Royal Caribbean Cruises announced an agreement this week to buy its next generation of giant ships — but they will likely not be quite as massive as the company’s record-holders.

The world’s second-largest cruise ship company has dubbed the project “Icon.” Beyond that, not many details have been released.

At least two ships are expected to be included in the order, though contracts typically come with an option to order more. The initial vessels are due to be delivered in 2022 and 2024.

They will sail for the Royal Caribbean International line, which already operates the world’s largest cruise ships. The biggest of the three Oasis-class ships, Harmony of the Seas, is 226,963 gross registered tons and holds 5,479 passengers at double occupancy.

According to the announcement from the cruise company, the Icon ships will likely hold about 5,000 passengers — smaller than Oasis, but larger than the recent Quantum-class ships that hold 4,180 passengers.

Before the newest vessels arrive, another Oasis-class ship is due in 2018 and Quantum sisters are expected in 2019 and 2020. Royal Caribbean said its capital expenditures through 2020 add up to $9 billion but did not name a price for the Icon ships.

The company has signed a memorandum of understanding with Meyer Turku, a ship builder in Finland. Contracts are expected to be finalized by the end of 2016.

For the newest ships, Royal Caribbean is devoting the most attention to a subject cruise passengers don’t spend a lot of time thinking about: The way the vessels will be powered.

The ships will run on liquefied natural gas, or LNG, and use fuel cell technology in powering some guest-facing operations. Those moves are efforts to reduce greenhouse gas emissions, a goal that cruise lines have sought to meet in recent years by using devices that “scrub” exhaust.

“With Icon class, we move further in the journey to take the smoke out of our smokestacks,” Richard Fain, chairman and CEO of Royal Caribbean Cruises, said in a statement. “We are dedicated to innovation, continuous improvement, and environmental responsibility, and Icon gives us the opportunity to deliver against all three of these pillars.”

Ships that use liquefied natural gas need larger tanks for storage, and may run into supply problems when they need to refuel since LNG use isn’t yet widespread. Other cruise companies, including Europe’s MSC Cruises and industry giant Carnival Corp., have announced plans to build ships that can run on LNG. Royal Caribbean’s ships will be able to run on traditional fuel so they can still visit ports that don’t have a setup to provide LNG.

“Increasing the commitment to LNG makes it easier for suppliers to make their own infrastructure commitments,” Fain said in the statement. “As more ships are built for LNG, the number of ports that support it will grow.”

The use of fuel cell technology is new to cruise ships, though not to shipping. Royal Caribbean said it will start trying out the technology on an Oasis-class ship next year and continue to test it in larger ways on Quantum-class ships that have yet to be built.

Royal Caribbean’s chief of ship design, Harri Kulovaara, said in the announcement that the technology would require the development of more regulatory standards.

“As the technology becomes smaller and more efficient, fuel cells become more viable in a significant way to power the ship’s hotel functions,” he said. “We will begin testing those possibilities as soon as we can, and look to maximize their use when Icon class debuts.”

Photo Credit: Quantum of the Seas, the first of Royal Caribbean International's most recent class of ship, sails into New York. The cruise line's parent company announced that its next generation will debut in 2022. Royal Caribbean