IT IS not the invasion of Normandy, but by peacetime standards the flotilla stationed about 65km (40 miles) off the Louisiana coast is a mightily impressive one. Where once the ill-fated Deepwater Horizon drilling rig floated in solitary splendour, there are now two similar rigs, along with the Discoverer Enterprise, a drilling ship; the Viking Poseidon, which knows how to install things on the sea floor; four mother ships for remotely operated underwater vehicles; various barges and supply vessels; and the Q4000, a rig that specialises in repairing and closing wells. If the well that the Deepwater Horizon was in the process of closing off four weeks ago continues to spray oil into the sea for months to come, it won't be for a lack of expensive, sophisticated and improbable-looking hardware a mile up above it.

It is that mile which is the problem. The oil industry has been fixing blowouts for more than a century. The challenge is doing it under 150 atmospheres of pressure with the tools and lights of a robot mini-submarine that gets its power and instructions by way of a cable. Under these conditions well-laid plans can come to naught, as they did when icy methane hydrates that form when natural gas gets mixed up with cold water at high pressure scuppered plans to funnel the leaking oil up to Discoverer Enterprise. The hydrates did not just clog the pipes, they also buoyed up the 125-tonne cofferdam that had been lowered over the leak, lifting it right off the sea bed.

On May 16th, though, oil did start to be collected, thanks to a subtler intervention. Oil is currently escaping from two leaks, one at each end of the well's riser. The riser connected Deepwater Horizon to its blowout preventer, a stack of valves on the sea floor which marked the top of the well proper (see illustration). When the rig sank, the riser broke near the top while remaining attached to the blowout preventer at the bottom, bending itself like a pretzel in its subsequent collapse. Some oil is now flowing from where the riser and the blowout preventer meet; most is coming from the broken end of the riser, which has ended up about 300 metres away on the sea floor. It is from a tube slipped into that distal end that oil is now being pumped up to Discoverer Enterprise and its attendant barges.

The insertion device, about a fifth of the diameter of the riser itself, is not supposed to block the flow of oil completely. If it did, the pressure of the oil would blow it out of the riser like a cork in a hose. Instead it sucks at the oil flowing around it, but gauging how hard to suck is tricky. Without enough suction more oil than necessary would continue to leak out; too much and it will let in water which will make the formation of those pesky hydrates more likely. Other anti-hydrate measures include a small pipe feeding methanol, an antifreeze, into the maw of the riser, and hot water circulating through a sleeve to warm the pipe bringing the oil to the surface. According to Kent Wells, vice-president of BP, the oil company in overall charge of the project, the amount of oil coming up through the pipe had risen to 2,000 barrels a day by May 17th.

What fraction of the whole that represents is unclear. The American government has estimated that about 5,000 barrels a day are pouring out of the well. Other analysts reckon the flow rate may be five or ten times higher, on the basis of video images from mini-submarines and the area and thickness of the slicks at the surface. A well might produce 50,000 barrels a day. BP is keeping mum, but it does say that the plume of leaking oil looks significantly different now that suction is being applied. That suggests 2,000 barrels a day is a noticeable fraction of the whole.

Full fathom five hundred

The next step is to try to staunch the flow proper with drilling mud, a mixture of water and clay minerals. The well is gushing because of the pressure the oil is under in its reservoir 4,300 metres below the sea bed. If drilling mud can be forced into the well under even greater pressure—a technique called “top kill”—it will eventually reach a depth where the weight of the column of drilling mud exerts enough pressure to stop any oil flowing upwards. The tough bit is getting the requisite amount of drilling mud far enough down the well.

To do this, BP has been replumbing and rewiring the blowout preventer, paying particular attention to its choke pipe and the kill pipe. These provide access to the central bore of the well underneath most of the heavy valves which should have closed off the flow of oil, but for some reason did not. The two pipes, which originally went up to the Deepwater Horizon, have been reconnected with heavyweight hoses to a metal framework called a manifold that has been installed nearby. Above it the Q4000 rig and three attendant vessels have 50,000 barrels of peculiarly heavy drilling mud and pumps capable of providing 30,000 horsepower with which to muscle that mud into the well.

The blowout preventer's control pod, which was also originally connected to the lost rig, has been taken up to the Q4000, where it has been tested and attached to new cables. The pod should soon be on its way back down for reattachment to the preventer. The Q4000 will then have control over the valves that connect the choke and kill lines to the well proper. One reason why all this is taking time is that there are now up to 14 remotely operated underwater vehicles working around the well. A “simultaneous operations” unit is needed to choreograph the complex dance of vessels, submersibles and rigs.

Mr Wells says that once the control pod has been reinstalled on the blowout preventer and the manifold is attached to the Q4000, the top kill could start. The control pod will open the valves that allow the drilling mud to be forced into the well. At that point the fluid-dynamics equivalent of a titanic arm-wrestling match will get under way, with the surface vessels' pumps trying to push the mud down the well while the rising oil tries to push it out.

If the oil wins, then the team will try a “junk shot”. The manifold has two containers full of various sorts of rubber and plastic that are particularly good at gumming things up (Mr Wells prefers to call this a “bridging agent”). Open and close a few valves on the manifold and the drilling mud from the surface can squirt one of these junk shots down the pipes and into the blowout preventer. The oil pressure will force it up into the heart of the stack of valves. There, by making it harder for anything to get out of the top, the junk will give the drilling mud a better chance in a second bout of arm wrestling. The second container of junk provides another shot.

If both barrels fail there is the possibility of putting a new blowout preventer on top of the old one. That would mean cutting the existing riser, but now that the siphon inserted into it is bringing oil up to the barges it means risking a serious setback. So it may be wiser to wait for the relief wells that are being drilled to get down to the point, 4,000 metres below the sea bed, where they will intersect the existing well. At that depth stopping the flow with a deadweight of drilling fluid should create few problems, once the exceedingly difficult challenge of hitting the well is met. So far, the first relief well is only about 1,000 metres or so below the sea bed, and the second has only just been started.

Underwater spraying

Not all the remotely operated vehicles at the site have been involved in replumbing kill lines, hooking up manifolds and supporting the relief wells. Some have been swanning around spraying chemical dispersants into the oily waters. If the top kills do not work this spraying may also help limit the damage done by the oil before the relief wells are finished.

Dispersants consist of surfactants (which are like detergents) in a solvent. When applied to a slick of oil they are meant to break it up into tiny droplets which disperse widely and are broken down by bacteria. The use of dispersants reduces the chances of direct exposure to oil by birds, fish, sea animals and everything ashore, but it may increase the risks for things on the sea floor in some circumstances. This is why they cannot be used everywhere. They are also toxic, though a lot less so than oil itself. In the early days of the spill Corexit 9527, the manufacture of which requires a solvent that has been linked with health problems in workers spraying the stuff, was used because there was not enough of its less toxic version, Corexit 9500, to hand.

BP has been criticised for its exclusive use of these two chemicals, which have been around for decades. Inevitably companies have claimed to offer products that are more effective and less toxic. Although BP had at first said it was not considering any other dispersants, a source at the company says it is now evaluating four additional dispersants, already approved by America's Environmental Protection Agency (EPA), and assessing the reliability of supply as well as toxicity. By mid-week some 582,000 gallons of dispersant had been sprayed since the emergency began.

Spraying dispersants in new ways, rather than using new formulations, may make more of a difference. BP thinks spraying dispersants into the oil plume where it leaves the riser may be 20 times more effective than waiting until it reaches the surface. If so, the 45,000 gallons so far sprayed at depth may have done more good than all the spraying at the surface. Three trials of this technique have been made by the EPA, the first two being inconclusive due to logistical difficulties.

The EPA has been trying to find out whether oil treated with dispersants at depth behaves in the way one would expect if dispersed higher up. It also wanted to make sure there were no problems with oxygen levels. The bugs which eat oil use oxygen to do so, so slightly lower levels would be a good indicator that they are at work. There is a risk, though, that the bugs might eat up all the available oxygen—which would harm other things and then stop any further oil-eating.

On May 17th, after the third of the trials at the well, the EPA approved the deep-sea use of dispersants, and BP is now preparing a ship to provide the analysis and monitoring that continuous spraying will require. Research into dealing with spilled oil is limited, and breakthroughs are scarce. If spraying at source really is as good or better than surface spraying, and requires a great deal less dispersant, it will be a boon.

There will be plenty of lessons to learn from the Deepwater Horizon disaster. Once the leaks have been stopped, Mr Wells hopes to get the failed blowout preventer to the surface so that it is at last possible to see what really went wrong. It is not just the engineering response to future oil spills that will be affected, but also working practices, safety systems and regulation. The future safety of offshore drilling for both the roughnecks and the environment will be shaped by what happens a mile below.