1.1 Problem definition
Multiphase flows are usually encountered in oil and gas industries, commonly among these flows is slug flow in which liquid flows intermittently with gas along pipes or wells in a concentrated mass called slugs.
The existence of slug flows usually poses a major and expensive threat or problem to the oil industry, especially to the designer or the operator of multiphase systems. For example, slug flow in oil production pipeline has a significant deleterious impact on both the process operation and on the mechanical construction of piping systems. Also, it can cause large fluctuations in gas and oil flow rates entering the gas-oil separation plant. This sometimes results in oil carry-over, gas carry-under, or significant level deviations which consequently results in plant shut-down. Again, high momentum of the liquid slugs frequently creates considerable force as they change direction when passing through elbows or other processing equipment. Moreover, if the low frequencies of the slug flow resonate with the natural frequency of large piping structures, severe damage can take place in pipeline connections and supports unless this situation is considered in the design (Ahmed, 2011).
Slug flow is highly unsteady and can exist in a variety of situations of industrial importance where the flow configuration is that of an annulus. For instance, these conditions can be expected during drilling and logging operations in oil wells, In order to design such systems or to interpret their performance, it is necessary to model slug flows. A central problem in such modeling is the need to predict the rise velocity of the Taylor bubbles (Fernandes et al. 1983).
Pressure drop is also substantially higher in slug flow as compared to other flow regimes; pressure drop is dependent on the mixture density which is affected by liquid holdup (or void fraction). Therefore, the maximum possible length of a liquid slug that might be encountered in the flow system needs to be known (Abdulkadir et.al, 2014).
Identifying the slug length and slug velocity are important parameters in many practical applications. For instance, in the oil and gas industry, estimation of maximum slug size or length is crucial in the design of slug-catchers in the transportation of hydrocarbon two-phase flow (Ahmed, 2011). Therefore as part of slug characterisation, the maximum possible slug length or slug size to be anticipated must also be determined for proper design of separators and their controls to accommodate them.
Extensive work has been carried out on slug flow characterization, some of the most recent works are those carried by Abdulkadir et.al (2014) on ‘‘experimental study of the hydrodynamic behaviour of slug flow in a vertical riser using air silicone oil’’ and Ahmed (2011) on ‘‘experimental investigation of air-oil slug flows through horizontal pipes using capacitance probes, hot-film anemometer, and image processing’’.
Most models on slug flow characterisation established in literature are based on air and water, there are limited research works conducted on air and oil. Abdulkadir, (2014) noted that reports on the study of the behaviour of these slugs in more industry relevant fluids are limited. For that reason, it is important to study the behaviour of slug flow in great detail for the optimal, efficient and safe design and operation of two-phase gas–liquid slug flow systems.
Ahmed (2011) noted that pipe inclination effect continues to be an open question and recommended that more experimental studies for different pipe inclinations should be carried out to obtain more reliable slug flow models.
Also, in practice it is rare to have a perfectly horizontal or perfectly vertical pipe or well. There is some slight deviation from the true vertical or horizontal; therefore characterizing slug flow for such pipes or wells is worth pursuing.