A structure resting on the seabed or piled into it from which development wells are drilled, using directional drilling, to exploit an oil field or gas field. To date, these platforms are of two kinds, although several novel designs are in existence. Gravity base structures, either concrete or hybrid with concrete base and steel legs and superstructure, which rest on the seabed by virtue of their own weight, or steel, which are piled into the ocean floor.
The Gravity base Structure model is more popular in modern times since the oil fields in shallow water are more or less tapped. New discoveries tend to be in deep water making superstructure inpractical. While mostly designed for oil drilling, these facilities can also offload their crude oil and natural gas to oil tankers.
The most extensive of today's natural gas and oil drilling platforms, while not altogether ocean cities, certainly are ocean "villages." There is a North Sea oil ocean platform with 97.7 meters by 67.1 meters deck area capable of providing housing for 900 workers. The facilities for this platform include an onboard desalination plant to provide fresh water as well as a waste treatment plant, kitchens, bakery, mess hall, and an assorted cranes and hoists and the obligatory helicopter landing pad.
A single platform may dril multiple wells from a single location. It's also possible to drill for crude oil and natural gas at the same time.
These platforms are becoming more and more numerous in the North Sea, the Gulf of Mexico and off the coast of South America, such as Brazil.
Some platforms in Everything2 include the Ocean Ranger which was owned and operated by the Hibernia Project in the area of the Grand Bank off Newfoundland and the Piper Alpha which was owned by Occidental Petroleum and was located in the North Sea oil field.
Major areas where these platforms exist are over 4,000 in the Gulf of Mexico, more than 400 platforms in the North Sea, the Caspian Sea, off the coasts of Australia, Brazil, Africa, and in the Persian Gulf. There is active exploration and new fields found all the time.