Degassing, deballasting and operational discharge

Degassing is an operation which involves freeing fuel tanks and crude oil tanks of the gases which remain when the tanks have been emptied. Onboard a vessel, the most practical and cost-effective solution involves expanding the gases using vapour, ventilating and rinsing the tank with water.

Discharge at sea

Deballasting is an operation which involves emptying out the contents of a ballast tank, i.e. a reservoir that can be filled, or partially filled, with water to load down or lighten a vessel to improve her stability and trim.

Empty fuel tanks, or on an oil tanker crude oil tanks, constitute natural ballasts. Ballasting (filling of ballast tanks) with water rinses out the oil and waste they contain.


When degassing and deballasting occur anywhere other than in treatment plants they constitute discharge at sea, producing trails of sheen where onboard separation is insufficient. However degassing and deballasting are only part of operational discharge of hydrocarbons.

The majority of deliberate discharge is caused by hydrocarbon residues unfit for propulsion. This affects all vessels. In modern tankers, there are two different types of residues: waste waters and fuel residue. Waste waters from the bilge are pumped from the engine sump into settling tanks. They are then filtered and separated by an oily water separator to a limit of 15 ppm. If the concentration is higher than 15 ppm, they are returned to the bilge water drain tanks. Fuel residues (known as sludge) and engine oil must be discharged on land or burned onboard (and are stored in tanks called slop tanks on oil tankers).

More information

IMO regulations in the document Marpol 73-78 for crude oil washing, segregated ballast tanks and dedicated clean ballast tanks, in force since 1993