BP awaits test results over system to cap Gulf oil spill

BP started crucial pressure tests on a new, more sophisticated cap on the leaking well in the Gulf of Mexico yesterday, but results showing whether the system is working may not be available until Thursday morning.

The newly fitted "capping stack" was successfully manoeuvred into place by robots a mile down on the sea floor on Monday night. If the pressure tests prove successful, it will be the first time that the spill has been fully controlled since the fatal explosion on the Deepwater Horizon on 20 April damaged the well head and caused the largest oil slick in US history.

Optimism about the new cap sent BP's share price up by 2.9 per cent to 410.35p. The stock has risen by more than 35 per cent in the past two weeks, fuelled by much-needed good news on efforts to control the spill and a flurry of City rumours positing everything from outright takeovers to major injections of cash from the Middle East. But even with the recent improvements, BP's stock has still dropped by a whopping 37 per cent since the disaster.

A "lower marine riser package" – which collects the escaping oil and siphons it to a surface ship above – fitted in early June was the first remotely successful attempt to control the leak, which is estimated to be belching out anything between 35,000 and 60,000 barrels per day. But the capacity of the surface ship siphoning off the oil collected by the cap restricted the efficacy of the system to around 15,000 barrels per day.

The new system is much more technically complex. Not only is it attached to two ships with far greater capacity. It also includes hi-tech control systems now being used to assess the integrity of the actual well itself. If the tests prove successful, BP will have the option of shutting the capping stack's valves and thereby closing off the leak altogether – although the decision may be taken to continue to siphon off some oil in order to keep the pressure down.

Even if the pressure tests are successful, the capping stack is still only an interim measure. The well cannot be finally "killed" until nearby relief wells are finished.

There are two such wells being drilled and the most advanced is on track to intercept the leaking Macondo well at the end of July. At that point it can be used to pump a technical "mud" and ultimately cement down to block the well completely.

Efforts to stem the flow had several false starts. The initial plan to use robots to close the broken valves at the wellhead made no progress. And a first attempt to cap the leak failed when escaping gas reacting with the cold sea water formed hydrates which blocked the container. Attempts to plug the leak by squirting mud into the well itself and a "junk shot" of pelletised rubber into the broken valve equipment also came to nothing.

Against such a background, BP is keen to stress the technical risks associated with the new system. "The sealing cap system never before has been deployed at these depths or under these conditions, and its efficiency and ability to contain the oil and gas cannot be assured," the company said in a statement.

So far, the combined cost to BP of attempts to kill the leak, clean up the polluted environment and settle legal claims is running at $3.5bn (£2.3bn). Tony Hayward, the embattled chief executive, was in Abu Dhabi last week as part of an international charm offensive widely interpreted as an effort to drum up investment in the group's depressed stock and avoid a hostile takeover. The Gulf state's crown prince, Sheikh Mohammed bin Zayed Al-Nahyan, said yesterday that the emirate is still considering whether to invest in the company.