ABSTRACT
This study presents an investigation of the hydrodynamics behaviour
of slug flow in an inclined (80 degree inclination) and 67 mm internal
diameter pipe. The study provides a more rudimentary explanation into
the physical phenomenon that controls slug flows behaviour and the way
these parameters behave under variable flow conditions. Various
correlations for determining slug characterisation parameters have also
been presented and validated with the experimental data
The slug flow regime was generated using multiphase air-silicone oil
mixture over a range of gas (0.29 SG < 1.42 m/s) and liquid (0.05
< USL < 0.28 m/s) superficial velocities. Electrical capacitance
tomography (ECT) data was used to determine: the velocities of liquid
slugs and the Taylor bubble, the void fractions within the Taylor
bubbles and the liquid slugs. It is found that structural velocity as
reported earlier by Abdulkadir et.al (2014) was strongly dependent on
the mixture superficial velocity. A weak relationship was also found
between structure velocity and length of Taylor bubble buttressing
earlier report by Polonski et.al (1999).
The frequency of slugs was determined by power spectral density
method. Frequencies of liquid slugs were observed to be fluctuating
(i.e. increase and decrease) with gas superficial velocity depending on
the flow condition. The behaviour of the characterizing parameters for
this work which is for 800 pipe inclination except frequency, were found
to be in good agreement with that reported earlier by Abdulkadir et.al
(2014) which was for 900 pipe inclination.
CHAPTER 1
INTRODUCTION
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.