Long liquid slugs reaching a length of several hundreds of pipe diameter may appear when transporting gas and liquid in horizontal or nearly horizontal pipelines. These long slugs may cause system vibration, separator flooding, and operational problems for the downstream processing facilities. Although mainly short hydrodynamic slugs have been observed in offshore gas and oil production fields over the past years, the appearance of the long slugs is becoming more common as many production fields are now more mature and reach end of field life, giving reduced production rates and reduced operational pressure. In this paper we investigate the effect of gas pulsation at the pipeline inlet on the length and frequency of long slugs for gas-liquid flow in horizontal pipelines. Experiments have been carried out in a 137 m long air-water horizontal flow loop with an internal diameter of 0.052 m. The results show a strong relation between the frequency of gas pulsation and the resulting slug flow. Particularly gas pulsation can decrease the size of the long liquid slugs significantly. (C) 2011 Elsevier Ltd. All rights reserved.