The United States Chemical Safety and Hazard Investigation Board will conduct a full investigation of causes of a catastrophic fertilizer tank collapse at Allied Terminals in Chesapeake, Virginia, the CSB announced in a press statement.

The CSB is urging the company to take immediate steps to safeguard three other nearby fertilizer storage tanks from possible failure.

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.

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.

In the course of investigating the collapse of Tank 201 last week, 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.

The remainder of the CSB investigation will 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.

The CSB has identified similar oversight issues in other aboveground storage tank accidents. For example, the CSB previously investigated a sulfuric acid tank collapse in 2001 at the then-Motiva oil refinery in Delaware City, Delaware. Following that accident, which killed a contractor, injured eight others, and polluted the Delaware River, the Delaware state legislature enacted an extensive regulatory system for aboveground storage tanks, under the Jeffrey Davis Aboveground Storage Tank Act.

The Board does not issue citations or fines but does make safety recommendations to plants, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA.