Shell Oil, one of the largest oil and gas operators along the US Gulf of Mexico, said on Tuesday it shut in several key crude pipelines and terminals ahead of Hurricane Isaac and was in the process of an orderly shutdown at two other plants. Shell also said it was starting an orderly shutdown of process units at its manufacturing facilities in Convent and Geismar, Louisiana ahead of Isaac, a massive, slow-moving Category 1 hurricane expected to strike the Louisiana coast early Wednesday.