Abstract: In order to decrease the possibility of blowouts and explosions in deepwater drilling and for early detection of spills,the early monitoring of gas cut to deepwater riser was investigated.Based on the discipline of wellbore gas cut distribution in the deepwater drilling,the Untouched Ultrasonic Doppler Measuring System(UUDMS)was applied in a deepwater environment to detect the variation of gas cut in deepwater riser.In case that the continuous phase consisted in a water-based drilling fluid,and by changing the flow velocity of drilling fluid and gas void fraction,it was possible to analyze the changing characteristics of UUDMS signal.By averaging the voltages of signals under different void fraction conditions,the relationship between the UUDMS signals and gas void fraction was obtained.Results showed that the average voltage of the signals from UUDMS increased with increasing gas void fraction.When the void fraction was lower than 1%,the distance between the mid-value of the probability distribution of signals with gas cut and that without gas cut was 55 mV.When the void fraction was larger than 3%,the distance was 200 mV;and the distance increased with increasing void fraction.Results from experiments showed that by using the Untouched Ultrasonic Doppler Measuring System,gas cut to deepwater riser could be detected in time,which could save a significant amount of time in well control.