Large-Scale Experiments Examine Slug-Length Evolution in Long Pipes
The complete paper presents a set of two- and three-phase slug-flow experiments conducted in a 766-m-long, 8-in. pipe at 45-bara pressure.
The complete paper presents a set of two- and three-phase slug-flow experiments conducted in a 766-m-long, 8-in. pipe at 45-bara pressure. The results show that the mean slug length initially increases with the distance from the inlet but that this increase slows and the mean slug length typically reaches a value between 20 and 50 diameters at the outlet. At low flow rates, slug-length distributions tend to be extremely wide. At higher flow rates, slug-length distributions are generally narrower. The experiments demonstrated that slug flow often requires a long distance to develop.
Slug-length distributions tend to evolve substantially as the slugs travel along the pipe so that the length and frequency measured in one location will generally not be the same at another location. Ultimately, flow-assurance engineers could be more interested in slug lengths than slug frequencies because the slug-length distribution determines the liquid-handling capacity required at the receiving facilities, as well as the magnitude of the load variations in unsupported pipe sections. The authors write that full-scale transient simulations are necessary to achieve any real predictive power for large, complex systems.
In their review of the literature, the authors find that a disconnect exists between events observed in laboratory experiments and in field studies. The aim of the complete paper is to close this knowledge gap by gaining a better understanding of the mechanisms leading to the observed evolution of slug lengths in long pipes. The pipe used in the current work was very long by laboratory standards but still much shorter than typical oil-transport lines. The authors make no attempt to model slug flow in the paper; however, they write that the findings from the paper can be of use to develop models that can predict slug-flow characteristics for real multiphase transport systems.
Experimental Setup and Conditions
The loop setup is shown in Fig. 1. The loop is made from 8-in. carbon steel piping with an internal diameter of 189 mm. It consists of two main test sections, one 359-m-long horizontal and one 366-m-long section with 0.5° inclination. These two sections are connected by a bend with a radius of 1.2 m. The first 30 m of the loop has a downward angle of 5° to promote stratified flow at the beginning of the horizontal section. The locations of the instruments are given as distance from the inlet/mixing point, which is directly upstream of the 30-m declined section.
Instrumentation, as well as the thermodynamic properties of the experimental fluid system (nitrogen as the gas phase, a commercial oil, and tap water) are detailed in the complete paper.
Slug-length and -velocity calculations used in the study are detailed in the complete paper.
Flow-Regime Development. The first 30-m stretch of the loop was declined at an angle of 5° to promote stratified flow in the beginning of the test section so that slugs would be allowed to appear spontaneously and not by any artificial mechanism at the mixing point. In particular, the authors suspected that, by not allowing slugs to appear naturally, the prevailing slug length and frequency might depend on the design or arrangement of the mixing point.
The first gamma densitometer was located 87 m from the mixing point, or 57 m downstream of the declined section. The results from the gamma densitometer showed that the flow was virtually never fully developed at this point. The conclusion was that slug flow was obtained somewhere between 57 and 157 m downstream of the declined section (translating to a normalized distance of 300 to 830 pipe diameters).
Physical Mechanisms for Slug-Length Development. Slugs Consuming Waves. The first important mechanism highlighted in the paper is the growth of slugs by consummation of waves. Slugs generally travel at a higher velocity than waves, so, once a slug is born, it will catch up to the waves traveling in front of it and consume them. This process allows the slugs to increase in length, and, near the stratified-slug transition, they can grow quite large because there are so many waves to consume.
Slug Death. Under certain circumstances, slugs can decrease in length to the point that they become a wave that is subsequently consumed by the following slug. The authors’ observations indicate that this phenomenon happens mainly at relatively low rates. The explanation for this phenomenon is that slug fronts are only conditionally stable at low rates. When the slug is close to the front stability limit, the height of the front will vary, potentially allowing gas pockets to enter the slug. When the gas pockets are close to the slug tail, they are believed to distort the local velocity profile, causing the trailing bubble nose to accelerate and subsequently shorten the slug. Ultimately, if the slug is or becomes sufficiently short, this process can lead to the slug being transformed into a wave that can be consumed by the trailing slug.
Slug Coalescence Caused by Gas Carry-Under. The final mechanism discussed is slug coalescence caused by gas carry-under between slugs. The authors have only observed this at relatively high rates where significant amounts of gas usually exist inside the slugs. At sufficiently high rates, the gas inside the slugs does not always have time to segregate from the liquid before the next slug arrives, especially if the slugs are close together. This carry-under effect causes an imbalance in the gas fluxes in and out of the gas volume between the two slugs, where the trailing slug absorbs more gas than the following slug releases. The result is that the gas volume between the slugs shrinks until the large bubble disappears completely; the slugs have at this point combined into one slug.
Slug-Length Measurements. In the experiments with oil, slug-length distribution is always wider at the end of the loop than at the beginning and the difference is most pronounced for the lowest gas rate. For the experiments with water, this is not always the case. In fact, for the three highest gas rates, the distribution becomes narrower at the end of the pipe.
In most cases, the mean slug length increases significantly in the beginning and later stabilizes to either a constant value or to a slower rate of increase. This initial slug length increase may be attributed largely to slugs consuming waves. After this initial slug-length increase, the mean slug-length evolution becomes less pronounced. However, in many cases, a small positive rate of slug-length increase is observed toward the end of the pipe. The authors write that they believe that the continued slug-length development is caused mainly by slug death at low rates and slug coalescence at high rates. The slightly positive slug-length development sometimes observed at the end of the loop may be of great importance. Indeed, even small slug-length development rates can add up to giant slugs if the pipe is sufficiently long.
For the high-rate gas/water experiments, slug-length distribution is observed to become narrower as the slugs approach the outlet, while this tendency is not observed for gas/oil experiments. A possible explanation for this difference might be that the gas carry-under mechanism, which leads to merging of slugs at high rates, is more important for oil than for water.
- Prediction of slug lengths in long pipes requires a rigorous model that incorporates all relevant physical mechanisms that affect slug evolution.
- A long distance was needed to obtain slug flow—between 300 and 830 pipe diameters.
- In circumstances where the flow consists of both slugs and large waves, the slugs consume the waves and subsequently increase in size.
- At low flow rates, slugs frequently die and become waves. The authors write that this phenomenon is likely related to unstable slug fronts that allow pockets of gas to enter the slugs.
- At high flow rates, pairs of slugs sometimes coalesce into one.
- The mean slug length initially increases quickly with the distance from the inlet, but this development slows down after some distance.
- The mean slug length often appears to continue to increase slowly even after most of the waves have been consumed.
- The longest slugs observed were more than 250 diameters (50 m).
- At low flow rates, slug-length distributions tend to be very wide, sometimes with standard deviations approaching 100%.
- The effect of the water cut on slug-length distribution is significant but complex, and it is difficult to establish any general trends regarding this relationship.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30864, “Large-Scale Experiments on Slug-Length Evolution in Long Pipes,” by Jørn Kjølaas, Tor Erling Unander, and Marita Wolden, SINTEF, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission.