BP on Wednesday began a long-awaited operation intended to seal off its ruptured well in the Gulf of Mexico, raising hope after five weeks of massive crude oil seepage into the sea.
The operation, called top-kill, involves pumping heavy mud down into the damaged well head in the hopes of counteracting the pressure of the crude oil pouring into the Gulf since April 20.
It marks the first time that BP has tried to seal off the well, a precarious operation that, as BP chief Tony Hayward pointed out, could make the leak even worse. Earlier attempts to contain the situation focussed on siphoning off the oil.
It could be anywhere from half a day to several days before engineers know if the procedure is effective, BP vice president Kent Wells has noted. The operation began at 1800 GMT, about an hour and a half after the US Coast Guard gave the go-ahead.
Up until the last minute, BP engineers studied whether they should even begin the procedure, said Hayward, who was up through the night evaluating the situation as underwater robots took pressure readings in the valves.
Weeks of talk about top-kill have built up expectations. BP officials warned against hopes of a quick fix, but gave themselves an up-to 70-percent chance of success. Engineers planned to carefully ramp up the forced mud pressure to make sure it does not burst through possible weak points further down the well casing, a federal mining official said.
All previous efforts to reduce the flow have fallen short as thick heavy oil washes onto 110 kilometres of Gulf marshes and beaches.
There is only one permanent solution to the oil disaster now arriving on shore, where fish suffocate, oiled birds die slow deaths and soupy crude oil swamps fragile marsh grasses: that's the tedious drilling of two parallel relief wells, not expected to be finished until August.
The top-kill method has a successful past, but has never been tried at this depth, 1.6 kilometres beneath the surface. The well burrows another 6 kilometres below the sea floor, explaining the strong pressure forcing oil and gas out of two leaks.
Engineers are to force up to 50 barrels of heavy mud a minute into valve openings on the five-storey damaged blowout preventer - the very one that failed to shut down when a sudden rush of gas ignited and blew up the drill rig, killing 11 workers, April 20.
If the specially produced kill mud succeeds and "outruns" the pressure of the well, engineers will then pump cement into the well casing.
To prepare for top-kill, BP deployed multiple underwater robots that could withstand human-crushing pressure, fiddle with the stuck valves and take pressure measurements.
Hayward admitted Wednesday that one of the "big lessons from the incident" was that BP was woefully unprepared for the disaster. It took weeks to assemble a fleet of sub-sea intervention equipment.
"With the benefit of hindsight, it would clearly have been good to have that ready to go from day one," Hayward told CNN. He said this was something that industry "will undoubtedly need to, should do and will probably be required to have in the future."
US President Barack Obama, who noted a wide "sense of despair" about the unfolding environmental disaster, was to head to the oil-slicked coast for a second time Friday to assess efforts.
"We're going to bring every resource necessary to stop this thing," Obama vowed at a solar panel manufacturer in California. "We will not rest until this well is shut, the environment is repaired and the clean-up is complete."
The words were little balm to angry Gulf residents and elected officials whose fishing and tour boating livelihoods are at stake.
They have the equipment to start a sand-dredging plan and build barrier islands to catch the oil. But federal officials want weeks of study before they grant permission.