HOUSTON (Reuters) - Salvage efforts continued on Monday to remove a sunken drydock in an industrial portion of the Houston Ship Channel, home to major oil refineries and petrochemical plants, officials said.  Port operations across the U.S. Gulf Coast oil and gas industry are slowly coming back, in many cases with restrictions on vessel draft, according to U.S. Coast Guard updates.  It was unclear on Monday when the sunken drydock near Brady Island in the upper shipping channel would be cleared, said Alberto Hernandez, a watch supervisor at the Coast Guard’s Vessel Traffic Service in Houston.  “There is no timeframe” for removing the drydock debris, he said. The area affected is the main stem of the ship channel. “This is the main port of Houston City Docks, they are blocked due to these salvage operations,” he said.  The storm dumped as much as 50-inches of rainfall over the Gulf Coast, forcing officials to close or restriction operations at ports from Corpus Christi to Lake Charles, Louisiana.   It is unclear when the drydock will be cleared from the Houston Ship Channel, which links the busiest petrochemical port to the Gulf of Mexico, and restrictions in that area of the channel lifted, said Alberto Hernandez, a watch supervisor at the U.S. Coast Guard.  He said the Houston shipping channel is open to just past Exxon Mobil Corp’s Baytown refinery to vessels up to a 40-foot draft. Hernandez said there were ships exiting and entering the channel on Monday.  