool would identify not only pipe\ncrack defects that would fail during hydrostatic pressure testing but also considerably\nsmaller defects that could then be repaired or removed before they could grow and lead to\nfailure of the pipe.\nIn 1995, Enbridge began inspecting its 34-inch pipeline with the Elastic Wave in-\nline inspection tool and found that the tool was identifying more pipe crack defects than\nhad been identified by previous hydrostatic pressure testing. Twice during 1995 and again\nin early 1996, PIIís tool was used to inspect the pipeline section that contained the crack\nthat ruptured in this accident, but various mechanical problems with the inspection tool\nresulted in unusable data. PII acquired usable data in a May 1996 inspection. (The details\nof this inspection are discussed later in this report.)\nIn the 4 years from 1995 through 1998, 216 miles (66 percent) of the 325 miles of\n34-inch pipe from Gretna, Manitoba, to Superior, Wisconsin, had been inspected with the\nElastic Wave tool, and pipeline repairs were made according to the pipeline operatorís\npolicy. All crack defects identified by the inspections were repaired with pipe sleeves, and\nnone were removed and subjected to metallurgical examination. During this period of\ntime, in-line inspections were performed on all U.S. Steel manufactured DSAW pipe. As a\nresult of these inspections, the operator excavated the pipe at 74 locations. An evaluation\nconcluded that none of the defects found with the Elastic Wave tool would have failed a\npressure test to 100 percent specified minimum yield strength. Following completion of\nthe Elastic Wave tool inspections in the 34-inch U.S. Steel pipe, Enbridge submitted an\nassessment report dated April 28, 1998, that proposed reinspecting the pipeline\napproximately 10 years from the previous inspection. A number of reviews were made by\nRSPA before closure of the consent order on May 5, 1999. After the consent order was\nclosed, Enbridge operated the pipeline up to the pressures allowed by 49 CFR Part 195.\n\n<<<PAGE 17>>>\n\nFactual Information 11 Pipeline Accident Report\nBefore the accident, Enbridgeís unwritten defect inspection practice for Elastic\nWave data was to excavate all crack-like indications that were found by the Elastic Wave\ntool. Enbridge ran Elastic Wave tool inspections in all of its 34-inch pipeline sections in\nthe United States between 1995 and 2001. Based on the results of these inspections, the\ncompany excavated 23 crack-like features; 23 weld/manufacturing defects; 16 other\ndefects, including corrosion and laminations; and 41 spurious13 indications and made\nrepairs where needed.\nElastic Wave In-Line Inspection at Rupture Location\nThe in-line inspection company, PII, performed a computer analysis of the May\n1996 Elastic Wave inspection tool log data as part of its interpretation process after the\ntool was run. An indication was present at the point where the pipe ruptured on\nJuly 4, 2002. PII interpreters reviewed the indication in their initial screening of the data\nin 1996, but the indication did not exhibit the diamond-shaped signature signifying a crack\nand did not meet PIIís standard that an anomaly must meet at least 6 of 10 feature\nselection criteria in order to be identified as a crack. After the accident, PII stated that, at\nmost, the indication would have met two of the feature selection criteria. An important\nfeature selection criterion that the indication did not meet was confirmation of the signal\nfrom both the clockwise and counterclockwise views as the tool records data while\nmoving downstream through the pipe. PII representatives stated that during the May 1996\ninspection run, one of the toolís two sets of wheel sensors was close to the longitudinal\nweld, which placed the weld in proximity to the source of the toolís ultrasonic signal and\ncould have resulted in the masking of the signal.\nPIIís postaccident review of the May 1996 data also evaluated the size of the\nindication at the rupture and determined that it was below the detection level specification\nfor a reportable defect (25 percent of pipe wall thickness and 2.5 inches long). The data on\nthis indication have been recorded in a database, and PII and Enbridge have worked to\ndetermine how this information will be used to improve the feature selection criteria. Also\nafter the accident, RSPA had an independent consultant and PII analyze the May 1996\ninspection log data for the area from 0.5 mile upstream to 0.5 mile downstream of the\nrupture location. No indications were found with characteristics similar to those of the\nJuly 4, 2002, rupture.\nIn addition, PII personnel reviewed the log data from two 1995 Elastic Wave tool\ninspections that had shown no significant defect at the point of the 2002 rupture. They found\nthat on the first run, the clockwise sensor was functioning properly and was not on the\nlongitudinal weld at the point that ruptured. The counterclockwise channel was working but\nwas electronically noisy and provided a weak signal at the point that ruptured. Thus the\nsignal on this run did not meet feature selection criteria for confirmation of the signal from\nboth the clockwise and counterclockwise views. The signal on this run also did not exhibit\nthe diamond-shaped crack signature. On the second 1995 log, the clockwise channel was not\nproviding acceptable quality data when it was in the area of the point of rupture.\n13 Spurious features were those that did not have a corresponding defect associated with them, had\nqualities not considered a defect (for example, weld profile), or were under sleeves and could not be assessed.\n\n<<<PAGE 18>>>\n\nFactual Information 12 Pipeline Accident Report\nAll of the 1995ñ1996 in-line Elastic Wave tool inspections were performed by the\nMark II version of the device. In 1997 the tool was upgraded to the Interim Mark III,\nwhich contains an additional set of wheel sensors that are offset so at least one set of\nsensors is not riding on the longitudinal seam weld.\nBoth before and after the accident, Enbridge provided PII with feedback on its\nfindings from actual excavations and field inspections.","truncated":true,"body_characters":106948}