This spill is a fairly spectacular example
of the power of hydrostatic balance in the pre-Marpol single hulls.
The ship was nearly fully loaded with 55,000 tons of crude.
Whet it hit the rock, 8 of 15 cargo tanks were breached.
However, the ship flooded in a manner
that resulted in her sinking 2 or 3 meters with only a slight list to port,
putting the port gunnel just underwater.
See the first picture at the AMSA site.
This sinkage improved the hydrostatic balance immensely.
The AMSA report says a total of 12 tanks flooded
Apparently the weather was very calm throughout,
keeping the sagging moment at a level that the hull could handle,
although the picture seems to show noticable deflection.
The crucial fact here is that the Oceanic Grandeur was able
to withstand the over-design sagging moment
associated with flooding the midships permanent ballast tanks while loaded
with her bottom all torn up.
The fact that this 61,000 tonner was built in 1965
meant that she had more strength than the later 1970's built pre-Marpol ships,
not to mention far more strength than the tankers built in the 1980's and later.
If the hull had failed, we would have spilled the better part of 50,000 tons
on the Great Barrier Reef.
In the event, the Australian report indicates that less than 1%
of the cargo was spilled as a result of the initial damage.
The Australian report further indicates nil oil was lost
during the subsequent three days despite tidal currents of up to 6 knots.
It appears that the bulk of the 1100 tons spilled was lost on the 7th day of lightering
as the ship rose out of the water, reducing the external hydrostatic pressure.
If they had done a really careful job of lightering,
this spillage could have been prevented;
but probably, at the cost of a longer lightering,
which would have entailed its own risks.
The AMSA report talks about stability problems;
but CTX's guess is that their real worry was longitudinal strength,
which is another reason why they would have wanted
to get liquid out of the middle of the ship as soon as possible,
even at the cost of some spillage.
Even so the Oceanic Grandeur, with over half her cargo tanks breached, lost just 2% of her cargo.
The Australians later located the offending rock and blasted it to a depth of 12 m,
so it is easy to accept that poor charting was indeed the cause of this casualty.