Vessels resumed moving inbound along the Houston Ship Channel on Thursday afternoon following a 20-hour halt caused by dense sea fog, the U.S. Coast Guard said, the fifth stoppage this week along the waterway to the busiest U.S. petrochemical port.

Exxon Mobil Corp said on Thursday that its 560,500 barrel per day (bpd) Baytown, Texas, refinery, the nation’s second largest, has seen minimal effects on production because of the fog stoppages that have effectively closed the Houston Channel for nearly four days.

Ship pilots halt vessel movement along the 53-mile (85-kilometer) channel when dense sea fog reduces visibility to unsafe levels.