The tanks are prone to float and break during floods, and Harvey's unprecedented rainfalls revealed a new vulnerability when the roofs of some storage tanks sank under the weight of so much water.Federal and state rules require companies to be prepared for spills, but mandate no specific measures to secure storage tanks at refineries, chemical plants and oil production sites.Although Florida has no oil refineries, it has more than 20 petroleum product storage terminals in coastal communities and about 30 chemical companies with a presence in the state, including a significant number of facilities in the Tampa Bay area, according to the American Chemistry Council and U.S. Energy Information Administration.

"Tampa Bay is one of the most vulnerable cities in the country" to hurricanes, said John Pardue, a Louisiana State University professor who has researched problems with storage tanks during storms.

"The record rainfall also exposed problems among almost 400 large storage tanks in the Houston area that have "floating roofs" that go up or down depending on how much fuel is inside the containers.

The unprecedented rains that came with Harvey caused 14 of those roofs to sink, in some instances allowing the chemicals inside them to escape, according to company reports and Padgett of Rice.There are no government rules dictating how tanks are designed.

All the gasoline that was released was contained on-site, Shell spokesman Ray Fisher said.Exxon Mobil spokeswoman Charlotte Huffaker said safety was a priority for the company and it was able to lessen environmental damage from Harvey by shutting down equipment in advance.