Gulf of Mexico oil well disaster

20 April: A surge in oil and gas causes an explosion on the Deepwater Horizon oil drilling rig. The blowout preventer system of valves (BOP) at the well head on the seabed is believed to have failed.

22 April: The oil rig sinks and the riser pipe that connected it to the well falls to the seabed. Oil and gas continue to flow from the pipe and blowout preventer, causing a five mile (8km) oil slick.

The blowout preventer is meant to be the ultimate fail safe against pressure surges. Its valves should have closed, shutting off the oil and gas from the reservoir and sealing the well.

The initial failure of the blowout preventer, and subsequent efforts to remotely shut it down, result in a constant leak of oil and gas from holes in the bent riser pipe on the seabed and above the BOP.

As oil continues to flow into the sea, initial efforts are made on the surface to contain the leak using booms and dispersants. The response grows daily and soon hundreds of vessels are involved, including skimmers, tugs and recovery ships as well as dozens of aircraft and multiple mobile offshore drilling units.

2 May: BP starts drilling the first of two relief wells. The aim is to connect with the original well and then pump in heavy liquid to stem the flow of oil. Drilling for the second starts on 16 May. Both are expected to take two to three months to complete.

5 May: BP successfully stops the flow of oil from the end of the drilling pipe - one of three leak points. Around the same time, a huge containment dome is lowered over the main leak but this fails on 8 May as the dome is blocked by frozen hydrate crystals caused by leaking gas.

16 May: A tube is inserted into the leaking pipe to funnel off leaking oil and gas to a ship on the surface.

26 May: BP starts its "top kill" procedure in an attempt to plug the well by pumping mud into the blowout preventer from a vessel on the surface.

A manifold system of pipes and valves is connected to the BOP and a drill pipe from the vessel.

Pipes from the manifold are attached to the "choke and kill" bypass systems inside the blowout preventer. This gives access to the BOP system's main valves.

Heavy mud with a large proportion of clay is pumped into the BOP under high pressure. The aim is to force enough mud into the well to stop it flowing.

A "junk shot" mixture of materials such as rubber, golf balls and rope is injected in an effort to help block the flow.

29 May: BP announces that the top kill system has failed and the oil spill continues.

2 June: BP starts the next procedure - to lower a cap over the blowout preventer to capture the leaking oil and funnel it to a surface vessel.

The riser pipe is cut and removed and the cap is lowered onto the top section of the blowout preventer, known as the lower marine riser package or LMRP. The cap is not a tight fit and oil is still leaking around it, but it does collect about 15,000 barrels of oil a day.

16 June: Engineers open a second route to the surface by connecting the lines used in the top kill procedure to a floating rig called Q4000. This collects about 10,000 barrels a day.

10 July: The LMRP cap is removed to be replaced with a tightly fitting capping stack, designed to seal the well.

15 July: With the sealing cap fitted, all three rams - or valves - inside are turned off and the flow of oil is stopped for the first time since 20 April. In the following days, tests are conducted to check whether pressure will cause oil to leak from elsewhere in the BOP or well. The tests are successful.

Work begins on the next process: a "static kill" which involves pumping drilling mud through the blowout preventer into the well and reservoir.

5 August: Engineers follow up the mud with cement, blocking the well. US Government scientists announce the total estimated amount of oil spilt into the Gulf of Mexico is 4.9 million barrels. Work continues on the relief wells.

The plan is to use at least one relief well to pump more mud and cement into the main well to totally seal or "kill" it. The well can then be abandoned although BP says it will continue clean-up work in the area for "as long as it takes".

The cost of cleaning up the oil spillage in the Gulf of Mexico now runs into billions of dollars, according to BP estimates.

The company has tried a number of methods to shut down the leak from the damaged oil well - often innovating as subsequent attempts fail.

Although some of the methods have been tried and tested at incidents elsewhere in the world, they have not been carried out at such great depth as the Deepwater Horizon leak - 5,000 feet (1.5km) below the surface.

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