Several hundred workers at Motiva refineries in Convent and Norco and at a Shell Chemical plant in Norco joined workers striking at other refineries around the U.S. A work stoppage was launched at midnight Friday at the Motiva Enterprises refinery in Port Arthur, Texas. Motiva said its three refineries will continue operating under pre-arranged contingency plans.Some 1,350 union workers are employed at the three Louisiana facilities and the Port Arthur refinery.