Oil tankers

All high sea vessels are equipped with ballast tanks, which can be filled with seawater to improve stability when carrying little or no cargo, to correct the vessel’s trim or to reduce her list. Previously, the fuel tanks and some of the cargo tanks of oil tankers used to carry out this job. When these tanks were emptied, significant quantities of oil mixed with seawater were discharged into the ocean. Operational discharge from a vessel can thus be compared to oil from a number of cars or trucks being released into the environment when changing the engine oil.

The attention paid to the pollution of the sea by oil tanker accidents triggered an awareness of the importance of operational discharge. In 1973, the volume of deliberate discharge was five times greater than accidental spills according to the US National Academy of Sciences. Such high volumes required worldwide control measures. These measures were brought in by the International Maritime Organization within the framework of the Oilpol Convention (1954) and reinforced by the Marpol Convention (1973, 1978), which also extended them to non-tankers.

Crude oil washing

The crude oil washing method involves cleaning out tanks not with water but using crude oil, i.e. the cargo itself. The solvent action of the crude oil makes the cleaning process far more effective than when water is used. The process is usually completed by a final water rinse, but only a small volume of water is used. This method is accepted as a possible alternative to the system of segregated ballasts onboard existing tankers and constitutes an additional requirement onboard new tankers.

Crude oil washing introduces dangers due to the accumulation of explosive gases in the cargo tanks as the cargo is offloaded. This is why the protocol relating to the 1974 SOLAS Convention,
adopted at the 1978 International Conference on Tanker Safety and Pollution Prevention (TSPP) and brought into force in May 1981, stipulates that an inert gas system must always be used when crude oil washing is carried out.


Oilpol banned the release of polluted waters in certain zones, so-called special zones, such as the Mediterranean Sea, and established authorised discharge thresholds elsewhere. Marpol, which came into force in 1983, introduced even more restrictive discharge standards and imposed new ship-building rules for new vessels. All new tankers with a deadweight tonnage equal to or greater than 20,000 tonnes had to be equipped with segregated ballasts, situated defensively to contribute to the protection of cargo tanks in the event of collision or grounding. Depending on their size and age, and according to a predefined timescale, existing vessels were to be equipped with segregated or dedicated ballasts (oil tanks equipped with a crude oil washing system to be put into action before using tanks as ballasts), with the transfer of washing residues into specific reservoirs (slop tanks).

Oil tanker

Discharge at sea was restricted to less than 1/30,000 of the cargo volume per journey for vessels built after 1996, with the introduction of load-on-top techniques. This system involves pumping oily mixtures resulting from tank cleaning operations into special slop tanks. The mixture is then separated by settling and the water portion is pumped into the sea, leaving only crude oil. At the loading terminal, fresh crude oil can then be loaded on top of the remaining oil.

These regulations and the investment of certain countries in remote sensing of operational discharge led to positive results. According to the US National Academy of Sciences, operational discharge from oil tankers fell from 1,080,000 tonnes in 1973 to 700,000 tonnes in 1981,and then to 159,000 tonnes in 1989, with a slight increase to 163,000 tonnes in 2000. It is however important to note that these figures are not statistical data but estimations of the most likely situation, based on the proportion of oil tankers which fulfil the Marpol Convention criteria and on the waste reception facilities in ports.