A multimillion-dollar deal with a major national contractor will allow Pittsfield-based Cianbro Corp. to help create the largest oil refinery in the United States.

Cianbro’s Eastern Manufacturing Facility in Brewer has been awarded the contract with two California companies, San Francisco-based Bechtel Corp. and Pasadena-based Jacobs Engineering Group, for the Motiva Enterprises LLC oil refinery expansion project in Port Arthur, Texas. Cianbro has been asked to construct approximately 50 large modules weighing in excess of 500 tons altogether, Cianbro President and CEO Peter G. Vigue said in an interview at his office earlier this week.

“This is a huge happening in this country, and it is for us as well,” Vigue said. “I have confidence in Maine that we can compete with other companies.”

The modules will be composed of steel, pipes and refinery equipment and will become components of the refinery structure, according to Motiva project manager Mick Heim.

Vigue declined to reveal the contract’s exact value, but Peter Foster, a Farmington resident who has been appointed project manager at the Eastern Manufacturing Facility, said it would take 15 months to fill Motiva’s order. The Brewer facility should be up and running in April and is expected to employ at least 500 people, Vigue said.

Motiva Enterprises is a joint venture between Saudi Refining Inc. and Shell Oil Co. The project will add 325,000 barrels per day by 2010 to the capacity of Motiva’s refinery. The facility will process 600,000 barrels a day of crude oil, making it the largest refinery in the U.S. and one of the 10 largest in the world, according to a statement Motiva released last month.

“Cianbro’s commitment to the local work force makes them a perfect fit for our project team,” Heim said in a prepared statement. “This contract is significant on many levels. It is just as important to our expansion project in Texas as it is to Cianbro’s team in Maine. It is also just as important to strengthening our nation’s energy supply as it is to creating jobs here in Maine.”

The financial news Web site Bloomberg.com reported last month that the refinery expansion will cost $7 billion.

The expansion is designed to strengthen the U.S. supply of gasoline, diesel, aviation fuels and high-quality base oils. Motiva plans to be a top supplier of conventional fuels and biofuels, according to its Web site.

Motiva project team representatives will visit Brewer on Thursday and meet with people from across the ranks at Cianbro, the city of Brewer and other project stakeholders.

Motiva has hired Bechtel and Jacobs to contract work from companies such as Cianbro. Bechtel is known for overseeing work on the Hoover Dam, the Tacoma Narrows Bridge in Washington state and the Central Artery, or “Big Dig,” project in Boston.

Cianbro is one of several module fabrication companies Bechtel and Jacobs have hired, according to Bechtel.

With the contract, Cianbro has also moved one step closer to its larger goal of revitalizing the region’s economy, Vigue said. He had hinted at the news at a talk on Aug. 21 in Orono, where he said he had confirmed his first client for the Brewer facility. At the time, Vigue declined to name that client.

Vigue is an outspoken advocate for large investments in Maine’s economy and at the same talk unveiled his proposal for an east-west toll highway from Calais to Coburn Gore.