Slug flow could pose serious threat to oil and gas production facility. The objective of the study was to gain better insight into the behaviour of slug flow in large pipe diameter pipeline-riser system. The influence of geometry configuration on the slug characteristics was also investigated. The understanding of these are very important in the development of effective slug control strategy. Numerical simulations were carried out on a 3.7 km long horizontal pipeline leading to a 0.13 km vertical riser using an industrial software package. The pipeline and riser are both of 17 internal diameter. Slug envelopes were developed for the pipeline-riser system and its constituents pipes. A total number of 572 data points were investigated, covering superficial velocities ranging from 0.01 to 44.28 m/s for gas and 0.02 and 8.25 m/s for liquid. The results showed three distinct slug flow regions: region due to horizontal pipeline slugging (H) where slugs formed in the horizontal pipeline are transported through the riser pipe nearly unchanged, region due to both horizontal and vertical pipes slug contributions (I) where the slugs formed in the horizontal pipe keeps growing even through the riser pipe and region due to vertical pipe slugging (V) where slug formation was predominantly due to the vertical pipe. The observed phenomenon is in consonance qualitatively with the experimental studies published in another paper, but quantitatively different and this may be due to diameter effect. The results also showed that choking can indeed be used to mitigate slug flow in all the regions but at considerable cost. The valve must be choked down at various degrees depending on the regions (flow conditions). There is therefore, the need to seek a better way of stabilizing slug flow bearing in mind the distinct behaviours of the identified regions.