Severe slugging is an undesired multiphase phenomenon in offshore pipeline/riser systems. Previous experimental studies have been limited to relatively short pipelines (no more than 400 m) compared with the subsea fluid transportation system. Here, a much longer pipeline/riser system (46 mm ID) was built to investigate severe slugging. The test loop consists of a 1657 m long horizontal pipeline; 30 m long, 7-degree downward inclined pipeline; and 11.2 m high S-shaped riser. Based on the pressure cycling characteristics, the flow patterns were classified into three broad categories. It was found that the long pipeline resulted in a longer slug production stage during severe slugging flow, eventually leading to longer liquid slugs. The critical gas velocities for the severe slugging region were larger compared with short pipeline/riser systems, while the critical liquid velocities were barely influenced by the long pipeline. The main factors that influenced the flow pattern transitions on the critical gas and liquid velocities for the severe slugging region were different. Based on the existing models, two criteria for severe slugging and unstable flows were developed, and more accurate predictions were obtained when compared with experimental results in the present study and the literature. (C) 2017 Elsevier Ltd. All rights reserved.