Gulf spill: BP prepares for key oil well test

Oil firm BP is preparing to test the strength of its leaking well in the Gulf of Mexico, in its latest bid to stop the flow.

BP began shutting valves linked to the new cap, but a "small leak" in a pipe has delayed the tests. The firm said it would take a "short while" to fix.

The tests will find out if the well can handle the pressure of the flow.

BP hopes the cap will eventually stem the flow entirely - three months after the Deepwater Horizon rig exploded.

The crucial testing phase of the cap was due to begin on Tuesday, but anxious politicians asked experts to review BP's plans amid fears that the procedure could make things much worse if it went wrong.

"I was gung-ho for this test and I remain gung-ho for this test," said Adm Thad Allen, the US incident commander, on Wednesday.

But he warned that nobody wanted to make an "irreversible mistake".

After 24 hours of deliberation, officials gave BP the go-ahead, and the firm began shutting off the valves that were allowing some of the oil to flow from the well on to the surface, where it was being collected by container ships.

Undersea robots then began to close the three openings in the cap that let oil pass through.

Live video footage from BP's cameras showed the first two valves had closed successfully, but engineers discovered a leak in a line attached to the third valve.

"It will take them a short while to fix this particular problem and then we'll get the well test itself under way," said a BP spokesman.

Once the leak is fixed, engineers will begin to test whether oil will leak out of other areas if all the valves are closed.

The experts will monitor seismic, acoustic and temperature changes in the well and the surrounding area, as well as the pressure of the oil in the well.

High pressure will mean the oil has been contained inside the wellhead; low pressure could indicate that oil is leaking elsewhere in the well.

If the pressure proved to be low, BP officials said that the valves would not be kept shut.

If the well is not strong enough to hold back the flow, BP hopes it will be able to channel the oil to container ships on the surface.

The White House said in a statement that the tests would provide "valuable data" about the condition of the well, and also a better understanding of how the well could be closed down during hurricanes.

Meanwhile, BP continues to face political pressure in the US.

A Congressional committee has agreed measures that would ban the firm from new offshore drilling for seven years.

And in a separate move, Secretary of State Hillary Clinton has said she will look into a request by four senators to investigate allegations that BP lobbied for the release of Lockerbie bomber Abdelbaset al-Megrahi while attempting to finalise an oil deal with Libya.

The 1988 bombing of Pan Am flight 103 killed 270 people - most of them were American.

Megrahi, who has terminal prostate cancer, was freed by Scottish Justice Secretary Kenny MacAskill on compassionate grounds in August 2009 after serving eight years.

In a statement on Thursday, BP admitted it had expressed concern to the UK government about the slow progress of a prisoner transfer agreement between the two countries.

But the firm said it had taken no part in discussions on the decision to free Megrahi.