We have surprisingly little info on this very big spill.
The ship had a GRT of 8997,
so the 18,000 ton figure is not possible.
P. D Sharma in Environmental Microbiology says 3,000 tons.
Fairly clear entire cargo was lost,
so going with Hooke's number of 13,000 tons.
Assuming bulk of the cargo was diesel,
given National Institute of Oceanography comment,
but this could easily be wrong.
We have no cause info,
but it sounds like the weather was pretty bad.