Gulf of Mexico Oil Spill

In the United States, most offshore drilling takes place in the Gulf of Mexico, where, on April 20, 2010, an explosion and subsequent oil spill occurred on an offshore rig operated by British Petroleum. The spill introduced about 4.9 million barrels (about 206 million gallons) of oil into the Gulf’s waters over several months. More than 3 million liters (about 800,000 gal) of chemical dispersants were used to combat the spill.

Offshore Drilling

The majority of the world’s liquid petroleum (oil) and natural gas resources take the form of deposits known as traps that are buried under incredibly thick layers of dirt and rock. These traps have been formed through millions of years, as organic materials like the bodies of plants and animals are “cooked” by heat and pressure into hydrocarbons: organic compounds containing carbon and hydrogen atoms.

As the world’s demand for energy grows and as oil companies begin to exhaust the supply of land-based oil reserves that are easily recoverable, more companies have turned to offshore drilling to increase supply. Offshore drilling is the search for and extraction of petroleum or natural gas reserves from beneath the ocean floor, usually at locations on the continental shelf, the area of seabed that surrounds a large landmass.

In the United States, offshore drilling occurs anywhere from about 60 meters (about 200 feet) to about 320 kilometers (about 200 miles) off the country’s coastline. The wellbores, or drilled holes, which have been created in the process, have been of varying depths. Some have extended as far as 5.5 km (3.4 mi) below the ocean’s surface. Deeper wellbores are becoming increasingly more common as the search for viable traps expands.

To extract oil from undersea deposits, a trap must first be located. This is often done through a technique known as seismic surveying, in which shock waves emitted by compressed air guns and explosives are sent down into the seabed. When the reflected waves bounce back, they are analyzed to measure the specific geophysical properties of the rocks below and to determine the likelihood of oil deposits in that region. Next, a mobile drilling platform is established over a likely site; exploratory drilling then occurs. If core samples reveal what is called a show, or clear evidence of a petroleum deposit, a production well or wells are drilled.

The wells are affixed to an enormous platform, usually attached directly to the ocean floor by a foundation made of metal, concrete, and cables. The entire rig is designed to last for the time it takes to deplete the deposits, which may be ten or twenty years or longer; the structure also must be sturdy enough to weather storms and large enough to accommodate supplies, equipment, and housing for the platform workers.

Drilling Hazards

The oil industry in the United States maintains that offshore drilling’s benefits (which include stabilizing the price of oil and reducing the United States’ dependence on foreign petroleum imports) outweigh any serious risks to the environment, particularly in the wake of technological improvements such as advanced safety valves and sensitive temperature and pressure sensors. However, opponents of offshore drilling outline many potential hazards.

The most severe of these hazards is the risk of an oil spill. Even a smoothly functioning rig may have detrimental effects on the environment and may also pose health risks to oil workers because of the danger of fires, electrocutions, and other accidents. Air pollutants such as carbon monoxide, nitrogen oxide, methane, and volatile organic compounds are discharged during routine activities, such as loading and shuttling oil from the rig to shuttle tanks. Heavy metals and other contaminated sediments released in drilling fluids also are likely to be picked up from the surfaces of the rig by ocean currents and dispersed to surrounding areas.

The physical infrastructure associated with offshore drilling platforms, including pipelines and wellbores, also causes permanent changes to the ecosystems that exist at and below the ocean floor (in the benthic and abyssal zones). Finally, when rigging equipment, pipelines, and ships are taken from one place to another, invasive species such as mussels and barnacles can be spread to new environments, where they may disrupt the balance of the native ecosystem.

The Deepwater Horizon Oil Spill

A 2009 report by the Energy Information Administration, part of the U.S. Department of Energy, estimated that about 50 percent of oil consumption in the United States was met by domestic production. About one-third of that production came from offshore drilling, the majority of which was attributable to oil rigs in the Gulf of Mexico—namely those off the coast of Louisiana, Mississippi, Alabama, and Florida.

At about 10 p.m. Eastern Standard Time on April 20, 2010, an explosion rocked an offshore oil rig in the Gulf of Mexico that was operated by British Petroleum (BP); the rig was known as the Deepwater Horizon. Deepwater Horizon’s wellbore ran about 5.5 km (about 3.4 mi) below sea level.

Because the weight of the rocks above an oil trap creates pressure, and because this pressure increases with depth, deeper wells present greater risks of blowouts, or explosions. The Deepwater Horizon explosion caused gas, oil, and concrete to burst violently up the wellbore onto the deck of the platform, starting a fire. Eleven platform workers were killed and seventeen were injured in the incident. Drilling of the rig’s primary wellbore had begun in February of that year, and the final placement of the cement designed to seal it had been completed just before the accident. The fire raged for two days, after which the entire rig sank below the surface of the water.

According to the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, an independent U.S. presidential commission that studied the disaster, an intricate array of factors led to the explosion and subsequent spill. Technically, the reason the explosion occurred was straightforward: The cement that had been pumped down to the bottom of the wellbore failed to seal the wellbore from the highly pressurized hydrocarbons around it.

Once the wellbore was declared to be completely prepared, the drilling mud inside it was replaced by seawater, reducing the pressure inside the well. When the pressure of the hydrocarbons became greater than the pressure in the well, oil and gas began to flow into the ring-shaped space around the well casing, a phenomenon known as a kick. The stream of volatile hydrocarbons then traveled up to the wellhead, where it burst through the blowout preventer and then ignited. The commission cited pervasive failures of management, communication, and safety procedures as responsible for these technical problems.

Strategies for Sealing the Leak

On April 24, underwater cameras detected a leak in the wellbore that was releasing oil into the ocean; estimates of the rate at which oil was spilling from the rig varied dramatically in the next weeks and months, but official figures eventually settled at a rate of approximately 5,000 barrels (about 210,000 gallons) of oil per day.

The oil continued to gush into the Gulf for several months, even as a large number of different techniques were employed to try to stem the flow. For example, attempts were made to properly activate the blowout preventer: a series of safety valves designed to prevent an explosion of oil, but which were never fully deployed during the disaster. Two relief wells were dug that were used to inject mud and cement into the leaking well; golf balls, rubber, and other objects were also used as attempted plugs. A large box made of steel, known as a containment dome, was lowered over the leak. A tube was inserted into the broken pipe to divert the oil to a ship on the surface. Several different caps were placed on top of the blowout preventer and used to stop the leakage of oil and simultaneously siphon oil and gas to tankers on the surface.

Eventually, a combination of mud and cement, introduced slowly into the well, provided a complete seal. Officials declared the well permanently sealed on September 21, 2010. Ultimately, an estimated total of 4.9 million barrels (about 206 million gallons) of oil was released into the Gulf. To reduce the amount of oil that washed to shore, floating devices called booms and artificial sand walls called berms were deployed as physical barriers.

Chemical Dispersants and the Effects of the Weather

Cleanup efforts were occurring at the same time as attempts to seal the well. Chemical dispersants are most commonly used to deal with oil spills. They consist of substances that break up large amounts of surface oil into smaller droplets that disperse; the droplets can then be more easily weathered or biodegraded. (Oil slicks that are not dispersed present little surface area to which natural underwater microbes like bacteria can attach.) Dispersed oil also is less likely to wash up in coastal wetlands or to form tarballs, which are dense, sticky blobs of weathered oil that can travel great distances and can persist for some time after a spill.

Dispersants typically consist of a solvent, or a component that dissolves other substances, and a surfactant, a surface active agent that reduces the surface tension of a liquid in which it is dissolved. Surfactants consist of long molecules with a chain-like structure. One end of the chain is hydrophilic, meaning that it has a tendency to be drawn to and dissolve in water. The other end is oleophilic, meaning that it has a tendency to be drawn to and dissolve in oil. Normally, oil and water do not mix. As the surfactant molecules attach themselves to water on one end and to oil on the other, they form a kind of bridge between the two substances. This lowers the tension of the surface between them and allows the oil to break up into droplets.

To deal with the Deepwater Horizon spill, BP used more than 3 million liters (about 800,000 gallons) of chemical dispersants. Mainly, the dispersants consisted of a particular dispersant known as Corexit 9500A, one of whose main ingredients was a surfactant called dioctyl sodium sulfosuccinate (DSS). The Corexit and other dispersants were not simply applied at the surface of the ocean; they were directly injected into the flow of oil and gas that was leaking from the wellhead in the Gulf of Mexico at depths of about 1,200 to 1,500 m (4000 to 5000 ft). This substantial use of dispersants in the deep ocean was unprecedented, and a later analysis revealed that a detectable level of DSS remained in the deep waters of the Gulf four to five months later. Scientists did not know if the DSS lingered for long enough or at high enough levels to be toxic to deep-water organisms.

One factor that worked in favor of the cleanup efforts was the warm weather in the Gulf of Mexico. Higher water temperatures allow certain volatile compounds in the oil to evaporate faster; higher temperatures also increase the rate at which bacteria can break down the compounds into harmless carbon dioxide and water.

The region’s tropical weather systems also caused concern among scientists, however. On the one hand, the strong winds and turbulent seas would help to accelerate the biodegradation and weathering of oil. On the other hand, a hurricane could distribute the oil farther than it would otherwise travel, bringing it farther inland toward coastlines that would otherwise not have seen much oil. If, for example, winds drove oil into the loop current, the oil could have been carried into the Gulf Stream around Florida and partway up the East Coast of the United States. Ultimately, no major hurricanes made contact with the Deepwater oil slick.

Environmental Impact

Oil spills can have serious effects on marine wildlife and habitats. The spills kill fish and other ocean life, especially if the spills occur during breeding season when eggs are vulnerable. The Gulf spill had a negative impact on the shrimp, crab, and oyster industries in Louisiana for this reason. In 2021, the National Oceanic and Atmospheric Administration (NOAA) estimated that trillions of fish hatchlings had been killed, 4 billion oysters had been lost, and 100,000 to 200,000 sea turtles had been injured or killed due to the 2010 spill. NOAA also noted that the spill greatly reduced the Gulf of Mexico's bottlenose dolphin population.

Oil spills are notorious for killing seabirds and shorebirds. Oil that coats feathers can prevent birds from flying, can destroy their natural temperature regulation mechanisms and waterproofing, and can poison birds as they attempt to clean themselves. Oil that directly reaches marine mammals, such as whales, otters, and seals, has similar harmful effects. Oil also can contaminate the food chain as it washes up to beaches and coastal wetlands like marshes and mangrove forests, making its way into grasses and other plants. This can damage or destroy essential feeding, breeding, and nesting grounds for many species. Also, the cleanup efforts themselves can do harm to delicate wetlands.

After the Gulf spill, long stretches of the Louisiana coastline, particularly in the south, did experience heavy surges of oil, which blanketed the marshes there. NOAA estimated that the spill killed tens of thousands of seabirds.

Multiple analyses performed about one year after the spill found that a surprising amount of the oil that had leaked from the wellhead did not travel with ocean currents outside the Gulf, but had instead remained in deep water and been broken down by microbes. Because of favorable weather conditions, the worst predictions about how much oil might wash up on the coastline were not fulfilled. In particular, oil never flowed around the tip of Florida up the East Coast of the United States. However, the long-term environmental effects of the Deepwater Horizon spill itself will take years, if not decades, to fully assess, especially when it comes to the impact the disaster had on marine populations such as bluefin tuna, dolphins, starfish, and coral. By 2024, fourteen years after one of the world’s worst environmental disasters, the Gulf of Mexico's ecosystems and wildlife had still not recovered. Evidence of oil from the accident remained evident on the deep sea floor.

In January 2025, President Joe Biden ordered a ban on new offshore oil and gas drilling in the majority of US coastal waters. While the ban aimed to protect coastlines in Florida and other states, it did not apply to large portions of the Gulf of Mexico, where most of the nation's offshore drilling continued.

Principal Terms

berm: an artificial ridge or wall, usually made of sand; normally used to prevent flooding but employed after the Gulf spill as a barrier to keep oil from polluting beaches

blowout preventer: a large series of safety valves installed at the wellhead of an oil rig to seal the well in case of a problem and to prevent the uncontrolled release of oil

boom: a floating device, usually made of either plastic or cloth, which is linked with other booms to form a flexible barrier intended to prevent oil spills from moving inland

dispersant: a chemical, usually consisting of a surfactant and a solvent, which is used to break up an oil slick into smaller droplets that will then disperse into the water and be weathered more quickly

hydrocarbon: any of a large group of organic compounds containing various combinations of carbon and hydrogen atoms; a common substance in petroleum products, including crude oil and natural gas

loop current: an ocean current that transports warm water from the Caribbean Sea into the Gulf of Mexico

surfactant: a substance that reduces the surface tension of a liquid in which it is dissolved

tarball: a dense, sticky blob of weathered oil that can travel great distances and is difficult to break down

weathering: the disintegration and degradation of a substance, such as oil, through the action of wind, water, and waves

wellbore: a hole drilled into the earth to look for or extract a natural resource, such as oil, water, or gas

wellhead: a general term for the equipment installed at the surface of a wellbore; designed to provide a pressure seal for the oil or other substance being extracted

Bibliography

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