That has sparked concerns that the Genmar Companion, which is carrying a 54,000-tonne load of vacuum gas oil, could have been more of a danger to the Donegal coast than first feared.
The ship has been forced to anchor outside Belfast lough since.
The 228m-long tanker had been off the north-west coast of Donegal on December 16, en route from Rotterdam to New York, when the captain reported a crack in the hull.
Now its cargo is to be unloaded at sea to prevent any danger inside Belfast harbour.
All 23 crew members are reported to be in good health and have remained on board.
The ship was inspected by experts who decided that, as a precautionary measure, the cargo should be removed and the ship repaired.