BP's new plan to stop Gulf oil spill

'Top kill' process pumps mud mixture into well forcing oil back down – but could make leak worse if it goes wrong

Interactive: the 'top kill' illustrated
Dispersed oil floats on the surface of the Gulf of Mexico waters close to the site of the BP oil spill as Discoverer Enterprise drill ship is seens on the horizon approximately 42 miles off the coast of Louisiana May 18, 2010. Photograph: Hans Deryk/REUTERS

BP's latest plan to stop oil leaking from its subsea well is called a "top kill". The procedure, which could last two days, has been successful in the past, but has never been attempted at such depths before.

The process works by forcing "heavy mud" into the well to overcome the rising oil and shut down the flow. If the mud does the job, engineers will follow up by pumping concrete into the well to plug the leak.

BP has several boats in the region holding a total of 50,000 barrels of heavy mud, far more than the company believes it will need. Heavy mud is made by mixing a natural mineral called barite with water and sometimes polymers to thicken the mixture.

To kill the well, the heavy mud is pumped from one of the surface vessels to BP's offshore platform that floats above the well. From here, it is channelled down a drill pipe into two narrower hoses, which pass into a manifold on the seafloor. The manifold controls the flow of mud through two more hoses into the "blowout preventer" unit that sits on top of the well on the sea floor.

BP plans to pump mud into the well at a rate of 50 barrels a minute, enough, the company hopes, to overwhelm the pressure of the rising oil, forcing it back down into the well.

If the procedure goes wrong, it could make the leak worse. Mud that is pumped into the well might be forced out of the top of the blowout preventer and into the riser, from where the oil is leaking. The sudden rush of mud may cause more oil to leak out.

If engineers think too much mud is being forced out of the top of the blowout preventer, instead of going down into the well, they can use the manifold to pump in what is called a "junk shot". The junk shot contains pieces of tyre, golf balls and bits of rope, which should clog up the blowout preventer and stop the mud and oil leaking out.

BP puts the chances of the top kill working at somewhere between 60 and 70%. One difficulty is that the leak is a mile underwater, far too deep for divers to do the complex work of checking that all the pipes are connected properly.

If the top kill fails, BP will move on to its next plan, which is to use what is called a lower marine riser package (LMRP) cap. In this option, engineers cut the damaged riser pipe from the top of the blowout preventer and lower the LMRP on top. Provided the cap forms a good enough seal, the LMRP can direct oil coming up from the well through a fresh section of piping to the Discoverer Enterprise ship on the surface.

Another option BP is investigating involves lowering a new blowout preventer onto the old one to shut off the leak.