Abstract: Severe slugging is an undesirable flow phenomenon for apipline-riser system with the gas-liquid mixture transportation in oceanengineering. In present paper experiments were performed for the two-phaseflow characteristics in a horizontal/ declination pipeline-catenary risersystem with the gas-liquid mixture transportation, and flow patterns wereidentified by series combinations of gas and liquid superficial velocities,including severe slug, intermittent and oscillation flows. The formationmechanisms of these flow patterns were presented and the conditions wherethe severe slug flow can be produced in the catenary riser were obtained.The results show that the severe slug flow in a catenary riser hasremarkably periodic characteristics and consists of four stages in a circle,including slug formation, slug production, slug blowout and liquid fallback,respectively, where the characteristics of the flow parameters in each stagewere given. Moreover, the formation mechanisms of severe slug flows in bothcatenary and vertical risers were compared and analyzed, and a remarkabledifferentiation in the slug formation stages for the two types of risers wasobserved, where the formation procedure of a mixture liquid slug with bothgas and liquid first occurs before the pure liquid slug in the catenaryriser can be produced, however, there is no occurrence of such a procedureof the mixture liquid slug for the vertical riser.