transportation facilities. In an offshore oil and gas production facility, pipeline-riser systems are required to transport two-phase hydrocarbons from subsurface oil and gas wells to a central production platform. Severe slugs reaching several thousands pipe diameters may occur when transporting gas and liquid in these pipeline-riser systems. Severe slugging creates potential problems in the platform facilities, e.g. separators, pumps, and compressors. Severe slugging may cause flooding and overpressurization of the separator, rupture of the pipe, and an increased back pressure at the wellhead. All of these might lead to the complete shutdown of the production facility. Therefore, the accurate predictions of severe slugging characteristics, e.g. slug length, oscillatory period, are essential for the proper design and operation of two-phase flow in the pipeline-riser systems. Pipelines used for the transportation of hydrocarbons in an offshore production facility, are laid out over the seafloor. The uneven seafloor topography forms different pipeline-riser configurations. In this dissertation, we described the severe slugging characteristics in a long downward inclined pipeline-riser system. We carried out experiments in a relatively long pipeline-riser configuration, and also performed numerical simulations using a one-dimensional two-fluid model. It was found experimentally, as also reproduced numerically, that transient slugs were generated in the pipeline upstream of the riser base. These transient slugs effectively contributed to the initial blockage of the riser base. Furthermore, an existing analytical model for the prediction of the flow behaviour in the pipeline-riser system was modified. The modified model, which was tested against our experimental results, showed a better performance than previously published models. We developed a transient drift flux model to simulate the severe slugging characteristics in a pipeline-riser system. The model was tested against experimental data and interestingly, could predict the occurrence of severe slugging in a horizontal pipeline-riser system, which is a subject of debate in the open literature. That motivated us to conduct experiments in a horizontal pipeline-riser configuration. It was observed that severe slugging can develop even in the horizontal pipeline-riser configuration. Moreover, a new class of severe slugging was found and referred to as dual-frequency severe slugging, which corresponds to dual-frequency pressure and flow rate fluctuations. It was found that dual-frequency severe slugging evolves when the pipeline length exceeds a certain threshold. In this dissertation, we also described the severe slugging characteristics in a hilly-terrain pipeline-riser configuration. A hilly-terrain pipeline consists of interconnected horizontal, downhill, and uphill sections. It was observed that, the existence of a hilly-terrain unit in a pipeline-riser system induces a more severe type of slugging, which exhibits longer slugs than that of a horizontal pipeline-riser system. So far we have summarized our work on the characteristics of severe slugging in a pipeline-riser system. In this dissertation, we also discuss the occurrence of severe slugging in an extended reach well. In response to meet the world energy demand, the oil and gas industry has also moved towards development of resources in scattered, isolated oil and gas pockets. Snake wells and fish-hook wells are extended reach wells, which have been used to develop these small hydrocarbon deposits more efficiently than conventional vertical or horizontal wells. The extended reach well resembles the pipeline-riser configuration. The flow conditions, e.g. pressure, and the pipe specifications, e.g. diameter, at the bottom of a well are generally different than the pipeline laid out over the seafloor. It is expected that severe slugging at the bottom of the well is less likely to occur. In this dissertation, we performed numerical simulations to study the possible formation of severe slugging at the bottom of an extended reach well. It was found that severe slugs were initiated at the bottom of the extended reach well. This teaches one to study the well hydrodynamics more carefully when designing an extended reach well.