The corrosion was due to El Paso Natural Gas Co.'s failure to prevent, detect or control internal corrosion within the company's pipeline, noted the report. Ineffective federal inspections of the company's internal corrosion program that did not identify deficiencies shared in the blame.
On Saturday, Aug. 19, 2000, a 30-inch-diameter natural gas transmission pipeline operated by El Paso Natural Gas Co. ruptured adjacent to the Pecos River near Carlsbad. The released gas ignited and burned for 55 minutes. Twelve members of the same family who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed. Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged.
During the investigation, NTSB investigators found the rupture was a result of severe internal corrosion that caused a reduction in pipe wall thickness to the point that the remaining metal could no longer contain the pressure within the pipe. Furthermore, the corrosion was likely caused by the combination of microbes and such contaminants as moisture, chlorides, oxygen, carbon dioxide and hydrogen sulfide, the report noted.
One of the major issues of this investigation involved the use of cleaning pigs, a mechanical device that is used to clean the pipeline. These devices, which may include scrapers or brushes on the pig body, are inserted in a pipeline, or launched, and traveled downstream with the gas flow.
In its report, NTSB noted that periodic use of cleaning pigs could remove water and other liquid and solid contaminants that may cause corrosion in a pipeline. However, because the section of pipeline that ruptured could not accommodate pigs, cleaning pigs were not run in that section.
Another related issue the board emphasized in the report was the partial clogging of the drip, a stub line that branched off the bottom of the gas pipeline. Its purpose is to collect liquids and solids that may have built up in the pipeline during normal transportation of gas or after pigging operations.
The investigation revealed that as a likely result of the partial clogging of the drip upstream of the rupture location, some liquids bypassed the drip and continued through the pipeline to the eventual rupture site. At the rupture site, a bend in the pipe had created a low point in the pipeline where liquids and other residue accumulated and caused corrosion.
Consequently, the board found that because of the configuration of the piping, including the location of the pig receiver and the design of the drip, cleaning pigs could not run in the section of pipeline that ruptured and therefore removal of substances was incomplete. The board also concluded that if the accident section of pipeline had been able to accommodate cleaning pigs, and if cleaning pigs had been used regularly with the resulting liquids and solids thoroughly removed from the pipeline after each pig run, the internal corrosion that developed in this section would likely have been less severe.
The board stated in the report that before the accident, EPNG did not have in place an internal corrosion control program that was adequate to identify or mitigate the internal corrosion that was occurring in its pipelines. Had EPNG effectively monitored the quality of gas entering the pipeline and the operating conditions in the pipeline and periodically sampled and analyzed the liquids and deposits for corrosivity that were removed from the line, it would likely have detected the potential for significant corrosion to occur within the pipeline.
NTSB found that the current federal pipeline safety regulations do not provide adequate guidance to pipeline operators or enforcement personnel in mitigating pipeline internal corrosion.
As a result of this accident investigation, the board made the following recommendations to the Research and Special Programs Administration (RSPA) and NACE, International (formerly known as National Association of Corrosion Engineers):
Research and Special Programs Administration
- Develop the requirements necessary to ensure that pipeline operators' internal corrosion control programs address the role of water and other contaminants in the corrosion process.
- Evaluate the Office of Pipeline Safety's pipeline operator inspection program to identify deficiencies that resulted in the failure of the inspectors, before the Carlsbad accident, to identify the inadequacies in EPNG's internal corrosion control program.
- Implement the changes necessary to ensure adequate assessments of pipeline operator safety programs.
- Establish an accelerated schedule for completion of an industry standard for the control of internal corrosion in steel pipelines that will replace or update NACE standard RP-01-75.
A summary of this report is available now on the NTSB Web site at www.ntsb.gov/default.htm, under "Publications." The complete report will be available on the Web site in about one month. Printed copies of the report may be purchased later this spring from the National Technical Information Service (NTIS) (800) 553-NTIS.