The blast killed seven workers.
The federal investigators also said problems they found with cracked, corroded steel at refineries in Anacortes and elsewhere "strongly suggest an industry-wide problem."
On the first anniversary of the April 2010 blast, the Chemical Safety Board released preliminary findings: Microscopic cracks in the steel of a poorly maintained piece of equipment called a heat exchanger led to the explosion.
Nearly four years after the blast, the board's long-awaited inquiry said a variety of deficiencies led to the fireball that engulfed and killed seven workers.
Root causes included unreliable inspections, weak regulations, and faulty industry standards that led Tesoro to operate equipment at unsafe temperatures and pressures, according to the long-delayed report.
The report said the refinery had ineffective safeguards, hazard identification and control of hazards for 15 years before the blast.
Leaks of hot, explosive fuels occurred frequently at the Tesoro plant, according to the Chemical Safety Board, and Tesoro did not always investigate what caused them. The report said Tesoro management was "complacent" about leaks.
One of management's responses to the frequent leaks was to have more workers on hand when restarting one of the refinery's heat exchangers, an especially dangerous procedure. That practice contributed to the large number of deaths in the blast, according to the safety board.
Tesoro spokeswoman Tina Barbee declined to be interviewed. In a brief emailed statement, she acknowledged the efforts of the investigation teams and "the significant and successive turnover in the agency’s assigned personnel."
"We respectfully disagree with several findings in the draft report," she wrote, "and, most importantly, take exception to CSB’s inaccurate depiction of our process safety culture."
Tesoro-Anacortes machinist and United Steelworkers Union Local 12-591 president Steve Garey said safety problems in the petroleum industry stretch far beyond Tesoro or Anacortes:
The refining industry in general, they know what they're supposed to do, and we believe that they don't always do it. That's why we lost seven at Tesoro. That's why we lost 15 at Texas City eight years ago. It's why we lost 11 people in the Gulf and polluted the entire Gulf of Mexico. It's the reason Cherry Point burned a few years back, and it's the reason Chevron Richmond plant burned and nearly killed 20 people and put thousands in the hospital.
The report also said the understaffed Washington Department of Labor and Industries was unable to enforce workplace safety in complex facilities like oil refineries. Labor and Industries has only four specialists to regulate and inspect nearly 270 hazardous chemical facilities in the state, including five oil refineries.
The safety board said Tesoro-Anacortes has improved some safety procedures since the blast and upgraded the steel in the equipment that failed.
The Chemical Safety Board is taking public comment at a meeting in Anacortes Thursday night, and for the next 45 days, before finalizing its report.
In December, Tesoro reached a $39 million settlement with the families of the seven workers killed by the blast.
The company is currently appealing its $2.4 million workplace-safety fine from the Washington Department of Labor and Industries.
It is the biggest fine in state history for a workplace accident. It is also the equivalent of one hour's worth of Tesoro's revenue the year of the explosion.
Last fall, in preliminary judgments that could be reversed when the Tesoro appeal has its formal trial before the the state's Board of Industrial Insurance Appeals, Judge Mark Jaffe reduced the penalty by nearly two-thirds, to $859,000.