Very large floating structures (VLFSs) or, as some literature refers to them, very large floating platforms (VLFPs) can be constructed to create floating airports, bridges, breakwaters, piers and docks, storage facilities (for oil & natural gas), wind and solar power plants, for military purposes, to create industrial space, emergency bases, entertainment facilities (such as casinos), recreation parks, mobile offshore structures and even for habitation. VLFS for habitation could become reality sooner than one may expect. Currently, different concepts have been proposed for building floating cities or huge living complexes.[1]
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Unlike the top surface of vessels, that of a VLFS is used as the ground, and therefore the floating structure should provide a very large surface area. It can be constructed by joining the necessary number of floating units together. The design of the floating structure must obey safety and strength requirements, operating conditions, etc. Steel, concrete (prestressed or reinforced hybrid) or steel-concrete composite materials may be used to build the floating structure. The motions of the floating structure must be less than those allowed to ensure the safety of people and facilities on a VLFS. Generally, floating structures shall be moored at the same site for a long time.
VLFSs may be classified under two broad categories, namely the pontoon-type and the semi-submersible type. The former type is a simple box structure and features high stability, low manufacturing cost and easy maintenance and repair. In open sea, where the wave heights are relatively large, it is necessary to use the semi-submersible VLFS type to minimize the effects of waves while maintaining a constant buoyant force. VLFSs of the semi-submersible type are used for oil and gas exploration in sea and other purposes. They are fixed in place by column tubes, piles, or other bracing systems.
In contrast, the pontoon-type VLFS just floats (lies) on the sea surface. The pontoon-type VLFS is very flexible compared to other kinds of offshore structures, so that the elastic deformations are more important than their rigid body motions. Thus, hydroelastic analysis takes center stage in the analysis of the pontoon-type VLFSs. Together with the motion of the floating structure, the response of the structure to water waves and the impact on the entire fluid domain have to be studied.
Pontoon-type VLFS' are also known in the literature as mat-like VLFSs because of their small draft in relation to the length dimensions. Very large pontoon-type floating structure is often called Mega-Floats. As a rule, the Mega-Float is a floating structure having at least one length dimension greater than 60 meters. Horizontally large floating structures can be from 500 to 5000 meters in length and 100 to 1000 meters in width, while their thickness can be of the order of about 2-10 meters.
Aircushion supported Mega-Floaters are mega floaters that are supported by an air cushion. They were invented by Jan Van Kessel of TU Delft.[2]
The largest offshore structure built so far is the Mega-Float, a floating airport prototype constructed in Tokyo Bay from 1998 to 1999.[3]