A new, tighter cap is expected to be placed on top of BP's busted oil well in the Gulf of Mexico on Monday, a few tantalizing steps shy of knowing whether it will be enough to finally stop crude from gushing into the Gulf.

The new cap, known as "Top Hat 10," was about 90 metres from the point where it's supposed to connect with the leaking well, said BP Chief Operating Officer Doug Suttles.

The BP executive was careful to keep expectations grounded, stressing that once the cap is in place, it will take days to know whether it can withstand the pressure of the erupting oil and feed it through pipes to surface ships. The cap and vessels together make up BP's plan to stop oil from spewing into the Gulf for the first time since April 20.

"Until we have the cap on, securely fitted in place, and know it's operating per the design, we have to recognize this is a complex operation," Suttles said.

Once the cap is firmly in place, the company will begin "shutting in" the well by closing perforated pipe at the top. The company will be looking to see if the pressure rises under the cap. If it does, that means there are no other leaks, and the cap is stopping oil from leaking into the Gulf.

But lower pressure readings may indicate leaking elsewhere in the well. In that case, Suttles said, the company will work to collect the leak with surface vessels and by dropping yet another cap on top of the stack.

The testing should last about 48 hours, Suttles said.

Even if the tests show the cap is blocking oil from getting into the waters of the Gulf, the leak will continue to spew until a permanent fix is reached.

BP still has to finish work on a relief well that's intended to allow the company to pump heavy drilling mud and cement into the busted wellhead, plugging it for good.

Work on the new cap has been moving briskly, with the old, leaky cap coming off the well Saturday. One snag in the operation has been a delay in the startup of a vessel called the Helix Producer, which is supposed to connect to the well by a link below the cap, ultimately collecting 3.8 million litres of oil a day.

The ship was supposed to begin collection Sunday, but two minor technical glitches prevented that, Suttles said. He expects the ship to begin Monday, reaching full capacity within two days or so.

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