What Is Backflow? Backflow from the drill pipe is a normal occurrence when a connection is broken at the rig floor. Backflow can result from the density differential of warm (low density) surface water pumped down the pipe against cold (denser) water in the ocean, air that has been trapped during connections and pumped down the pipe, dense cuttings or mud in the annulus flowing back ("U-tubing") to equalize hydrostatic pressure, and so on. Backflow into the pipe is usually reduced by the closure