By Erwin Seba and Terry Wade
HOUSTON (Reuters) - Massive barge gates shut to prevent flooding in New Orleans were reopened on Sunday and energy firms started returning workers to oil platforms in the Gulf of Mexico, after officials lifted evacuation orders as Tropical Storm Karen faltered.
The U.S. Army Corps of Engineers was reestablishing normal port traffic on waterways, including the Inner Harbor Navigation Canal-Lake Borgne Surge Barrier and the Seabrook Floodgate Complex in New Orleans, built after Hurricane Katrina devastated the low-lying city in 2005.
Karen was downgraded to a tropical depression late on Saturday by the U.S. National Hurricane Center after its top sustained wind speeds dropped to 35 mph. All tropical storm warnings and watches were scrapped.
BP Plc, Marathon Oil Corp, and Chevron Corp were returning workers to offshore facilities by helicopter after earlier evacuations. The vital Louisiana Offshore Oil Port, the only U.S. facility that can dock the world's largest oil tankers, reopened. Other companies were working to restore operations.
Storm warnings had prompted energy firms to shut in nearly two-thirds of oil output and half of natural gas production as of Saturday, the government said. The shut-ins started on Thursday.
The U.S. Gulf produces about 1.3 million barrels of oil a day (bpd), according to the U.S. Energy Information Agency.
The U.S. Gulf normally churns out about 19 percent of U.S. oil production and 6 percent of natural gas output.
The Gulf Coast is also lined with crude oil refineries that together equal 45 percent of U.S. refining capacity, with plants producing everything from gasoline, to diesel, jet fuel and asphalt for roads.
A third of U.S. gas processing capacity is also located along the coast.
At least one refinery reduced rates on Friday owing to a lack of crude oil supplies - Motiva Enterprises' 235,000 barrels per day Norco, Louisiana plant.
Chevron's Pascagoula refinery also had reportedly shut a crude distillation unit, apparently for work unrelated to the storm.
No other refineries reported disruptions.
(Reporting by Erwin Seba and Terry Wade; Editing by Terry Wade and Diane Craft)