Corrosion fatigue cracks that grew under loose polyethylene tape coating, along with external loading that caused bending stress, led to a pipeline rupture that released more than 108,000 gallons of liquid anhydrous ammonia. The toxic release killed a nearby homeowner and closed a stretch of highway in rural Nebraska for several days.

The National Transportation Safety Board (NTSB) reported in late January on its findings into the October 2016 accident involving an 8 in underground pipe that was owned and operated by Magellan Midstream Partners.

The pipeline was operated and monitored from a control center in Tulsa, Oklahoma. The NTSB said that low pressure alerts were received in the control center shortly after 9:00 p.m. on Oct. 17, 2016, and that a rupture alarm first occurred soon after. Within minutes, control center personnel had received a third-party report of strong ammonia odor and a vapor cloud some 8.5 miles north of Tekamah, Nebraska.

Accident timeline

According to the NTSB, during the early evening of October 17, no operational issues were reported by the on-duty controller at shift change, and operational records indicated no operating irregularities. About 9:00 p.m., the controller was pumping ammonia north for a customer delivery. At 9:03:45 p.m., the ammonia pipeline pressure dropped 42 psi on the suction side of a pump located near where the release occurred.

Seconds later, at 9:03:49 p.m., a 68 psi pressure decrease occurred on the discharge side of the same pump. The NTSB said these events triggered a control center alert and led to the pump's automatic shutdown.

At 9:11:38 p.m., the on-duty controller contacted a leak detection analyst after receiving additional alerts and notices from the control system. The controller told NTSB investigators that he did not initially think that a rupture had occurred. Rather, he said that he contacted the analyst to seek an opinion on why the system appeared to be losing pressure and if a software problem was to blame.

The pipeline rupture closed a section of federal highway for several days. Source: Lincoln Journal StarThree minutes later, at 9:14:40 p.m., the on-duty controller received an alarm indicating that a rupture had occurred. The SCADA controller immediately began to shut down the ammonia pipeline. At 9:17:15 p.m., another leak alert was received after the controller had initiated a pipeline system shutdown.

Fatigue indicators

Three days after the accident, the ruptured section of the ammonia pipeline was unearthed. The NTSB said the pipe had a fracture on its underside (at the 6 o’clock position) and exhibited downward bending near the fracture.

The NTSB said the fracture followed the spiral seam pattern of field-applied polyethylene tape wrap. The fracture ran roughly 45° from the longitudinal axis of the pipe and in a spiral path.

Investigators said the fracture had sawtooth-like features, with numerous short cracks merging into the main fracture. The total fracture length was 11 7/8 in, and the widest opening of the fracture was 0.19 in.

Portions of the protective polyethylene tape coating adjacent to the fracture were displaced from the pipe but recovered during the investigation. When loose pieces of tape were positioned on the pipe surface, the NTSB said the fracture aligned with the edges of successive windings of the tape, where the tape overlapped.

Also, areas on the pipe segment exhibited where the tape wrap had visibly shifted. One area of the tape had its edges folded over, exposing the pipe surface. Brown-colored corrosion was visible on the exposed pipe surface.

The NTSB said that numerous thumbnail-shaped cracks consistent with fatigue cracks were visible on the fracture surfaces. The patterns were oriented in a way that the thumbnail-shaped cracks emanated from the outside diameter of the pipe. The presence of these cracks "was consistent with multiple initiation fatigue cracking," the NTSB said, with the deepest thumbnail-shaped fatigue crack extending through roughly 75% of the pipe wall thickness.

"Overall, the fracture features were consistent with corrosion fatigue," the NTSB said.

Inspections and shutdown

Days after the pipeline accident, the Pipeline and Hazardous Materials Safety Administration directed Magellan to shut down a 49 mile section of the anhydrous ammonia pipeline for repairs. After repairs were made, Magellan was ordered to operate the pipeline at a 20% reduction.

Following the accident, Magellan told the NTSB that it had re-examined data to screen the pipeline through the affected area and found additional indications of pending failures, which Magellan repaired.

Even so, in January 2019, Magellan said it would decommission the 1,100 mile Texas to upper Midwest ammonia pipeline system. The company started the shutdown in September and by the end of 2019 had decommissioned around 588 miles of pipeline. Magellan indicated that it expected to finish anhydrous ammonia displacement activities in the summer of 2020.