Big oil companies are returning to ambitious offshore projects that languished during the slump in crude prices, but they are working on tighter budgets as investors urge them to rein in spending.

About 30 major new projects are expected to launch in 2018, more than double the average annual number approved in the global oil industry for 2014 through 2016, according to Wood Mackenzie, the Scottish energy consultancy. They are likely to include multibillion-dollar developments off the coasts of the U.S., Brazil and Nigeria,...