Royal Caribbean will buy 15 million gallons of biodiesel from Seattle-based Imperium Renewables in 2007 and 18 million gallons annually for four years after that, The Seattle Times reports. The cruise line has four vessels that call in Seattle.

“We believe this is the single-largest long-term biodiesel sales contract to an end user in the U.S.,” the company said in a filing with the Securities and Exchange Commission, the article reports.

Imperium also said it had reached an agreement to purchase Royal Caribbean Cruises’ seven percent stake in the Grays Harbor facility.