An oil platform exploded and burned off the Louisiana coast Thursday, the second such disaster in the Gulf of Mexico in less than five months. This time, the Coast Guard said there was no leak, and no one was killed.The Coast Guard initially reported that an oil sheen a mile long and 100 feet wide had begun to spread from the site of the blast, about 200 miles west of the source of BP's massive spill. But hours later, Coast Guard Cmdr. Cheri Ben-Iesau said crews were unable to find any spill.