A theoretical and experimental study of a normal (hydrodynamic) slug flow in a pipeline-riser pipe system is presented in this paper. Experimental flow visualization revealed that long hydrodynamic slugs generated in the pipeline do not dissipate in the riser pipe. It was shown that slug dynamics in the riser pipe is controlled by two phenomena: the gravitational forces and the compression of gas in the trailing bubble. It was found that slugs can accelerate in the riser to a velocity more than five times exceeding the average slug front velocity in the flowline. The results obtained can be used to define operating strategies for offshore production facilities with large diameter flowlines.