A four-person investigative team from the U.S. Chemical Safety Board continues to investigate the April 26 explosion at the Husky Energy oil refinery in Superior, Wis.

The refinery was shutting down in preparation for a five-week maintenance turnaround when the incident occurred. The initial explosion caused property damage and loss of containment of an asphalt tank. The contents of the tank spilled out and the material combusted. Several workers sought medical attention, and thousands of residents near the refinery were asked to evacuate due to heavy smoke.

The CSB investigation team arrived in Superior the evening of April 26 and has photo-documented a large percentage of the refinery. The investigation team has also conducted interviews with approximately 50 Husky Energy employees and contractors who were working at the facility on the day of the incident. Husky Energy has worked cooperatively with the CSB to provide access to the site and relevant information.

The initial explosion took place within the refinery’s fluid catalytic cracking (FCC) unit. The FCC unit “cracks” heavy, high boiling point hydrocarbon molecules into smaller molecules with lower boiling points. The main product produced by the FCC unit is gasoline. The CSB investigation team is currently narrowing its focus to equipment within unit to determine the cause of the initial explosion. The investigation team is requesting and reviewing documentation pertaining to the FCC unit.

Metallurgical testing will be conducted in order to determine the cause of the initial explosion. The CSB has retained a metallurgical expert to help identify the testing necessary and will work with Husky and other agencies to determine the protocols for the testing.

The explosion created a large amount of debris which was scattered in and around the refinery complex. This debris is being collected and stored for future analysis as needed.