I think the main difference with with a VLCC (Very Large Crude Carrier) is that Crude Oil is a Uniformly Distributed Load of Low Density, whereas Iron Ore is a Concentrated Load of High Density.
It is possible that the ship was an Old single skinned Tanker, that was re purposed when Double Hulls became mandatory, in this case as part of the conversion a Double Bottom should have been installed and this should have increased the Longitudinal strength of the Hull.
Iron ore has a wide range of densities from 2,100 kg/m3 to 5.100 kg/m3 ( according to my on line research) Crude Oil of course has a density of less than 1,000 kg/m3.
Therefore she would have been designed so that a full Cargo of Crude would fill the Tanks Volume wise, and the Centre of Gravity (of the cargo) would be half way up the tank, but, Iron Ore would only fill them to between 20% and 50% by Volume. making the C of G between 10% and 25% of the tank height above the bottom, the Structural Strength and Stability of the Ship would have been calculated for crude. Substituting Iron Ore means the Centre of Gravity of the Cargo is much lower, making the ship too stiff, it would roll like a bastard in bad weather, unless the bottoms of the Tanks were raised considerably, or Ballast Tanks were installed under the Main Deck to raise the C of G, but if these were used it would reduce the Deadweight capacity of the ship.
Because of the difference in Densities of different ores, many purpose built ore carriers have heavily strengthened Tank Tops in ALTERNATE Holds, combined with a lot of additional Longitudinal strengthening, so that they can load the strengthened holds full and leave the unstrengthened ones empty to overcome the stability problems and when they
Iron Ore Carriers offer suffer structural damage because of the heavy weight of Iron Ore dropped into them from a great height in the loading Ports, often at tens of thousands of tons an hour, it can be very difficult to load the cargo evenly, unlike with a Tanker, it may be that the ship has to be moved up and down the jetty, delaying operations, in order to load cargo in a different part of the ship, and a lot of pressure can be put on the ships crew to load the cargo in an unsafe manner. Poorly trained crew often do not understand the importance of CONTINOUSLY monitoring the load condition so that AT ALL TIMES the STABILTY and LONGITUDINAL STRESSES are within Limits, it is NOT OK if the stresses are within limits on arrival in Ballast and again on Departure fully loaded, in between the ship could have Hogged or Sagged badly, causing severe structural damage, leading to the ship breaking up at Sea.
However, many Third World Crew would not enter the intermediate states of load into the Load computer nor would they be assertive enough in dealing with the shore side people managing the loading.
I suspect that any Tanker conversion would have minimal safety margins of Structural Strength when loading when compared to a purpose built Iron Ore Carrier, and may have already suffered from wastage of the steelwork in the areas of the old tanks. This could be aggravated by the excessive loading stresses (Stress Corrosion).
I don't have any personal experience of Ore Carriers so if anyone out there knows more, I won't be upset by any criticism, this is what I have read over the years..
It is always better to ask a stupid question than to do a stupid thing.