* Gustav weakens to a tropical storm

* New Orleans levees hold so far, rural levees fragile

* Pentagon authorizes up to 50,000 troops for storm

* Oil, gas prices fall as threat to platforms eases

By Matthew Bigg and Tim Gaynor

NEW ORLEANS, Sept 1 (Reuters) - Hurricane Gustav weakened to a tropical storm by late Monday after crashing into the Louisiana coast and menacing New Orleans, where rebuilt levees managed to hold floodwaters out of the city devastated by Katrina three years earlier.

Gustav weakened before hitting land, easing fears it would be another Katrina, whose floodwaters burst protective levees in 2005, swamping 80 percent of New Orleans and stranding thousands.

The storm roared through the heart of the U.S. Gulf oil patch but oil and natural gas prices plunged as Gustav weakened before landfall and spared key Gulf oil installations, easing fears of serious supply disruptions. [ID:nSIN255334]

Gustav's powerful storm surge pushed tons of water into the Mississippi River, Lake Pontchartrain and New Orleans canals, putting pressure on barriers that were repaired or reconstructed after failing three years ago and prompting a tense watch for signs it would happen again.

The U.S. Army Corps of Engineers closed massive new floodgates intended to keep Lake Pontchartrain waters from surging back into the city and over the banks of two canals.

Water flowed over flood walls and spurted through cracks, but a barrier system which officials had warned left New Orleans vulnerable appeared to hold up as of late Monday.

Six inches (15 cm) of water pooled in streets near the New Orleans Industrial Canal, but new cement-girded floodwalls protecting the city's Ninth Ward held off the a repeat of the flooding that devastated that neighborhood during Katrina.

New Orleans Mayor Ray Nagin said residents could begin to return to the city later this week. With the city still under curfew, officials will assess hurricane damage on Tuesday and begin allowing businesses to return as soon as Wednesday.

"Reentry is only days away and not weeks away," Nagin said.

Some residents emerged from boarded up homes relieved to find only broken tree branches and toppled signs.

DODGED A BULLET

"We'll still get some nasty weather but we've dodged a big-time bullet with this one," said stockbroker Peter Labouisse, sitting on the porch of his home, which was shuttered and without power.

Louisiana officials reported six storm-related deaths, including an elderly couple in Baton Rouge who were killed when a tree fell on their home.

In contrast to the widespread lawlessness that followed after Katrina, New Orleans police said they had only arrested two people for looting during the storm.

Oil companies had shut down nearly all production in the region, which normally pumps a quarter of U.S. oil output and 15 percent of its natural gas.

Exxon <XOM.N> said it was shutting down its Baton Rouge refinery, the second largest in the United States, although the storm weakened to a Category 1 hurricane with 75 mph (120 kph) winds as it moved inland

Louisiana Gov. Bobby Jindal said Exxon would ask for crude oil from the U.S. emergency Strategic Petroleum Reserve and Shell Oil Co <RDSa.L> was expected to make a similar request, as refiners look to ensure gasoline supplies in the wake of Gustav.

Mindful of the ravages of Katrina, which killed some 1,500 people, nearly 2 million people fled the Gulf Coast as Gustav approached and only 10,000 were believed to have remained in New Orleans.

More than 14,000 National Guard troops and pilots were deployed to the Gulf Coast and the Pentagon authorized up to 50,000 troops. Soldiers are routinely deployed in U.S. disasters for rescue and clean-up and to prevent looting.

Homeland Security Secretary Michael Chertoff warned residents it was too early to sound the all-clear.

"This is not over. It's still hitting parts of the state very hard," he said.

Underscoring continued concern about the fragile flood barriers, officials in rural Plaquemines Parish told the handful of residents remaining to flee as a levee protecting 200 homes had been weakened by water surging over the top.

Some officials recalled that catastrophic breaches in the city's levees occurred a day after Katrina departed.

BUSH RESPONSE

Gustav stole the limelight from the Republican Convention to nominate presidential candidate John McCain. It opened on Monday with a bare-bones program.

President George W. Bush, who was heavily criticized for the slow Katrina relief efforts, canceled his appearance at the convention and went to Texas to oversee relief effort.

A dangerous Category 4 hurricane a few days ago, Gustav hit shore near Cocodrie, Louisiana, about 70 miles (115 km) southwest of New Orleans, as a Category 2 storm, one step below Katrina's strength at landfall.

EQECAT Inc., which helps insurers model catastrophe risk, said it estimated Gustav's insured losses at $6 billion to $10 billion. Katrina's insured losses were more than $40 billion and total damage was more than $80 billion, making it the costliest hurricane in U.S. history.

Before landfall in Louisiana, Gustav killed at least 97 people in the Dominican Republic, Haiti, Jamaica and Florida. Cuba, swatted by Gustav on Saturday, said on Monday that more than 90,000 houses were damaged or destroyed in the storm.

By late Monday, the U.S. National Hurricane Center called off coastal warnings for the storm. Gustav, now a tropical storm spinning toward Texas, will continue to lose power over the next day, even as it dumps heavy rain parts of six states, forecasters said.

Gustav knocked out power to some 750,000 Entergy <ETR.N> customers across Louisiana and the utility said it could take as long as two weeks to restore power in some areas.

In New Orleans, some 77,000 customers were without power as of Monday night, down from a peak of about 107,000 just after the hurricane hit, the utility said.

As U.S. fears over Gustav eased, Tropical Storm Hanna grew to hurricane strength near the southeast Bahamas, threatening the U.S. east coast from Florida to the Carolinas, and Tropical Storm Ike formed in the Atlantic Ocean. (Additional reporting by Tom Brown in Miami, Lilla Zuill in New York, David Alexander in Washington, and Bruce Nichols, Chris Baltimore and Erwin Seba in Houston; Writing by Jim Loney and Kevin Krolicki; Editing by Mary Milliken and Sandra Maler)

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