As you know, in the case of Costa Concordia, it was a straightforward case of the hull being breached by a rock - hence flooding the engine room - and ending up beached.
Correct. The ultimate problem was that there was water where there wasn't supposed to be water. In terms of buoyancy, that's why any ship sinks. In terms of stability, that's why there's no righting moment. The center of gravity is no longer fixed; it changes depending on flooding. And it changes in a way that the righting moment is nulled in different orientations, not just when the ship is visibly upright. This is why we throw out the intact-hull model as soon as flooding enters the picture.
And this is why Björkman's claims don't hold water, so to speak. The claim that a ship must turn turtle once it rolls past a critical angle is based on the observation that the righting moment reverses. That doesn't occur when the righting moment has been nulled by a shifting center of gravity due to flooding. And there are other reasons, but you've resisted every effort I've made to help you learn them.
It wasn't the hole in the hull that doomed
Costa Concordia. It was all that water being where it's not supposed to be. The hole was merely the medium of trespass. It's
flooding that matters, not the circumstances of how flooding occurred. That's why we can cut holes in ship hulls when they're in dry dock. The hole
per se is not dangerous. Water ingress
is per se dangerous.
In the case of the Estonia the JAIC said the hull was intact.
[...]
However, if the hull WAS breached, that puts a different complexion on the whole matter.
No. Breaching the hull is not a requirement—if by that you mean a hole below the waterline. It's the flooding that alters the ship stability equation, not the circumstances that caused the ship to flood. You may remember the
USS Lafayette, formerly the ocean liner
SS Normandie. Her hull was completely intact too, but that didn't stop all the water from the firehoses trying to put out her fire from creating a heeling moment. There have even been cases of ships developing an unresisted heeling moment simply due to heavy rainfall. The question is not whether there's a hole in the hull. The question is whether flooding is a significant term in the center-of-gravity reckoning.
The JAIC is correct to say
MS Estonia's hull was intact. It was, until it slammed into the sea floor. There were no holes below the nominal waterline. That doesn't mean it hadn't taken on significant amounts of water, and that this rampant and increasing flooding wasn't dominating the stability calculation. It doesn't matter whether the water gets in through the stairwells and downpipes, through a hole in the hull, or through a missing bow section. It's the presence of the water that matters, not the flooding mechanism.