Frequency characteristics and slug velocities of severe slugging in pipeline-riser systems

The frequency characteristics and slug velocities of severe slugging are crucial for assessing fatigue damage and ensuring flow safety in offshore pipeline-riser systems. The flow pattern distributions and flow characteristics of severe slugging were obtained for the small-scale (L_H = 20 m, L_I = 3 m, H_R = 2 m) and kilometer-scale long-distance (L_H = 1657 m, L_I = 30 m, H_R = 29 m) pipeline-riser systems. Due to the periodicity of severe slugging, the local flow pattern in the riser was a composite pattern of alternating slug flow, bubble flow, and liquid flow. The significant decrease of the differential pressure in the riser induced slug flow in the horizontal pipe. A quantitative correlation was established among structural parameters, dimensionless gas and liquid Froude numbers, and severe slugging frequency. The average percent absolute relative error of the prediction was reduced from 58.31 % to 28.42 %. The peak velocities of liquid slugs in the small-scale and kilometer-scale systems can reach 7.5 and 22.9 times the inlet mixture velocities, respectively. A mechanism prediction model for liquid slug peak velocity considering the average void fraction in the riser was proposed. Approximately 85 % of the predicted values were within the error range of ±20 %.