Look up the definition of "slug" in the dictionary, and you will find everything from the familiar garden gastropod and bullets to a unit of mass. Everything, that is, other than the two-phase flow hydrodynamic regime of slug flow, which consists of alternating high-speed liquid slugs and slower moving gas bubbles.

Slug flow is a common flow regime encountered in the operation of oil and gas flowlines and generally results from flowline sizing pressure and velocity constraints. The very nature of slug flow gives rise to time variations in fluid density and velocity. The slug is pushed along the flowline at high velocity by the gas pressure behind it, and this can give rise to liquid flow rates in the slug that are an order of magnitude greater than the average. Slug impact loads can be very high. There are also fatigue concerns, which emanate from the density variation between the slug body (liquid dominant with gas bubbles) and the bubble region (gas dominant with a liquid film). These can be a significant issue due to the relatively high frequency of slugging.

Designing for the effect of slugging requires an understanding of the interaction between the internal multiphase flows and the containing conduit, a subject which is in its infancy relative to external fluid structure interactions. Recent advances in computational power, numerical techniques, 3-D multiphase flow simulation, and detailed experimental measurement techniques have the potential to facilitate significant advances in the safe design of mechanical restraining systems in terms of both the maximum allowable loads and fatigue life.

Shown here are the excursions of a wave catenary riser with a slugging velocity of 10 m/sec (33 ft/sec). (Images courtesy of Cranfield University)

Forces caused by density and velocity variations

Forces due to multiphase flows in offshore risers began to receive increased attention in the 1980s with the advent of flexible risers. It was realized that the shape of the riser could be affected by the fluid density variations and could lead to riser clashing and increased static loads, particularly at the touch-down point and the riser top.

In addition, slug flows can lead to high-frequency oscillations that may excite structural vibrations at a similar frequency to vortex-induced vibration and generate high stresses due to the change in flow direction. The high frequency of slugging (one per minute or less) can lead to fatigue concerns, even if the magnitude of the loads is not that large. Slugs can be so large that the riser could be completely liquid and then gas-filled. While unlikely in the case of normal hydrodynamic slug flow, it could easily happen during transient events and pigging operations. These effects lead to the need for coupling between the modeling of the internal multiphase flows with the riser mechanical design packages.

Phase and pressure are distributed as the slug tail leaves the bend center.

Another manifestation of the damaging effects of the density variations caused by slug flow has been the so-called "bouncing pipe" that has been observed at buckle initiator sites. Here, the production pipe is laid along "sleepers" at various points to promote lateral buckling during initial startup. This creates a "hump" in the pipeline, and its buoyancy changes during the passage of the dense liquid slugs and the lighter gas bubbles. The result is a pipeline that bounces and rocks with such a high frequency the fatigue life can be shortened dramatically.

Momentum forces due to impacts and change of flow direction

Any fluid or moving object that is constrained to change direction requires a force to be applied to make it change direction. If the fluid has a steady density and is moving at a steady velocity, the force required is constant and may be evaluated from a force/momentum balance.

This is the case for stratified, bubbly, and annular multiphase flows. However, for the case of intermittent slug flow, the velocity and density of the flow varies with time and can give rise to large variations in the forces experienced as the flow is made to change direction.

The important thing to note is that the force is proportional to the velocity. The intermittent nature of the flow means that the force is pushing the bend out as the slug front hits the bend, which then snaps back as the tail of the slug passes.

The University of Cambridge designed and built the test rig shown to measure slugging forces.

If it is a long slug, acceleration can occur, giving rise to a higher slug tail velocity compared to the slug front, meaning that the maximum load on the bend may be due to the recoil, which is somewhat counter-intuitive.

There are many cases in industry of the destructive forces of slug flow at bends causing them to move, ultimately jumping off pipe supports or failing. Bends near separator inlets can generate a twisting torque, leading to cracks. Separator internals have been damaged.

Topside separators can experience large loads if a long slug exits the riser due to the large accelerations at the slug tail. This is caused by the reduced hydrostatic pressure required, which is then available to accelerate the slug. However, the friction length is reduced as the slug is consumed by the separator, leading to a type of domino effect where the main restraint on the maximum velocity is the backpressure created in the separator.

Flowlines that operated satisfactorily early in their life may start to experience movement as the field matures and the water cut increases. This can increase the loading due to the greater density of the liquid.

In terms of dynamic impact factor, its value is the subject of debate, but it is generally regarded as a maximum of 2, mainly from field experience. Whether it is 1 or 2 can make quite a lot of difference, and this debate has led to several experimental attempts to measure the slug forces on pipe bends. This has been quite difficult to do, with one of the most successful attempts achieved at the University of Cambridge in 2002. The sophisticated testing rig used a three-axis load cell to measure the forces due to slugging on a 70-mm (2.75-in.) pipe bend and generally corroborated the use of a dynamic impact factor of two as a maximum design value.

These data were subsequently analyzed using a 1-D transient multiphase flow code linked to a 3-D computational fluid dynamics (CFD) code; in this case the STAR-OLGA coupling was used. In the coupling model, the horizontally oriented 90° bend is modeled using the 3-D CFD code STAR-CCM+, with the flowline upstream of the bend modeled using the 1-D transient multiphase flow code OLGA. The predictions were able to provide reasonable estimates of the slug flow-induced forces on the bend and were able to generate force distribution contour plots that illustrated the large forces that act on a certain area of the bend wall, which is the part most vulnerable to mechanical damage.

This type of simulation represents the current state of the art in trying to get a handle on speedy slugs. In the future, the industry needs to attempt experiments at a more representative scale and with hydrocarbon fluids or even consider making the slugs less damaging in the first place.

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