US & Canada

BP will continue drilling a relief well in the Gulf

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Media captionAdm Thad Allen: "Everybody is in agreement we need to proceed with the relief well"

The US government has said BP will continue drilling a relief well to permanently plug the blown-out well in the Gulf of Mexico.

But the government's incident commander, Adm Thad Allen, said drilling would not proceed until they were sure it was safe to do so.

Scientists had been analysing tests to see if mud and cement pumped in the top of the well had "killed" it.

When they decide to drill again, it will take 96 hours for crews to resume.

"The relief well will be finished," Adm Allen said. "We will kill the well."

BP said last week its "static kill" procedure, in which mud and cement was pumped into the top of the well, had worked.

But Adm Allen said it was not clear whether that plug that had formed was enough to block the damaged well permanently.

Drilling of the relief well was put on hold this week because of poor weather.

Meanwhile, Alabama's attorney general is suing BP, Transocean, and other companies connected with the oil spill for damages the state has sustained from the disaster.

Image caption Drilling of the relief well was put on hold this week because of bad weather

Attorney General Troy King filed the lawsuits in federal court on Thursday against the wishes of Alabama Governor Bob Riley, who wishes to reach a settlement with the companies outside the courtroom.

The lawsuits claim the companies damaged the state's coast and economy through failing to "adhere to recognized industry standards". BP has not commented on the move.

'Bottom kill'

The relief well is intended for a "bottom" kill" procedure, in which cement would be pumped into the bottom of the damaged well in the hopes of permanently stopping the leak.

The bottom kill procedure follow's BP's static kill operation, which forced special drilling fluid known as mud through the top of the well to try to seal the leak.

President Barack Obama is set to travel to Panama City, Florida, for a family trip on Saturday. The Gulf Coast town witnessed tar balls washing up on its shores after the oil spill.

Mr Obama, his wife, and their two daughters will be visiting the area during a time when summer tourism is down compared with previous years along the coast.

Oil leaked into the Gulf from 20 April when the Deepwater Horizon rig explosion killed 11 workers. The flow was stopped on 15 July, when a cap was used to seal the top of the wellhead.

An estimated 4.9m barrels of oil leaked into the waters of the Gulf over the course of 87 days, with only 800,000 barrels being captured.

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".

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