Voluntary discharge of oil at sea is a result of various maintenance operations as illustrated in the diagram below. Discharging oily waste at sea is not always illegal: as show below, discharging sludge or waste oil is prohibited under all circumstances, however bilge water can be legally discharged in the open sea if the hydrocarbon content is no higher than 15 parts per million (ppm) whatever the quantity involved.
A ship can therefore leave a permanent stream of oily water in its wake for several hours, or even several dozen hours, as long as the concentration of oil in the discharged waste does not exceed 15 ppm. However, operational discharge is illegal, whatever the concentration, in marine areas protected by national or international regulations. The MARPOL 73/78 Convention thus bans all discharge in certain "special areas", including the Mediterranean Sea, the Baltic Sea, the Black Sea, the Red Sea, the Gulf of Aden, the Antarctic area and the Atlantic waters of north-west Europe.
Voluntary discharge is often referred to as “deballasting” or “degassing”. Strictly speaking, degassing is an operation which involves freeing the gases and traces of product which are left in the bunker tanks or crude oil tanks once the tanks have been emptied. Deballasting is an operation which involves emptying the contents of a ballast tank, i.e. a reservoir that can be filled with water, to a greater or lesser extent, to make the vessel heavier or lighter in order to ensure better stability and a better angle of trim (balance of the mass over the length of the vessel). Empty bunker tanks or, on an oil tanker, crude oil tanks, are used as ballast tanks. Their ballasting (filling) with water rinses out the oil and waste left in the tanks. This forms waste waters containing oil, which will then be “deballasted” (emptied) in port facilities or before arriving in port, preferably in compliance with the regulations in force.
Today oil tankers are rarely found in illegal discharge reports: they are now among the most environment-conscious vessels, placing their slops in a single tank and loading their cargo on top (followed by deballasting in port).
Deballasting in port is down to a fine art. As the tank is being emptied, an inert gas is injected to prevent explosion. Once the tank is empty, it is rinsed with a heavy fuel oil. It is then rinsed again, this time with seawater, reusing the same water for all the tanks. This water is settled, filtered and stored to be discharged in the open sea, in compliance with the 15 ppm rule. The residue is retained and will be incorporated with the following cargo. In the tanks, before inspection, the inert gas is replaced with air: this is the so-called “degassing” phase.
The quality, maintenance and adjustment of the oil-water separator are, along with appropriate consideration of special zones, the keys to environmentally-friendly shipping.