A visualization study on characteristics of severe slugging in the S-shaped riser with a kilometer-scale pipeline

Severe slugging is a hazardous multiphase flow phenomenon in the process of offshore oil and gas transportation. Few visualization image data are obtained in experimental riser systems with kilometer-scale pipelines. Therefore, the interface distribution characteristics of severe slugging are visually investigated in a 29.1 m high S-shaped riser connecting a 1657 m long horizontal pipeline. Based on the image data of the gas–liquid interface structure and the pressure fluctuation of the riser, the flow pattern map is plotted. The high riser increases the accumulation space of severe slugging and prolongs the cycle period. A new prediction correlation of the slugging period suitable for experimental systems with different pipeline lengths is proposed. The long pipeline provides sufficient gas compression space and intensifies the blowouts of liquid slugs. The maximum velocity of the slug blowout in the riser reaches 23 times the gas–liquid mixed velocity. The gas expands sharply during the gas blowdown stage of severe slugging, which causes the instability of the gas–liquid interface in the horizontal pipeline and induces a large number of liquid slugs to flow into the riser; thus, the continuity blowdown of slugs appears in the riser.