The Washington Department of Labor and Industries (L&I) is investigating the fatal explosion. The investigation could take up to six months. In addition, the L&I inspection team will work closely with the US Chemical Safety Board, a federal agency that investigates serious chemical accidents, and the federal Occupational Safety and Health Administration.

L&I most recently inspected Tesoro in 2009, citing the company for 17 “serious” process safety management violations, with fines totaling $85,700. A “serious” violation is cited when there is the potential for death or serious physical injury from the violation.

L&I reached a settlement in November with Tesoro Refining and Marketing Co. that required them to correct hazards identified in the L&I inspection and to hire a third-party consultant to do an independent process-safety audit and to outline methods to correct safety issues identified by the audit. It is not known at this time whether any of the hazards noted in the previous inspection were involved in today’s incident at the plant.

Tesoro Corporation is an independent refiner and marketer of petroleum products. Through its subsidiaries, it operates seven refineries in the western United States with a combined capacity of approximately 665,000 barrels per day. Tesoro’s retail-marketing system includes more than 880 branded retail stations, of which more than 380 are company-operated under the Tesoro, Shell, Mirastar and USA Gasoline brands.

Tesoro, which originally was an E&P company (exploration and production), exited the E&P business in the late 1990s to focus on refining and marketing in the Western US. In 1998, it purchased the Anacortes Refinery from Shell. (Shell operates another refinery in Anacortes today.)

Tesoro’s Anacortes refinery is located about 70 miles north of Seattle on the Puget Sound. With a total crude-oil capacity of some 120,000 barrels per day, the refinery primarily supplies gasoline, jet fuel and diesel to markets in Washington and Oregon, and manufactures heavy fuel oils, liquefied petroleum gas and asphalt. It receives crude feedstock via pipeline from western Canada and by tanker from Alaska and foreign sources.

The refinery was built in 1955; it currently carries a complexity index number of 8.2. The Nelson Complexity Index assigns a complexity factor to each major piece of refinery equipment based on its complexity and cost in comparison to crude distillation, which is assigned a complexity factor of 1.0. The complexity of each piece of refinery equipment is then calculated by multiplying its complexity factor by its throughput ratio as a percentage of crude distillation capacity. Adding up the complexity values assigned to each piece of equipment, including crude distillation, determines a refinery’s complexity on the Nelson Complexity Index.

The Nelson Complexity Index indicates not only the investment intensity or cost index of the refinery but also its potential value addition. Thus, the higher the index number, the greater the cost of the refinery and the higher the value of its products.

In the second edition of the book Petroleum Refinery Process Economics (2000), author Robert Maples notes that US refineries rank highest in complexity index, averaging 9.5, compared with Europe’s at 6.5.

The Oil and Gas Journal calculates refinery complexity as measured by the Nelson Complexity Index annually.