ENSCO Inc., an $18-million-a-year research and development firm in Springfield which conducted an unusual search for a new president, has found one -- Francesco A. Calabrese.

In July, ENSCO, launching a nationwide search for a new president and chief operating officer, surprised the high-tech industry with a somewhat unorthodox campaign identifying itself as well as the vacancy in ads in national newspapers and "drive time" radio spots. It also offered cash awards to employes if their referral became a finalist or was selected as president.

Applicants responded from a variety of industries and from every region of the United States and several foreign countries.

From a field of over 1,300 candidates, Calabrese, who saw the company's ad in The Washington Post, was selected as the unanimous choice of the search committee and the ENSCO board of directors.

Calabrese had been with the Planning Research Corp. in McLean. He was appointed president of a subsidiary company in 1974 and was most recently executive vice president for PRC's $100-million government information systems group.

Before joining PRC in 1965, he spent 10 years with the Army Corps of Engineers in both military and civilian positions.

Calabrese, who came to the United States from Italy in 1934, is also a member of the President's Private Sector Survey on Cost Control (PPSSCC), which President Reagan established in February to provide a private sector view on management improvements and cost reductions in the federal government.