I leave the project of a supporting structure for someone else. Whatever the supporting structure, it must have sufficient mass to overcome the varying buoyancy of the upwelling device (which depends on the temperature and salinity of the water it contains). The FLIP (Floating Instrument Platform) ocean research vessel demonstrates that it is possible to maintain stable elevation in the deep ocean.
It is more durable than most any man-made object placed out in the ocean just by virtue that no part of it extends above sea level. The pipes that do extend from the depths to sea level are made from polypropylene which is tough, and also has a tendency to flex. I imagine that they will sway somewhat with the waves, and if a boat runs over them, will give way. The device will still continue to function even if a few pipes are chipped from propeller damage. The supporting structure will no doubt in part extend above sea level and should be designed with durability in mind.
Indeed, maintenance because of biofouling may well be a major expense. To keep the big critters out, gratings need to be installed at the top entrances. As for algae and bacteria that may form a coating of scum on the heat transfer surfaces, there are a few things that work in the device's favor. In the deep ocean there is a dearth of primary production to begin with, hence the problem cannot be as severe as near the coast. And, there is no light in the heat exchanger and there are no photosynthesizing organisms at the depth of the source of the up-flow at a depth of 300 meters or so. The up-flow creates a region conducive to primary production, but I imagine the prevailing current carries the effluent away from the device. A coating of tributyltin on the heat conducting surface would certainly be effective, though use of it may be undesirable due to environmental concerns, even though the upwelling is far from any shore. For more information, see: Antifouling Paint Biocides by Ioannis K. Konstantinou. At any rate, the upwelling device provides a unique environment in the ocean and the biofouling problem cannot be assessed without experimentation.
For one thing, I did not have the burden of having to use Microsoft software. Also, my dad is Cornell professor of Mechanical Engineering, Emeritus, and when I needed a formula for convective heat transfer, he told me what it was.