Marpol Regulation 34

Discharges outside special areas

Any discharge into the sea of oil or oily mixtures from the cargo area of an oil tanker shall be prohibited except when all the following conditions are satisfied:

.1 the tanker is not within a special area;
.2 the tanker is more than 50 nautical miles from the nearest land; (This is to make quite certain that the very small amounts of oil permitted to be discharged in compliance with requirements (.4) and (.5)will not reach the shore.)
.3 the tanker is proceeding en route; (This eliminates the possible concentration of the permitted discharge from a stationary vessel or from a vessel steaming in tight circles, and ensures that the discharge is well distributed).
.4 the instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile; (The “instantaneous rate” means the rate of discharge of oil content in litres per hour at any instant divided by the speed of the tanker in knots at the same instant. Experiment has shown that 30 litres per mile discharged in this way disperses rapidly and leaves no trace of oil on the surface of the water within 2 or 3 hours of discharge.)
.5 the total quantity of oil discharged into the sea does not exceed for tankers delivered on or before 31 December 1979 = 1 / 15,000 the total quantity of the particular cargo of which the residue formed a part and for tankers delivered after 31 December 1979, 1/30,000 of the total quantity of the particular cargo of which the residue formed a part; and (This puts a limit on the total amount of oil that can be discharged even at the reduced rate set out in (.4)).
.6 the tanker has in operation an oil discharge monitoring and control system and a slop tank arrangement as required by regulations 29 and 31 of Annex of Marpol.

Discharges in special areas

.1 Any discharge into the sea of oil or oily mixture from the cargo area of an oil tanker shall be prohibited while in a special area.
.2 This shall not apply to the discharge of clean or segregated ballast.

*Nothing shall prohibit a ship on a voyage, only part of which is in a special area from discharging outside the special area in accordance with above i.e “discharges outside special areas”

Requirements for oil tankers of less than 150 gross tonnage

The requirements of regulation 34 do not apply to oil tankers of less than 150 gross tonnage. The total quantity of oil and water used for washing and returned to a storage tank shall be discharged to reception facilities unless adequate arrangements are made to ensure that any effluent which is allowed to be discharged into the sea is effectively monitored to ensure that the provisions of this regulation are complied with.