In the oil and gas industry the occurrence of slug flow in flowlines and risers can cause operational problems. Such flowline-riser systems are used to transport the oil and/or gas from the location of the wells to a production platform, where the fluids might be separated into single phases. Slug flow conditions imposefluctuations in the production rate, which may lead to the flooding of the separators, trips of compressors or pumps, and to increased loads on the supports of the pipeline and piping sections at the production platform.Therefore, measures have to be taken to mitigate the slugging, which can be a reduction of the production rate, making adjustments to the pipeline system, or adding surfactants to the flow. For vertical flow in production wells, the use of surfactants is a proven technology. Here the creation of a foam through adding asurfactant can increase the production life time, as the accumulation of liquid, which typically occurs when the reservoir pressure has decreased at the end of field life, is prevented. Far less is known yet about the effect of using surfactants for the mitigation of hydrodynamic slugs in a nearly horizontal flowline or the mitigation of severe slugs in risers. From a previous Master’s Thesis project by Pronk [1], in which lab experiments were carried out in the same flowline-riser facility as used in the present study, it was concluded that growing slugs could be suppressed by adding a surfactant to create foam. The surfactants also influence the characteristics of the severe slugging cycle, but they are not able to fully suppress it. Re-analysis of the measurement data by Pronk hasturned out that instead of growing slugs, severe sluggingwas measured and therefore that severe slugging can be suppressed using surfactants and no conclusions can be drawn on growing slugs or hydrodynamic slugsfrom her research. There is limited literature on the effect of adding a surfactant to hydrodynamic slug flow in horizontal flowlines, but there are some indications that hydrodynamic slugs can be suppressed, though requiring a higher concentration of the surfactant than vertical flow. Suppressing slug flow using surfactants should eliminate the fluctuations in the flow and can lead to a lower pressure drop. For this research experiments have been carried out in the Severe Slugging Loop at the Shell Technology Centre Amsterdam. This flow loop consists of a horizontal flowline of 50 m, a U-turn and a 15 m section. From there onward the pipe starts declining at an angle of -2.54° over a length of 35 m. Thereafter there is a short horizontal section of 5 m before arriving at the base of the riser, which has a height of 16.8 m and an internal diameter of 32 mm. The other sections have an internal diameter of 50.8 mm. For this research the configuration was used where air and water are connected through a Y-sprout at the start of the flow line. The air is supplied as pressurized air of 6 barg. The water is supplied by a pump, and it is separated from the airat a section at the top of the riser and recirculated through the system. The used surfactant is the household detergent Dreft. At several points along the flowline and riser the flow conditions have been measured using pressure indicators and at two locations along the flowline the pressure difference over 3 m has been measured using differential pressure indicators. Through the inspection windowat the end of the flowline the flowis recorded by using a GoPro camera. Furthermore, over a distance of 40mthe acoustic energy of the flow has been measured by a Distributed Acoustic Sensing (DAS) system. The DASmeasurements have been used to determine the flow behaviour of the flow. In the case of slug flow, the velocity of the slugs was deduced from the DAS data. Adding a 2000 ppm concentration of the surfactant Dreft Original has resulted in the full suppression of hydrodynamic slug flow. This has been verified using the pressure data, the video data and the DAS data in combination with the velocity tracking tool. The waves that were present without surfactant are decreased to a much smaller wave height when the surfactant is added; therefore, the flow pattern is changed from slug flow to wavy stratified flow. The severe slugging cycle can also be suppressed using 2000 ppm concentration of the surfactant Dreft Original. At the same flow conditions, the gas is able to lift more liquid through the riser. The measurements by the DAS system were quantified using a velocity tracking algorithm. The measurement data can now be used to accurately measure the slug velocity and to determine whether slug flow or stratified flow is present in the system. This measurement method is ready for scale-up. When the concentration of surfactant is sufficient to suppress slug flow, the flow can be described from visual observations as a threephase flow, containing gas, foam and liquid. The foam absorbs the stresses imposed by the other phases, because of its non-Newtonian fluid properties. The waves are no longer able to grow until they reach the top of the pipe and are diminished into slow moving dampened waves of the foam layer.