Realism: Unable to dive

[Adour]

Fellow subsimers,

I have been trying to find info on what may prevent u-boats from diving. I have heard of boats having to return to base on the surface and the inherent risks therein but this is not well represented in SH3.

From what I can work out as your hull integrity drops your crush depth decreases but you can always get to periscope depth right up until instant death at 0% integrity.

I get the impression being rammed or large calibre gun fire which breached the pressure hull would keep u-boats on the surface.

So my questions are:

What kind of damage would constitute a critical failure in the pressure hull?

How do any of you simulate this in game?

LJ