How to add internal supports/localized infill, preferably in Cura? I'm currently designing a very large and complicated model, because it's large and should be strong enough completely hollow I want to print it with no infill to save material.

However, there are some points in the model where there are indentations in the top shell.

The bottom of those indentations are lower then the surrounding shell (pretty mush the dictionary definition of indentation) and so they are printed earlier with nothing to connect to.

For example, in the image below from Cura's layer view you can see two round surfaces just hanging in mid air



Obviously this can't be printed.

What I'm currently doing is designing internal spaces inside the model where the internal space shell is holding the "detached" surfaces.

What I want is a mostly automatic process that will generate a support structure inside where needed or that will generate infill but only between the floating surface and the nearest solid shell below it.

Reorienting the model is not an option because a. it wouldn't fit on the build plate and b. even if it did it has a lot of internal parts with different orientations so it would just move the problem from one part of the model to another. 

I usually use SketchUp for designing and Cura for slicing so I prefer something I can do in Cura - but since I strongly suspect this can't be done in Cura I'll accept answers that work with other tools
I'm not really sure if i get you right but it seems like you gotta switch on support material. That's all. Correct me if I'm wrong but it sounds like very common situation where some parts are hanging like your yellow disks.

Otherwise if your model is going to be closed at the bottom and at the top you can mock support on your own to have better control of it. How? Just add hollow cylinders under your disk so in fact cylinder will stay on the bottom layer then its walls will be support and finally your yellow disk will be a top cover of the cylinder. I hopw you can imagine that properly.

Another way is to design support elements wich would grow out of walls and this way support your disks



Maybe if you would explain a bit more, what the model is then I could support you more.

[edit]

To make it more automatic, you could try to use parametric CAD apps. I would say, Autodesk Inventor is one which I can suggest. You can use for example its Ribs functionality as support (varsion C).

[edit2]

After Tormod Haugene's and tbm0115's comments I decided to fill some gaps in my answer. Here go pros and cons of options A, B and C.


option A


The simplest one and more or less automatic one. Which means when user add support then this option will be a result of it. Of course if user needs to have support only for the disk but over the disk there will be... a "ceiling" then option A cannot use regular support as it will generate a support also for the ceiling. For such situation user needs to add a cylinder manually (filled one) or


option B


User can add hollow cyliner (just its walls). This option can give similar or higher stiffness as regular support because solid walls increase endurance tahn thin support grid.


option C


This option (my personal choose) has some advantages unavailable in A or B option. First - support doesn't have to have "ground" base. It means the yellow disk can really hang even if there is no bottom layer at all. Second - it joins walls with disk and walls together so the stiffness concerns other surfaces.

Depending on needs the cylinder (option A or B) can be transformed into a cross or pillar (thinner than cylinder itself). It can be more taper expandint to the top or to the bottom.

There could be even option where the above one would be joined with option C. So these (3) ribs would raise from the bottom but not from the walls.