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Investigation into causes of fertiliser tank collapse

09 December 2008

The U.S. Chemical Safety Board (CSB has issued urgent safety recommendations following the collapse of a two-million-gallon liquid fertiliser storage tank at the Allied Terminals distribution facility in Chesapeake, VA, in November 2008, urging the company to take immediate steps to safeguard three other nearby fertiliser storage tanks from possible failure.

Investigation into causes of fertiliser tank collapse
Investigation into causes of fertiliser tank collapse

According to the text of the urgent recommendations which were unanimously approved by the Board, “The potential for collapse of a tank poses an unacceptably high risk of causing substantial property damage or a number of injuries or possibly deaths among the general public.”

The November 12 tank collapse seriously injured two contract workers, who were hospitalised. Two members of the public who tried to aid the injured men required treatment likely related to exposure to ammonia vapour from the released fertiliser. The fertiliser overtopped a containment dike and flooded sections of a nearby residential neighbourhood, requiring ongoing remediation of the soil. At least 200,000 gallons of spilled fertiliser could not be accounted for, and some reached the nearby Elizabeth River, which flows into the Chesapeake Bay.

“The urgent recommendations we released are designed to protect the safety of workers, the public and the environment,” said CSB Chairman John S. Bresland. “We are calling on Allied Terminals to immediately reduce the hazard from the remaining tanks by lowering the maximum safe fill height and to retain a qualified tank engineering firm to assess the tanks’ safety. The independent engineering analysis should be conducted promptly, within 30 days, and its results provided to the city.” The recommendations further call on Allied Terminals to develop and implement a corrective action plan for any identified deficiencies in the tanks.

CSB investigators concluded that the November 12 collapse of Tank 201, which contained an aqueous solution of urea and ammonium nitrate fertiliser, likely resulted from defective welds on the tank wall. The welding was performed in 2006 as part of a project to strengthen four fertiliser tanks that were constructed around 1929 by replacing vertical riveted seams.

“We found a number of welding defects where the modifications were made, including incomplete penetration of the welding metal into the joints,” said CSB Lead Investigator Robert J. Hall, P.E. “These welding defects likely weakened Tank 201 and led to its failure when the liquid was raised to a level slightly below the tank's recommended safe fill height.”

In the course of investigating the collapse of Tank 20, CSB investigators determined that three other large fertiliser tanks, which were welded during the same time period, likely have welding defects similar to Tank 201—including insufficient reinforcement, porosity, and weld undercut—that could cause the tanks to fail. The closest of the three large tanks is located 250 feet from homes.

Investigators said that the level of risk could not be quantified based on their external visual examination of the welds and that a thorough, independent engineering analysis should be conducted, including testing to check for the internal defects in the welds.

Following the welding of the four fertiliser tanks, and before the collapse of Tank 201, Allied Terminals had hired HMT Inspection, a Texas-based tank engineering firm with offices worldwide, to examine each tank in accordance with existing industry safety guidelines for petroleum tanks. HMT’s report did not identify the welding defects that led to this failure; it recommended a “safe fill height” for each tank. However, the November 12 collapse of Tank 201 occurred while the tank was being filled to a level about three inches below the 27-foot safe fill height recommended by HMT.

Bresland said the remainder of the CSB investigation would focus on understanding why the welding defects occurred, why the tank deficiencies were not detected and corrected, and whether improvements are needed in the oversight of above ground storage tank safety.

“At this stage in the investigation, we are concerned there is an apparent lack of clear regulations covering the safety of non-petroleum aboveground storage tanks,” Bresland said. “Because of the hazard such tanks can pose, the CSB will examine whether additional safeguards are necessary at the national and state levels.”

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