A proposed offshore wind power project in Vietnam could have total capacity of 3.4 gigawatts and require investments of $11.9 billion, according to one of the developers, setting it on course to be the world’s biggest.

The Ke Ga project offshore Binh Thuan province will be developed in 600-megawatt phases, according to a statement from Singapore-based Enterprize Energy Group Dec. 7. It will make use of 9.5 megawatt wind turbines initially, though more powerful ones are expected over time, according...