You can try to get a permanent anchor permit as a bouy for this and promise to run and maintain the lights of a “reefmarker” to facilitate the coast guards work. This makes you a “desireable piece of infrastructure” instead of a piece of “undesireable development”. The picture on the right is my prototype. In your case i would not apply for vessel status applying for this status has a lot of downsides
And now you say
Sure mobility, make the concrete shell blimp shape and put a small engine in it – but that would already be the “Captain nemo float out”
To picture the Oceanic Concrete Sphere Habitat – this is the optical appearance of a ocean sphere sticking slightly out of the water. Definitly Draupner Wave impact and hurricane safe.
Interior of the sphere structure is something like that…
That “optical appearance of a ocean sphere sticking slightly out of the water” highly resembles a concrete cap over an island used for nuclear bomb testing. The concrete cap was built between 1977 and 1980 at a cost of $239 million. As you can see in your own photo, it’s not taken weather well, it’s got cracks and holes in it, and plants growing in the cracks. To repeat, $239 million for just the top part, not floating, and it has holes and cracks in it in under 40 years.
From what you say i assume that you try to make a point for “concrete structures do not stand in marine environment” This discussion is not really on in “well informed marine concrete engineering” since the following structures have performed perfect marine service records for decades. In short – if there is a failure – the culprit is not the concrete – the build was flawed from the biginning – the problem is the builder and his not performance of basic concrete engineering guidelines… the following link leads to 50 city sized concrete structures in the north sea – perfect service record. http://concretesubmarine.activeboard.com/t46256297/concrete-structures-in-the-north-sea/ this is how things need to be done. Concrete dome shells are the most durable thing that mankind has built EVER – the PANTEON in Rome is a 2000 year old concrete shell structure and it is still in top shape today.
To compare: Marine steel is structurally dead after 1-3 years if maintenance shedule stops. You really need to put things into the right context…