Answer
As per MARPOL Annex II (Regulation 13), the discharge of Noxious Liquid Substances (NLS) into the sea is strictly prohibited unless it complies with the following criteria:
Outside Special Areas:
The ship is proceeding en route at a speed of at least 7 knots (self-propelled) or 4 knots (not self-propelled).
The discharge is made below the waterline through the underwater discharge outlet(s).
The discharge is made at a distance of not less than 12 nautical miles from the nearest land in water depth of not less than 25 meters.
Categories X, Y, and Z:
Category X: Must be pre-washed in port; residues discharged to shore reception facilities until the concentration is $\le 0.1\%$ by weight. Remaining tank washings can be discharged at sea as per the above speed/distance criteria.
Categories Y and Z: Tanks must be stripped to minimum residual quantities as per the Procedures and Arrangements (P&A) Manual. If residues exceed limits, a pre-wash is mandatory before leaving port.
Antarctic Area (Special Area): Total prohibition of any discharge of NLS.