On your first point, I'm sorry, I meant ecological disaster, which I think more or less implies manmade.
On your second point, fair enough, although I would like to note that how bad the ecological problem is does not depend solely on the magnitude of the spill but on other factors such as the location and the water and air currents.
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How did you calculate the spilling rate? Wikipedia says 35000 to 60000 barrels per day, which is more or less 5000 to 10000 tonnes per day, isn't it?
(7 barrels per ton) This gives more than one million tonnes per year, by the lowend estimate (1,850,000).
EDIT: Easier math at the link: 1 barrel per day=50 tonnes per year which gives 1,750,000 tonnes per year.