Coast Guard Capt. Ed Cubanski told a news conference in New Orleans the well was not producing at the time and no oil was leaking. A small amount of oil spilled from the rig when workers using a torch cut into a 75-foot (22-meter)-long, 3-inch (76-millimeter)-wide line on the platform. Cubanski said a sheen one half-mile (800-meters) long and 200 yards (meters) wide was reported in the area.
"It's not going to be an uncontrolled discharge from everything we're getting right now," Cubanski said.
The fire had since been extinguished, said Coast Guard spokesman Drake Fore. He said Coast Guard aircraft and boats were searching for two missing people. Nobody was believed killed in the fire.
The platform is for oil production from an established well, unlike the Deepwater Horizon rig, which was drilling an exploratory well for oil giant BP in mile-deep water when it blew up and triggered a massive oil spill in 2010. That site is well to the east of Friday's explosion.
Taslin Alfonzo, spokeswoman for West Jefferson Medical Center in suburban New Orleans, said four injured workers were brought to the hospital in critical condition with second- and third-degree burns over much of their bodies. Three arrived by helicopter at 9:55 a.m. local time and one by helicopter at 10:10.
Two were sent by ambulance to the Baton Rouge Burn Center. Two others were to be sent later. She could not release identities or any other information.
The production platform owned by Black Elk Energy is about 25 miles southeast of Grand Isle, Louisiana. The Coast Guard said 26 people were aboard the platform at the time of the explosion.
Cubanski said the platform appeared to be structurally sound. After the April 2010 explosion on the Deepwater Horizon, that rig burned for about 36 hours before suffering structural collapse and sinking to the Gulf floor.
The Black Elk platform is in 56 feet of water. Cubanski said 28 gallons of oil were in the broken line.
The Coast Guard got the call about the fire at 8:42 a.m. CST.
A federal official in Washington said a team of environmental enforcement inspectors was flying to the scene.
David Smith, a spokesman for the Interior Department's Bureau of Safety and Environmental Enforcement, said the team was dispatched from a Gulf Coast base by helicopter soon after the Coast Guard was notified of the emergency. Smith said the team would scan for any evidence of oil spilling and investigate the cause of the explosion.
Black Elk is an independent oil and gas company headquartered in Houston, Texas.
The company's website says it holds interests in properties in Texas and Louisiana waters, including 854 wells on 155 platforms.