Oil is a critical resource for the world. Millions of barrels of oil are shipped from where the oil is taken from the Earth to where it is processed (refined) into substances varying from fuel to plastics. Because the world’s demand for crude oil is great, oil must be shipped in large quantities by oceanic tankers, barges on inland waters, and pipelines that run over the land and under the sea. When there is a mistake or accident that causes oil to spill from any of these means of transport, damage to the water, beaches, and economy can be devastating.
Oil (also known as petroleum) is a naturally occurring thick liquid mixture of the elements hydrogen and carbon combined to form chemicals known as hydrocarbons. The oil taken from the ground (or seafloor) is called crude oil. Crude oil is then pumped or shipped to plants (refineries) where the crude oil is converted into fuels such as gasoline and home heating oil, or turned into chemicals that are used in hundreds of other products such as plastic.
In addition to accidental spills, oil can also be spilled during routine processes of cleaning ships and pipelines as well as when loading and unloading ships. Evidence is mounting that the combined effects of these small spills in ports or local waters can cause substantial damage over time. More spectacular and damaging, however, are the sudden spills associated with the sinking or damage to an oil tanker or oil drilling platform at sea.
Oil pollution of waters
The total spillage of petroleum into the oceans through human activities is estimated to range from one million tons to two million tons of oil every year. Although this is less than about one-thousandth of the amount of oil shipped every year (0.1%), the effects can be very damaging if the concentration of oil (for example, the amount of oil in a liter or gallon of water) in a small area becomes too large.
The most damaging oil spills arise from disabled ocean tankers or drilling platforms, from barges or ships on inland waters, or from blowouts of wells or damaged pipelines. Damage is also caused by the relatively frequent spills and discharges from refineries. Large quantities of oil are also spilled when tankers clean out the petroleum residues from their huge storage compartment, sometimes dumping the oil and water mixture directly into the ocean.
Accidents and spills
Some the largest oil spills from ocean supertankers include:
• Amoco Cadiz, which went aground in the English Channel in 1978, spilling 253,000 tons (230,000 metric tons) of crude oil
• Torrey Canyon, which ran aground in 1967 off southern England, spilling about 129,000 tons (117,000 metric tons) of crude oil
• Braer, which spilled 93,000 tons (84,000 metric tons) of crude oil off the Shetland Islands of Scotland in 1993
• Prestige, which split in half off Galicia, Spain, in November 2002, spilling about 67,000 tons (61,000 metric tons) of crude oil
• Metula, which wrecked in 1973 in the Strait of Magellan and spilled 58,000 tons (53,000 metric tons) of petroleum
• Exxon Valdez, which ran onto a reef in Prince William Sound in southern
Alaska in 1989 and discharged 39,000 tons (35,000 metric tons) of petroleum.
All of the tankers involved in these spills were of the older single hull (the frame or body of a ship) design. New-double hulled tankers are designed to reduce the chances of a spill following an accident.
Prestige Oil Spill Near Spain
Despite rescue efforts, the oil tanker Prestige sank off the coast of Spain in November 2002. Oil washed ashore along the coasts of Spain, Portugal, and France. The spill threatened hundreds of miles of wildlife-rich coasts. Scientists and environmentalists compared the damage from the Prestige spill to the damage caused by the 1989 Exxon Valdez spill in Alaska.
Although the final estimate on the size of the spill is debated, most petroleum engineers estimate that about 67,000 tons (61,000 metric tons) of fuel oil spilled. The tanker broke apart as it was being towed to deeper waters. Engineers initially hoped that most of the oil might solidify (form tar balls) inside the sunken Prestige tanker’s compartments in the colder and deeper water, but later estimates showed that most of the oil escaped.
The oil spilled was far more toxic (poisonous) than the type of oil carried by the Exxon Valdez.
Experts predict that marine and bird life will suffer death and disease caused by the Prestige spill well into 2012. In addition to those species directly harmed, other species in the food chain will also suffer, either from lack of food or from eating other poisoned animals.
The preliminary cost for cleanup and lost economic activity is estimated at $42 million.
A worker releases a sorbent boom in an attempt to contain an oil slick from the 1989 Exxon Valdez oil spill.
Significant oil spills have also occurred from offshore drilling or production platforms. In 1979, the IXTOC-I exploration well had an uncontrolled blowout that spilled more than 551,000 tons (500,000 metric tons) of petroleum into the Gulf of Mexico. Smaller spills include one that occurred in 1969 off Santa Barbara in southern California, when about 11,000 tons (10,000 metric tons) were discharged, and the Ekofisk blowout in 1977 in the North Sea off Norway, which totaled 33,000 tons (30,000 metric tons) of crude oil.
Wartime spills
Enormous quantities of petroleum have also been released during warfare. Because petroleum and its refined products are critically important to economies and industry, enemies have commonly targeted tankers and other petroleum-related facilities during wars. For example, during World War II (1939-45), German submarines sank 42 tankers off the east coast of the United States, causing a total spillage of about 460,000 tons (417,000 metric tons) of petroleum and refined products. There were 314 attacks on oil tankers during the Iran-Iraq War of 1981-87, 70% of them by Iraqi forces. The largest individual spill during that war occurred when Iraq damaged five tankers and three production wells at the offshore Nowruz complex, resulting in the spillage of more than 287,000 tons (260,000 metric tons) of petroleum into the Gulf of Arabia.
The largest-ever spill of petroleum into the marine environment occurred during the brief Gulf War of 1991. In that incident, Iraqi forces deliberately released an estimated 0.6-2.2 million tons (0.5-2 million metric tons) of petroleum into the Persian Gulf from several tankers and an offshore tanker-loading facility known as the Sea Island Terminal. An additional, extraordinarily large spill of petroleum to the land and atmosphere also occurred as a result of the Gulf War, when more than 700 production wells in Kuwait were sabotaged and ignited by Iraqi forces in January 1991. The total spillage of crude oil was an estimated 46 to 138 million tons (42 to 126 million tons).
Exxon Valdez
The most damaging oil spill ever to occur in North American waters was the 1989 Exxon Valdez accident.
More than most tanker accidents, this one was very preventable. It was caused when the captain (who was later found to have been drinking alcohol) gave temporary command of the supertanker to an unqualified and inexperienced subordinate, who quickly made a mistake in navigation and ran the ship aground onto a well-known reef. The spilled oil affected about 1,200 miles (1,900 kilometers) of shoreline of Prince William Sound and its vicinity, causing especially great ecological damage.
Large numbers of sea mammals and birds were also affected in offshore waters. An estimated 5,000-10,000 sea otters were present in Prince William Sound, and at least 1,000 of these charismatic mammals were killed by oiling. About 36,000 dead seabirds of various species were collected from beaches and other places, but the actual number of killed birds was probably in the range of 100,000300,000. At least 153 bald eagles died from poisoning when they consumed the carcasses of oiled seabirds.
Great efforts were expended in cleaning up the oiled shoreline, almost entirely using manual and physical methods, rather than dispersants and detergents. In total, about 11,000 people participated in the cleanup, and about $2.5 billion was spent by the ship owners and $154 million by the U.S. federal government. This was by far the most expensive cleanup ever undertaken after an oil spill. Within a year of the spill, the combined effects of the cleanup and winter storms had removed most of the residues of the Exxon Valdez spill from the environment. However, in August 2002, the Exxon Valdez Trustee Council released a report stating that many fish and wildlife species injured by the spill had still not fully recovered.
Much of the spilled petroleum burned in spectacular atmospheric infernos, while additional, massive quantities accumulated locally as lakes of oil, which eventually contained 5.5-23 million tons (5-21 million tons) of crude oil. Large quantities of petroleum vapors were dispersed to the atmosphere. About one-half of the free-flowing wells were capped (closed) by May, and the last one in November 1991.
After the spill
After oil is spilled into the environment, it spreads out or sinks (dissipates) in a number of ways. Spreading refers to the process by which spilled petroleum moves and disperses itself over the surface of water. The resulting slick can then be transported away from the initial site of the spill by currents and winds. The rate and degree of spreading are affected by the thickness (viscosity) of the oil, wind speed, and waves.
If a spill is near enough to land, a mixture of oil and water called a mousse can wash up on the shore. The mousse combines with sand on the shore to form sticky patties that can harden into asphalt like lumps (material similar to that used to make roads).
At sea, the mousse eventually forms tar balls and in the vicinity of frequently traveled tanker routes world-wide, tar balls can be commonly found floating offshore and on beaches.
Ecological damages of oil spills
Even small oil spills can cause important change in ecologically sensitive environments. For example, a small discharge of oily bilge (wastewater) washings from the tanker Stylis during a routine cleaning of its petroleum-storage compartments caused the deaths of about 30,000 seabirds, because the oil was spilled in a place where the birds were abundant.
Studies made after large oceanic spills have shown that the ecological damage can be severe. After the Torrey Canyon spill in 1967, hundreds of miles of the coasts of southern England and the Brittany region of France were polluted by oily mousse. The oil pollution caused severe ecological damage and many different life forms suffered from exposure to petroleum. The ecological damages were made much worse by some of the cleanup methods, because of the highly toxic detergents and dispersants that were used.
The effects of oil spills can be harmful both immediately and over time. For example, the Torrey Canyon spill caused the deaths of at least 30,000 birds, but it also resulted in a large population of surviving birds that experienced difficulty in laying eggs for many years after the spill.
The damage caused by detergents and dispersants (chemicals used to break up spills) during the cleanup of shorelines polluted by the Torrey Canyon spill provided an important lesson. Subsequent cleanups of oil spills involved the use of less toxic chemicals.
In 1978, the Amoco Cadiz was wrecked in the same general area as the Torrey Canyon. Considerable ecological damage was also caused by this accident. However, the damage was less intense than that caused by the Torrey Canyon because less-toxic detergents and dispersants were used during the cleanup.
WORDS TO KNOW
Detergent: A chemical used as a cleaning agent because it encourages the formation of an oil-in-water emulsion.
Dispersant: A chemical agent that reduces the surface tension of liquid hydrocarbons, encouraging the formation of an oil-in-water emulsion. This reduces the volume of residual oil on shorelines or the water surface after a spill.
Mousse: A water-in-oil emulsion that is formed by turbulence of the surface water after a petroleum spill to the aquatic environment.
Petroleum: A naturally occurring, liquid mixture of hydrocarbons that is mined and refined for energy and the manufacturing of chemicals, especially plastics. Also known as crude oil.
