What considerations does a city built on an oceanic platform have to make?

Question

In a world without enough land for a massive (and growing) population but plenty of water surface, experimental cities have started to spring up along the edges of various continents. The first is a city that is built into a cliff and is supported by oceanic platforms. As their technology and engineering strategies improve, these cities would inch further away from land masses and would have submerged livable space. I imagine these cities look like large towers or skyscrapers.

What are some of the engineering hurdles that such cities would face (besides withstanding violent storms)? For example, would their maximum height be more limited than a traditional skyscraper based on land?

Answer 1

I think building construction on an ocean platform won't be too different from land, advantage is that you don't have land limitations as you can just build more onto the platform.

That being said, i did see a documentary about some skyscraper in India where they had to design it in such a way as to avoid the wind catching it and toppling it over. I'd dare say the same thing would apply when we talk about height.

The main things to consider with ocean platforms involve energy and food, but these supplies can be obtained from the sea.
Solutions: Fishing for food; hydro-electricity or wave generation for energy; water treatment (desalinisation plants) for turning sea water into drinkable water.

The other thing is of cause the sea itself; well yes, you did touch on this slightly when you talked about storms, but also, if the platform is stationary or on a single/multiple pillars these will need to be protected from eventual decay (metal will rust, stone will erode).
Solutions: Pillars made from layers of material that won't rust or erode; consistently maintenance and treatment.

If the platform is not stationary, then how will it handle waves? Small ones might not be a problem, but what about larger ones? If the platform floats then you should also consider whether it will move with the currents and what happens if it approaches land or rocks.
Solutions: The platform can be steered; wave machines produce counteracting waves, to cancel out incoming waves.

Also there's weight considerations; a floating platform will start to sink the heavier it becomes, whilst a platform atop a pillar won't, but that said, how much stress can the platform take with such a big weight on top of it, regardless if it's on a pillar or sitting/floating on the sea?
Solutions: Very think platform which can sink into the sea more; giant walls so the platform can go below sea-level; a dome to cover the top.

Building on the platform won't be all that different than building on land really, it's the building of the platform you should focus on, since that's just man-made land, and if anything happens to the platform then the buildings mounted to it are sunk.

Answer 2

There has actually been a substantial amount of research put into this subject, and real proposals to create this in our world in the near(ish) future. For example take a look at Seasteading and specifically http://www.seasteading.org/floating-city-project/

There is a more general look at the subject on wikipedia https://en.wikipedia.org/wiki/Seasteading which has a very relevant section:

Retrofitted cruise ships

The first seasteads are projected to be cruise ships adapted for semi-permanent habitation. Cruise ships are a proven technology, and they address most of the challenges of living at sea for extended periods of time. The cost of the first shipstead was estimated at $10M.

Spar platform

The Seasteading Institute has been working on communities floating above the sea in spar buoys, similar to oil platforms. The project would start small, using proven technology as much as possible, and try to find viable, sustainable ways of running a seastead. Innovations that enable full-time living at sea will have to be developed. The cruise ship industry's development suggests this may be possible.

A proposed design for a custom-built seastead is a floating dumbbell in which the living area is high above sea level, which minimizes the influence of waves. In 2004, research was documented in an online book that covers living on the oceans.

András Győrfi's "The Swimming City"

At the Seasteading Institute Forum, an idea arose to create an island from modules. There are several different designs for the modules, with a general consensus that reinforced concrete is the most proven, sustainable and cost-effective material for seastead structures

Other

Many architects and firms have created designs for floating cities. Marshall Savage also discussed building tethered artificial islands in his book The Millennial Project: Colonizing the Galaxy in Eight Easy Steps, with several color plates illustrating his ideas. Some design competitions have also yielded designs, such as those produced by Evolo and other companies.

So as you can see a sea city is entirely feasible. Whether you go the "floating" or "fixed to the floor" route really would depend on local environmental conditions, in the middle of the deep ocean for example floating would be the only option.