Hurricane Isaac hits land in Louisiana

Hurricane Isaac crashed ashore in southeast Louisiana on Tuesday, bringing high winds and soaking rains that pose the first test for multibillion-dollar flood defenses put in place in New Orleans after Hurricane Katrina devastated the U.S. Gulf Coast seven years ago.

Flood-control systems and levees failed when Katrina hit in 2005, leaving parts of the New Orleans underwater. Troops and law enforcement officials have been deployed throughout the city this week to prevent a repeat of the chaos and looting that followed in the days and weeks after Katrina.

Isaac killed at least 23 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday.

The effects of the large, slow-moving storm were felt along the coastlines of Louisiana, Mississippi and Alabama. Storm surge caused flooding in Louisiana and winds gusted to 62 miles per hour (96 kph) in New Orleans, the National Hurricane Center said in an advisory.

New Orleans Mayor Mitch Landrieu assured residents "your city is secure," and said emergency services were ready for search and rescue missions. This raised the specter of Katrina, when Coast Guard helicopters plucked stranded residents from the roofs of their flooded houses.

"We're in the heart of this fight," Landrieu told an evening news conference. "We are in the hunker-down phase."

About 1,000 U.S. National Guard troops in military vehicles took up positions on the city's mostly deserted streets, brandishing automatic assault rifles to ward off any threat of looting.

Isaac spared Tampa, Florida, where the Republican National Convention began on Monday. But it forced party leaders to revamp their schedule. They may have to make further revisions so as not to be seen celebrating Mitt Romney's presidential nomination while Gulf Coast residents struggle through the storm.

President Barack Obama urged residents to take cover and heed warnings, saying that now was "not the time to tempt fate." . He issued emergency declarations for Louisiana and Mississippi earlier this week.

On Tuesday morning, army engineers closed the massive new floodgate at Lake Borgne, east of New Orleans, for the first time. It is largest storm-surge barrier in the world.

In other preparations, oil production in the U.S. Gulf of Mexico nearly ground to a halt, and ports and coastal refineries curtailed operations.

At 7 p.m. CDT (2400 GMT), the Hurricane Center said Isaac was centered about 90 miles (140 km) southeast of New Orleans with top sustained winds of 80 miles per hour (130 kph).

The storm was traveling at a relatively slow 8 mph (13 kph). The sluggish pace is a concern for people in its path since slow-moving cyclones can bring higher rainfall.

Isaac was about 370 miles (595 km) wide. Hurricane-force winds extend outward up to 60 miles (97 km) from the center of the storm. Heavy rains and big storm surges were also forecast for parts of Mississippi, Alabama and the Florida Panhandle.

Nearly 70,000 people in Louisiana were without electricity.

Isaac comes almost exactly seven years to day after Katrina hit New Orleans on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage.

Multibillion-dollar defense 

The Army Corps of Engineers has since built a $14.5 billion defense system of walls, floodgates, levees and pumps designed to protect the city from a massive tidal surge like that which swamped New Orleans in Katrina's wake.

Most of the city's Lower Ninth ward, still scarred by the devastation of Katrina, was deserted on Tuesday. Residents who had not evacuated stocked up on water, food and fuel.

"We've got all kinds of eats and treats," Arthur Anderson, 61, who was trapped in the attic of his house during Katrina before he escaped by boat.

Authorities have urged thousands of residents in low-lying areas to leave, warning that the storm could flood towns and cities in Mississippi and Alabama, as well as Louisiana, with a storm surge of up to 12 feet (3.7 meters).

Rainfall accumulations, potentially totaling as much as 20 inches (50 cm) in some areas, could trigger widespread flooding. Customers in Louisiana's coastal parishes were already without power.

Isaac has not been forecast to strengthen beyond a Category 1 hurricane, the lowest on the five-step Saffir-Simpson scale. Its winds have been forecast at up to 80 mph (129 kph). While this would be well below the intensity of Katrina, which was a Category 3 storm, the size of Isaac's slow-moving system has had forecasters predicting widespread flooding.

Residents of Louisiana's low-lying Plaquemines Parish, where some flooding was already happening on Tuesday, were anxious about their homes.

Avenal Terrance, 52, who was evacuated early on Tuesday, is hoping the levee holds up. "I'm living in an old trailer, not a new one, and I just hope and pray that the storm doesn't take it," she said.

U.S. energy output disrupted 

With more than 90 percent of offshore U.S. Gulf of Mexico oil production shut down and nearly half of natural gas output offline, energy companies along the Gulf Coast refining center braced for the storm's impact, shuttering some plants and running others at reduced rates ahead of Isaac's landfall.

Intense hurricanes such as Katrina -- which took out 4.5 million barrels per day of refining capacity at one point -- have flooded refineries, keeping them closed for extended periods and reducing fuel supplies.

The U.S. Department of Energy estimated that about 12 percent of the Gulf Coast's refining capacity had gone offline. Louisiana usually processes more than 3 million barrels per day of crude into products like gasoline.

Although no damage to offshore installation was reported, some energy experts said the sweeping disruption of oil production, refineries and key import terminals could make it more likely that the U.S. government would release oil supplies from its nearly 696-million-barrel Strategic Petroleum Reserve.

A release, which had previously been under consideration, is still on the table, White House spokesman Jay Carney told reporters on Tuesday.

International benchmark Brent crude traded down slightly to $112 a barrel on Tuesday.