Around the world, offshore oil and gas platforms are generally built to survive without serious damage a so-called 100-year storm - a hurricane so powerful that it typically occurs only once every hundred years.
Hurricane Ivan roared through the Gulf of Mexico a year ago, generating the highest waves ever recorded there in a storm considered likely to occur only once every 2,500 years. Given the scale of the hurricane, it was inevitable that it would wreak havoc in the gulf, America's biggest energy-producing region, uprooting miles of underwater pipelines, destroying platforms and crimping production for months.
But when industry officials, engineers and oceanographers gathered at an American Petroleum Institute conference in Houston in July to discuss ways of improving the gulf's infrastructure, they expected to have plenty of time to work on the problems. Then Katrina struck.
"We're seeing more 100-year events happening more often, even every few years," said Jafar Korloo, who has designed, engineered and managed offshore platforms for Unocal, the oil company recently acquired by Chevron. "The bar has to be higher."
The stakes, too, are higher than before. Older production basins in Texas and Oklahoma have been on a gradual decline for years; some potential oil-producing regions on land elsewhere in the United States are out of bounds.
In the meantime, more oil and gas has been gushing out of the gulf, which was first tapped half a century ago, amounting now to nearly a third of domestic output. And the bulk of that production is concentrated at no more than a couple of dozen platforms, each costing $1 billion to $2 billion.
As the petroleum industry confronts the challenge of recovering as quickly as possible from Katrina, officials are just beginning to assess the bigger, longer-range questions. But clearly, they cannot count on nature being predictable.
"Most definitions of a 100-year event were calculated before Ivan and Katrina," said Bob Hamilton, a vice president at the Woods Hole Group, an ocean engineering group in Massachusetts. "At this point, are the 100-year criteria good enough?"
Hurricane Katrina cut deeply into oil and gas production, shut down major refineries, sent gasoline prices to record highs and set off fears of an energy crisis. Oil companies, just as they did after earlier devastating gulf storms - Betsy in 1965, Camille in 1969, Ivan in 2004 - are rethinking how they operate offshore. But now they are asking themselves tougher questions.
"We've never seen anything like Katrina," said Tim Sampson, one of the July conference organizers and the coordinator for drilling and production operations at the petroleum institute. "We have to consider the data and look at whether we need to update our standards."
According to the Coast Guard, Katrina destroyed, damaged or sank about 50 of the 4,000 gulf platforms.
When it passed over the gulf's crowded waters, Katrina was at its peak, a Category 5 storm, the most powerful on the Saffir-Simpson scale. Winds of 175 miles an hour snapped mooring lines, sending some platforms adrift. Waves toppled steel structures, and underwater slides shifted the ground under pipelines, probably causing widespread damage that has yet to be fully assessed.
Four installations owned by Royal Dutch/Shell, the gulf's largest operator, suffered extensive damage. These included the biggest offshore facility in the region, a tension-leg platform called Mars, which is expected to be out of commission for months after being shaved by the hurricane's winds and waves.
Shell said its output - which usually amounts to 450,000 barrels a day, or nearly a third of the gulf's oil production - would be down 40 percent until next year.
At its peak, the hurricane caused nearly the entire gulf oil region to be shut down. A few days before Katrina struck, oil companies closed off wells, evacuated the platforms and stopped production as a safety precaution. From daily output of 1.5 million barrels a day, the region is now producing an average of 650,000 barrels a day, according to the Interior Department.
But since Aug. 26, when the platforms were shut down, 20 million barrels of oil production has been lost, the equivalent of a day's consumption for the United States. The loss in natural gas production, now back at 65 percent of its level before the storm, was 99 billion cubic feet.