Deepwater Horizon oil spill summary

Oil spills are probably one of the worst environmental hazards, which can cause deep and devastating damage to nature. Fortunately, due to the high standards imposed on oil industry, they occur rarely. But in 2010, the world could feel just how little does it take for crisis to create.

Deepwater Horizon was an ultra-deepwater, dynamically positioned, column-stabilized, semi-submersible mobile offshore drilling unit. The rig was 396 feet long and 256 feet wide. It was designed to operate in waters up to 8,000 feet, with a maximum drill depth of 30,000 feet. Built by Hyundai Heavy Industries in South Korea and completed in 2001, the rig was owned by Transocean Ltd. and leased to British Petroleum until September 2013.

On the evening of April 20, 2010, a well control event allowed hydrocarbons to escape from the Macondo well onto Transocean's Deepwater Horizon, resulting in explosions and fire on the rig. Eleven people lost their lives, and 17 others were injured.

The fire, which was fed by hydrocarbons from the well, continued for 36 hours until the rig sank. Hydrocarbons continued to flow from the reservoir through the wellbore and the blowout preventer for 87 days, causing a spill of national significance for the United States.

Events on the offshore drill unfold
The Deepwater Horizon crisis involved a well integrity failure, followed by a loss of hydrostatic control of the well. The day before the accident, cement had been pumped down the production casing and up into the wellbore annulus to prevent hydrocarbons from entering the wellbore from the reservoir. However, there were weaknesses in cement design and testing, quality assurance and risk assessment. Prior to temporarily abandoning the well, a negative-pressure test was conducted to verify the integrity of the mechanical barriers (the shoe track, production casing and casing hanger seal assembly).

The test involved replacing heavy drilling mud with lighter seawater to place the well in a controlled underbalanced condition. Test was deemed successful and well integrity had been supposedly established. However, the shoe track barriers did not isolate the hydrocarbons as they should. The rig crew did not recognize the influx and did not act to control the well until hydrocarbons had passed through the blowout preventer and into the riser.

Well control response actions failed to regain control of the well. Diversion to the mud gas separator resulted in gas venting onto the rig. The heating, ventilation and air conditioning system probably transferred a gas-rich mixture into the engine rooms, causing at least one engine to over speed, creating a potential source of ignition. Three methods for operating the blowout preventer in the emergency mode were unsuccessful in sealing the well.

Oil spill shocks the world
The arrival of the oil onshore was different than the iconic images from the Exxon Valdez spill where crude oil from a tanker spilled onto the surface of an enclosed body of water close to a rocky, static shoreline. The Deepwater Horizon spill was pouring millions of gallons from the floor of the Gulf 5,000 feet below in an open sea, and 50 miles from the nearest land, which is composed of broken marshes, river deltas, open bays and barrier islands.

It was the largest release of oil in the history which released approximately 4,900,000 barrels of oil into the sea. The resultant oil spill continued until July 15 when it was temporarily closed by a cap. Relief wells were used to permanently seal the well, which was declared "effectively dead" on September 19, 2010. British Petroleum began performing the first controlled burn of surface oil. The Environmental Protection Agency and Coast Guard approved the use of dispersants, a group of chemicals designed to be sprayed onto oil slicks to accelerate the process of natural dispersion.

Oil spill dispersants do not actually reduce the total amount of oil entering the environment. Instead, they change the inherent chemical and physical properties of oil. British Petroleum announced that it supports the plan to construct the six sand berms in order to stop the oil from progressing to the shore. The company will fund the estimated $360 million cost of construction. The six approved barriers, four west of the Mississippi River and two to the east, would rise 6 feet above sea level. They will be 300 feet wide at their base and 25 feet wide at their crown.

Next: Deepwater Horizon oil pollution