BP is using a "top kill" procedure to stop the Gulf of Mexico oil spill. But what exactly is "top kill"?

One way to think about "top kill" is to imagine something sitting on a spring.

Put a heavy enough weight down on the spring and it will be squashed down. But put something on that's too light and the object will be thrown off.

In this case, the "spring" is oil and gas coming out of the reservoir, beneath the sea, at the site of the well drilled by the Deepwater Horizon and at pressures exceeding 5,000lb (2,268kg) per square inch.

THE PROCEDURE Drilling mud pumped from surface

Goes into blowout preventer

If pressure and density sufficient, oil and gas flow stopped

Well then filled with cement

The "top kill" operation involves pumping "heavyweight drilling mud" from a number of ships through the command vessel on the surface, the Q4000.

The mud has been pumped down a "drill pipe" from the Q4000 and into a manifold - a unit with converging pipes - on the sea bed.

From there it has been primarily pumped into the kill and choke pipes that connect to the blowout preventer - the unit that sits on top of the oil well.

If the density of the mud - and the pressure of the pumping - is right, it will pass through the blowout preventer and then inside the top of the well and with its downward pressure will stop the flow of oil and gas gushing from the reservoir beneath.

Permanent seal

"It is like two forces fighting with each other. If the force downwards exceeds the force upwards, you are going to succeed," said Iraj Ershaghi, director of the petroleum engineering programme at the University of Southern California.

If this has happened, the well is "killed". The leak will be completely stopped, but this will only be a prelude to a permanent seal using cement.

When you have 5,000 barrels of fluid coming, the camera cannot see Prof Iraj Ershaghi, University of Southern California

The procedure has been carried out many times around the world but, as BP admits, never at anything approaching this depth.

In 50ft of water, divers would be doing much of the complicated work of checking pipes and connections.

But all the work on the Deepwater Horizon's well is being carried out by Remote-operated Vehicles (ROVs), making everything much more difficult.

And there could be a major difficulty, said Prof Ershaghi. The assumption behind the "top kill" plan is that the leakage is all coming out of the riser connected to the top of the blowout preventer.

"If this was not the case, such as if there are leaks around the conductor casing, or if the intermediate casing strings are not cemented in place adequately, a burst of a certain portion of the casing could occur and the 'top kill' may fail," said Prof Ershaghi, who spent the early years of his career as an engineer in the Bahregansar oil field in the Middle East.

"Some of us suspect that the problem is more serious than simply a kill operation."

Further danger

The well itself has been drilled into a geological formation. Inside the "hole" there is a steel casing, and the gap between the casing and the formation - the annulus - is filled with cement.

If the cement job is not adequate, leaks can occur behind the casing and oil and gas can find their way to the sea floor, or can also cause a collapse of unprotected parts of the casing at shallower depths.

If that cement has leaks, then oil and gas could be escaping into the water to the side of the blowout preventer, said Prof Ershaghi.

GLOSSARY Barrel: 42 US gallons or 159 litres

Drilling mud: Dense liquid used in drilling

Riser: Steel pipe coming out of blowout preventer

Blowout preventer: Device that sits on top of well with valves to stop flow

Casing: Steel sheath inside drilled well

It would not be easy to see if this was happening, he explained.

"When you have 5,000 barrels (795,000 litres) of fluid coming per day, you need better ROV camera shots otherwise you cannot pinpoint the sources of leaks."

Even if BP's experts are right and all of the oil is coming through the blowout preventer, the "top kill" could still fail.

It could even start increasing the leakage of oil, by as much as 5-15%, if the high density mud ends up bursting segments of the casing.

For some engineers, it is simply impossible to speculate on the chances of success.

"I would hate to venture a guess," said Lloyd Heinze, professor of petroleum engineering at Texas Tech University.

"[It is] a procedure described in training scenarios… [an] established procedure, not well-known, not the ideal way of trying to kill a well because it is not an easy way to do it."

But he said only BP would know the chances of success, as engineers around the world would not know the calculations the firm's experts had undertaken.

If the "top kill" fails, BP will then move on to other plans.

It has prepared something called a lower marine riser package (LMRP). This device would sit on top of the blowout preventer.

But every engineer involved will be crossing their fingers that the "top kill" works, and makes engineering history