Mobil and its partners--Monsanto Oil, 25%, Diamond Shamrock, 15%, and Kerr-McGee, 5%--announced plans to develop an oil field in the Gulf of Mexico at a cost of more than $250 million. Mobil Oil Exploration & Producing Southeast, a Mobil subsidiary, is the operator of the field and has a 55% interest. Bechtel Petroleum was given a letter of intent from the owners to start engineering for the project in the Green Canyon 18 field, which involves installing a platform in 760 feet of water about 160 miles southwest of New Orleans. Production is scheduled to begin in two years at a rate of 3,000 barrels a day from three wells already drilled at the site. The platform's total capacity is rated at 20,000 barrels a day.