Oil refineries across the country have been plagued by a record number of fires, power failures, leaks, spills and breakdowns this year, causing dozens of them to shut down temporarily or trim production. The disruptions are helping to drive gasoline prices to highs not seen since last summer’s records.

These mechanical breakdowns, which one analyst likened to an “invisible hurricane,” have created a bottleneck in domestic energy supplies, helping to push up gasoline prices 50 cents this year to well above $3 a gallon. A third of the country’s 150 refineries have reported disruptions to their operations since the beginning of the year, a record according to analysts.

There have been blazes at refineries in Louisiana, Texas, Indiana and California, some of them caused by lightning strikes. Plants have suffered power losses that disrupted operations; a midsize refinery in Kansas was flooded by torrential rains last month.

American refiners are running roughly 5 percent below their normal levels at this time of the year.

“You have a system that is taxed to the limit,” said Adam Robinson, an energy research analyst at Lehman Brothers. “This is what happens when spare capacity is eroded.”

After Hurricanes Katrina and Rita disrupted the nation’s energy lifeline two years ago, oil companies delayed maintenance on many of their plants to make up for lost supplies and take advantage of the high prices. But, analysts say, they are now paying a price for deferring repairs.

As a whole, refining disruptions have been considerably higher than in previous years: they averaged 1.5 million barrels a day in the first quarter, compared with 700,000 to 900,000 barrels a day from 2001 to 2005. In the days after the hurricanes, refiners were forced to briefly halt as many as five million barrels of production.

In 2006, when refiners were still reeling from the impact of the hurricanes, disruptions in the first quarter averaged 1.35 million barrels a day.

Many factors have led to the rise in gas prices, including disruptions in oil supplies from places like Nigeria and Norway. But analysts say the refining bottleneck in North America has been one of the main drivers of higher energy prices this year.

The refining crunch has pushed wholesale gasoline prices up 35 percent this year and has contributed to a 23 percent gain for crude oil prices. Oil futures in New York closed at $75.57 a barrel on Friday.

Some critics of the industry have theorized on Internet blogs that the squeeze on gasoline and other refined products points to a deliberate effort among oil companies to bolster profits by keeping supplies tight. But experts point out that the companies have little incentive right now to hold back on fuel supplies.

“Every refinery would like to run as much crude as possible but they simply can’t,” said David Greely, senior energy economist at Goldman Sachs, who in a recent report compared the drop in domestic refining with an “invisible hurricane.” “These are more complex systems. There are more chances for things to go wrong. And when things go wrong, they tend to back up the system.”

Meanwhile, refiners have been scrambling to meet a raft of environmental regulations, phase out toxic additives, add ethanol to the fuel mix and introduce new ultralow sulfur standards for gasoline and diesel. Industry insiders attribute much of the fragility of refining operations to the difficulty of making these cleaner fuels.

Refiners spent $9 billion from 2002 to 2006 to make low sulfur diesel. But producing these cleaner fuels means processing crude oil more intensely through the refining process, at higher pressures and temperatures. This, in turn, leads to more chances for glitches or breakdowns, refiners say.

“It’s a marvel we can continue to run refineries the way we do these days given the many requirements and specification changes we have,” said Charles T. Drevna, executive vice president of the refining industry’s main trade group, the National Petrochemical and Refiners Association. “There comes a time when the piper has got to be paid.”

This year’s problems have raised alarms about the safety of refining operations, especially after a deadly accident at a BP refinery in Texas two years ago that killed 15 workers. The federal Chemical Safety Board issued a highly critical report blaming a broken safety culture at BP. But the board’s chairwoman, Carolyn W. Merritt, who has spoken out about safety problems at refineries, said there was a pattern in many other refinery incidents that the board had investigated.

“There is a lack of investments in modern equipment,” Ms. Merritt said. “The overwhelming preponderance is that if you have inadequate engineering and equipment, poor process safety management, and poor staffing, you’re set up for a catastrophe.”

Ms. Merritt, who was appointed by President Bush and will retire after her five-year term ends in August, also said the Occupational Safety and Health Administration does not conduct enough inspections. “There is no enforcement,” she said.

OSHA defended its record and said it had inspected almost 500 refineries from 1994 to 2004. The agency also said it would inspect all refineries under its jurisdiction within the next two years. “OSHA inspections of refineries have proven to be effective,” the agency said.

Meanwhile, demand has been rising relentlessly, providing little respite to the nation’s aging energy infrastructure. Even as consumers complain loudly about high prices, they show no signs of scaling back. Gasoline consumption reached 9.66 million barrels a day in the first week of July, the second-highest level on record.

“The cushion that used to be available five to seven years ago for these unplanned perturbations is no longer there,” said Jeet Bindra, Chevron’s president of global refining. “When a refinery has a hiccup, there are consequences on supplies.”

Part of the problem, analysts and refiners said, stems from the hurricanes two years ago. In Louisiana and Mississippi, many refineries were flooded, and about a quarter of the nation’s refining capacity was shut for weeks.

“Since refining has become such a wonderful business, refiners have delayed maintenance,” Mr. Robinson said. “But when they do go down, they stay down for longer and they discover all sorts of problems.”

In late March, for example, a fire at a large compressor at a BP refinery in Whiting, Ind., caused a hydrogen-treating unit that removes sulfur from some oil products to shut. That meant BP had to turn off a crude oil unit for early maintenance. Two weeks later, a brief power disruption damaged another distillation tower. And in July, a third crude oil tower was shut briefly so operators could fix a small leak. Since the first incident, the 405,000 barrels-a-day refinery has been running at about half its capacity.

Not all refining disruptions are the result of similar incidents. Refineries typically schedule yearly maintenance that sometimes requires them to halt production entirely. But even these long-scheduled shutdowns can now take longer to complete.

No refineries have been built in the United States in over three decades, because refiners say they are too costly. Instead, they have been expanding their existing refineries.

All this is happening as the industry goes through another golden age. After 20 years in the doldrums, the refining business has never been so good for oil companies. Refining margins — the difference between the price of crude oil and the value of refined gasoline made from it — have shot up as much as $25 a barrel for some types of crude oil, compared with about $5 a barrel just a few years ago.

But with a third summer of high gasoline prices, lawmakers are debating legislation they claim would punish oil companies for exploiting the tight supply situation and engaging in “price gouging.” At the same time, they are pressing refiners to produce more fuel.

“Refiners want to keep running in today’s economic environment,” said Mr. Drevna of the refiners association. “But when they shut down they are accused of gouging the system. When they don’t, they are criticized for overrunning their facilities.”