BP OIL CO. plans an early 1992 start of construction for a $52 million diesel desulfurization unit at its 95,000 b/d Ferndale, Wash., refinery. BP also agreed to buy Exxon Co. U.S.A.'s service stations in the Seattle-Tacoma region. Rebranding is expected in April 1992. OCCIDENTAL CHEMICAL CORP. let contract to Jacobs Engineering Group Inc. to provide engineering and procurement services for a methyl tertiary butyl ether plant at its Chocolate Bayou complex in Alvin, Tex. (OGJ, Aug. 5, p.