The U.S. National Transportation Safety Board (NTSB) said that a November 2017 pipeline rupture near Amherst, South Dakota, was due to a fatigue crack likely caused by mechanical damage to the pipe exterior from a metal-tracked vehicle during the pipeline's installation. An NTSB report said the fatigue crack grew and extended to a critical size, resulting in the rupture.

The NTSB limited it investigation to a metallurgical evaluation of the ruptured pipe, part of TransCanada Corp.'s Keystone Pipeline. Materials engineers used optical and scanning electron microscopes and energy dispersive spectroscopy to examine the fracture surface and adjacent exterior pipe surfaces to determine the mode of fracture.

Keystone’s Supervisory Control and Data Acquisition system detected the leak and Keystone’s Operational Control Center (OCC) in Calgary, Canada, shut down the pipeline, but around 5,000 barrels of crude oil were spilled. There were no injuries or fatalities associated with the incident.

The NTSB investigation found that at the time of the rupture, Keystone was conducting an in-line inspection (ILI) using a cleaning tool (pig) and an acoustic leak detection tool (SmartBall). Just before the rupture, both the cleaning pig and the SmartBall leak detection tool had traveled past the nearest block valve downstream of the rupture location.

(Click to enlarge.) Pipeline and Hazardous Materials Safety Administration photo showing the rupture. Credit: NTSB (Click to enlarge.) Pipeline and Hazardous Materials Safety Administration photo showing the rupture. Credit: NTSB To prepare for the tools to bypass the next downstream pump station, a bypass operation at that station was was started at 5:03 a.m. The bypass was fully executed at 5:24 a.m. The pump station bypass resulted in a gradual and anticipated pressure increase at the discharge of the upstream station. The NTSB says that pressure records indicated that the pressure rose from 1,170 pounds per square inch gage (psig) to 1,352 psig when the rupture occurred.

The first indication of the rupture occurred at 5:33 a.m., when an abrupt drop in discharge pressure and a corresponding increase in flow rate were observed at the affected pump station. In addition, at 5:34 a.m., a corresponding pressure drop was observed at the downstream pump station.

The controller at Keystone’s OCC initiated an emergency shutdown of the pipeline at 5:36 a.m. and began isolating the pipeline. By 5:45 a.m., the failure location had been isolated by using remotely operated valves. Keystone personnel were dispatched to investigate the pipeline right-of-way for signs of a release and confirmed oil on the ground about 9:15 a.m.

The NTSB report says that the ruptured pipe was 30 in. in diameter with a 0.386 in. wall thickness and was manufactured by Berg Pipe to American Petroleum Institute Specification 5L grade X-70 product specification level 2, using a double submerged arc-welded longitudinal weld seam and fusion bonded epoxy coating. The pipeline was constructed and operated under a special permit, which allowed operation at pressures up to 80 percent of specified minimum yield strength.

NTSB metallurgists found midway along the length of the fracture face (a region 5.52 in. long), ratchet marks, crack arrest lines and other features consistent with a multiple-origin fatigue fracture that originated at an external groove in the pipe wall.

The fatigue origin area was located near the top of the pipe about 6.5 in. away from the longitudinal seam. The pipe exterior exhibited areas of exposed metal and grooves formed by sliding contact that aligned nearly parallel to the pipe axis on the exterior surface, coinciding and adjacent to the length of the fracture. Coating material was present and intermixed with the sliding contact marks, and many edges of the remaining coating adjacent to individual contact marks were curled and rounded consistent with sliding contact deformation.

In addition, a larger cluster of grooves was present toward the upstream end of the fracture.