WASHINGTON (Bloomberg) ? Hurricane Ophelia may cost insurers as much as $800 million after skirting North and South Carolina, a storm modeller said.

Ophelia never became stronger than a Category 1 hurricane, the weakest on the five-tier Saffir-Simpson scale of intensity. While the storm didn't make landfall, its slow movement up the US East Coast brought tropical storm force winds to parts of the Carolinas for more than a day, modeller AIR Worldwide Corp. said in a statement on Friday.

"This continued pounding weakens building components and ultimately leads to more damage than would normally be expected at these wind speeds," Jayanta Guin, AIR Worldwide's vice president for research and modelling, said in the statement.

Ophelia weakened on Friday as it headed northeast toward the coast of New England and Nova Scotia. Its sustained winds slowed to 60 mph from a peak of 85 mph on Thursday.

Hurricane Katrina, which flattened the Gulf Coast and flooded 80 percent of New Orleans, may cost insurers $40 billion to $60 billion, according to Risk Management Solutions Inc., another storm modeler. That would make Katrina the most costly disaster ever.

AIR Worldwide said on August 30 that it expected claims from Katrina to be $17 billion to $25 billion. The estimate excludes flooding.