Four years after the Deepwater Horizon disaster, giant new oil projects are returning to the Gulf—bigger and more expensive than ever. This platform, owned by Royal Dutch Shell PLC, is a floating hive of human activity, 130 miles off the Louisiana coast. Larger than a New York City block and weighing more than an aircraft carrier, Olympus is among roughly a dozen new multibillion-dollar platforms that are or will be pumping oil in the deep waters of the Gulf by the end of next year. The resurgence could be short-lived if the decline in crude-oil prices, down about 30% since June, continues and prompts companies to delay substantial investments in the Gulf. For the near term, though, the activity promises to return the Gulf to prominence as a major source of U.S. energy. In 2001, the waters produced about a quarter of all American oil and gas. Since then, production has fallen by half as wells petered out and the government issued fewer permits in the aftermath of the 2010 Deepwater Horizon explosion and oil spill. Last year, the Gulf accounted for less than 10% of the country’s energy production, in part because of soaring output from wells drilled in onshore shale formations.