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The effect of the surfactant additive on the upward intermittent flows in a pipeline-riser system is studied experimentally,in a 3 m long horizontal pipe connected to a Perspex pipe of 2.0 m long and 25 mm in diameter,inclined to the horizontal plane by 7°,followed by the vertical PVC riser of 3.5 m high and 25 mm in diameter,operating at the atmospheric end pressure.Based on the analysis of the pressure signal and the visual observation of the riser,it is shown that the additive of surfactant to the carrying liquid makes bubbles smaller in size but much larger in number in the upward intermittent flows.In addition,the additive of surfactant to a two-phase flow does not have a significant impact on the in-situ gas fraction,the pressure drop and the frequency of the liquid slug,but it reduces significantly the velocity of the liquid slug.When the superficial liquid velocity is set,an exponential relationship between the dimensionless velocity of the liquid slug and the Webber number can be obtained.These results might be used for estimating the characteristic parameters of the upward intermittent flow based upon the input operating conditions.