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Advances and Future Challenges of Wax Removal in Pipeline Pigging
Operations on Crude Oil Transportation System
Article in Energy Technology · April 2020
DOI: 10.1002/ente.201901412
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Advances and Future Challenges of Wax Removal in
Pipeline Pigging Operations on Crude Oil Transportation
Systems
Weidong Li, Qiyu Huang,* Wenda Wang, and Xuedong Gao
flow assurance challenge, that is, when
oil temperature drops below wax appearance temperature (WAT), wax components would precipitate and deposit onto
the pipe wall,[1–3] which narrows the flow
passage and lowers the transportation
capability.[4–6] In worst cases, the bulk flow
could be clogged so as to cause complete
production shutdown, the remediation
treatment an issue of millions of dollars
of economic loss.[7,8] This problem is
particularly severe when moving toward
deep-offshore reservoir production due to
the low sea water temperature.[9–12] For
wax prevention and remediation, varieties
of techniques have been developed,
including mechanical pigging, biological treatment,[13,14] chemical inhibitor
injection,[15–20]
exothermic
chemical
reaction,[21–24] cold flow,[25,26] and thermal
managements (inductive heating and pipe
coating),[27–29] among which mechanical
pigging is the most popular one.[30,31]
Although it has been a regular practice for most waxy crude
oil pipes, field pigging operation is still surprisingly empirically
operated due to the elusiveness in wax removal mechanism.[32]
Experience often matters more than scientific specifications in
decision-making, which sparks pig stalling and wax blockage
every now and then.
Wax breaking force and wax removal efficiency are governing
parameters for pig stalling and wax blockage accidents. Wax
breaking force characterizes wax deposit resistive force and
determines pig motion. Pig stalling easily occurs when wax
breaking force is underestimated, as was a case of an offshore
pipeline in the Gulf of Mexico.[33] By contrast, overestimating
wax breaking force means pigging frequency beyond actual
demand, the cost of consequent deferred production is a heavy
financial burden.[34] Wax removal efficiency measures the
removed wax. It matters in forecasting the rheological property
of wax-in-oil slurry and scheduling the next pigging program.
Accurately predicting the wax breaking force and wax removal
efficiency in pigging is therefore urgently demanded by the crude
oil piping industry.
Estimating the wax breaking force and wax removal efficiency
is strongly premised on wax layer thickness and strength.[35] To
this end, multiple mechanisms have been proposed, including
molecular diffusion, Brownian diffusion, shear dispersion,
gravity settling, and so forth.[36] Researchers have reached a
Wax deposition is a severe flow assurance challenge that threatens waxy crude
oil production and transportation. For wax remediation, pipeline pigging is the
most widely used technique. However, the elusiveness of wax removal mechanism and the lack of reliable methods to evaluate wax breaking force and wax
removal efficiency easily trigger pig stalling and wax blockage in field pigging
operations. Modeling wax breaking force and wax removal efficiency, therefore,
promotes the pigging confidence. This Review seeks to clarify the current picture
of wax removal research in crude oil pipeline pigging. Relevant wax deposit
properties including wax layer thickness and strength are discussed. Wax
removal mechanisms are summarized from perspectives of wax–pig interaction,
macroscopic force response, and scenarios with oil flow. Prediction models of
wax breaking force and wax removal efficiency are analyzed comprehensively.
Pig geometry optimization using this model is given. In addition, the key roles
of wax deposit strength, viscoelasticity and thixotropy, foam pig investigation,
and wax plug prediction are highlighted for guiding future endeavors in
this area.
1. Introduction
Serving as the blood of modern industry, petroleum has been
one of the most important primary energy for decades and will
continue to play this role in the foreseeable future. Crude oil,
together with natural gas, is an existence form of petroleum and
is mainly convoyed in pipe. This method faces an intractable
Dr. W. Li
College of Chemical Engineering
Fuzhou University
No. 2 Xueyuan Road, University Town, 350116 Fuzhou, China
Prof. Q. Huang, X. Gao
National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory
of Urban Oil and Gas Distribution Technology
China University of Petroleum-Beijing
No. 18 Fuxue Road, Changping District, 102249 Beijing, China
E-mail: ppd@cup.edu.cn
Dr. W. Wang
PetroChina Marketing Company
No. 9 Dongzhimen North Street, Dongcheng District, 100007 Beijing,
China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/ente.201901412.
DOI: 10.1002/ente.201901412
Energy Technol. 2020, 8, 1901412
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consensus that molecular diffusion is the determinant one.
Several models for predicting wax deposition rate have been
accordingly developed,[8,37–43] but method for estimating wax
deposit strength is still lacking.[44]
A good knowledge of wax removal mechanism in pigging is
another prerequisite to predict the wax breaking force and wax
removal efficiency. In early works, compression and shear
mechanisms were intuitively used to explain wax removal, and
corresponding models for calculating the wax breaking force
were proposed.[45–47] But the shear angle observed in wax breakage by Southgate seems to suggest that the wax removal process
is not that simple.[48] Orthogonal cutting, a theory widely used in
metal cutting, was then introduced to explain wax removal.[48–50]
The overwhelming effect, where the wax layer failure stress
always outweighs its yield stress,[51,52] also indicates that the
compression and shear mechanisms might have oversimplified wax breakage. In response, Li et al.[53,54] used orthogonal
cutting to describe the wax removal process and utilized
slip-line field theory to conduct stress analysis. Wax breaking
force and wax removal efficiency of disk and cup pigs were
modeled, with wax elasticity and pig scraper deformation
incorporated.
In this article, a comprehensive review on wax removal in
crude oil transportation system pigging operations is presented,
aiming to provide a clear clue to understand the current research
status and guide the future direction. In outline, this article proceeds sequentially by reviewing several issues concerning wax
breaking force and wax removal efficiency. Initially, we briefly
discuss wax deposit thickness and strength as key roles affecting
wax removal. At the next section we give an overview of previous
studies from perspectives of wax removal mechanism, force,
and efficiency. Subsequent work is done by summarizing how
current efforts have experimentally, empirically, and theoretically
proceeded in wax removal research. A practical method for
wax deposition validation and pigging evaluation is also given.
The final section contemplates some key concerns expected to
be supplemented, intending to provide guidelines for future
endeavor in this field.
2. Wax Properties Affecting Wax Removal
For waxy crude oil pipes, the major incentive for pigging is to
remove the wax deposit and maintain the transportation capacity.
What operators concern most are whether the pig would be stuck
in pipe and how much deposit could be scraped off. These issues
are strongly correlated to wax layer thickness and strength.
For example, too thick and hard of wax deposit generally requires
large wax removal force, and soft wax deposit often indicates low
pigging force and good efficiency. Therefore, a good knowledge
of wax layer thickness and strength is significantly essential for
scheduling pipeline pigging plan.
2.1. Wax Deposition Mechanisms for Modeling Wax Layer
Thickness
Molecular diffusion is widely accepted as the dominant wax
deposition mechanism.[36,55] It is assumed that when oil temperature is lower than WAT, wax molecules would precipitate from
Energy Technol. 2020, 8, 1901412
Weidong Li received his Ph.D. degree
from China University of Petroleum,
Beijing. He holds an M.S. degree in
oil and gas storage and transportation
engineering from the same university.
Now he works at the College of
Chemical Engineering, Fuzhou
University. His research interests
include wax deposition and pipeline
pigging.
Qiyu Huang is a professor at China
University of Petroleum, Beijing. He
focuses his research interests in flow
assurance challenges such as wax
deposition, oil-gelling, and complex
flow in pipes. He has devoted
research into pipeline transportation
for more than 20 years. He holds a
Ph.D. degree in oil and gas storage
and transportation engineering from
China University of Petroleum, Beijing.
Wenda Wang is a process/flow
assurance engineer at PetroChina. His
research interests include wax and
asphaltene deposition and
remediation, complex flow in
wellbores and pipelines, and rheology
and non-Newtonian fluids. During
2014–2015, he served as a visiting
scholar in the Department of
Petroleum and Geosystems
Engineering at the University of Texas at Austin. He holds
a Ph.D. degree in oil and gas storage and transportation
engineering from China University of Petroleum, Beijing.
oil due to the impaired wax solubility. Also, the temperature gradient establishes a concentration gradient, which drives wax
molecules to diffuse from the oil and continuously deposit
onto the pipe wall. Another wax deposition theory is that
Brownian motion drives wax crystals to the pipe wall from
high concentration areas. Many researchers have taken it as
insignificant,[37,40,56–58] but Azevedo and Teixeira[55] claimed that
there is no sufficient evidence to ignore this mechanism because
the drop of solid wax concentration profile from peak to near
zero in the laminar sublayer[37] may create a Brownian deposition
flux toward the pipe wall. Shear dispersion, where oil shear
triggers lateral motion of wax particles, is another theory for
wax deposition.[59,60] Experimental evidence, however, seems to
indicate that shear dispersion is also ignorable.[37,55,61] Beyond
the aforementioned mechanisms at play, shear stripping,[39]
Soret diffusion,[62] and some other theories are also incorporated
in wax deposition explanation. With the consensus that molecular diffusion is the governing mechanism, multiple models have
been accordingly proposed to estimate wax layer distribution
along the pipe.[8,37–43]
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2.2. Wax Deposit Strength
Wax deposits are themselves wax–oil gels structured by wax
crystals with liquid oil entrapped inside.[63,64] Therefore,
wax deposit strength is determined by the strength of solid
wax network. Authors have proposed that the yield stress and
solid wax content of wax deposit are exponentially correlated,
which can be given by[65–67]
τy ¼ a · φbs
(1)
where τy is the yield stress of wax sample (Pa); φs is the solid
wax content. For different wax samples, the values of a and b
are different.
It was reported that there exists a threshold solid wax fraction
delineating the transition of wax yielding from cohesive breakage
at low solid wax fraction to adhesive breakage at large solid wax
fraction.[68] To incorporate this transition, the following correlation may be more appropriate for calculating the wax layer yield
stress
τy ¼ a · ðφs φT Þb
(2)
where φT is the threshold solid wax content.
Apart from wax content, wax deposit strength is also dramatically affected by wax crystal morphology. Li et al.[67] argued that
the increase in boundary fractal dimension (an index characterizing wax crystal complexity) and decrease in aspect ratio are the
root causes for wax strength enhancement when wax content
increases. Coutinho et al.[69] and Masoudi et al.[70] found that
the increase in wax deposit strength over time is accompanied
by changes in wax crystal morphology. Bai and Zhang[44] proposed that the increasing average carbon number of wax crystals
ratchets up the aspect ratio and down the boundary fractal
dimension and average size, consequently, the strength of
oil-wax gel decays. Some authors put their attentions on how
wax deposit strength is affected by flow condition, including
velocity,[71–73] temperature,[40,71,74,75] pressure,[76] and deposition
time.[40,73,77,78] These works present the qualitative correlation of
wax deposit strength to the aforementioned factors. Modeling
wax deposit strength still has enormous scopes for advancement.
3. Overview of Wax Removal Research
3.1. Wax–Pig Interaction
Mendes et al.[45] proposed two wax–pig interactions in pigging,
i.e., compression and shear models. In compression model
(Figure 1A), rigid pig acts axially on the cross section of wax layer,
pushing it until the maximum resistive shear stress is reached. In
shear model (Figure 1B), deformable pig rides on wax layer, thus
wax is supposed to be damaged by pig shear. It seems that these
two wax–pig interactions are distinguished by pig hardness, but
the boundary between rigid and deformable pigs is quite ambiguous. Nevertheless, most early wax breaking force models were
based on the compression and shear assumptions.[45–47]
In contrast, inspired by metal cutting, Southgate[48] executed
wax cutting experiments on paraffin wax with a self-designed
metal cutting equipment. He observed that the paraffin wax
Energy Technol. 2020, 8, 1901412
Figure 1. Load models on wax layer by pig: A) compression model
and B) shear model. Reproduced with permission.[45] Copyright 1999,
American Society of Mechanical Engineers ASME.
breaks on a shear plane and thus used orthogonal cutting to
describe wax–pig interaction. The angle between shear plane and
pipe centre line is called shear angle (φ in Figure 2, 3, and 4).
Direct observation of wax–oil gel scraped by polyurethane disk
(Figure 2) supported the employment of orthogonal cutting in
wax–pig interaction explanation.[49,50] Although real wax deposit
was missing in their works, application of orthogonal cutting in
wax removal research is an inspiring contribution.
3.2. Macroscopic Force Response
Instead of investigating wax–pig interaction, Wang et al.[79]
focused on the macroscopic force response in pigging and
initiated indoor simulation experiments. Schematic of the
experimental setup is shown in Figure 5. Wax layers of different
thicknesses and oil contents were formed from wax and oil
mixture. Pigs in different geometries were pulled by the electric
winch through steel wires. The force transducer was used to measure the pigging force and the data acquisition system to record
the force data. This design strategy provides significant references for subsequent researches.
Figure 6 shows that the wax removal force profile generally
presents four distinct stages regardless of pig geometry and
wax layer thickness and strength. Other authors also confirmed
a force profile of this kind in their respective experiments.[32,51,52]
The wax removal force in the second phase was defined as wax
breaking force. Moreover, Wang et al.[79] claimed that the total
wax removal force is composed of baseline force (i.e., the frictional force between the pipe and the pig), wax breaking force,
and plug transportation force.
3.3. Experimental and Field Pigging at the Presence of Oil Flow
In a following research to the winch-driving experiment,
Wang et al.[80] proceeded to investigate wax breaking and plug
transportation under oil flow conditions where the pig was
directly driven by oil flow to scrap the wax layer off the pipe.
No measurable wax plug transportation force was observed for
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Figure 2. Orthogonal cutting in wax–pig interaction during pigging. Reproduced with permission.[50] Copyright 2015, Elsevier.
Figure 3. Wax removal for disk pig. Reproduced with permission.[53] Copyright 2018, Elsevier.
Figure 4. Wax removal for cup pig. Reproduced with permission.[54] Copyright 2019, Elsevier.
bypass pigs because the removed wax particles were scoured
downstream by the jet flow. For pigs with no bypass holes,
the scraped wax debris accumulated in front of the pig to form
wax plug. The wax plug transportation force gradient was found
to be independent of plug length. This agrees well with field experience that none-bypass pigs easily generate wax plug, as was the
case in a North Sea pipeline pigging program.[81]
In field pigging operations, the schemes are carefully designed
to avoid formation of wax plug. It involves various types of pigs
Energy Technol. 2020, 8, 1901412
such as foam pig, cup pig, and brush pig (Figure 7). Initially,
foam pigs from small to large in diameter are successively
launched to remove the soft oil gels at the outer layer of wax
deposit. Then regular cup and disk pigs expanding from small
to big are put into the pipe to clean the hard part. Brush pigs
are last sent, to clean the toughest residual wax. All the pigs have
small bypass holes, allowing the liquid to flush through to help
blast the removed wax particles and disperse them into the bulk
oil flow. For pipes that have not been pigged for a long time, the
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Figure 5. Schematic of pigging apparatus: 1) electric winch; 2) steel wire; 3) pulley; 4) force transducer; 5) test section; 6) pig; 7) camera; 8) videoimagescopeTM;
9) data acquisition unit; 10) platform. Reproduced with permission.[79] Copyright 2005, American Society of Mechanical Engineers (ASME).
the FEM results of compression load model presented the differential pressure on pig
0.50
τmax
t
(3)
¼ 3.11
D
Δp
Figure 6. Typical force versus distance behavior: 1) buildup phase;
2) preplug phase; 3) plug phase; and 4) production phase.
Reproduced with permission.[79] Copyright 2005, American Society of
Mechanical Engineers (ASME).
whole pigging program usually involves a large number of
different types of pigs and can last for over a month.
3.4. Wax Breaking Force Models
3.4.2. Hovden Model
3.4.1. Mendes Model
[45]
where τmax is the maximum loaded shear stress on wax layer (Pa);
Δp is the differential pressure on pig (Pa); t is the wax layer
thickness (m); and D is the pipe inside diameter (m).
Substantially, this model is an empirical correlation regressed
from calculation results, but not a strictly derived mathematical
method. As a first attempt to tackle wax removal, Mendes model
adopted coaxial compression in wax removal explanation, which
easily corresponds with our intuitive assumption. Even though
coaxial compression and simple shear oversimplify the actual
wax removal process, they still provide foundation for most subsequent works in this field.
Mendes et al.
initially investigated the pigging force and
described wax removal as coaxial compression and/or simple
shear. The pressure differential on pig was calculated with finite
element method (FEM). As baseline force and wax plug transportation force were not involved in their work, the pressure differential actually represented wax breaking force. Least-square fit to
Hovden et al.[47] regarded wax breakage as compression. Taking
wax removal efficiency and pig geometry into consideration, wax
layer breaking force was given by
F wbf ¼ Cpw · τy ðC o Þ · δwl · π · dip · η · ð1 ΦÞ
(4)
where F wbf is the wax layer breaking force (N); Cpw is a tuning
factor; τy is the wax layer yield stress ¼ 1.25 106 · ð1 C o Þ4
Figure 7. Different types of pigs with bypass holes: A) foam pig; B) cup pig, and C) brush pig.
Energy Technol. 2020, 8, 1901412
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(Pa); C o is the wax porosity, i.e., the volume fraction of oil (%); δwl
is the wax layer thickness (m); dip is the inside diameter of pipe
(m); η is the wax removal efficiency (%); and Φ is a pig form
factor.
In this model, the yield stress equation is actually used for
viscosity prediction.[47] Moreover, the assumption that wax
breaking force fits with wax layer yield stress in a linear relation
is somewhat arbitrary. The impact of pig geometry was considered; nevertheless, it was merely expressed as an empirical
parameter while no determination principle was given.
Likewise, the tuning factor is another decisive but arbitrarily
determined empirical parameter.
3.4.3. Kleinhans Model
From perspective of force balance, Kleinhans et al.[46] compared wax breaking force to wax layer shear force on pig. An
equation to calculate the pressure differential was accordingly
proposed
π · D2pc · ðP u P d Þ=4 ¼ ðP id Dp Þ · π · Dpc · Lpc · Sp =2
and excess in empirical parameters limited application of these
models.
3.5. Wax Removal Efficiency
Wax removal efficiency is an essential parameter to arrange
pigging program and avoid wax blockage. However, scarcely
can we find any specialized researches on this topic in open
literature. The few existing works are primarily qualitative
descriptions. For example, in Hovden model, wax removal efficiency was a parameter to calculate wax breaking force,[47] but
its determination principle was not mentioned. Wang et al.[79]
reported that wax removal efficiency is significantly affected
by pig shape and material. For the pigs used in their work, disk
pig showed the best pigging efficacy but required the highest
driving force, whereas foam pig gave the poorest cleanup performance and the lowest driving force. Barros et al.[32] also
experimentally confirmed the impact of pig geometry on wax
removal efficiency.
(5)
where Dpc is the pig diameter (m); P u and P d are the upstream
and downstream pressures, respectively (Pa); P id is the pipe
inside diameter (m); Dp is the inside diameter of wax layer (m);
Lpc is the pig-wax contact length (m); Sp is the wax shear
strength (Pa).
Mathematically, this equation is the simple product of Hovden
model and pig-wax contact length, which causes dimensional
inconsistency. Moreover, the compression effect was completely
missing, therefore the physical implication of wax removal force
faces significant ambiguity.
In short, these wax breaking force models are generally
empirical correlations or simple force balances with pig
geometries undistinguished. Lack of theoretical foundation
4. Current Development
4.1. Experimental Plan
The pigging facility is shown in Figure 8. Multiple factors, including pipe wall temperature, scraper hardness, pig geometry and
velocity, and wax layer thickness and strength, can be well controlled. Actual wax deposit and crude oil were mixed to get wax
layer. The test section comprises two coaxial pipes. Water baths
were used to control the temperature. During pigging, the servo
motor drives the pig and the tensile force transducer detects the
force. Wax removal efficiency is determined by weighing the
loaded and removed wax. Detailed information about the pigging
facility can be found elsewhere.[51–53]
Figure 8. Schematic of pigging experiment. Reproduced with permission.[53] Copyright 2018, Elsevier.
Energy Technol. 2020, 8, 1901412
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4.2. Test Results
4.2.1. Pig Dynamics
The experimental results verified the four stages of wax removal
proposed by Wang et al.[79] As shown in Figure 9 and 10, at
the very beginning, the pig compresses the wax layer thus
wax removal force dramatically rises, corresponding to buildup
phase. Then wax begins to yield and wax removal force stays stable. As the pig moves forward, wax plug gradually forms and
finally gets discharged out of the pipe, the force profile accordingly increases again and then decreases. Wax breaking force is
the average wax resistive force in the second phase.
4.2.2. Comparison of Wax Failure Stress and Yield Stress
Wax failure stress was defined as wax breaking force per unit
cross area of wax layer.[82] Figure 11 shows that wax failure stress
fits with its yield stress in a linear manner, regardless of pig
geometry and scraper hardness. It was also revealed that the failure stress of wax layer in pigging is always larger than its yield
stress, especially when the wax layer is hard.[51,52] This new
Figure 10. Force profile in pigging. Reproduced with permission.[51]
Copyright 2015, Society of Petroleum Engineers (SPE).
Figure 11. Comparison of wax failure stress and yield stress for different
pigs. Reproduced with permission.[51] Copyright 2015, Society of
Petroleum Engineers (SPE).
finding challenges our past understanding that the wax layer
would break once the exerted load reaches the yielding point.
4.2.3. Sensitivity Study on Wax Breaking Force and Wax Removal
Efficiency
Figure 9. Pig dynamics during pigging. Reproduced with permission.[51]
Copyright 2015, Society of Petroleum Engineers (SPE).
Energy Technol. 2020, 8, 1901412
Impacts of several factors on pigging were investigated.[51,67] Wax
breaking force was found to increase with: increasing wax layer
thickness and wax mixing ratio, decreasing pipe wall temperature.
Wax removal efficiency improves with test temperature and wax
layer thickness, and decays with wax mixing ratio. Irregular trends
were observed for wax breaking force and wax removal efficiency
against scraper hardness. Moreover, the determinant impact from
pig geometry was also confirmed. Disk pig generally has larger
wax breaking force and higher wax removal efficiency than cup
pig at the same wax layer characteristics and scraper hardness.
It compares well with the experimental results by Wang et al.[79]
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4.3. Empirical Models
4.3.1. Wax Breaking Force
Based on the fact that wax failure stress is linearly correlated
to yield stress,[51] an empirical wax breaking force model was
developed[52]
F w ¼ Adδw τy þ Bd
(6)
where F w is the wax breaking force (N); A is a dimensionless
coefficient; d is the inside diameter of pipe (m); δw is the wax
layer thickness (m); τy is the wax layer yield stress (Pa); and B
is used for describing the impact from pig feature on wax breaking force (N m1).
Values of A and B for different pig geometries and scraper
hardnesses were determined by regression analysis. This model
was the first of its kind to incorporate scraper hardness in wax
breaking force calculation. The authors claimed that it has a
better prediction accuracy than that of Hovden model.
4.3.2. Wax Removal Efficiency
Li et al.[67] introduced nondimensional analysis in modeling wax
removal efficiency. Initially, they constructed two dimensionless
coefficients, π 1 and π 2 , to incorporate multiple factors affecting
wax removal, including pig velocity v, inside diameter of the pipe
d, wax density ρ, wax layer thickness δw , and yield stress τy ,
qffiffiffiffiffiffiffiffiffi
w
where π 1 ¼ v ρ=τy and π 2 ¼ dδ
d η. Pigging experiments
were next conducted, and it was found that ln π 1 and ln π 2 can
be well fitted in a linear relation, where ln π 2 ¼ a ln π 1 þ lnb.
Thus, wax removal efficiency can be given by
qffiffiffiffiffiffiffiffiffia
b · d v ρ=τy
η¼
(7)
d δw
The values of coefficients a and b were experimentally
determined with a single pipe/pig size, which might elicit
some uncertainties to scalability of this model to different
pipe/pig sizes. Nevertheless, this is the first quantitative work
on wax removal efficiency compared with previous qualitative
descriptions. Also, the nondimensional analysis is an interesting
approach that gives some theoretical foundation to this model.
4.4. Theoretical Model
deformation zone, and then shear angle can be derived. Based
on this, stress distribution on the boundaries of plastic deformation was obtained. Next, wax elasticity was incorporated with
an assumption that the plastic deformation zone compresses
the undamaged wax and therefore shrinks the shear angle.
The change of shear angle can be calculated with knowledge
of the strain–stress response of wax layer. To acquire scraper
deformation, the scraper was assumed to be circumferentially
divided into infinite many parts. Each part was taken as a cantilever beam. By calculating the internal bending moment and
the second moment of area, the deflection angle and radial displacement of scraper were obtained. By definition, wax breaking
force is actually the axial components of wax–pig interaction,
and wax removal efficiency is the ratio of wax area swept by
pig to its original sectional area. Then a predicting model of
wax breaking force and wax removal efficiency of disk and
cup pigs was established
8
>
< F w ¼ jσ n jA cos γ þ jτn jA sin γ
(8)
ðδw ΔhÞð2R Δh δw Þ
>
:η ¼
δw ð2R δw Þ
where F w is the wax breaking force (N); σ n is the normal stress at
pig–wax interface (Pa); τn is the shearing stress (Pa); A is the contact area between wax and pig (m2); γ is the disk scraper rake
angle; δw is the thickness of wax layer (m); Δh is the scraper-pipe
gap width (m); and R is the pipe radius (m).
This model features the incorporation of scraper deformation
and wax rheology in the absence of adjustable parameters.
Verification using the pigging facility in Figure 8 showed that
the predicted and experimental results match well. It is worth
noting that due to the disparities in pig geometry, expressions
of some parameters in Equation (8) are different for disk and
cup pigs.
4.4.2. Application in Pig Design
Li et al.[54] utilized the developed model (Equation (8)) to improve
pig design by investigating the impact of pig geometry on wax
removal. Variations of calculated wax breaking force and wax
removal efficiency of disk and cup pigs against these geometrical
parameters in different hypothesized cases are shown in Table 1.
Recommendations on pig design are aimed to decrease the wax
breaking force and improve the wax removal efficiency.
4.5. Experiment with Real Wax Deposit
4.4.1. Model Development
Based on the fact that there exists a shear plane in wax
breakage,[48] Li et al.[53,54] introduced orthogonal cutting to
explain wax removal in pigging. Physical models of wax removal
using disk and cup pigs were established in Figure 3 and 4.
The wax area was divided into wax chip, plastic deformation
zone, and undamaged wax. Shear plane is the interface of plastic
deformation zone to undamaged wax. Shear stress reaches its
maximum on the shear plane.
Slip-line field theory was used for stress analysis. Initially, the
slip-line field was proved to uniformly distribute in the plastic
Energy Technol. 2020, 8, 1901412
Artificial wax samples instead of real wax deposit were widely
used in previous pigging experiments. Due to the difference
in physical properties, some conclusions drawn from artificial
wax may not be necessarily feasible to real wax deposit. For example, thickness, density, and yield stress of artificial wax uniformly
distribute in axial and radial directions of the pipe, whereas for
real wax deposit these parameters are uneven in both directions.
Therefore, force response of the pig in wax removal may be different for artificial wax sample and real wax deposit. To address
this, Li et al.[83] constructed a facility that allows pigging experiments with naturally deposited wax. It mainly consists of a flow
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Table 2. Physical property measurement of crude oil in front of the pig
during pigging.[86]
Standard density
[kg m3]
Gel point
[ C]
Viscosity
[mPa s]a)
Remark
08:40
842.2
20
10.01
Blank oil
10:30
847.8
22
–
11:00
848.8
9
10.47
11:20
867.1
14
396.60
11:23
883.3
15
459.10
11:28
880.0
16
722.10
11:33
890.2
23
1601.00
11:37
891.9
23
2075.00
11:40
893.9
24
2360.00
11:45
890.1
26
2834.00
11:50
892.8
26
2729.00
11:52
895.3
21
1524.00
18.24
Sampling
time
12:00
853.6
5
12:10
848.5
3
13.22
12:29
848.0
13
10.06
13:00
848.1
13
10.95
14:00
847.8
17
11.00
Depositcontaminated
oil
Blank oil
Viscosity test temperature is 25 C, shear rate is 20 s1.
a)
loop apparatus with a detachable wax deposition section and the
wax removal equipment in Figure 8 for pigging simulation.
It was verified that the four stages of wax removal drawn from
artificial wax still apply for naturally deposited wax.
5. Field Validation of Wax Deposition by Pigging
Validation of predicted wax layer distribution along the pipe is
essentially important for planning transportation and pigging
schemes. Conventionally, it is realized by calculating the pressure drop and flow rate. However, this method has some
shortcomings. For one thing, the obtained wax layer thickness
is an average of the whole pipe, whereas actual wax deposit usually distributes along the pipe in a nonuniform manner. Thus,
the calculated result does not reflect the actual volume of deposited wax.[84] For another, if wax deposition is weak, the increase
in pressure drop and decrease in flow rate caused by flow area
shrinkage might be so insignificant as to be covered up by normal fluctuation and nonconstant condition during production.
In response, Wang et al.[85] proposed a practical method to validate wax layer distribution by measuring the volume of scraped
wax deposit in pigging. Specifically, the contaminated crude
before the pig was densely sampled at intervals of few minutes
during pigging operation. Oil sample density, gel point, and
viscosity were tested. As shown in Table 2, these properties vary
dramatically for samples collected at different times. After pigging, wax deposit particles were sampled from the pig receiver,
and then added into uncontaminated oil with different wax/oil
ratios. Viscosities of the obtained wax-in-oil slurries were next
measured. By checking the viscosities of field-collected contaminated crude oils to those of the prepared wax-in-oil slurries,
wax/oil ratios of the collected oil samples can be calculated.
With information of flow rate and time interval, the volume
of scraped wax deposit was determined to be 176.4 m3. The
volume predicted by a wax deposition model developed by
Huang[42,43] was 195.6 m3. Given that a pigging operation could
not achieve a complete clearance and some wax particles remain
undissolved and suspend in the bulk flow, this result is quite
satisfactory. In another work, Wang et al.[86] used gel point as
indicator. The measured and predicted volumes of deposited
wax were 65 and 73 m3, respectively.
6. Future Challenges
Wax deposit strength is of paramount importance for pipeline
pigging. Until now, feasible wax deposit strength model is still
lacking. Indeed, the mechanical hardness of wax deposit is
mainly determined by flow condition and oil property. Current
attention is mostly paid on estimating the yield tress of wax–oil
gel from wax content.[65–67] The impacts of wax crystal morphology and carbon number distribution were just qualitatively
investigated. Quantitatively modeling wax deposit strength from
Table 1. Summary of variations of wax breaking force and wax removal efficiency of disk and cup pigs against pig geometries and corresponding pig
design recommendations.
Geometrical parameter
Disk pig
Variation
Cup pig
Design recommendation
Variation
Design recommendation
Force
Efficiency
Force
Efficiency
Increase first then decrease
Increase
Appropriately thick scraper
Increase
Increase
Avoid thick scraper
Clamping rate
Irregular
Increase
Determined by wax breaking
force-clamping rate profile
and oil pressure
Increase
Increase
Appropriately large clamping rate
Original rake
angle (cup pig)
–
–
–
Decrease
Decrease
Small original rake angle for high pressure pipe,
large original rake angle for low pressure pipe
Straight part
length (cup pig)
–
–
–
Increase
Increase
Long straight part length for high pressure pipe,
short straight part length for low pressure pipe
Scraper thickness
Energy Technol. 2020, 8, 1901412
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flow condition and oil property is urgently required by industry,
but remains a long way off.
Moreover, wax deposit rheology including viscoelasticity
and thixotropy is completely missing in current wax removal
research. Yield stress has long been taken as the only rheological
property that affects wax–pig interaction in pigging. This causes
some unrealistic problems. For example, authors have implemented the assumption of flat shear plane in wax removal,[48,53,54]
but if the irreversible thixotropy of wax layer is incorporated, we
can easily conclude that the shear surface should be a curve.
Therefore, comprehensive consideration of wax deposit rheology
in pigging needs to be vastly improved.
Third, as aforementioned, foam pig is widely used in field
pigging operations. It is initially sent into the pipe from small
to large to remove the soft oil gels before launching other types
of pig to clean the hard wax deposit. However, advances in this
regard are rather limited. We found no theoretical method to
calculate the wax breaking force and wax removal efficiency of
foam pig. Despite the tremendous success of foam pigs in field
practice, they were performed just heavily based on experience
and the risk of pig stalling and wax blockage remains high.
Therefore, understanding the wax removal mechanism of foam
pig, incorporating its complex deformation, and accurately predicting its wax breaking force and wax removal efficiency, should
never be absent from future wax removal research.
Another demanding problem to be considered is to evaluate
the critical condition of wax plug formation, i.e., the point when a
pig gets stuck by wax plug. This is the chief culprit for the high
risk of wax blockage in field pigging operations. During pigging,
the removed wax would suspend in the bulk flow, forming
wax-in-oil slurry. Under some specific conditions, the removed
wax would settle down and accumulate within the pipe, forming
wax plug. If the pump cannot provide high enough pressure to
push the plug to the next station, wax blockage occurs. To reduce
the risk of wax settling and buildup, bypass pigs with jet holes are
often used to flush the removed wax particles. This process is
highly correlated to fluid rheology and flow condition.
Unfortunately, although some models have been developed to
predict the viscosity of suspension systems,[87–89] knowledge
on rheological property of wax-in-oil slurry before the pig
is rather limited. Moreover, lack of information of factors affecting wax particle motion, such as fluid rheology, wax particle
geometry, and property, makes simulating wax–oil interaction
quite challenging, especially at the existence of bypass jet.
Evaluating the wax plug formation condition, thereby, should
be a demanding concern.
7. Conclusions
This article presents a comprehensive review of development of
wax removal research in pipeline pigging operations on crude oil
transportation system. In general, the ultimate goal of works in
this field is to reduce the risks of pig stalling and wax blockage;
therefore, wax breaking force and wax removal efficiency become
key concerns. Based on the aforementioned reviews and analysis,
the following can be concluded: 1) Wax layer thickness and
strength are decisive properties affecting wax removal. It is
widely accepted that molecular diffusion governs wax deposition.
Energy Technol. 2020, 8, 1901412
Multiple models were accordingly developed to estimate wax
deposition rate. But modeling wax deposit strength from flow
condition and oil property is still lacking. 2) In early works,
wax removal was intuitively taken as a compression or shear process. Quite a few wax breaking force models were accordingly
proposed, but they were mostly empirical correlations or simple
force balances. The shear plane observed in wax breakage suggests that orthogonal cutting is suitable to describe wax removal
in pigging. 3) Based on experimental results, the four stages of
wax removal proposed by Wang et al.[79] were confirmed by later
researchers. It was reported that wax layer failure stress is linearly
correlated to its yield stress, which formed the basis for an
empirical wax breaking force model. In addition, nondimensional analysis was adopted in modeling removal efficiency. In
our following works, orthogonal cutting and slip-line field theory
were introduced in a theoretical wax breaking model; no adjustable parameters were involved. Application of this model in pig
geometry optimization was also given in detail. Lastly, we present
a practical wax deposition validation method. It is realized by analyzing the changes in viscosity and gel point of wax-in-oil slurry
in front of the pig. 4) The current picture of wax removal research
is far from perfect. Several aspects are expected to be supplemented in future work, including modeling wax deposit strength
from flow condition and oil property, incorporating wax layer viscoelasticity and thixotropy, investigating wax removal with foam
pigs, and evaluating the critical condition of wax plug formation.
Acknowledgements
The authors gratefully acknowledge the financial support by National
Natural Science Foundation of China (NNSF, grant no. 51534007). The
permission statements were updated on June 8, 2020 after initial online
publication.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
deposit properties, pipeline pigging, wax breaking forces, wax removal
efficiency, wax removal mechanisms
Received: December 7, 2019
Revised: March 1, 2020
Published online: April 28, 2020
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