Hydrodynamic slugs are a feature of the slug flow regime and are formed by the instability of waves at certain gas-liquid flow rates. The instability of waves leading to the formation of a liquid slug is shown in the following schematic animation:

 

 Slug Growth AVI (654 KB)

 

The animation is shown from a Lagrangian perspective, i.e. following the slug along the pipeline. Initially the flow is stratified, but as gas passes over the wave there it is forced to accelerate (due to continuity) which leads to a local reduction in the pressure (Bernoulli effect) over the crest of the wave.  This local reduction in the pressure above the wave provides a force, which if sufficient, can cause the wave height to increase and eventually bridge the pipe.  The instability of the wave which causes it to grow in size is known as the Kelvin-Helmholtz instability.  The effect is analogous to the lift force exerted on an aerofoil due to its convex wave-like shape.

Once the wave reaches the top of the pipe, it forms into the familiar slug shape, with a ‘nose’ at the front (right hand side) and a ‘tail’ at the back (left hand side).  The slug is pushed by the gas and so usually travels at a greater velocity than the liquid film.

Liquid enters the slug through the front, but also drains under gravity through the tail.  The difference in rates determines the rate at which the slug grows or decays and ultimately determines the size of the slug.

Behind the nose of the slug is a turbulent region that can exert significantly greater wall shear stresses than the rest of the flow regime and can therefore be a significant contributor to the total pressure drop through a flowline. The rates of liquid ingress and shedding, and the turbulence within the slug, determine whether the slug will persist or not.  These depend on the local flowing conditions, fluid physical properties and pipeline inclination.  In particular, the evolution of slugs is very sensitive to the pipe inclination and changing the inclination by less than a degree can be sufficient to tip the balance causing a flow regime transition.  Thus, peaks and troughs along the pipeline profile of relatively small amplitude (for example less than a pipe diameter) can have a very significant effect.

An added complication is the effect of the gas bubbles within the slug that are entrained by the liquid ingress into the slug nose. Under certain conditions, it is possible to have a higher volume fraction of gas in the slug than liquid!  A slug is therefore a complex three-dimensional phenomenon making it very difficult (arguably impossible) to model exactly.

The fact that there are multiple slugs in the pipeline introduces a further complication.  Large flowline systems may have many tens of slugs, generated at different locations, at different times and travelling at different velocities.  These slugs interact with each other either directly or indirectly, meaning that individual slugs cannot usually be considered in isolation.

A flowline operating in the slug flow regime, can behave like a chaotic system exhibiting sensitivity to the initial conditions.  This suggests that in order to predict the exact behaviour of a slugging flowline, one must define the initial and boundary conditions perfectly.  However, this being said, it is usually not necessary to predict the behaviour exactly.  Fortunately, observations indicate that slugging pipelines often exhibit ‘chaotic attractors’ and do not normally follow wild excursion.  In design, it is possible to establish the location and extent of the attractor (for given parametric conditions) by completing a simulation with a sufficient number of cycles.  Having established the extent of the attractor, it is then possible to design the downstream processing equipment.