# Rupture of Enbridge Pipeline and Release of Crude Oil - **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. Event Type: Accident Event Date: 2002-07-04 Event City: Cohasset Event State Or Region: MN Event Country: USA Pipeline Operator: Enbridge Energy Partners L.P. Pipeline Type: Hazardous Liquid - Regulated Accident Type: Rupture Completion Status: Completed Report Number: PAR-04-01 Probable 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. Tier1Name: System operating Tier2Name: Product leak/release Tier1Name: System operating Tier2Name: Pipe structural malfunction/failure Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Oversight Finding Modifier Name: Federal agency Finding Report Text: Organizational - Support/oversight/monitoring - Oversight - Federal agency Finding Tier1Name: Organizational Finding Tier2Name: Development Finding Tier3Name: Design Finding Modifier Name: Other institution/organization Finding Report Text: Organizational - Development - Design - Other institution/organization Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline structure Finding Tier3Name: Joint/weld/seam Finding Modifier Name: Fatigue/wear/corrosion Finding Report Text: Pipeline - Pipeline structure - Joint/weld/seam - Fatigue/wear/corrosion Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: (general) Finding Tier4Name: (general) Finding Modifier Name: Damaged/degraded Finding Report Text: Pipeline - Pipeline handling/service - (general) - Damaged/degraded Official NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations. What Happened About 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. What We Found We 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. The following safety issues were identified during this investigation: The effectiveness and application of line pipe transportation standards. The adequacy of Federal requirements for pipeline integrity management programs. What We Recommended As a result of this investigation, we made the following new safety recommendations. To the Research and Special Programs Administration: Remove 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) Amend 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) Evaluate 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) To the American Society of Mechanical Engineers: Amend 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) Amend 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) To 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) PAR-04-01 <<>> Rupture of Enbridge Pipeline and Release of Crude Oil near Cohasset, Minnesota July 4, 2002 Pipeline Accident Report NTSB/PAR-04/01 PB2004-916501 Notation 7514A National Transportation Safety Board Washington, D.C. <<>> this page intentionally left blank <<>> Pipeline Accident Report Rupture of Enbridge Pipeline and Release of Crude Oil near Cohasset, Minnesota July 4, 2002 NTSB/PAR-04/01 PB2004-916501 National Transportation Safety Board Notation 7514A 490 LíEnfant Plaza, S.W. Adopted June 23, 2004 Washington, D.C. 20594 <<>> National Transportation Safety Board. 2004. Rupture of Enbridge Pipeline and Release of Crude Oil near Cohasset, Minnesota, July 4, 2002. Pipeline Accident Report NTSB/PAR-04/01. Washington, DC. Abstract: About 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. The safety issues identified in this accident are the effectiveness and application of line pipe transportation standards and the adequacy of Federal requirements for pipeline integrity management programs. As a result of its investigation of this accident, the Safety Board issues safety recommendations to the Research and Special Programs Administration, the American Society of Mechanical Engineers, and the American Petroleum Institute. The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine, pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews. Recent publications are available in their entirety on the Web at . Other information about available publications also may be obtained from the Web site or by contacting: National Transportation Safety Board Public Inquiries Section, RE-51 490 LíEnfant Plaza, S.W. Washington, D.C. 20594 (800) 877-6799 or (202) 314-6551 Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To purchase this publication, order report number PB2004-916501 from: National Technical Information Service 5285 Port Royal Road Springfield, Virginia 22161 (800) 553-6847 or (703) 605-6000 The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. <<>> iii Pipeline Accident Report Contents Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv Factual Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Accident Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Accident Narrative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Emergency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Postaccident Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Tests and Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Preaccident Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Fatigue Cracking in Enbridge Pipe Manufactured by U.S. Steel . . . . . . . . . . . . . . . . . . 8 Operational Reliability Assessments of the Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Elastic Wave In-Line Inspection at Rupture Location . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Pipe Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Railroad Transportation of Thin-Walled Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Railroad Transportation of Accident Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Safety Board Materials Laboratory Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 RSPA Postaccident Corrective Action Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Enbridge Postaccident Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 American Society of Mechanical Engineers Pipeline Codes . . . . . . . . . . . . . . . . . . . . . . . . 23 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Transportation of Accident Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Transportation Fatigue Cracking in Line Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Natural Gas Pipeline Safety Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Liquid Pipeline Safety Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Marine Transportation of Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Truck Transportation of Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 ASME Pipeline Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Pipeline Integrity Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Probable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 <<>> iv Pipeline Accident Report Executive Summary About 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. 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. The following safety issues were identified during this investigation: ï The effectiveness and application of line pipe transportation standards. ï The adequacy of Federal requirements for pipeline integrity management programs. As a result of its investigation of this accident, the Safety Board issues safety recommendations to the Research and Special Programs Administration, the American Society of Mechanical Engineers, and the American Petroleum Institute. <<>> 1 Pipeline Accident Report Factual Information Accident Synopsis About 2:12 a.m., central daylight time, on July 4, 2002, a 34-inch-diameter steel pipeline owned and operated by Enbridge Pipelines (Lakehead), LLC1 ruptured in a marsh west of Cohasset, Minnesota. (See figure 1.) Approximately 6,000 barrels (252,000 gallons) of crude oil were released from the pipeline as a result of the rupture. No deaths or injuries resulted from the release. Figure 1. Enbridge pipeline system. Accident Narrative The crude oil pipeline involved in the accident originated at Edmonton, Alberta, Canada, and terminated at Superior Terminal in Superior, Wisconsin. The 34-inch- diameter pipeline, designated line no. 4 at the time of the accident, was operated by pipeline controllers in the Enbridge control center in Edmonton using a supervisory control and data acquisition (SCADA) system.2 About 2:12 a.m. on July 4, 2002, the 1 Enbridge Pipelines (Lakehead), LLC is the operator of the pipeline system formerly named Lakehead Pipe Line Company. 2 Pipeline controllers use a computer-based SCADA system to remotely monitor and control movement of oil through pipelines. The system makes it possible to monitor operating parameters critical to pipeline operations, such as flow rates, pressures, equipment status, control valve positions, and alarms indicating abnormal conditions. <<>> Factual Information 2 Pipeline Accident Report controller operating the line observed a SCADA system indication of a loss of suction and discharge pressure at the Deer River pump station. (See figure 2.) At 2:13 a.m., the Floodwood pump station suction pressures began dropping, and then audible and visual alarms were received for an invalid suction pressure. The controller initially suspected an inaccurate pressure transmitter at Floodwood, because the suction pressure had gone to zero. Subsequently, he noticed that the discharge pressure for Floodwood was also dropping and realized that he had an abnormal condition. The controller showed the shift coordinator the situation, and, suspecting a possible leak, they agreed at 2:14 a.m. to shut the pipeline down. At 2:15: a.m., the controller initiated closure of the pipeline injection valve at the Clearbrook Terminal and began shutting down pumps and remotely closed valves to isolate the suspected leak. The upstream valve at Deer River and the downstream sectionalizing valve at milepost (MP) 1017.9 were remotely closed by 2:21 a.m., which isolated the ruptured section. All remotely controlled valves on the pipeline from Clearbrook to Superior Terminal were closed by 2:32 a.m. Figure 2. Enbridge pipeline facilities and rupture site. About 2:25 a.m., the Enbridge control center notified the Deer River and Floodwood police departments of the suspected leak, and about 2:30 a.m., Enbridge field personnel were notified. About 5:20 a.m., Enbridge field personnel dispatched to investigate along the pipeline right-of-way detected the odor of crude oil in a marshy area near Blackwater Creek and manually closed the closest valve to the failure. This valve was near MP 1007.32, about 4 1/2 miles downstream (east) of the rupture. <<>> Factual Information 3 Pipeline Accident Report At 7:00 a.m., after Enbridge field employees verified the release, Enbridge notified the National Response Center of a crude oil leak in the companyís 34-inch pipeline. This notification indicated that an unknown amount of crude oil had been released. The pipe was found to have ruptured at MP 1002.73, about 7 miles downstream of the Deer River pump station. The company then contacted local, State, and Federal officials, as well as Enbridge spill response contractors, who proceeded to the spill site. Enbridge also had right-of-way representatives contact landowners in the vicinity of the spill. At 12:09 p.m., Enbridge called the National Response Center again and updated the spill volume to 6,000 barrels of crude oil. At the time of the accident, Enbridge had not designated the area where the rupture occurred as a high-consequence area3 based on the criteria defined in 49 Code of Federal Regulations (CFR) Part 195, ìTransportation of Hazardous Liquids by Pipelines.î Emergency Response Booms were placed in Blackwater Creek as a precaution to prevent crude oil from moving away from the spill site toward nearby waterways, including the Mississippi River. Enbridge started building a 1/4-mile-long road along the right-of-way to the spill site using wood mats. With heavy rain forecast, responders were concerned that the crude oil might spread farther and contaminate the Mississippi River. The unified command for the accident response was established and included the Cohasset Fire Department, Enbridge, the Minnesota Pollution Control Agency, the Minnesota Department of Emergency Management, and the Forestry Division of the Minnesota Department of Natural Resources. The unified command decided that the best way to prevent the crude from entering nearby waterways was to perform a controlled burn. As a precaution, the command designated 12 homes in the local area to be evacuated, and seven residents were evacuated. Later in the afternoon, the Minnesota Department of Natural Resources coated the spillís perimeter with chemical fire retardant from tanker planes. After the chemical was placed, flares were shot into the crude oil to ignite the oil. The controlled burn was ignited about 4:45 p.m. (See figure 3.) The burn created a smoke plume about 1 mile high and 5 miles long. (See figure 4.) The controlled burn lasted until about 5:00 p.m. the next day, July 5. While they monitored the fire, Enbridge personnel, firefighters, and environment authorities also monitored the spill perimeter to ensure that no crude was getting into area waterways. Reportedly, no free-flowing product reached any of the boomed areas. 3 High-consequence area refers to commercially navigable waterways, high population areas, concentrated population areas, or unusually sensitive areas that might be affected by an accident involving the pipeline in that area. Title 49 CFR 195.450, 195.452, and 195.6 contain the criteria for designating an area a high-consequence area for hazardous liquid pipelines. <<>> Factual Information 4 Pipeline Accident Report Figure 3. Controlled burn surrounded by white fire retardant. Figure 4. Smoke plume 1 mile high and 5 miles long. <<>> Factual Information 5 Pipeline Accident Report Damage The cost of the accident was reported to the Research and Special Programs Administration (RSPA) Office of Pipeline Safety to be approximately $5.6 million.4 Enbridge recovered 2,574 barrels of oil and estimated that the in situ burn consumed approximately 3,000 barrels, with the remainder being lost to evaporation or entrapment in the soil. Postaccident Inspection On July 6, after vacuum trucks had removed the remaining oil and water, the ruptured pipe was exposed. The pipe was fractured along the edge of a longitudinal weld. When the pipe that failed was installed, the longitudinal weld was at the 5:30 clock position when viewed facing downstream (eastward). The rupture was about 69 inches long and gapped open about 6 1/4 inches at the center. (See figure 5.) At the rupture location, the pipeline was rated for a regulatory maximum operating pressure of 687 pounds per square inch, gauge (psig). The pressure at this location at the time of failure was calculated to be 526 psig. The United States Steel Corporation (U.S. Steel) manufactured the pipe at its National Tube Works in McKeesport, Pennsylvania. Figure 5. Rupture in accident pipe. 4 This total includes estimated property damage, including cost of cleanup and recovery, value of lost product, and damage to the property of the pipeline operator and others. <<>> Factual Information 6 Pipeline Accident Report Tests and Research Two sections of pipe, one containing the rupture and one from the same length of pipe, were removed and sent to the Safety Boardís Materials Laboratory for metallurgical examination. The pipe that ruptured was manufactured in accordance with American Petroleum Institute (API) standard 5L, grade X52, indicating that the steel had a specified minimum yield strength5 of 52,000 pounds per square inch (psi). The 34-inch outside diameter pipe was specified as 0.312-inch nominal wall thickness with a double submerged arc weld (DSAW) longitudinal seam weld. The pipe had a diameter-to-wall thickness (D/t) ratio of 109:1. The pipe was coated with a spiral wrap tape that was applied in the field during construction in 1967. Surface corrosion was visible on the outer surface of the pipe adjacent to the rupture, but no dents, scratches, or gouges were present at any location on the pipe sections examined. The corrosion was assessed as light, with no apparent pitting and little apparent loss of wall thickness. Both pipe sections were ultrasonically inspected for cracks along the longitudinal seam weld, and, other than the rupture that caused the accident, no additional cracks or discontinuities were uncovered. Fatigue cracking6 has been shown to initiate at seam welds because of changes in geometry, residual stress, and material properties associated with the weld. Metallurgical testing and examination of the ruptured area found no material or manufacturing defect in the steel or the welded seam of the pipe. Initial examination of the rupture revealed a preexisting fatigue region at the center of the rupture. The fatigue region was 13 inches long adjacent to the inside surface of the pipe and did not extend all the way through the pipe wall. (See figure 6.) More detailed examination showed that the fatigue cracking initiated at multiple locations along the inside surface (see figure 7) at the toe of the longitudinal weld bead. (See figure 8.) Examination of the cleaned fracture surface revealed a darker, more heavily oxidized band adjacent to the inside surface of the pipe that extended the entire length of the fatigue area. The more heavily oxidized portion of the fatigue area penetrated a maximum of about 0.04 inch deep at the center of the rupture. The oxidized band was visible for almost the entire length of the fatigue area. Near its ends, the oxidized portion of the fatigue crack extended about 0.010 inch into the pipe wall. The remainder of the fatigue crack was less oxidized and extended more deeply into the pipe wall over the central 6 inches of the fatigue region. Along approximately 2.5 inches in the central region, the fatigue crack almost penetrated the pipe wall. At its maximum depth, the fatigue crack penetrated through 0.270 5 Yield strength is a measure of the pipeís material strength and is the stress level, expressed in pounds per square inch, at which the material starts to exhibit permanent deformation. Although yield strength is expressed in pounds per square inch, this value is an expression of a pipe materialís strength, which is not equivalent to a pipeís internal pressure. 6 The term fatigue cracking is used to describe a progressive cracking of structural material that occurs under repeated loading and may eventually lead to failure. The fatigue crack grows with cyclic loading until the crack reaches a critical length at which the stresses cause it to grow unstably leading to structural failure. Fatigue cracks can initiate at microscopic flaws or weak spots in the material. Once initiated, cracks can grow at stress levels that are quite low in comparison to the materialís yield strength. <<>> Factual Information 7 Pipeline Accident Report inch of the 0.297-inch measured wall thickness.7 Measurement and testing of the pipe showed that it met thickness and strength requirements. The pipe fracture beyond the fatigue crack contained features typical of overstress fracture. Figure 6. View of top fracture surface of 13-inch-long crack, showing penetration nearly through pipe wall in center. Figure 7. Face of fracture in accident pipe. 7 The 0.297-inch measured wall thickness is within the allowable range for a pipe with 0.312-inch specified nominal wall thickness. <<>> Factual Information 8 Pipeline Accident Report Figure 8. Fatigue initiating at toe of weld on interior surface of pipe. Preaccident Events Fatigue Cracking in Enbridge Pipe Manufactured by U.S. Steel Enbridgeís 34-inch U.S. Steel DSAW pipe had a documented history of longitudinal seam weld failures due to fatigue cracks. Metallurgical analysis reports of longitudinal seam weld failures in Enbridgeís U.S. Steel pipe in 1974, 1979, 1982, 1986, 1989, and 1991 identified the causes as fatigue cracking at the toe of the weld. Enbridgeís 34-inch pipeline system also used A.O. Smith flash-welded pipe, Canadian Phoenix electric resistance welded pipe, and Kaiser Steel submerged arc welded (SAW) pipe. All of the longitudinal seam weld failures caused by fatigue cracks in this pipeline have occurred in pipe manufactured by U.S. Steel. Operational Reliability Assessments of the Pipeline After the 1991 pipe rupture at the toe of the weld in the 34-inch pipeline resulted in the release of 40,500 barrels (1,701,000 gallons) of crude oil, Enbridge signed a consent order with RSPAís Office of Pipeline Safety to conduct an operational reliability assessment of the 34-inch pipeline from Gretna, Manitoba, Canada, to Superior, Wisconsin. The assessment was to include a review of pipeline operating conditions and an analysis of the previous pipe failures. The operator was also required to restrict <<>> Factual Information 9 Pipeline Accident Report allowable operating pressures, to hydrostatically pressure test8 the pipeline to establish that the line was safe to operate, and to develop a program to ensure that the line would continue to be safe in the future. In December 1992, Enbridge performed an operational reliability assessment9 of the 34-inch pipeline in the United States. As a result of the study, changes were made in pipeline operations that reduced the number of pressure cycles10 and their associated pressure ranges. Among other actions it took as a result of the 1991 rupture, Enbridge financially and technically supported British Gasís development of the Elastic Wave in- line inspection tool to identify pipe cracks before they precipitate a failure. British Gas did the inspections in 1995 and 1996. PII North American, Inc. (PII), the successor to British Gas, currently provides the inspection tool data report of the Elastic Wave inspection tool in the United States. The pipeline section in which the 2002 rupture occurred was pressure tested to 835 psig after its construction in 1967. Enbridgeís first longitudinal seam weld in-service failure of U.S. Steel pipe from a fatigue crack occurred in July 1974. The entire pipeline, including the pipe joint11 containing the failure, was pressure tested between 1974 and 1976 at a test pressure of 764 psig. The entire 34-inch pipeline was pressure tested in 1991 and 1992 at higher stress levels than had been used before. Because of variations in pipe wall thickness and changes in elevation in each section of the pipeline, the test pressure range was from 85 percent to 105 percent of the specified minimum yield strength of the pipe, or up to 1,002 psig.12 The 1991 test pressure at the point of the July 4, 2002, rupture was 937 psig. The operator agreed in 1991 to pressure test the pipeline again in 5 years unless an in-line inspection tool capable of identifying cracks in the longitudinal seam of the pipe was developed. RSPA did not allow the operator to raise the pressures above those in effect at the time of the 1991 accident while the consent order was in effect. During the 1991 and 1992 pressure testing program, Enbridge found four crack- like/manufacturing defects, four corrosion defects, and one blister. Two subsequent leaks occurred that resulted from pressure-cycle-induced growth of fatigue cracks in U.S. Steel pipe. The two in-service leaks occurred in the first 6 months of 1994 at the site of fatigue cracks that had survived the pressure test levels of the 1991ñ1992 program. A reassessment report was completed in December 1994 following those two failures. Enbridgeís metallurgical report indicated that the initiating fatigue cracks were readily apparent adjacent to the inside pipe wall and had been introduced during the transportation of the pipe, as they were smoother and darker than subsequent fatigue crack growth. The report 8 A hydrostatic test of a pipeline involves filling the pipeline with water or similar liquid, gradually increasing the pressure of the liquid to a predetermined maximum, and examining the line and/or test records for indications of a leak. 9 The 1992 assessment was updated in 1994, 1995, and 1998. 10 One pipeline pressure cycle is the pressure variation from a minimum to a maximum pressure and to the minimum again. 11 A joint is a single length of pipe, nominally 40 feet long. 12 Using the internal design strength formula in 49 CFR Part 195, a test pressure of 954 psig is calculated at 100 percent of specified minimum yield strength for line pipe with the specification of the pipe that ruptured. <<>> Factual Information 10 Pipeline Accident Report noted that both defects at the point of failure showed evidence of having grown during the 1991ñ1992 pressure tests and concluded that ductile tearing of the metal caused the growth of these existing defects. Another Enbridge conclusion was that the operating histories of the upstream operating stations showed that pressure cycles also contributed to the failures. After Enbridge ran tests with the Elastic Wave inspection tool, the results were reviewed and recommendations were included in Enbridgeís 1995 integrity assessment report. As a result of the recommendations, Enbridge proposed to RSPA an in-line crack inspection program as the most appropriate means of reducing or eliminating the risk of pipeline failures. The detection level specification for the Elastic Wave tool stated that the tool would find a defect equal to or greater than 2.5 inches long with an accuracy of ±0.4 inch at 4.5 mph. The detection level specification for crack depth was 25 percent of the pipe wall thickness with a sizing accuracy of ±25 percent of the wall thickness. For an indication to be reported to the operator as a defect, both the crack length and the crack depth threshold requirements had to be met. RSPA agreed in 1995 to the use of the in-line crack inspection program in lieu of hydrostatic pressure testing. As a condition for accepting the proposal for 1996, RSPA stipulated that it would review the inspection program before deciding on future pressure testing. One of the reasons for conditional approval in RSPAís stipulations was that RSPA wanted to know whether the Elastic Wave inspection tool would identify not only pipe crack defects that would fail during hydrostatic pressure testing but also considerably smaller defects that could then be repaired or removed before they could grow and lead to failure of the pipe. In 1995, Enbridge began inspecting its 34-inch pipeline with the Elastic Wave in- line inspection tool and found that the tool was identifying more pipe crack defects than had been identified by previous hydrostatic pressure testing. Twice during 1995 and again in early 1996, PIIís tool was used to inspect the pipeline section that contained the crack that ruptured in this accident, but various mechanical problems with the inspection tool resulted in unusable data. PII acquired usable data in a May 1996 inspection. (The details of this inspection are discussed later in this report.) In the 4 years from 1995 through 1998, 216 miles (66 percent) of the 325 miles of 34-inch pipe from Gretna, Manitoba, to Superior, Wisconsin, had been inspected with the Elastic Wave tool, and pipeline repairs were made according to the pipeline operatorís policy. All crack defects identified by the inspections were repaired with pipe sleeves, and none were removed and subjected to metallurgical examination. During this period of time, in-line inspections were performed on all U.S. Steel manufactured DSAW pipe. As a result of these inspections, the operator excavated the pipe at 74 locations. An evaluation concluded that none of the defects found with the Elastic Wave tool would have failed a pressure test to 100 percent specified minimum yield strength. Following completion of the Elastic Wave tool inspections in the 34-inch U.S. Steel pipe, Enbridge submitted an assessment report dated April 28, 1998, that proposed reinspecting the pipeline approximately 10 years from the previous inspection. A number of reviews were made by RSPA before closure of the consent order on May 5, 1999. After the consent order was closed, Enbridge operated the pipeline up to the pressures allowed by 49 CFR Part 195. <<>> Factual Information 11 Pipeline Accident Report Before the accident, Enbridgeís unwritten defect inspection practice for Elastic Wave data was to excavate all crack-like indications that were found by the Elastic Wave tool. Enbridge ran Elastic Wave tool inspections in all of its 34-inch pipeline sections in the United States between 1995 and 2001. Based on the results of these inspections, the company excavated 23 crack-like features; 23 weld/manufacturing defects; 16 other defects, including corrosion and laminations; and 41 spurious13 indications and made repairs where needed. Elastic Wave In-Line Inspection at Rupture Location The in-line inspection company, PII, performed a computer analysis of the May 1996 Elastic Wave inspection tool log data as part of its interpretation process after the tool was run. An indication was present at the point where the pipe ruptured on July 4, 2002. PII interpreters reviewed the indication in their initial screening of the data in 1996, but the indication did not exhibit the diamond-shaped signature signifying a crack and did not meet PIIís standard that an anomaly must meet at least 6 of 10 feature selection criteria in order to be identified as a crack. After the accident, PII stated that, at most, the indication would have met two of the feature selection criteria. An important feature selection criterion that the indication did not meet was confirmation of the signal from both the clockwise and counterclockwise views as the tool records data while moving downstream through the pipe. PII representatives stated that during the May 1996 inspection run, one of the toolís two sets of wheel sensors was close to the longitudinal weld, which placed the weld in proximity to the source of the toolís ultrasonic signal and could have resulted in the masking of the signal. PIIís postaccident review of the May 1996 data also evaluated the size of the indication at the rupture and determined that it was below the detection level specification for a reportable defect (25 percent of pipe wall thickness and 2.5 inches long). The data on this indication have been recorded in a database, and PII and Enbridge have worked to determine how this information will be used to improve the feature selection criteria. Also after the accident, RSPA had an independent consultant and PII analyze the May 1996 inspection log data for the area from 0.5 mile upstream to 0.5 mile downstream of the rupture location. No indications were found with characteristics similar to those of the July 4, 2002, rupture. In addition, PII personnel reviewed the log data from two 1995 Elastic Wave tool inspections that had shown no significant defect at the point of the 2002 rupture. They found that on the first run, the clockwise sensor was functioning properly and was not on the longitudinal weld at the point that ruptured. The counterclockwise channel was working but was electronically noisy and provided a weak signal at the point that ruptured. Thus the signal on this run did not meet feature selection criteria for confirmation of the signal from both the clockwise and counterclockwise views. The signal on this run also did not exhibit the diamond-shaped crack signature. On the second 1995 log, the clockwise channel was not providing acceptable quality data when it was in the area of the point of rupture. 13 Spurious features were those that did not have a corresponding defect associated with them, had qualities not considered a defect (for example, weld profile), or were under sleeves and could not be assessed. <<>> Factual Information 12 Pipeline Accident Report All of the 1995ñ1996 in-line Elastic Wave tool inspections were performed by the Mark II version of the device. In 1997 the tool was upgraded to the Interim Mark III, which contains an additional set of wheel sensors that are offset so at least one set of sensors is not riding on the longitudinal seam weld. Both before and after the accident, Enbridge provided PII with feedback on its findings from actual excavations and field inspections. - **truncated:** true - **body characters:** 106948