Workers at the Husky Superior Refinery lacked expertise and training that could have helped prevent an explosion and fire that injured 36 employees in 2018, according to a Chemical Safety Board (CSB) investigation.
CSB noted that the refinery, located in Superior Wisconsin, relied on inadequate internal process knowledge rather than outside experts to manage the fluid catalytic cracking (FCC) unit during a planned-maintenance shutdown.
“As a result, at the time of the incident, Husky Superior Refinery employees did not adequately understand how to effectively control the FCC unit’s transient operation hazards,” wrote the CSB in a Dec. 29 news release.
“Transient operations” refer to when a process is not operating in its normal state, such as maintenance shutdowns.
CSB also concluded the refinery – now owned by Cenovus Energy after it merged with Husky Energy in 2021 – failed to maintain satisfactory process safety information, operating procedures, process hazard analyses and operator training for its FCC unit. In addition, the workers were not aware of a lesson trade groups distributed regarding FCC safety after a similar incident occurred in California in 2015.
Other actions the refinery failed to implement include transient operation safeguards, such as the separation of air from flammable hydrocarbons to prevent an explosive mixture. CSB also noted structural issues, including failure of the primary absorber and sponge absorber vessels due to a brittle fracture “or shattering like breaking glass, which sent more than a hundred pieces of metal debris throughout the refiner – some striking an asphalt tank.”
CSB determined the vessels were not constructed of newer, recommended grades of steel that are more durable than older materials.
Two vessels in the FCC unit exploded, sending metal fragments up to 1,200 feet away that punctured a nearby asphalt storage tank at the refinery, which eventually spilled about 17000 barrels of hot asphalt that ignited and caused multiple fires.
In addition to injuries, the explosion caused about $550 million in damage to the facility and released 39,000 pounds of flammable hydrocarbon vapor into the air, requiring the evacuation of 2,500 Superior residents and a shelter-in-place order for nearby Duluth, Minnesota, residents.
The final CSB report includes 16 safety recommendations, including actions the company should take to improve refinery operations and a recommendation that the Environmental Protection Agency adopt a program that prioritizes FCC inspections for HF alkylation units.
