Hurricanes like last year's Ivan and this year's Dennis and Emily can damage oil platforms and pipelines and disrupt supplies to fuel-producing refineries, boosting energy prices.

The United States pumps about 1.6 million barrels per day, or more than 25 percent of its domestic oil, from underwater fields in the Gulf of Mexico. That is up from 12 percent a decade ago, and the percentage "is going to increase pretty drastically because the onshore (oil production) is still declining," said Auggie Payne, an analyst at the US Energy Information Administration in Dallas.

US onshore production has been declining since the 1970s, and the only huge finds the country has left are offshore.

ATLANTIC CONVEYOR BELT

The fuel driving hurricanes is a decades-spanning ocean current cycle known informally as the Atlantic conveyor belt, climate scientists say.

The current, which has been in an active phase since 1995, brings warm water north from south of the equator. The warm water evaporates, sending energy from the ocean to the atmosphere and providing fuel for tropical storms to become hurricanes. As the water cools, it sinks and flows back to the south, maintaining the warm water conveyor process for up to three decades.

It also changes the atmosphere in ways that support hurricanes. "The trade winds slow down a little bit, we get lower pressures and less wind sheer tearing apart the hurricanes; all of which contribute to stronger hurricanes," said Chris Landsea, a hurricane researcher at the National Oceanic and Atmospheric Administration in Miami.

Since 1995, winds have mostly kept the more frequent hurricanes out at sea, but that has already begun to change. Indeed, NOAA predicted on Tuesday that this year's hurricane season could foster 11 hurricanes, two more than predicted earlier.

Nobody can predict if storms will enter the gulf and damage oil platforms, but hurricane damage to such operation since last year has been devastating.

Last year's hurricane Ivan took the heaviest toll ever on US oil and gas, tearing up as much as 10,000 miles (16,100 km) of older pipelines closer to shore. It shut in 45 million barrels of oil, or two-and-a-quarter days' worth of the entire amount of oil used in the United States.

So far this year, early into the season that ends in November, hurricanes have shut in more than 6 million barrels from the Gulf of Mexico. The storms shut even more in Mexican waters.

SELF-INSURED PLATFORMS

That could be bad news for oil companies building platforms in the Gulf of Mexico. Many of these platforms are so expensive and risky that no insurance company will cover them at a reasonable price, leading the industry to self-insure them.

"Hurricane season is one of realities of operating in the Gulf," said BP Plc.'s Gulf of Mexico spokeswoman Ayana MacIntosh-Lee.

BP's $1 billion Thunder Horse semi-submersible platform in the US Gulf will not start inaugural production this year, as previously planned. Thunder Horse workers evacuated by Hurricane Dennis in July returned to find the platform tilting by 20 to 30 degrees. The company is still investigating the cause of the accident at the platform, of which Exxon Mobil owns 25 percent. The platform carries no insurance.

BP's future in the Gulf of Mexico also includes the Atlantis platform, slated to start producing late next year and to reach 200,000 barrels per day of oil output. Thunder Horse and Atlantis combined are expected to produce 450,000 bpd or 22 percent of current output from the gulf.

MacIntosh-Lee said Ivan caused almost no damage to BP's platforms in the gulf, even though the eye of the storm passed directly over several of its facilities. She said the company's offshore facilities were built to withstand hurricanes and that BP has not changed designs for future platforms.

"The facilities proved robust and that's exactly what we engineer for," she said.

But risk remains as the search for oil in the gulf goes to ever deeper waters. "It's really the coratid artery of oil flowing out of the Gulf of Mexico " said Steve Grape, EIA analyst in Dallas about deep water production in the gulf. Only a few pipelines take that oil to refineries. Grape said more capillaries should be built to lower the risks.