Not only are Rules ships designed to handle only a unrealistically restrictive range of loading conditions, they are not designed to handle even the most likely damage scenario. Double hull tankers have a glaring weakness: their exposure to flooding of the ballast tanks when loaded. The idea of the double hull is that it's a buffer between the sea and the cargo. Whatever the merits of this idea, if it has any real purpose, you expect to use this buffer from time to time. Once the tanker fleet is converted to double hulls, it is nearly guaranteed that there will be multiple such floodings per year. Yet the Rules ignore this risk.
It is nuts that IMO Reg 25 and IMO Reg 16 Raking go to great length to require that the ship be able to withstand a whole range of flooding scenarios from a stability point of view, many of which scenarios the ship cannot withstand structurally.
The Rules should require that any double hull tanker withstand the flooding of any single ballast tank and any single contiguous pair, trio and quartet of ballast tanks when loaded to scantling draft in the design wave without going plastic. For a standard sized VLCC, this will result in a sagging moment allowable of just over 1,000,000 ton-m. The IACS required sagging moment allowable for this ship is about 620,000 ton-m.