Pirates operating from the Somali coast had picked off huge targets before: a French cruise liner; a Ukrainian freighter bearing military hardware for a still-mysterious destination. But the Saudi-owned MV Sirius Star is the biggest ship ever commandeered by privateers: the supertanker is more than 1,000 feet long and three times as large, by displacement, as a U.S. Nimitz-class aircraft carrier — the largest naval ships in the world.