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ON THE GULF OF MEXICO (AP) — Engineers prepared to abandon their vigil over BP's broken oil well Friday as ships and rig workers evacuated the Gulf of Mexico ahead of Tropical Storm Bonnie.

The mechanical plug that's throttled the oil for a week will be left closed, even if the undersea robots monitoring the well's stability leave. The only way BP would know if the cap had failed would be satellite and aerial views of oil gushing to the surface.

But retired Coast Guard Adm. Thad Allen said he's confident the cap will hold, despite a few leaks that raised concerns last week. Scientists say even a severe storm shouldn't affect the plug, nearly a mile beneath the ocean surface 40 miles from the Louisiana coast.

Bonnie made landfall in Florida south of Miami Friday morning with top sustained winds of 40 miles per hour. The storm was on a track to pick up strength as it crosses the Gulf of Mexico, reaching the site of the massive oil spill by Sunday.

The National Hurricane Center in Miami said the eye of Bonnie came ashore Friday midday near Cutler Bay, about 20 miles south of Miami. There were no immediate reports of damage.

The rough weather could delay by another 12 days the push to plug the broken well for good using mud and cement, Allen and BP officials conceded. The storm already has pushed back efforts to kill the well. BP had hoped to finish drilling the relief tunnel on Friday, but had to plug it on Wednesday to prepare for the storm.

"Preservation of life and preservation of equipment are our highest priorities," Allen said.

The rigs working to plug the well were pulling up a mile of pipe and will start moving to safer waters later Friday, Allen said.

Ships carrying the robotic submarines watching the well will be the last to leave — likely for about two days — and the first to return.

"If conditions allow, they will remain through the passage of the storm," Allen said.

Audio surveillance gear left behind could tell BP whether the well is unstable, but they won't be able to listen to the recordings until the ships get back into the area.

The delay in work would be worse if BP had to open the cap while the ships closely monitoring the well head left. More oil would have been allowed to spew into the Gulf until they returned.

Shell Oil is also evacuating its operations in the Gulf, moving out more than 600 workers and shutting down production at all but one well.

Seas already were choppy in the Gulf, with waves up to five feet rocking boats as crews prepared to leave, and more of the smaller boats involved in the coastal cleanup were called into port, Coast Guard Rear Adm. Paul Zukunft said.

At the spill site, the water no longer looks thick with gooey tar. But the oil is still there beneath the surface, staining the hull of cutters motoring around in it.

Before the cap was attached and closed a week ago, the broken well spewed 94 million to 184 million gallons into the Gulf after the BP-leased Deepwater Horizon rig exploded April 20, killing 11 workers.

BP is likely to be fined per gallon spilled, although determining that could be difficult. Concentrations of underwater oil at least doubled last month, according to the National Oceanic and Atmospheric Administration.

Researchers at the University of South Florida said Friday they have the first scientific proof that two giant plumes of oil beneath the surface of the Gulf came from the broken well. BP initially denied the plumes even existed.

Work on plugging the well came to a standstill Wednesday, just days before authorities had hoped to complete the relief shaft. Allen said Thursday he has told BP to go ahead preparing for a second measure called a static kill that would pump mud and cement into the well from the top, a move he said would increase the relief well's chances for success. BP will have to get final approval from Allen before starting the procedure.

Vice President Joe Biden visited cleanup workers in southern Alabama, and said he was cheered the cap could remain on.

"After the storm's passage we will be right back out there," Biden said.

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Associated Press writers Colleen Long in Port Fourchon, La., Michael Kunzelman in New Orleans and Melissa Nelson in Theodore, Ala., contributed to this report.

Copyright 2010 The Associated Press.

For more news & multimedia on the Gulf oil crisis, visit our Oil Spill section.