Discharges of oil from shipping, offshore extraction of oil, and transport of oil in pipelines is the result of either accidents or "normal", deliberate operational discharges. Accidental discharges (oil spills) occur when vessels collide or come in distress at sea (engine breakdown, fire, explosion) and break open, or run aground close to the shore, or when there is a blowout of an offshore oil well, or when a pipeline breaks. Much can be done to avoid accidents, but there will always be unfortunate circumstances and situations that cause accidents to happen. Operational discharges, on the other hand, are mostly deliberate and "routine", and can to a very large extent be effectively controlled and avoided. It is much a question combining available technical solutions with information, education and a change of attitude among ship-owners, mariners, offshore platform and pipeline operators.

Accidents involving oil tankers or offshore platforms or oil pipelines have caused many and sometimes very large oil spills. Such spills are the most obvious, visible and dramatic causes of acute oil pollution of the marine environment. However, the largest oil spill ever was caused by Iraq deliberately released about 240 million gallons (about 800,000 tonnes) of crude oil into the Persian Gulf during the 1991 Gulf War and burnt oil wells in Kuwait.

According to estimates made of the contribution of oil to the sea from different sources (NRC), accidents involving oil tankers and offshore installations account for some 10 per cent of the annual total amount of oils entering the marine environment. As reported by INTERTANKO, "tankers carry close to 40 per cent of the world's seaborne trade. In 2001, 57 per cent of the oil consumed in the world was transported by sea, approx. 2,000 million tonnes.

As pointed out by U.S. NOAA, "if oil spills of all sizes are considered, tankers and offshore installations do not account for most of these spills. Tanker accidents and accidents with offshore installations do, however, account for most of the world's largest oil spills. Accidental spills are less frequent than other kinds of oil spills, but typically involve large volumes of spilled oil relative to other kinds of oil spills".

"Analysts for the Oil Spill Intelligence Report track oil spills of at least 10,000 gallons (34 tonnes). In their annual International Oil Spill Statistics report for 1999, they reported that in that year about 32 million gallons of oil spilled into the water or onto land, in 257 incidents. Of those incidents, only 11 were spills from tankers, accounting for about 6.6 million gallons, or about one-fifth of the total volume of oil spilled. Twenty-five of the 257 spills were from barges and other kinds of vessels, such as freighters (totaling 1.5 million gallons). Eighteen spills were from trucks or railroad trains (totaling about half a million gallons). The largest number of spills, and the largest volume of oil spilled were from accidents involving pipelines or fixed facilities (131 pipeline spills, totaling about 18.8 million gallons; 66 spills from facilities, totaling about 4.7 million gallons). The percentages of oil spilled from different sources vary greatly from year to year; in some years, tanker accidents represent the largest single source of spilled oil, but only in a very few years is it the case that most of the oil spilled (in significant spills) during that year came from tankers. However, tanker accidents have been the cause of most of the very largest oil spills. The Cutter Information Corporation analysts also have found that of the 66 spills in which at least 10 million gallons (34,000 tonnes) of oil were lost, 48 were from tankers. Eight were from fixed facilities, especially storage tanks, five were from production oil wells, three were from pipelines, and two were from other kinds of cargo vessels."

"Oil spills happen all around the world. According to the Oil Spill Intelligence Report, spills in the size range of at least 34 tonnes have occurred in the waters of 112 nations since 1960. However, oil spills happen more frequently in certain parts of the world. They identified the following 'hot spots' for oil spills from vessels: the Gulf of Mexico (267 spills); the northeastern U.S. (140 spills); the Mediterranean Sea (127 spills); the Persian Gulf (108 spills); the North Sea (75 spills); Japan (60 spills); the Baltic Sea (52 spills); the United Kingdom and English Channel (49 spills); Malaysia and Singapore (39 spills); the west coast of France and north and west coasts of Spain (33 spills); and Korea (32 spills)".

However, one should not merely compare figures — the size of the spill is certainly not the only factor of importance in terms of what environmental damage can be caused by a spill. One example only, from the waters off South Africa: "There is rather little correlation between the tonnages of oil released in spills and the impacts on the marine ecosystems. For example, a collision between two oil tankers in 1977 released 31,000 tonnes of oil and polluted 47 African Penguins, but in the Apollo Sea sinking of 1994, about 2,000 tonnes of oil impacted about 10,000 penguins. After the Apollo Sea, we generally believed that this was the maximum amount of penguin mischief that 2,000 tonnes of oil could achieve. However, when the Treasure sank on 23 June 2000, half this amount of oil threatened four times as many penguins! In round figures, 20,000 penguins were oiled, and 20,000 penguins were prevented from becoming oiled by removing them off their breeding colonies on Dassen and Robben Islands."

Leakages from sunken, grounded or abandoned ships is another potential (and often very real) source of oil to the marine environment. These can be merchant or military vessels. Sunken vessels means just that — ships that have sunk to the bottom of the sea, generally due to an accident but sometimes also as the result of a deliberate action to get rid of them. Regarding grounded and abandoned vessels, a distinction is made in the NRC report between derelict vessels and historic wrecks. "Derelict vessels are generally unseaworthy or are no longer useful and have been tied up and abandoned. Others are mothballed, are awaiting repair or dismantling, or are intentionally grounded as a result of illegal activity. --- These abandoned vessels become potential sources of oil pollution, from either chronic leaks or a large release once oil storage areas fail." And: "From an oil pollution perspective, wrecks sunken during and since World War II pose the greatest risk because of the presence of residual fuels. -- World War II wrecks are of particular concern because they can contain large volumes of oil, and corrosion after nearly 60 years underwater can lead to chronic leaks and the potential for catastrophic releases.


As described by APPEA, "the weight of the drilling fluid/drilling mud acts as the first line of well control by keeping underground pressures in check. If an influx of pressurised oil or gas does occur during drilling, well control is maintained through the rig's blowout prevention system (BOP). This is a set of hydraulically operated valves and other closure devices (rams) which seal off the well, and route the wellbore fluids to specialised pressure controlling equipment. Trained personnel operating this highly reliable equipment minimise the possibility of a blowout, or an uncontrolled flow of fluids from a well".

However, it does not always work. As pointed out in the book Environmental Impact of the Offshore Oil and Gas Industry, the most typical causes of blowouts "include equipment failure, personnel mistakes, and extreme natural impacts (seismic activity, ice fields, hurricanes, and so on). Their main hazard is connected with the spills and blowouts of oil, gas, and numerous other chemical substances and compounds. The environmental consequences of accidental episodes are especially severe, sometimes dramatic, when they happen near the shore, in shallow waters, or in areas with slow water circulation. Broadly speaking, two major categories of drilling accidents should be distinguished. One of them covers catastrophic situations involving intense and prolonged hydrocarbon gushing. These occur when the pressure in the drilling zone is so high that usual technological methods of well muffling do not help. Lean holes have to be drilled to stop the blowout. Drilling accidents are usually associated with unexpected blowouts of liquid and gaseous hydrocarbons from the well as a result of encountering zones with abnormally high pressure. No other situations but tanker oil spills can compete with drilling accidents in frequency and severity."

The largest blowout, so far, was the Ixtoc I accident off the Mexican coast in 1979-1980 (about nine months). With a total oil spill of about 475,000 tonnes, it was the second largest oil spills of all times. There have been other large blowouts since then, among them one in Nigeria in 1980 with a total spill of over 54,000 tonnes (destroying over 340 hectares of mangroves).


"The causes of underwater pipeline damage can differ greatly. They range from material defects and pipe corrosion to ground erosion, tectonic movements on the bottom, and encountering ship anchors and bottom trawls. --- Depending on the cause and nature of the damage (cracks, ruptures, and others), a pipeline can become either a source of small and long-term leakage or an abrupt (even explosive) blowout of hydrocarbons near the bottom. The dissolution, dilution, and transferring of the liquid and gaseous products in the marine environment can be accompanied in some cases by ice and gas hydrates formation. --- Modern technology of pipeline construction and exploitation under different natural conditions, including the extreme ones, achieved indisputable successes. However, pipeline oil and gas transportation does not eliminate the possibility of serious accidents and consequences." (Environmental Impact of the Offshore Oil and Gas Industry)

Thus, pipelines can continuously leak oil for a long time, which is bad enough, but when a pipeline ruptures it causes a large, acute spill. The best-known examples of large spills occurred in the Guanabara Bay, off the Brazilian coast in 2000, when about 1,300 tonnes of oil were released into the sea, and in Nigeria in 1998, when a pipeline broke and 14,300 tonnes of oil were spilt.