All designs don't use ballast-

TLP's (tension leg platforms) don't.  They use the buoyancy of the structure against tight mooring lines.

http://www.atlantia.com/semisub/images/semisub_FPU.jpg

In this picture all the lines you see going to the bottom are risers (vertical pipelines). You can see the anchorlines (under tension) leading off at about a 30deg angle.

Here's a better pic- http://www.atlantia.com/semisub/images/semisub_upclose.jpg

It's simple, very stable and resistant to failure. For instance a flooding of one pontoon results in a minor list that direction. The buoyancy of the structure and the opposing tensioned anchor lines preserve stability while you deal with the problem.

The TLP is a very nicely stable platform too. Think of it as an upside down pendulum. Now make it a box with 4 strings instead of one, cut the sides out to let waves through, put a lot of weight in it and immerse it in a viscous fluid.  The TLP is more stable in all dimensions than a floating platform.

The TLP configuration would allow all occupied spaces to be above the waterline. Improved safety, lower cost, and more high value space are a few advantages. Some mechanical spaces could be located in the upper legs but basically ballast tanks aren't suitable for much else.

I think durability would be measured in decades not years. Regular maintenance like biofouling removal and anchor replacement would be rather easy.

It's simple to build and deploy. How I'd do it is build the base and the legs up to about 40' and launch it. Move it to a spot about 30' deep and ballast (flood it) down to the bottom. Then working from a barge, continue construction. The legs would be cylindrical, steel (or frp rebar) reinforced poured aggregate concrete. The deck pre stressed concrete like a parking garage. I think a truncated pyramid superstructure would work well.

Deployment is simple. Pump out the ballast water, tow it to location, ballast down (pump water in), connect the tension legs (anchor lines), and pump the water out tensioning the legs.

The anchors would be suction piles or "SEA" (suction embedded anchors). See www.sptoffshore.com  go to products, SEA anchors.

Simple and stable, probably the most square footage per dollar possible. It could be constructed by the owners without a shipyard. Shipyards don't build ships, people build them and the shipyard is a middle man so to speak. A middle man trying to cover overhead between projects by adding it to your ship. pet peeve there.

A smaller design could be steel, but again it could be self constructed after launching the base. Or the base could be self constructed in cofferdam or drydock.

Here's an interim solution for construction housing and support. It's an ex Navy YRST II dive support barge that accomodates 100. I've inspected it personally. Cheap at twice the price and much nicer than the pics would indicate.

http://www.freewebs.com/riversummers/yrst2forsale.htm

 A related post- Design