How did top management at BP show a concern for maintaining high ethical standards following the Deepwater Horizon incident?

Why was there an explosion and fire on Deepwater Horizon oil rig?

According to BP's September 2010 report, the accident started with a "well integrity failure". This was followed by a loss of control of the pressure of the fluid in the well. The "blowout preventer", a device which should automatically seal the well in the event of such a loss of control, failed to engage. Hydrocarbons shot up the well at an uncontrollable rate and ignited, causing a series of explosions on the rig.

How many people were killed?

Eleven, from Texas, Louisiana and Mississippi.

Why did it take so long to stem the oil flow?

According to the final progress report from the Deepwater Horizon study group, 10 different techniques were used to try to plug the leak. This began with efforts to close the blowout preventer with a remotely operated vehicle. Successive efforts involved capturing oil spewing from the riser by lowering a "top hat" over it. Next, engineers attempted to "kill" the well by injecting heavy mud into the blowout preventer. All of these efforts failed. Finally, engineers were successful by bolting a sealing cap on top of the blowout preventer. This provided a temporary fix until engineers could pump heavy kill mud and cement into the well to reduce pressure at the well head and permanently seal off the flow paths.

The leak began on 20 April. The well was capped on 15 July and it was permanently sealed on 19 September 2010.

What was the size of the area affected by the oil spill?

It depends who you ask. BP contracted Polaris to assess the area affected and provide recommendations for cleanup. The shoreline cleanup technical adviser, Ed Owens, believes all of the oil is on the surface and only 10% of the oil actually reached the shoreline. His team surveyed about 4,000 miles of coastline, and on their first assessment they found that about 1,000 miles of shoreline had been affected. Of this stretch, about 200 miles were heavily oiled – meaning that the oil covering them was more than three-foot wide and covered 50% distribution. This included about 80 miles of heavily oiled wetland. Now, he says, 15 miles of heavily oiled beach remain, along with "some tens of miles" of moderately oiled coastline.

This correlates with reports from a spokesman for the National Oceanic and Atmospheric Agency (NOAA), who told the Guardian earlier this month that about 60 miles of coastline remain oiled.

Not everyone agrees. Samantha Joye's research team from the University of Georgia believes that more oil is lurking beneath the surface of the water. They found a 22-mile plume of oil and gas droplets in the depths of the Gulf of Mexico in May last year. Their findings were recorded in a report published in the peer-reviewed journal Science.

In December, Joye discovered a thick coating of oil, dead starfish and other organisms over an area of 2,900 square miles on the bottom of the ocean. She told the Guardian earlier this month: "I think it is not beyond the imagination that 50% of the oil is still floating around out there."

Some independent scientists dispute Joye's claims. Simon Boxall, an expert in the Deepwater Horizon disaster from the National Oceanography Centre in Southampton, said that various organisations - "almost too many to mention" - have taken samples from the sea bed and have not found any evidence for Joye's claims. He added: "She's talking about huge areas being affected, but she's basing it on one or two samples she's taken."

Who was responsible for the accident?

BP's report placed most of the blame on Halliburton and Transocean, but its findings have been heavily criticised. Ed Markey, a senator investigating the spill in Congress, said at the time: "Of their own eight key findings, they only explicitly take responsibility for half of one. BP is happy to slice up blame, as long as they get the smallest piece."

Tony Hayward, who was BP's chief executive at the time of the explosion, said in a statement that the initial well integrity failure was down to a "bad cement job" by oil and gas equipment company Halliburton. Halliburton said that it had noticed "a number of substantial omissions and inaccuracies" in the BP report, and said that it was "confident that all the work it performed with respect to the Macondo well was completed in accordance with BP's specifications."

In its report on the disaster, BP blamed the rig owners, Transocean, for failing to adequately maintain the blowout preventer. The report said: "The BOP [blowout preventer] maintenance records were not accurately reported in the maintenance management system. The condition of critical components in the yellow and blue pods and the use of a non-OEM [original equipment manufacturer] part, which were discovered after the pods were recovered, suggest the lack of a robust Transocean maintenance management system for Deepwater Horizon BOP."

Transocean responded with a vigorous rebuttal: "The BP report is a self-serving attempt to conceal the critical factor that set the stage for the Macondo incident: BP's fatally flawed well design."

It has been argued by commentators that the failure of Deepwater Horizon was the inevitable result of years of deregulation of the oil industry. However, the independent Deepwater Horizon study group final report, published in March this year, suggests that the real root of the problem was BP's own laissez-faire approach to safety.

Were BP's safety standards adequate?

No. According to the Deepwater Horizon final report: "This disaster was preventable if existing progressive guidelines and practices been followed", but BP "did not possess a functional safety culture."

The report, which was independently compiled by an international group of 64 experienced professionals, experts and scholars, gave a damning analysis of BP's failings. It said that "as a result of a cascade of deeply flawed failure and signal analysis, decision-making, communication, and organisational - managerial processes, safety was compromised to the point that the blowout occurred with catastrophic effects."

There were also reports that BP knew about a fault in the blowout preventer – the piece of equipment that ultimately failed, triggering an explosion – but did nothing to fix it.

In addition, BP's contingency plan for dealing with a catastrophic oil spill contained many errors and miscalculations, according to an analysis by the Associated Press. The mistakes included listing animals not found in the Gulf region (including seals and walruses) as potential victims of an oil spill, and the recommendation of a long-deceased scientist as an expert on wildlife contamination.

Did BP play down the severity of the leak?

BP was accused by a senior US politician of lying to Congress to reduce its liabilities, after an internal document showed that the company's own worst-case assessment of the size of the oil leak in the Gulf of Mexico was 20 times its public estimate.

Markey, the Democratic head of the House sub-committee on energy and the environment, said at the time: "This document raises very troubling questions about what BP knew and when they knew it. It is clear that, from the beginning, BP has not been straightforward with the government or the American people about the true size of this spill."

BP's PR machine also went into overdrive. In the three months after the spill, BP tripled its advertising budget to £60m, targeting local and national newspapers, magazines, television and social media. It has run a series of full-page newspaper adverts to coincide with the one-year anniversary of the spill.

How much has the Deepwater Horizon disaster cost BP?

BP's own most recent estimate of the total cost of the disaster was announced in November 2010 as nearly $40bn.

The spill temporarily wiped out half the company's value. BP's share price plummeted from 653 pence per share on 15 April to a nadir of 303 pence per share on 29 June. Recovery has been steady since then but has stalled at around 480 pence per share since January. This can be partly attributed to its floundering efforts to partner with Rosneft, the state-controlled Russian oil major, and the related threat of a $10bn lawsuit from joint venture TNK-BP.

BP claims to have spent more than £7.9bn on the cleanup so far. Shoreline cleanup technical advisor Owens estimates that the cleanup operation is currently costing BP "probably a couple of million dollars a day."

BP set aside a $20bn fund to compensate people and businesses for damages relating to the spill. It has paid out $3.8bn for more than half a million claims, which included over $600m to the fishing industry.

At what point will all the oil have been removed from the area?

A spokesman for the NOAA said there was "no basis to conclude that the Gulf recovery will be complete by 2012."

Polaris, which is coordinating the cleanup, have put plans in place up until the end of December. Owens said: "We fully expect to be here for a few months yet."

One of the major concerns for the cleanup team at the moment is oil buried beneath the sand. With hurricane season approaching, Owens is concerned "that storms might remobilise some buried oil that we haven't yet found".

The cleanup operation will not be able to remove every last drop of oil, however. Polaris will rely upon natural processes of weathering, microbial activity and evaporation to break down residual oil.

How has the spill been cleaned up?

The NOAA produced an "oil budget" in November 2010 that showed that almost one-quarter of the oil evaporated or dissolved; 17% was sucked up by the "top hat" lowered onto the broken riser pipe or otherwise directly recovered; 16% was chemically dispersed by more than 8m litres of chemical dispersants; a further 13% was naturally dispersed; 5% was burned (which equated to up to 11,000 barrels a day), and 3% was skimmed. On the coast, teams used beach-cleaning machines and flushing techniques in the marshes.

How have birds and marine life been affected?

State officials are currently in the process of tallying and logging the harm done to wildlife; however, the indications are that the spill has not been as catastrophic as the worst predictions. According to data published by the US Fish and Wildlife Service in November 2010, the spill has affected thousands of birds and dozens of sea turtles. State and federal workers and contractors for BP recorded more than 6,100 dead birds, 2,200 of which were visibly oiled. They also recorded more than 600 dead sea turtles, 18 of which were visibly oiled. Not all of the deaths were thought to have been caused by the oil spill.

153 dead dolphins have washed up on the shore across the Gulf, at least eight of which were smeared with crude oil that has been traced to BP's well. It is yet to be established whether the oil killed them.

It is not clear how much life on the sea bed has been affected. Joye's research has produced images of mounds of dead organisms including corals, and has found areas where the ocean floor is coated in a dark brown slime about 4cm deep. A comprehensive survey of the sea bed has not been completed.