12 February 2003
Corrosion main cause of pipeline blast
The cause of an El Paso Natural Gas Co. pipeline rupture in Carlsbad, N.M., was a significant reduction in pipe wall thickness and severe internal corrosion due to the company's failure to prevent, detect, or control internal corrosion, according to the National Transportation Safety Board (NTSB). Contributing to the accident were ineffective federal inspections of the company's internal corrosion control program that did not identify deficiencies.
On 19 August 2000, a 30-inch-diameter natural gas transmission pipeline operated by El Paso ruptured next to the Pecos River near Carlsbad. The gas ignited and burned for 55 minutes. Twelve people camping under a concrete-decked steel bridge that supported the pipeline across the river perished in the blast, and three vehicles were destroyed. Two nearby steel suspension bridges for gas pipelines crossing the river suffered extensive damage.
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 likely occurred within the pipeline by the combination of microbes and such contaminants as moisture, chlorides, oxygen, carbon dioxide, and hydrogen sulfide, according to the report.
One of the major issues of this investigation involved the use of cleaning "pigs." A pig is a mechanical device that cleans the pipeline. These devices, which may include scrapers or brushes on the pig body, insert into a pipeline and travel downstream with the gas flow.
Periodic use of cleaning pigs could remove water and other liquid and solid contaminants that may cause corrosion in a pipeline, according to the report. However, because the section of pipeline that ruptured could not accommodate pigs, the company did not run them through this section.
Another related issue the NTSB emphasized in the report was the partial clogging of the drip. The "drip" is 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 NTSB 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 NTSB 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 NTSB said that before the accident, El Paso did not have in place an adequate internal corrosion control program to identify or mitigate the internal corrosion that was occurring in its pipelines. Had El Paso effectively monitored the quality of gas entering the pipeline and the operating conditions in the pipeline, and had it periodically sampled and analyzed the liquids and deposits for corrosivity taken from the line, it would likely have detected the potential for significant corrosion within the pipeline, the NTSB determined.
Overall, the NTSB found 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 NTSB made recommendations to the Research and Special Programs Administration (RSPA) and NACE International (formerly known as National Association of Corrosion Engineers).
- 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 to identify the inadequacies in El Paso's internal corrosion control program.
NACE International recommendations:
- 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 at www.ntsb.gov/default.htm, under "Publications."