(10:45 a.m. EDT) -- After a March announcement that it plans to build nine new cruise ships by 2022, Carnival Corp. has said four of those ships will be 180,000-ton environmentally friendly vessels, slated to be the largest in the world in terms of passenger capacity.

Two of the four will sail under AIDA, Carnival Corp.'s German line, which is already launching two new ships -- one this year and one in 2016. Roger Frizzell, senior vice president of corporate communications and chief communications officer for Carnival Corp., says the other two "are being built for the Costa Group," which includes AIDA and Italian line Costa. German shipbuilder Meyer Werft will handle the construction; two of the ships will be built at its shipyard in Papenburg, Germany, and the other two will be built at its facilities in Turku, Finland.

When asked why the corporation chose AIDA to receive two of the ships, Frizzell attributed it to the burgeoning German cruise market: "It's one of the fastest growing markets in the world. It's number-two in the world right now, and as a result, there's pent-up demand for cruising. This is part of our plan to meet that demand and increase our fleet."

Frizzell said two of the four ships will definitely serve the German market; the market(s) for the other two haven't yet been decided. Because Frizzell says the ships will be tailored to their respective markets, it's possible the onboard amenities will differ, even though the vessels will be sisters.

Each ship would have a 6,600-passenger capacity, which would make them the largest ships in the world by number of passengers. (Royal Caribbean's Oasis-class ships max out at about 6,450.) However, the ships will still be small enough to call on most ports, as told to Bloomberg by Carnival Corp. President and CEO Arnold Donald in an interview.

"While it's a lot of people, it won't feel congested," Donald said.

In addition, each of these new vessels will employ engines that run on liquefied natural gas, the cleanest burning fossil fuel, which Donald says will be stored onboard.

Frizzell tells Cruise Critic that these will be the first cruise ships to sail under LNG power. When asked how the fuel system compares with the systems currently using scrubber technology, he said it "will exceed ECA requirements." He also says that the cost of LNG fuel isn't expected to impact cruise fares.

--By Ashley Kosciolek, Editor