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Flow Assurance & Optimisation of Oil & Gas Production

Severe Slugging

The severe slugging phenomenon is illustrated in the animation below. The phenomenon is a cyclical production of liquid and gas coupled with cyclical flowline pressure fluctuations. The first phase of the cycle is referred to as ‘slug formation’. Here the base of the riser has become blocked with liquid preventing free passage of gas. The pressure in the pipeline then increases as more liquid runs down to the base of the pipeline increasing the size of the liquid slug. The system continues in this fashion until the pressure has built sufficiently to overcome the gravitational head associated with the liquid slug. The system is then hydrodynamically unstable and the liquid slug is discharge rapidly up the riser followed immediately by a gas surge as the pipeline blows down. The pressure in the pipeline then returns to a low value, leading to insufficient gas velocities to carry the liquids up the riser, and the process is repeated.

The animation below shows results of a transient multiphase flowline simulation in the slug flow regime. The animation shows how the liquid periodically builds up in several dips near the riser base and are then purged, dropping the flowline pressure.

Severe Slugging (Animated GIF 222K)

The normalised limit cycles for such an unstable system is shown in the figures below. The flowline inlet pressure changes by ~35% and the liquid content by ~25% during the cycle, however the sudden changes in gas and liquid flow rates could cause serious difficulties for downstream equipment.

For a more detailed discussion of how to analyse the effect of the severe slugging region on the operability of a flowline system,
please see the Life of Field Stability in a Deepwater Development case study.