For a long time the largest ships at sea were warships. But in the last few decades this has changed. As early as 1976 there were 414 meter (1,360 feet) long super tankers displacing over 600,000 tons. The largest warships, since the 1960s, have been American aircraft carriers displacing 100,000 tons. Before that there were some battleships during World War II that displaced over 70,000 tons. The first tanker displacing 100,000 tons appeared in the late 1950s.
Now there’s a new type of large ship, the FLNG (floating liquefied natural gas) which is a 488 meter (1,601 feet) long LNG (liquid natural gas) facility that takes natural gas in its gaseous state, lowers its temperature until it is a much more compact (by a factor of 600:1) liquid that can be carried to consumers aboard special refrigerated ships. The first FLNG ship displaces over 600,000 tons and can produce 3.6 million tons of LNG a year. This will be done at offshore natural gas fields in remote areas, where pipelines are impractical to LNG facilities on land are not yet available.
Putting factories or large industrial operations aboard ships is nothing new, but there’s rarely a reason for it. But such factory ships provide the commercial shipbuilding knowledge and experience for building ever larger military ships. Some navies, like the U.S. and China, are always looking at what commercial shipping they have access to and how they can use it in wartime.
The military has long been thinking about making use of the new technologies that have made these huge ships practical and affordable. Late in the Cold War there were proposals to take one of these huge ships and fill it with cruise missiles and call it an “Arsenal Ship”. More recently the U.S. has modified tanker designs to produce cheaper and more effective seagoing transports/command ships. Many construction, design and automation ideas are moving from commercial to military ships, a trend that has been going on for centuries.