A special [[coupling]] device. inserted one or two joints above the bottom of the casing string, that contains a check valve to permit fluid to pass downward but not upward through the casing. The float collar prevents drilling [[mud]] from entering the casing while it is being lowered, allowing the [[casing]] to float during its descent and also decreasing the load on the derrick. The float collar also prevents a backflow of [[cement]] during the cementing operation.