CSB investigators found a number of welding defects on aboveground storage tanks at Allied Terminals, including incomplete penetration of the welding metal into the joints. The defects, introduced when the tanks were modified in 2006, likely resulted in the tank collapse that occurred on November 12, 2008, according to the CSB.

As a part of its investigation of the November 2008 collapse of a two-million-gallon liquid fertilizer storage tank at the Allied Terminals distribution facility in Chesapeake, Va., the U.S. Chemical Safety Board (CSB) issued urgent safety recommendations to the company to prevent other potential failures.

According to the text of the urgent recommendations, which were unanimously approved by the board and released in December, "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 hospitalized. Two members of the public who tried to aid the injured men required treatment, likely related to exposure to ammonia vapor from the released fertilizer. The fertilizer overtopped a containment dike and flooded sections of a nearby residential neighborhood, requiring ongoing remediation of the soil. At least 200,000 gallons of spilled fertilizer could not be accounted for, and some reached the nearby Elizabeth River, which flows into the Chesapeake Bay.

"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," said CSB Chairman John S. Bresland. "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. Allied Terminals has been cooperating with the CSB investigation.

As a result of its ongoing examination of the incident, CSB investigators concluded that the November 12 collapse of Tank 201, which contained an aqueous solution of urea and ammonium nitrate fertilizer, likely resulted from defective welds on the tank wall. The welding was performed in 2006 as part of a project to strengthen four fertilizer 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 Robert J. Hall, P.E., the CSB lead investigator on this incident. "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 201, CSB investigators determined that three other large fertilizer 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 fertilizer 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.

Chairman 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 aboveground storage tank safety.