At constant inflow conditions, large-amplitude pressure and flow rate fluctuations may occur in a pipeline-riser system operating at relatively low liquid and gas flow rates. This periodic flow instability has been referred to as severe slugging. In this experimental study, three different orientation angles of the pipeline upstream of the riser base were investigated. The experiments were carried out in a downward inclined pipeline, in a horizontal pipeline and in a hilly-terrain pipeline followed by a vertical riser. Air and water were used as the experimental fluids. For each pipeline-riser configuration, different types of flow instability were found.
Pipeline-riser configurations in an offshore oil and gas production facility are required to transport multiphase hydrocarbons from a subsurface oil and gas reservoir to a central production platform. The diameter of the pipeline and the riser ranges from typically 0.1 to 0.8 m. The length of the pipeline can vary from a few kilometres to more than hundred kilometres. The height of the riser depends on the water depth, which can be more than two kilometres (in deepwater areas). At relatively low flow rates, liquid accumulates at the bottom of the riser, creating a blockage for the gas, until sufficient upstream pressure has been built up to flush the liquid slug out of the riser. After this liquid surge, and subsequent gas surge, part of the liquid in the riser falls back to the riser base to create a new blockage. This transient cyclic phenomenon is called severe slugging. Severe slugging can significantly reduce the production from the reservoir (due to an increased back pressure) and also can damage or even lead to a shut down of the platform facilities, downstream of the riser, like separators, pumps, and compressors.