Clean ballast. A charterparty can include a clause requiring the vessel to arrive in a loading port with only "clean ballast". In many ports there are inadequate reception facilities for dirty ballast before loading and ships are restricted from discharging dirty ballast at sea.
Clean ballast. A tank on board an oil tanker may be so cleaned since oil was last carried in it that the effluent from it would not produce visible traces of oil on the surface of the water or on nearby shorelines or cause a sludge or emulsion to be deposited beneath the surface of the water or on nearby shorelines. When the effluent is discharged, the vessel must be stationary and the discharge must be into clean, calm water on a clear day for the above criteria to apply.
The ballast may also be discharged through suitable oil discharge monitoring and control systems but the oil content must not be more than 15 parts per million for the effluent to be considered to be “clean ballast”, despite visible traces of oil being present.
In the attempts to stem pollution by oil from ships, the MARPOL Convention was adopted in 1973 and a Protocol added in 1978 which aimed at preventing pollution by oil from ships. Under MARPOL, existing ships were permitted to discharge oily mixtures into the sea under very strictly controlled circumstances. Such ships had to be fitted appropriately to prevent operational pollution. They were permitted to have either Dedicated Clean Ballast Tanks (CBT) or Segregated Ballast Tanks (SBT) or operate Crude Oil Washing (COW) depending on size. For crude oil tankers over 70,000 deadweight tonnes, the Clean Ballast Tank option ended in 1985 and for those over 40,000 it ended in 1987.
“New tankers” could have only “Protectively located” SBTs and COW.
If a tanker is not a “new tanker” there are requirements for control of operational pollution and control of the discharge of oil. One criterion permitting discharge from these tankers is that the effluent discharged must meet the description of “clean ballast”.
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