”Oasis-4” (6:35 a.m. EDT) -- Royal Caribbean will launch a fourth Oasis-class ship in spring 2018.

Royal Caribbean's CEO Richard Fain announced the news today (May 9) at a keel laying ceremony for Oasis 3, Royal Caribbean's third Oasis-class ship, at the STX shipyard in France.

The two Oasis-class ships currently sailing -- Oasis of the Seas and Allure of the Seas -- share the title of world's largest cruise ship, and both of Royal Caribbean's upcoming newbuilds in the class will be built on the same blueprint.

Speaking at the keel laying event, Fain said: "Oasis and Allure turned out to be so good that we are only talking about tweaks with Oasis 3 and 4. When you are talking about something so successful as the original Oasis ships, you don't want to screw it up."

Royal Caribbean has not yet announced any new features for Oasis 4, which will be built at the same STX France shipyard where Oasis 3 is currently under construction.

The line did say, however, that the new Oasis-class ships would be more fuel efficient than their predecessors.

Harri Kulovaara, who oversees Royal Caribbean's shipbuilding operations explained: "We made a number of tweaks for Oasis 3, but basically it's the same ship. Oasis and Allure already are the most efficient ships when it comes to fuel; Oasis 3 will be 20 percent more efficient and Oasis 4 possibly even more efficient than that."

Royal Caribbean's third Oasis-class ship will launch in spring 2016 and the fourth will be delivered two years later. --by Jamey Bergman, UK Production Editor; reporting by Carolyn Spencer Brown, Editor-in-Chief