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Drag Reducing Agents in Multiphase Flow Pipelines: Recent Trends and Future
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Article  in  Petroleum Science and Technology · February 2005

DOI: 10.1081/LFT-200038223

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Petroleum Science and Technology, 23:1403–1424, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1091-6466 print/1532-2459 online
DOI: 10.1081/LFT-200038223

Drag Reducing Agents in Multiphase Flow

Pipelines: Recent Trends and Future Needs

B. A. Jubran
Department of Aerospace Engineering, Ryerson University,
Toronto, Ontario, Canada

Y. H. Zurigat
University of Jordan, Amman, Jordan

M. F. A. Goosen
School of Science and Technology, University of Turabo, Puerto Rico

Abstract: In this paper, recent work on drag reducing agents in single and multiphase
flow pipelines is reviewed. Focus is placed on theories of drag reduction, the influence
of drag reduction agent types, and hydrodynamic and heat transfer characteristics of
flows in the presence of drag reducing additives. Questions are raised, shortcomings
are assessed, and future research needs are outlined.

Keywords: drag reducing agents, heat transfer, multiphase flow, flow conditioner

INTRODUCTION

Drag reduction in pipe flow using polymeric drag reduction agents (DRAs)
is a problem of great practical engineering interest because DRAs reduce
pumping power and increase piping system capacity. DRAs have been used
in several engineering systems, such as district heating and cooling, oil pro-
duction and transportation pipelines, and others. Its first commercial use was
in the 1.2 m diameter Trans-Alaskan Pipeline in 1979, where a 50% drag
reduction was achieved, thereby increasing the capacity of the pipeline from
1.45 to 2.1 MBPD (Burger et al., 1982). This resulted in eliminating the
need for installing two pumping stations, which were planned to achieve the

Received 4 March 2004; accepted 23 April 2004.

Address correspondence to B. A. Jubran, Department of Aerospace Engineering,
Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3. E-mail:
bassamj@squ.edu.om

1403
1404 B. A. Jubran et al.

mentioned increase in capacity. Since that time, the DRAs have been used
in many petroleum product pipeline installations, such as the Iraq-Turkey oil
pipeline and Oseberg Field in the North Sea (Berge and Solvik, 1996). Thus,
the use of DRAs has the following advantages:

1. Increased pipeline capacity (throughput).

2. Savings in pumping power.
3. Pressure reduction with the associated reductions in pipe thickness and
pressure surge.
4. Reduction in pipe diameter in the design phase as well as the number or
size of pumping facilities.

The result of DRA application is a reduction in systems’ overall costs.

One further advantage of using drag reducing agents is that the DRAs can
be implemented immediately or temporarily, giving high operational flexibil-
ity. Typical dosage rates for 10–30% flow improvement in oil pipelines are
1–2 ppm of polymer per injection site. Berge and Solvik (1996) found that
the required DRA-injection rates for multiphase flows were four times higher
than those needed for stabilized crude oil. This was attributed to the higher
shear degradation that resulted from the higher degree of flow turbulence
in the multiphase system. The performance of DRAs is measured using the
effectiveness defined by:
Pwithout DRA − Pwith DRA
effectiveness (ε) = (1)
Pwithout DRA
The performance of DRAs is affected by several factors, such as pipe
diameter, temperature, fluid viscosity, and the presence of paraffin and/or
water. Comparisons of effectiveness and costs for new and conventional DRAs
are shown in Figures 1, 2, and 3 (Berge and Solvik, 1996). Over a 14-year
period (between 1980 and 1994) the effectiveness of drag reducing agents
had increased 14 times.
The aim of this paper is to review recent work on drag reduction in
single and multiphase flow in pipelines. Focus is placed on theories of drag
reduction, the influence of drag reduction types, and hydrodynamic and heat
transfer characteristics of the flows in the presence of DRAs. Questions are
raised, shortcomings are assessed, and future research needs are outlined.

THEORIES OF DRAG REDUCTION

Drag reducing agents (DRAs) are applied in pipelines with turbulent flow,
hence, they are not effective in laminar flows. The reduction is achieved by the
interaction between the polymer molecules and the turbulence components
of the flow. Polymers tend to stretch in the flow and absorb the energy in the
streak, which in turn stops the burst that produces the turbulence in the core
and results in a reduction in turbulence (Lester, 1985; Mizunuma et al., 1996).
Drag Reducing Agents in Multiphase Flow Pipelines 1405

Figure 1. Comparison of conventional gel-type DRA and new generation type.

Figure 2. Performance comparison of new generation type and conventional gel-type

DRA.

Figure 3. Cost comparison of conventional gel-type DRA with new generation type
DRA.
1406 B. A. Jubran et al.

Thus, the principal effect of DRAs is to reduce the velocity fluctuations in

the normal direction and Reynolds stresses thereafter. Cationic surfactants are
another class of DRA which form rod-like micelles. Under shear stress, mi-
celles line up in the direction of flow and build the so-called shear-induced
state, which leads to a damping of radial turbulence and a subsequent re-
duction in pressure loss. The various theories used to explain drag reduction
phenomena are summarized by Kostic (1994) (see Table 1).
The existence of multiphase flow (oil/gas and oil/water/gas mixture) in
pipelines is common in the oil and gas industry. This is due to the fact that oil
and gas wells are drilled far away from the separation site, which necessitates
transport by multiphase pipeline flow. Drag reducing agents have been used
for a long time to lower the friction component of the pressure in a single-
phase flow during the transport of oil or gas in pipelines. However, recently
it has been shown that DRAs are also effective in multiphase flow and work
very well on all components of pressure drop: frictional, accelerational, and
gravitational (Dass et al., 2000). This is because of the DRA’s ability to
modify the flow pattern, which will be discussed later in this paper.
Drag reduction phenomena in multiphase flow are still far from being
well understood in spite of the numerous investigations. This is due to the
dependence of such phenomena on a large number of parameters, such as oil
viscosity; pipe diameter; liquid and gas velocities; composition of oil, such
as the wax content, pipe surface roughness, water cut, pipeline inclination,
DRA concentration, types of DRA; shear degradation of DRAs, temperature,
and pH (Kang and Jepson, 2000).

DRAG REDUCING AGENTS

Drag reducing agents (DRAs) are high molecular weight, long chain poly-
mers, such as polymethacrylate (PMMA), polyethyleneoxide (PEO), and
polyisobutylene (PIB). DRA polymers commonly used are x-olefin polymers
and copolymers of very high molecular weight. A new generation of drag
reduction agents is now available commercially. In general, the new DRA
is characterized by high polymer content. The active component is still a
polyalphaolefin polymer with a fast dissolution rate and a slow degradation
rate. Moreover, they are characterized by low viscosity and are much easier
to handle. Berge and Solvik (1996) reported field results in crude oil and mul-
tiphase flows using the new generation DRA, which is an emulsified powder
product with a polymer content of 20–25%, as compared to conventional
gel-type product with polymer content of 5–8%. They reported that the new
DRA tends to be four times more effective than the conventional gel-type
DRA, with cost savings of 25%. Table 2 summarizes drag reducing additives
and their properties, while Table 3 lists the drag reduction and heat transfer
behavior as reported by Kostic (1994).
 Table 1. Theories of drag reduction phenomena

Theory Description

Shear thinning Originally it was speculated that near-wall-layer, by virtue of shear-thinning, may have extremely lower friction coefficient than pure
solvent. Later this theory was discounted since it was proved that shear-thinning friction is somewhat lower, but not nearly that of
drag-reduction friction.
Visco-elasticity and This may well be the most unfortunate theory. Drag-reducing polymer solutions are viscoelastic and show the normal-stress differences,
normal-stresses but for concentrations extremely high by drag-reduction standards. Very dilute solutions do not exhibit any measurable elasticity, nor
change of viscosity from pure solvent, still they are very strong drag reducers. Also, viscoelastic, cross-linked polyacrylic acid
(Carbopol) solutions do not show any drag-reduction, except for shear-thinning effect. It may well be that viscoelasticity does not play
any major role in drag reduction, but is merely an accompanying property of some drag-reduction fluids. It is known that both

1407
viscoelastic and non-elastic fluids may produce drag-reduction.
Molecular “stretching” Greatly extended linear macromolecules in shear direction interfere with turbulence, providing a stiffening effect, thus reducing friction
drag. Others postulate that molecular entanglements are responsible for interfering with and enlarging the sublayer eddies. Some have
argued that macromolecules’ elastic properties and continuous deformation, like a “yo-yo” effect, are responsible for damping small
turbulent eddies, storing and recovering otherwise dissipated turbulent energy. However, for extremely dilute solutions it seems
unlikely that such a hypothesis could be valid.
Decreased turbulence Some researchers suggest that polymer additives interfere with the production of turbulence, and that the reduction phenomena are not
production due to turbulence dissipation, but are driven by reduced generation of turbulence. Since the two have to be in balance, their roles may
be easily mistaken.

(continued)
 Table 1. (Continued)

Theory Description

Decreased turbulence Turbulence energy dissipation via finest eddies is greatly reduced (suppressed) by additives interference, to an extent equal to the
dissipation drag-reduction, while larger eddies and large-scale flow instability are present (still turbulent flow), but with different and more
favorable structure.
Vortex stretching It is postulated that resistance to vortex stretching reduces the mixing and energy losses. It is further shown that dilute polymer solutions
may have thousands of times higher extensional viscosity than the steady-state viscosity, which may have a strong influence on
drag-reduction mechanism, believed to play a major role in a region just outside the laminar sublayer (5 < y + < 50).
Non-isotropic properties Since viscosity is shear-rate dependent and the shear-rate is directional, the solution structure becomes anisotropic; hence viscosity
and turbulence (including dynamic and higher-order stress coefficients) has to be anisotropic: for shear thinning fluids, it is lower in the flow
direction and higher in cross-flow directions, thus suppressing considerably the cross-flow fluctuating velocity components (especially

1408
small-scale eddy fluctuations).
Laminarization of Turbulence is the “wasteful” dissipation of fluid energy via the finest turbulent eddies, thus it directly increases friction drag. Therefore,
turbulent flow drag reduction is a direct measure of partial flow laminarization. By definition, turbulence implies random fluctuations and energy
dissipation, otherwise flow instability will have some orderly secondary (and unsteady) flow patterns.
Unanswered questions:
• Does viscoelasticity have any direct relation with turbulent drag reduction?
• Is influence of wall crucial since polymers may profoundly modify jets and free turbulence?
• What is the influence on drag reduction of internal and external boundary layers and how can concepts be unified?
• Why is “Onset” of drag reduction present with some but not all drag-reducing fluids?
• Why do additives produce the maximum friction and heat-transfer reduction asymptotes, but cannot fully laminarize flow (Ultimate Drag Reduction)?
• Why is the asymptotic heat-transfer reduction stronger and occurs for higher polymer concentration than friction drag?
 Table 2. Drag reducing additives and their properties

Type of additive Characteristic properties

High-polymers Macromolecules—high-molecular weight (106 or higher), linear structure, with maximum extensivity, excellent solubility.
—Polyethylene oxide (the best)
—Polyisobutylene (oil-soluble)
—Polyacrylamide
—Carboxymethylcellulose
Soap and surfactant aggregates Low-molecular-weight alkali-metal and ammonium soap molecules form aggregates or “micelles” in long-chains.
Fibers Asbestos fibers are extremely long (hair-like). Nylon fibers are shorter (length-to-diameter ratio about 50). Wood pulp
—Asbeston suspensions in water reduce turbulent friction. Drag reduction is less in fiber-gas suspensions.

1409
—Nylon
—Wood pulp
Solid-liquid particles Pneumatic systems have higher flow rates when dust-laden than with clean air only. Suspension of thoria in water show drag
—Thoria reduction. Even droplets in gases reduce friction.
—Sand and dust particles
—Droplets in gases
Other natural sources Natural gums (like guar), algae, and bacteria usually produce copious, high-molecular-weight polysaccharide.
Principal properties of drag-reducing additives
• Extended length and/or sufficient mass (inertia) to interfere and suppress turbulent fluctuations, particularly transverse ones.
• Rigidity and/or elasticity to suppress and absorb turbulent fluctuations.
 Table 3. Known friction and heat-transfer behavior of drag reducing fluids

Characteristic phenomena

Friction factor High friction drag reduction for very small concentrations gives a friction reduction of 40%, which, with increase of polymer
concentration, reaches the limiting asymptotic value up to 80%.
Heat transfer Stronger heat-transfer reduction than friction drag reduction; over 90% of corresponding Newtonian values for the limiting asymptotic
case. Generally, this phenomenon is not useful, as in crude-oil pipelines. In contrast, heat transfer is increased in boiling and in
laminar flow through non-circular ducts.
Entrance lengths Much longer than the corresponding Newtonian values, on the order of 100 and 500 hydraulic diameters for hydrodynamic and thermal
entrance lengths, respectively.
Transition to turbulence Smoother transition from laminar to turbulent flow, as opposed to abrupt transition of Newtonian fluids. Also, higher transitional

1410
Reynolds number values (much higher than 2000, often 5000 or higher). In some cases the “onset” of drag-reduction is encountered.
Mean velocity profiles Flatter velocity profiles (in central region) than the solvent alone. That is quite the opposite from the influence of pipe roughness on the
profile.
Turbulence structure Fluctuating v  velocity component is reduced, while axial component u is less affected; though some results are conflicting. Spacing
between large-scale slow-streaks is more than doubled, and time between the “bursts” (fluid lumps) ejected from the wall region is
increased ten-fold.
Other Cavitation is of a different character and is often greatly reduced. Extensional flows through porous media (an application in
enhanced-oil-recovery) and jet flows have different characteristics than in pure solvent. Several other behaviors of more-concentrated
polymer solutions, such as die-swell, Weissenberg rod-climbing effect, tubeless siphon, inverse secondary flow, etc. are markedly
different from Newtonian flows.
Drag Reducing Agents in Multiphase Flow Pipelines 1411

Generally, higher molecular weight polymers perform much better than

identical but lower molecular weight polymers. A major drawback of polymer
solutions is the degradation in high shear flows. This degradation is caused
by the pump and piping system. Injecting the polymers downstream of the
pipeline booster pumps can minimize this effect. Choi and Kasza (1989) re-
ported the dependency of degradation on the flow temperature. They found
that dilute polymer solutions tend to degrade rapidly at 87.8◦ C while no
degradation was experienced at 7.2◦ C. Moreover, they reported a drag reduc-
tion of 50% for one month of circulation.
Kwack and Hartnett (1982) investigated the effect of degradation on the
friction factor and heat transfer in a recirculating flow system. They observed
no effect of DRA degradation on the friction factor, but there was an effect on
the heat transfer. The degree of degradation was presented using the critical
Weissenberg number. High concentrations were used to make up for the
degradation. The effectiveness obtained was very much dependent on the
type of drag reducing agent used.
Sitaramaiah and Smith (1969) reported experimental results on drag re-
duction in turbulent flow using several acrylamide based polymers. They
compared their effectiveness with that of polyethylene oxides and found that
drag reduction increased with higher molecular weight, concentration and
flow rate for all polymers approaching values of 70–80%. The main conclu-
sion was that low-salt content solvents should be used for better efficiencies
when polymers with ionic groups are used as fluid-friction reducers. The se-
lection of the drag reduction agent was very much related to the application
under consideration and the cost.
Virk (1975a, 1975b) and Hoyt (1984) identified two asymptotic, additive-
intensive flow regimes of zero and maximum drag reduction that envelope
a third polymeric regime wherein additives’ properties exert certain influ-
ences. The polymeric regime, based on Prandtl-Karman (P-K) coordinates,
consisted of two extremes of flow behavior called types A and B. Type A
was a family of additive solutions that produced a “fan” of friction factor
segments which radiated outward from a common “onset” point on the P-K
law (Figure 4) (Virk et al., 1997). Type B included a variety of additives, such
as polyelectrolytes and fibers, with a ladder of segments on the P-K law.
Wahl et al. (1982) reported field experimental results on two drag re-
ducing agents to increase the capacity of crude oil pipelines. The pipelines
tested varied in diameter and length, and were in the range of 8–48 in and 12–
167 km, respectively. Two DRAs were used: CDR drag reducer and a modi-
fied drag reducer that is a more viscous polymer solution. The performance of
the modified drag reducer increased by approximately 10-fold, that is, 2 ppm
of the modified polymer gave the same level of performance as 20 ppm of
the standard drag reducer for a pipe of 8-in diameter and 4–5 fold for a 48-in
diameter pipe. The most important conclusion of their work was that high per-
formance, low concentration modified polymers were very attractive for off-
shore production operations where space and deck loading are critical factors.
1412 B. A. Jubran et al.

Figure 4. (a) Type A “fan” for collapsed conformation of B1120, in 0.3 N NaCL
(b) type B “ladder for extended conformation of B1120, in 0.0003 N NaCl.
Drag Reducing Agents in Multiphase Flow Pipelines 1413

The effect of surface roughness of the pipe on drag reduction using

different types of DRAs was reported by Derrule and Sabersky (1974) and
Bewersdoff and Berman (1987). Derrule and Sabersky observed that when
polyethelyne oxide was used as the surface roughness of the pipe was in-
creased, drag reduction also increased. Bewersdoff and Berman (1987) ob-
served no change in the drag reduction effectiveness for rough pipe when
polyacrylamide was employed for both smooth and rough pipes. A summary
of effectiveness for different types of DRAs is shown in Table 4.

HYDRODYNAMICS OF PIPE FLOW IN THE PRESENCE OF

DRAG REDUCING AGENTS

Drag reducing agent performance is very sensitive to any shear generated

in the flow, as it results in the degradation of the agent. The hydrodynamic
characteristics of the flow, such as turbulence, single pass or recirculatory
flow, and single phase or multiphase flow have a significant impact on drag
reduction effectiveness. Reddy (1986) observed a reduction in effectiveness
in recirculatory flow compared to turbulent rheometer and single pass flows.
This was attributed to the adverse effect of pipe fittings on the flow of poly-
mer solution and the rapid degradation in recirculatory flow. This degradation
was generated by the resulting shearing effects, which increased as the piping
network became more complex. They further developed empirical correla-
tions that could be used for the prediction of drag reducing effectiveness of
polymers in recirculatory flow systems.
Gyr and Tsinober (1997) concluded that drag reducing fluids are essen-
tially non-Newtonian in the turbulent flow state and generally Newtonian in
many laminar flows. They presented a critical discussion of the momentum
deficit of drag reducing flows and a simple unequivocal demonstration for
the claim that the drag reduction phenomena in a number of fluid systems
are of rheological nature. Berge and Solvik (1996) reported that, in general,
a higher degree of fluid turbulence resulted in a higher drag reduction. If this
is to be related to the Reynolds number (Re), then this implies increasing ve-
locity and decreasing viscosity. They reported that when the DRA dissolved
rapidly in the fluid, it resulted in a modified structure of the turbulence and,
hence, better performance.
Su and Gudmundsson (1994) presented the basic equations used for the
calculation of the total pressure drop in perforated pipe flow as applied to
horizontal wells. They divided the pressure drop into two components: re-
versible and irreversible. The reversible pressure drop was due to acceleration
as more fluid entered the wellbore through perforations, while the irreversible
pressure drop was due to friction and mixing effects. They computed the ac-
celeration terms using both momentum and energy equations. Their compu-
tations showed that the acceleration terms were about one-third higher when
the momentum equation was used compared to that obtained when the energy
 Table 4. Performance and applications of various types of drag reducing agents

Concentration
Drag reducing agents Properties Applications Fluids (ppm) Effectiveness References

CDR polymer (water Injection concentration, wt% Horizontal-oil pipeline, Oil, single phase 5, 10, 20 6–23% Burger et al. (1980)
soluble polymers) 10%; solvent flash point, (field tests), diameter
PM, 60◦ C; density, g/cm3 48-in
0.814, K = 230 Pa.s
CDR polymers Injection concentration, wt% Horizontal-oil pipeline, Oil, single phase 10, 20% 14–23% Wahl et al. (1982)
10%; solvent flash point, (field tests), diameter 8,
PM, 60◦ C; density, g/cm3 12, and 48-in
0.814, K = 230 Pa.s
Modified CDR Injection concentration, wt% Horizontal-oil pipeline, Oil, single phase 5, 2% 23–46% Wahl et al. (1982)
10%; solvent flash point, (field tests), diameter 8,
PM, 60◦ C; density, 0.814, 12, and 48-in
K = 280 Pa.s

1414
Guargum (GM), Horizontal water pipeline, Water, single phase 250–1500 ppm 17% for CMC, 37% for Reddy (1986)
Xanthangum (XM), diameter 1-in, Re = GM, 40% for XM, 33%
Polyacrylamide (PAM), 20,000 to 60,000 for PAM, and 28% for AF
Carboxymethylcellulose
(CMC), and asbestos
fiber (AF)
Oil soluble DRA Horizontal 10-cm Multiphase, oil/gas 20 and 50 ppm 82% for slug flow and 47% Kang and Jepson
diameter pipeline for annular flow; slug (2000)
frequency decreased
significantly with addition
of DRA
GEM (Detergent) Horizontal 2.5–10 cm Single crude oil 10–500 ppm 10% (2.5 and 5 cm dia.), Mansour and Aswad
diameter pipelines 35% (7.5 cm dia.) and (1989)
50% (10 cm dia.)
New generation DRA Emulsified powder with a Horizontal 14-in dia, Single crude oil, 10–100 ppm 70%, new generation DRA; Berge and Solvik
polymer content of 9.5 miles, 28 in dia, multiphase flow 50%, conventional DRA (1996)
20–25% 75 miles
Drag Reducing Agents in Multiphase Flow Pipelines 1415

equation was used. Moreover, they conducted experimental investigations on

a perforated pipe with 144 perforations, geometrically similar to the wellbore
casing. They found that the total pressure drop consisted of 80% wall friction,
15% mixing effects, and 5% pressure drop due to acceleration.
It is interesting to note here that most of the work carried out so far on
performance of horizontal wells only considered the friction component of
the total pressure (Dikken, 1990; Landman, 1994). Little work has been done
on drag reduction other than that used by friction (Dass et al., 2000). The
main outcome of this work was that the semi-empirical relationship developed
for pipe junction in hydraulics cannot be used for flow in horizontal wells
because the flow ratio and perforation diameters are different.
The effect of pipe diameter on the performance of the drag reducing agent
is an important parameter which cannot be accounted for through Reynolds
number (Re) as was done for Newtonian fluids. A good account of the effect
of diameter on drag reduction fluids was shown in the work reported by Sellin
and Ollis (1983) and Matthys (1991). Matthys pointed out that the effect of
the diameter must be included in an additional parameter that is necessary for
the prediction and characterization of friction in the non-asymptotic regime.
However, Re may be used provided the viscosity of the solvent rather than
the viscosity of the actual solution is used in the calculation of Re. The
rationale behind this is that very dilute solutions tend to have a viscosity that
is independent of the shear rate in the high shear rate regime. However, if the
viscosity is much larger than that of the solvent, then the approximation using
solvent-based Re will be justifiable, particularly when the drag reduction
obtained is small (Matthys, 1991). It was also reported that using smaller
diameter pipes to predict the performance of drag reduction in larger diameter
pipes would not result in an accurate prediction (Jepson and Taylor, 1993).
Mansour and Aswad (1989) conducted an experimental investigation on
the effect of pipe diameter on DRA using a detergent called GEM in a re-
circulating system. They reported that increasing the pipe diameter increased
drag reduction, which was contrary to the findings of Lester (1985), who
found that increasing the pipe diameter decreased drag reduction. Jubran
et al. (1992) conducted an experimental investigation on the effect of pipe
diameter on drag reduction of GEM in a recirculating system. They found
that as the diameter of the thermoplastic pipe was increased, the drag re-
duction decreased. Gasljevic and Matthys (1993) investigated the effect of
drag-reducing surfactant additives on heat transfer exchangers. Their results
indicated that increasing the diameter of the pipe from 2 to 52 mm resulted
in a decrease in the drag reduction effectiveness. This effect was diminished
as Re increased beyond 105 . The general consensus was that increasing the
diameter of the pipe tends to decrease drag reduction effectiveness.
Another focus research area for drag reducing agents is their influence
as flow conditioners for two-phase flow in pipelines (i.e., effects on flow
structure). Again, the effect depends on the type of DRA used. Rosehart
et al. (1972) investigated the presence of DRAs on the structure of single and
1416 B. A. Jubran et al.

two-phase flow in horizontal pipes using visual observation. The addition of

DRAs to the flow did not change the slug transitional velocity and the slug
frequency at low polymer concentration. It was found to be the same for
the air/water system, but decreased at higher polymer concentration. Kang
et al. (1998) investigated DRA in three-phase flow and oil/water/gas flow.
They concluded that DRA was effective in reducing drag for different flow
patterns, such as stratified, slug, and annular flow. DRA was found to change
the flow patterns in horizontal pipes. Their results agreed well with those of
Rosehart (1972) which showed that for three-phase flow DRA concentration
did not affect the slug transitional velocity. The amount of drag reduction
obtained is very much dependent on the type of flow regimes, as can be seen
in Table 5.
Kang et al. (1999) conducted an experimental investigation on using
drag reducer agents in multiphase flow in vertical pipes. In addition to the
performance of drag reduction, they reported flow conditioning due to DRAs.
Adding DRAs shifted the transition to slug flow to higher superficial liquid
velocities. No effect was reported on the superficial gas velocity for the
flow to remain in transition. The effectiveness of DRAs tended to decrease
with increasing superficial liquid velocity at the same superficial gas velocity
(Table 5). Kang and Jepson (2000, 1999) reported experimental investigations
on using drag reduction as a flow conditioning agent in multiphase pipe flows.
They reported that DRAs did not change the slug transitional velocity, but
decreased the slug frequency and the height of the liquid film.
The effect of DRAs in two-phase flow in annular flow was investigated
experimentally by Al-Sarkhi and Hanratty (2001, 2001a) and Soleimani et al.
(2002). In air-water flow in a horizontal 9.53 cm diameter pipe the DRA
injection resulted in drag reduction of 48% with only 10–15 ppm DRA con-
centrations (Al-Sarkhi and Hanratty, 2001). It was noted that the DRA’s ef-
fectiveness is sensitive to the method of injection as well as the concentration
of polymer in the injected solution (maser solution). At maximum drag re-
duction the annular flow became stratified with smooth interface. Also, for
the same DRA concentration in the flow there is an optimum concentration of
the master solution that maximizes the effectiveness. A master solution con-
centration of 1000 ppm of Percol 727 was suggested (Al-Sarkhi and Hanratty,
2001). In the work of Al-Sarkhi and Hanratty (2001), two injection locations
along the pipe were used: one 0.6 m upstream of the air-water mixing tee
in the liquid line and one 5.5 m downstream of the tee where two-phase
flow exists. It was observed that when the DRA is injected in the upstream
injection location its effectiveness decreased with increasing the gas velocity,
while it was insensitive when injection took place in the downstream injec-
tion location. Thus, in annular flow the injection of DRAs should be in the
liquid film.
To investigate the effect of diameter size on drag reduction in annular
flow Al-Sarkhi and Hanratty (2001a) used a smaller diameter (2.54 cm) and
achieved drag reductions of 63% compared with 48% for the 9.53 cm pipe
 Table 5. Effect of drag reduction agent for different flow regime

Drag Pipe
Flow regime effectiveness inclination Flow conditions Reference

Full pipe flow (100% oil) 42% Horizontal pipe DRA: 10 ppm, superficial liquid: velocity 0.25 m/s Kang et al. (1998)
Stratified flow More than 40% Horizontal pipe DRA: 10 ppm, superficial liquid: velocity 0.03 Kang et al. (1998)
m/s, gas velocities 4–7 m/s

1417
67–81% Horizontal pipe DRA: 75 ppm, superficial liquid: velocity 0.03 Kang et al. (1998)
and 0.11 m/s 7 m/s
90% Vertical pipe Superficial liquid velocity: 0.5 m/s, superficial gas Kang et al. (1999)
velocity less than 4 m/s
Slug flow 50% Vertical pipe Superficial liquid velocity: all velocities; Kang et al. (1999)
superficial gas velocity more than 4 m/s
1418 B. A. Jubran et al.

used previously (2001). However, they found that compared with the large
diameter pipe, a larger concentration of polymer is required in the smaller
diameter pipe to achieve the maximum drag reduction (10 ppm in 9.53 cm
pipe and 30 ppm in 2.54 cm pipe). Differences in the resulting flow pattern
were also observed. At the large diameter pipe the resulting flow pattern was
stratified with smooth interface while at the smaller diameter pipe the pattern
was characterized by stratified-annular.
The study of Soleimani et al. (2002) investigated the effect of DRAs on
the transition form stratified to slug flow in a horizontal 2.54 cm pipe. It was
found that at gas superficial velocities greater than 4 m/s the DRAs delay
the transition to slug flow; i.e., transition occurs at larger liquid holdup. As
DRAs are added into a stratified flow, a higher thickness of the liquid layer
is required to initiate the slugging. In view of these findings, the addition
of DRAs to multiphase flow has potential in flow conditioning. In general,
limited work has been done on the role of DRAs as a flow conditioner and
more comprehensive work is needed.
Dass et al. (2000) reported a model to predict the components of pressure
drop in slug flow in a horizontal pipe. The aim of their work was to shed
light on the contributions of the frictional and acceleration components to
total pressure drop in horizontal slug flow in the presence of drag reducing
agents. The predicted and experimental results showed good agreement. The
DRA was active in reducing both components of the pressure drop. It was
found that the acceleration component was dominant and contributed more
than 80% of the total pressure. This increased significantly as the superficial
gas velocity was increased. Both components of the pressure were reduced
by 67% and 78% at DRA of 20 and 50 ppm, respectively. However, drag
reduction was decreased as the superficial gas velocity was increased. It is
interesting to note in their study that the drag reduction obtained was mainly
in the acceleration component, indicating that the DRA was effective in the
mixing zone of the slug flow. Fan and Hanratty (1993) developed a model to
predict the pressure drop across a stable slug flow. They treated the slug as a
hydraulic jump and assumed that the pressure change takes place at the rear
of the slug, where the change could be positive or negative.
Dukler and Hubbard (1975) developed a model to predict the frictional
and acceleration components of total slug pressure drop in an air-water sys-
tem. The model assumed that the two phases within the slug body were
homogeneously mixed with negligible slip. The frictional component of the
pressure was predicted using an equation similar to that used in a single phase
flow after modifying the density of the mixture and the friction factor. The
acceleration contribution was found by assuming a stabilized slug flow body
that is receiving and losing mass at equal rates. The acceleration pressure drop
was then calculated from the force required to accelerate the liquid to slug
velocity. Vlachos and Karabelas (1999) investigated shear stress circumfer-
ence in stratified flow. They used the momentum equations for both phases
to predict the liquid holdup, axial pressure gradient, and average liquid to
Drag Reducing Agents in Multiphase Flow Pipelines 1419

wall shear stress, for the wavy stratified and stratified/atomization gas/liquid
flow in a horizontal pipe.

HEAT TRANSFER IN PRESENCE OF DRAG REDUCING AGENTS

Drag reduction and heat transfer phenomena associated with drag reducing
fluids are far from being well understood. Certain applications for the uti-
lization of drag reduction agents necessitate a closer look at the heat transfer
process as well as the hydrodynamics process involved. However, it is inter-
esting to note that in the case of using drag reduction in crude oil pipelines,
the effect of these agents on the heat transfer process can be useful in keep-
ing the loss of heat to the atmosphere to a minimum, while keeping the oil
flowing at a lower pumping power. Moreover, in certain cases it brings down
the cost of thermal insulation of the pipelines.
Matthys et al. (1987) reported local and heat transfer measurements in
circular tubes for suspensions of betonite and for a combination of betonite
and polyacrylamide in water for both laminar and turbulent flow. It was found
that a viscosity model based on rheological measurements could represent the
results with a Newtonian relationship. It was also found that combining clay
and polymer in a fluid produced viscoelastic solutions that were very sen-
sitive to mechanical degradation. The local heat transfer results were well
correlated using the Colburn and Reynolds analogies, regardless of the con-
centration of bentonite. Yoo et al. (1993) investigated experimentally the heat
transfer characteristics of drag reducing polymer solutions in the thermal en-
trance region of circular tube flows. The tests were conducted in two stainless
steel tubes with length to diameter ratios of 710 and 1100. The fluids used
were aqueous poly-acrylamide solutions of Separan AP-273 with a concen-
tration range of 300 to 1000 wppm. The main finding of this investigation
was that the order of magnitude of the thermal entrance length of the maxi-
mum drag reducing polymer solutions was much higher than that of turbulent
Newtonian fluids in tube flows.
Gasljevic and Matthys (1994) reported local heat transfer results and
friction in the entry region of a circular pipe in the presence of a drag re-
duction surfactant. Two entrance arrangements were used: a cone contraction
and a wire mesh plug fitted to flatten the velocity profile. The main findings
of this work were the restructuring of the fluid itself due to high local en-
ergy dissipation in the inlet region, and the stronger coupling between the
hydrodynamic and thermal field development in the case of surfactant so-
lutions than in the case of polymer solutions. The Reynolds analogy and
the direct relation between the friction and heat transfer coefficients were
not valid for drag reducing fluids; i.e., the Reynolds and Colburn analogies
were not valid for this type of flow. The reasoning behind this is still not
clear and further research is needed (Matthys, 1991; Matthys and Sabersky,
1987).
1420 B. A. Jubran et al.

Toh and Ghajar (1988) and Matthys (1991) observed that the thermal
entrance and hydrodynamic lengths for drag reducing solutions were more
than that observed for Newtonian fluid flow with values of more than 20 and
100 diameters, respectively.
Matthys (1991) carried out a comprehensive survey on the most impor-
tant results and the current research needs of heat transfer, drag reduction,
and fluid characterization for turbulent flow of polymer solutions in pipes.
He investigated the problem of the reduction in convective heat transfer in
the presence of a drag reducing agent. It was pointed out that the reduction
produced by the addition of the agent was upset by the greater reduction pro-
duced in the convection heat transfer. He attributed the lack of investigations
on heat transfer of polymer solutions to the complexity of viscoelastic flows.
This required a more demanding experimental set up to accurately record the
data. Matthys (1991) indicated the availability of macroscopic and correla-
tion work for purely viscous non-Newtonian fluids, but not for viscoelastic
non-Newtonian fluids that cover flows with drag reduction agents.
Gasljevic and Matthys (1991) investigated the thermal and hydrodynamic
characteristics of drag-reducing surfactant solutions in the entry region of the
pipe, as well as after fittings. In addition, they provided an excellent literature
review on the subject. It was reported that for surfactant solutions the friction
coefficient and the Nusselt number were varying at the same rate beyond 300
diameters. Heat transfer downstream of an elbow tended to increase over that
obtained for fully developed flow, but it did not degrade the fluid.
Gasljevic et al. (1993) conducted a comprehensive experimental investi-
gation on the performance of various types of heat exchangers in the presence
of drag reducing surfactants in the working fluid. The working fluid used was
a solution of 2300 ppm of Ethoquad T/13 and 2000 ppm of NaSal in deionized
water. Pressure and heat transfer measurements were taken at an operating
temperature in the range of 312–319 K and fluid velocities of 0.2–3 m/s.
They compared their results with those obtained when tap water was used as
the working fluid and concluded that the thermal and hydrodynamic charac-
teristics are very much dependent on the geometry and flow conditions in the
heat exchanger. It was also noted that a significant drag reduction could be
achieved in heat exchangers with little penalty in the heat transfer process.
Gasljevic and Matthys (1993, 1991) reported an investigation to explore
the use of surfactant drag reducing additives to reduce the pumping power
in hydronic heating and cooling systems. Various issues were investigated,
namely the matching of the additives with system characteristics, drag reduc-
tion in fittings and valves, and the heat transfer process in the presence of
reduction agents. It was concluded that the use of drag reducing agents in
heating and cooling systems can be implemented at a small cost and would
lead to significant energy savings.
Kostic (1994) carried out a critical review on turbulent drag, heat transfer
reduction phenomena, and laminar heat transfer enhancement in non-circular
duct flow of non-Newtonian fluids. The review outlined peculiar behaviors
Drag Reducing Agents in Multiphase Flow Pipelines 1421

and applications of DRAs. Kostic reported that the underlying mechanism

that produces drag and heat reduction is far from being understood. He noted
that this should keep researchers busy for many years to come. Despite the
limited research to date on the heat transfer aspects of viscoelastic fluids,
there was enough evidence to conclude that such fluids tend to enhance heat
transfer in laminar non-circular duct flow. Moreover, he reported that flow
laminarization, due to flow-induced anisotropic fluid structure and properties,
was the predominant factor for the reduction phenomena rather than fluid
elasticity. On the other hand, fluid elasticity was responsible for laminar heat
transfer augmentation. Hartnett and Kwack (1986) reported that for a polymer
solution the reduction in friction was not accompanied by a reduction in heat
transfer. For a comprehensive review of research work related to heat transfer
in the presence of drag reducing agents, see studies by Dimant and Poreh
(1976) and Cho and Hartnett (1982).

CONCLUDING REMARKS

This paper has highlighted research conducted on drag reduction in single and
multiphase flows with particular reference to the oil industry. It has examined
work related to theories of drag reduction, the influence of drag reduction
types, and hydrodynamic and heat transfer characteristics of the flows in the
presence of a drag reducing agent. Moreover, it has raised questions and
shortcomings that need answers, as well as pin-pointing potential areas that
need further research.
Drag reduction phenomena and theories related to multiphase flow are
still far from being well understood. More work is needed in the areas of
shear degradation, and the effect of wax content, water cut, and pipe incli-
nation on the performance of drag reduction in smooth and perforated pipes
with emphases on oil wells. Most of the work carried out on the performance
of horizontal wells consider only the friction component of the total pressure
without taking into consideration the acceleration component. Limited work
has been done on the role of drag reducing agents as a flow conditioner, espe-
cially for large pipe inclinations with a high water cut. Further fundamental,
experimental, and analytical investigations are needed to better understand
the heat and hydrodynamic processes associated with drag reduction in sin-
gle and multiphase flows, since the Reynolds and Colburn analogies are not
valid for drag reducing fluids.

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