Dense sea fog was keeping about 40 ships from entering or exiting the Houston Ship Channel to the busiest U.S. petrochemical port on Friday morning, according to the U.S. Coast Guard.

The shutdown began on Thursday morning when ship pilots stopped steering vessels through the 60-mile waterway to refineries and petrochemical plants in Houston and Texas City, Texas, after fog reduced visibility to unsafe levels, the Coast Guard said.

Twenty-one ships were waiting to enter the ship channel from the Gulf of Mexico and 19 were waiting to exit on Friday morning.

Eight refineries in Houston and Texas City account for 12.5 percent of U.S. refining capacity.