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).