- OIL & GAS
The U.S. Chemical Safety and Hazard Investigation Board (CSB) and the California Division of Occupational Safety and Health (Cal/OSHA) have released a technical evaluation report on piping samples taken from the Chevron Refinery in Richmond, California, where a hydrocarbon release and massive fire occurred on August 6, 2012. Cal/OSHA participated in this technical evaluation as part of its enforcement investigation.
The report, prepared by Anamet, Inc., a metallurgical laboratory in Hayward, California, concludes that the 8-inch steel pipe, from a section designated as 4-sidecut which was installed in 1976, ruptured due to severe sulfidation corrosion, and that tested pipe samples showed a very low concentration of corrosion-inhibiting silicon.
Corrosion over time
CSB Chairperson Rafael Moure-Eraso said, “The report, resulting from a cooperative effort between the CSB, Cal/OSHA, the United Steelworkers (USW), and Chevron provides a solid, technical basis for the firm conclusion that the pipe corroded over time from sulfidation corrosion. We hope this report receives widespread attention throughout the petrochemical industry as a precaution to all refiners to carefully examine potential corrosion mechanisms and use the safest possible materials of construction to avoid failures. Refineries and other plants must incorporate strong mechanical integrity and inherently safer strategies in their process safety management programs.”
Cal/OSHA Chief Ellen Widess added “This reports confirms what Chevron already knew– that the pipe was severely corroded and should have been replaced – but failed to act on before the August fire. This failure to act was included among the multiple Serious and Willful Serious citations issued to Chevron. Chevron’s own metallurgists and pipe inspectors reached the same conclusion and recommended as far back as 2002 that Chevron take action to protect its workers, the community and the environment by replacing the pipe that finally ruptured in 2012.”
Employees engulfed in vapor cloud
The CSB determined that nineteen Chevron employees were engulfed in a vapor cloud formed by the hydrocarbon release. Eighteen employees escaped before the fire started and one employee escaped without injury after the fire ensued. The incident resulted in six minor injuries. Production at the crude unit has been suspended since the accident. More than 15,000 residents in the surrounding area sought treatment at area medical facilities as a result of the release and fire.
Dr. Moure-Eraso said, “Based on the CSB’s investigation, the corroded pipe should have been replaced when opportunities arose years earlier. On the day of the accident, Chevron should have shut down the crude unit as soon as a leak was observed and removed workers to a safe location. Continuing to troubleshoot the problem and having firefighters remove insulation searching for a leak --while flammable hydrocarbons were flowing through the leaking piping -- was inconsistent with good safety practice.”
The report cites wall thinning due to sulfidation corrosion as the cause of the piping failure. In crude oil distillation, the report notes, naturally occurring sulfur and sulfur compounds are available to react with steel components, particularly plain carbon steels. Corrosion rates vary according to the sulfur content of the oil being processed, temperature, and other factors, including silicon content and other materials in steel pipe.
The report concludes, “The reported service conditions of the #4 Crude Unit 4-sidecut piping, the composition of the carbon steel, presence of thick sulfide scale on the inside surfaces of the pipe, and generally uniform wall thinning indicate that sulfidation corrosion was the cause of the 8-inch 4-sidecut rupture.”
The report noted that the failed pipe section was subjected to a higher corrosion rate than neighboring components due to low silicon content in the pipe. The report states that “Experience has shown that silicon in carbon steel is known to inhibit sulfidation” when the concentration is above a threshold value. Chemical analysis of the Chevron pipe showed the silicon concentration of the ruptured section to be far below this value, resulting in a significantly higher rate of sulfidation corrosion.
The report, which contains numerous photographs of the corrosion and pipe rupture, also notes the presence of an inward deformation toward the inside surface of the pipe. The CSB has determined this deformation was likely caused by a fire pike used during the plant’s emergency response. A photograph depicts a tight fit between the fire pike tip and this inward deformation of the failed pipe.
The incident occurred when a combustible hydrocarbon liquid known as “gas oil” leaked from an 8-inch pipe connected to an atmospheric crude oil distillation column in the refinery’s crude unit. Workers initially noted the leak and were in the process of attempting to diagnose the source of the leak in the still-operating crude unit when the pipe ruptured catastrophically. Due to the high temperature, in excess of 600 degrees Fahrenheit, and physical properties of the material in the equipment, the gas-oil immediately formed a large hydrocarbon vapor cloud.
The Anamet report provides the following conclusions:
1. Rupture of the 8-inch 4-sidecut resulted from wall thinning caused by sulfidation corrosion.
2. Post rupture corrosion destroyed fracture morphology that could have indicated the rupture initiation site. However, rupture likely originated in the thinnest region and initially followed a longitudinal path driven by the hoop stress. Consequently, an area of likely rupture initiation was identified.
3. The wall thickness of the ruptured section was less than all the other sections of the 8-inch 4-sidecut that were evaluated.
4. Chemical analysis showed the silicon concentration of the ruptured section was 0.01-wt%.
5. Six of twelve specimens (50%) from unique sections of the 8-inch 4-sidecut, 12-inch 4-sidecut, and 2011 12-inch samples were found by chemical analysis to have a silicon concentration of less than 0.1-wt%.
6. Deformation of a small region of the 8-inch 4-sidecut rupture edge was consistent with mechanical force applied from the outside surface of the pipe, possibly from a fire pike, such as samples E-099 and E-082-2, or other pointed object. This deformed region was located within the area of likely rupture initiation.
The CSB investigation to determine the root causes of the incident is ongoing. The CSB will release a report detailing its findings and recommendations to key stakeholders later this year. Cal/OSHA has already issued 25 citations and civil penalties of $963,000. Chevron has announced it will appeal these citations. The CSB, Cal/OSHA, the USW, and Chevron are cooperating under an agreement to test 4-sidecut carbon steel piping from Chevron’s refinery in El Segundo, California, south of Los Angeles.