I think you've answered your own question. - "As per the explanation given in annex-L, this is related to when the dike wall is flooded."

If there is a possibility that the containment dyke or bund can fill with water due to rain, flood etc to a depth higher than the floor of the tank, then you need to consider flotation issues when the tank is empty.

It doesn't cover the situation when the tank leaks as there would be equal or higher fluid height in the tank as outside.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Tank flotation, (buoyancy effects) must, of course, also consider site flooding by rain, tidal surge etc ...

For API-650 style tanks, the flotation issue must be considered at the foundation design stage. Frequently, clients will dance into the engineers office as the tanks are being welded up and ask for a design change.

Concrete pad and ringwall foundations with an anchor bolted tank bottom are highly resistant to flotation. Other types of foundations and non-anchor bolted tanks are not.

Considering flotation under flooding conditions for propane and other tanks located underground is very important.

MJCronin
Sr. Process Engineer