Oil Spills

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Oil Spills

Petroleum is an important natural resource that is often produced in places far away from the regions where most of its consumption occurs. Accordingly, petroleum must be transported in large quantities, mostly by oceanic tankers, barges on inland waters, and both subsea and overland pipelines. Any of these transportation systems can release pollution through spills of oil, by operational discharges associated with cleaning of the storage tanks of tankers, or during unloading at refineries. Some accidental oil spills have been spectacular in their magnitude and their near-term ecological impact, involving losses of huge quantities of petroleum from wrecked supertankers or offshore platform facilities.

Oil pollution can also be caused by discharges of improperly handled hydrocarbon-laden waste water from petroleum refineries and in urban runoff. Although individual spills typically involve relatively small amounts of oil, frequent small spills can release a large amount of oil into the environment.

Characteristics of petroleum

Petroleum is a naturally occurring mixture of organic chemicals, the most abundant of which are hydrocarbons (molecules containing only hydrogen and carbon atoms). Petroleum is synthesized from biomass by complex, anaerobic reactions occurring at high pressure and temperature over long periods of time deep in sedimentary geological formations. Petroleum can occur as a liquid known as crude oil, which may also contain natural gas, and also as a semi-solid tar or asphalt in oil sands and shales. There are hundreds of molecular species in petroleum, ranging from gaseous methane with only 16 g/mole, to very complex substances weighing more than 20,000 g/mole.

Petroleum differs in its physical and chemical characteristics from deposit to deposit. Some crude oils are thick and viscous, while others are light and volatile. The lighter fractions of petroleum evaporate relatively quickly when spilled into the environment. This leaves behind residues of relatively heavy molecules that are more persistent in terrestrial or aquatic habitats, and cause longer-lasting effects.

Oil pollution

The total spillage of petroleum into the oceans through human activities is estimated to range from about 0.7-1.7 million tons (0.6-1.5 million tons) per year, equivalent to less than 0.1% of the quantity of petroleum transported by tankers. In comparison, the production of hydrocarbons by marine plankton is about 28.7 million tons (26 million tons)/year. These natural hydrocarbons contribute to background concentrations in the oceans, but they are well dispersed and not associated with ecological damage or pollution. In addition, natural oil seeps contribute about 6-13% of the total petroleum input to the oceans, sometimes causing local damage.

The largest and most consequential oil spills of petroleum or heavy bunker fuel come from disabled oceanic 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 operational discharges associated with coastal refineries and urban runoff. Large quantities of oil are also spilled when tankers clean out the petroleum residues from their huge storage compartments, often discharging the oily bilge washings directly into the ocean.

Some examples of disastrous oil spills include the following accidents involving oceanic supertankers: (1) the Prestige, which split in half off Galicia, Spain in November 2002, spilling about 67,000 tons (61,000 metric tons) of crude oil; (2) the Torrey Canyon, which ran aground in 1967 off southern England, spilling about 129,000 tons (117,000 metric tons) of crude oil; (3) the Metula, which wrecked in 1973 in the Strait of Magellan and spilled 58,000 tons (53,000 metric tons) of petroleum; (4) the Amoco Cadiz, which went aground in the English Channel in 1978, spilling 253,000 tons (230,000 metric tons) of crude oil; (5) the 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; and (6) the Braer, which spilled 93,000 tons (84,000 metric tons) of crude oil off the Shetland Islands of Scotland in 1993. All of the tankers involved were of the older single hull design. Such occurrences should decrease with the advent of double-hulled tankers.

Significant oil spills have also occurred from offshore drilling or production platforms as the result of mechanical or operational failure. 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.

A leak from a hole in an oil pipeline at Prudhoe Bay, Alaska, is estimated to have released between 200,000 and 300,000 gallons of crude oil in early 2006. Discovery and cleanup of the spill were complicated by the fact that the pipeline is heavily insulated, and oil may have been leaking into the insulation surrounding the pipe for some time. Sub-zero temperatures made repairs, in which a steel sleeve was place around the pipeline and welded into place, and cleanup difficult. Later that year, the discovery of extensive corrosion in the 30-year old Prudhoe Bay pipeline system, which sends oil to the Trans Alaskan Pipeline, led to a complete shutdown of that oilfield while 16 miles of the 22 mile long pipeline were replaced. Corrosion in oilfield pipelines can be monitored using tools known as smart pigs, which are computerized tools run through the pipeline to assess its condition. At the time of the shutdown, the Prudhoe Bay oil field produced about 400,000 gallons of oil per day, or about 8% of the United States daily consumption. One of the immediate results of the reduction in oil production, which was compounded by other events such as the U.S. occupation of Iraq and political instability in such oil producing countries as Nigeria, was an increase in oil prices to record levels.

Enormous quantities of petroleum have also been released during warfare. Because petroleum and its refined products are important economic and industrial commodities, enemies have commonly targeted tankers and other petroleum-related facilities during wars. For example, during World War II 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 1981–1987, 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. Sabotage of pipelines and petroleum facilities was also common in the post-2001 U.S. led invasion of Iraq, during which insurgents frequently attacked oil pipelines. The amount of oil that leaked into the environment as a result of the attacks is unknown. An Israeli air attack on an oil-fired power plant in Lebanon released about 15,000 tons of oil into the Mediterranean Sea during the 2006 Hezbollah-Israel conflict, causing extensive environmental damage.

The largest-ever spill of petroleum into the marine environment occurred during the 1991 Gulf War. In that incident, Iraqi forces deliberately released an estimated 0.6-2.2 million tons (0.5-2 million 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 46–138 million tons (42–126 million tons). Much of the spilled petroleum burned in atmospheric conflagrations, while additional amounts accumulated locally as lakes of oil, which eventually contained 5.5–23 million tons (5–21 million tons) of crude oil. Enormous quantities of petroleum vapors were dispersed to the atmosphere. About one-half of the free-flowing wells were capped by May, and the last one in November 1991.

After oil is spilled into the environment, it 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 by currents and winds. The rate and degree of spreading are affected by the viscosity of the oil, wind speed, and turbulence of the water surface. Evaporation is important in the initial reduction of the volume of an oil spillage, especially of relatively light and volatile hydrocarbon fractions. Evaporation typically accounts for almost 100% of spilled gasoline at sea, 30-50% of spilled petroleum, but only 10% of bunker fuel. Solubilization occurs when some fractions of the spilled oil dissolve into the water column, causing a contamination of subsurface waters in the vicinity of the oil spill. For example, beneath a petroleum slick in the North Sea the concentration of hydrocarbons in water was 4 g/m³(ppm), compared with about 1 mg/m³ (ppb) in uncontaminated seawater. Lighter hydrocarbon fractions of petroleum are much more soluble in water than heavier ones, and aromatics are much more soluble than alkanes. (Aromatic hydrocarbons such as benzene and naphthalene have an unsaturated ring structure, while alkanes such as octane have a linear structure.) In addition, some of the spilled hydrocarbons are slowly oxidized by ultraviolet radiation and microorganisms into simpler compounds, ultimately to carbon dioxide and water.

The combined influences of solubilization, evaporation, and oxidation are known as weathering. Weathering preferentially removes the lighter hydrocarbon fractions, leaving a residual material made up of relatively heavy hydrocarbons. Over the shorter term in aquatic environments, this residuum forms a stable water-in-oil emulsion known as mousse, which is the material that usually impacts shorelines after an offshore spill. The mousse combines with sediment particles on the shore to form sticky patties of oil and sand, which eventually form asphaltic lumps.

At sea, weathering of the mousse eventually results in the formation of a dense, semi-solid, asphaltic residuum known as tar balls. In the vicinity of frequently traveled tanker routes worldwide, tar balls can be commonly found floating offshore and on beaches. Tar balls are especially common in places where the oceanic circulation resembles a surface vortex. A well known example of this phenomenon is the oceanic gyre known as the Sargasso Sea, famed for its accumulations of natural debris such as floating seaweed, as well as such human debris as tar balls.

Ecological damages of oil spills

Even small oil spills can cause important changes in ecologically sensitive environments. For example, a small discharge of oily bilge 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. Even relatively small operational spillages of petroleum can have significant though, perhaps temporary, ecological impact, especially to seabirds and marine mammals. An ecosystem is dynamic—ever changing—and continues its natural cycles and fluctuations at the same time that it struggles with the impact of spilled oil. As time passes, separating natural change from oil-spill impacts becomes more and more difficult.

Studies made after large oceanic spills have shown that the ecological damage can be intense. After the Torrey Canyon spill in 1967, hundreds of kilometers of the coasts of southern England and the Brittany region of France were polluted by oily mousse. The oil pollution caused severe ecological damage, due to the physical and toxic effects of fouling of organisms with petroleum residues. Those direct ecological damages were made much worse by some of the cleanup methods, because of the highly toxic detergents and dispersants that were used. As is the case with many oil spills, seabirds were victims of the Torrey Canyon incident. This accident caused the deaths of at least 30,000 birds, causing a substantial decrease in their breeding populations in subsequent years.

The damage caused by detergents and dispersants during the cleanup of shorelines polluted by the Torrey Canyon spill were an important lesson. Subsequent cleanups used less toxic chemicals. In addition, their use became largely restricted to offshore locations and places of high value for industrial or recreational purposes, rather than natural habitats.

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, in much smaller quantities, and only in high-value places such as harbors.

The most damaging oil spill ever to occur in North American waters was the Exxon Valdez accident of 1989. It was caused when an intoxicated captain gave temporary command of the supertanker to a subordinate, who erred in his navigation and ran the ship aground on a reef. The spilled oil affected about 1,200 mi (1,900 km) of shoreline of Prince William Sound and its vicinity, causing ecological damages in tidal and subtidal habitats. Large numbers of sea mammals and birds were also affected in offshore waters. An estimated 5,000-10,000 sea otters (Enhydra lutris ) 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 100,000-300,000 birds. At least 153 bald eagles (Haliaeetus leucocephalus ) died from poisoning when they scavenged 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 that has ever been 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 while it is clear that many fish and wildlife species injured by the spill have not fully recovered, it is less clear what role oil plays in the inability of some populations to recover. Bald eagles, black oystercatchers, common murre, pink salmon, river otters and sockeye salmon have recovered. Wilderness area intertidal communities, killer whale, marbled murrelet, sea otters, clams and sediments are recovering but the common loon, three species of cormorants, harbor seals, harlequin duck,

KEY TERMS

Bilge washings —Hydrocarbon-contaminated water that results from cleaning of the petroleum-holding compartments of a tanker, and that may be discharged to the environment.

Bunker fuel —A relatively viscous, liquid hydrocarbon mixture, also known as bunker-C fuel oil, that remains after lighter hydrocarbons are distilled from petroleum during refining. Bunker C is used as a fuel by oil-fired generating stations, heating plants, and ships.

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.

Pacific and pigeon guillemot have shown little or no recovery.

See also Fossil fuels.