Bypass pigging is a promising strategy to improve pipeline flow assurance by eliminating pigging-generated slugs and reducing pig velocity. This paper provides a comprehensive analysis of the fundamentals, recent progress, and prospects of bypass pigging for enhancing pigging safety and efficiency in gas pipeline systems. A model of bypass pigging motion is developed based on momentum balance, incorporating key factors that affect performance. Recent studies of the influence of the bypass fraction, pressure drop coefficient, and friction force on pigging performance are discussed. The pressure drop coefficient, crucial for accurate dynamic pigging simulation, depends primarily on the pig bypass structure. The impact of variations in the bypass fraction on pig velocity, a significant factor affecting pigging performance, is analyzed. Higher bypass fractions lead to lower pig velocities, resulting in improved pigging efficiency. However, the risk of pig blockage increases owing to the decreased driving gas force at a higher bypass fraction. Therefore, the use of bypass pigs with anti-blocking capability is necessary to enhance overall flow assurance. The paper also highlights the quantifiable benefits of bypass pigging in reducing pig velocity and the pigging-generated slug volume. The prospects for further development of bypass pigging are also discussed. This study aims to comprehensively elucidate the bypass pigging strategy, promoting its wider implementation in natural gas pipelines to enhance pigging efficiency and safety.

Pipeline networks are considered to be the most important and cost-effective approach for energy transmission in the oil and gas industry.1,2 However, pipeline systems frequently encounter flow assurance problems due to corrosion, liquid loading accumulation, wax deposition, etc.,3,4 and pipeline flow assurance is therefore a crucial issue in oil and gas fields.5,6 In particular, with the extension of energy exploitation to deep waters,7 pipeline networks are becoming increasingly complex, leading to more complicated flow conditions.3 Regular cleaning of pipelines is therefore of great importance to ensure high transportation efficiency.8 Periodic pigging is one of the most effective ways of doing this, and it is widely implemented in the oil and gas industry. Generally, this method uses a cylinder-like or spherical pipeline inspection gauge (PIG), which is propelled from the rear by a driving fluid.9,10 However, it should be noted that in the conventional pigging process in a natural gas pipeline, as illustrated in Fig. 1(a), the pig moves with the same velocity as the driving gas, which can lead to an excessively high pig velocity, which can result in damage to the pipeline. In fact, pigging performance is closely related to the pig velocity. As well as posing a threat to the integrity of pipeline internal coatings, a high pig velocity can considerably reduce pigging efficiency. Another problem that is encountered in conventional pigging operations is the generation of a slug in front of the pig. The accumulated slug volume can exceed the processing capacity of terminal slug catchers, leading to accidental overflows and consequent interruption of normal production. In view of these shortcomings of traditional pigging, a new approach is necessary to reduce the pig velocity and improve the safety and efficiency of pigging in gas pipelines. With this aim in mind, the bypass pigging technique was proposed,3 in which a bypass port is incorporated into the pig body to allow a portion of gas to go into the downstream pipeline section as shown in Fig. 1(b). This can greatly decrease the driving force of the pig and reduce the pig velocity. With the reduction in pig velocity and the dispersion of the bypassing gas, pigging-generated slug accumulation can be considerably reduced or even eliminated. In this way, it is possible to avoid the overflow accidents at terminal slug catchers that occur with traditional pigging and eliminate the consequent production deferment.11,12

FIG. 1.

Comparison of (a) conventional pigging and (b) bypass pigging.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

FIG. 1.

Comparison of (a) conventional pigging and (b) bypass pigging.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

Close modal

In light of the promising benefits to be had from the application of bypass pigging in oil and gas pipeline systems, there have been numerous studies of this technique,11,13–20 and extensive research has evaluated the performance of bypass pigging in both the laboratory and engineering practice.21–25 As the pig velocity is one of the most important factors influencing pigging performance,26 there have been a number of studies of velocity control strategies for bypass pigging.22,27,28 For instance, an active velocity regulation method using a rotatable bypass valve to control the bypass fraction was proposed by Zhu et al.29 Chen et al.3,4,30 carried out a thorough study of the motion characteristics of bypass pigging by conducting extensive laboratory tests on single-phase gas and two-phase gas–liquid experimental pipeline systems. Potential benefits of bypass pigging in mitigating pigging-induced slug flow have also been investigated numerically.30 To provide guidance for velocity-controlled pig design, the oscillatory behavior of pigs was studied experimentally under low-pressure conditions.31 With regard to the key factors affecting pigging performance, the pressure drop coefficient, which primarily depends on the structure of the pig bypass, has an obvious influence on pig motion,32,33 and therefore needs to be determined in advance of detailed pigging simulations in numerical tools for engineering applications, such as OLGA34 and LedaFlow.35 Recently, Chen et al.8 evaluated pressure drop coefficients for a self-regulated bypass pig prototype with an internal regulating module, with the aim of providing guidance for engineering simulations of a new bypass pigging strategy. The friction force between the pig disk and the inner wall surfaces of the pipeline is another factor that has a great impact on pig motion. In this context, a static experiment was designed to evaluate the friction force of a sealing disk and investigate the effect of lubrication on this force.36 Zhu et al.37–39 employed both finite element (FE) calculations and pull tests to study the characteristics of the friction force in the pigging process. Recent studies have also examined the possibility of utilizing the bypass pigging strategy to eliminating severe slugging in offshore riser systems,40 with the aim of enhancing pipeline flow assurance and improving the overall production efficiency of deep-water oil and gas exploitation.

Despite the progress that has been achieved so far, there is still a lack of comprehensive assessments of bypass pigging with regard to its working principles, recent development, and potential to improving pigging safety and efficiency for natural gas pipelines. It should be noted that the major problems associated with bypass pigging that hinder its wider application are related to the issue of pigs being stuck in pipeline systems, which can cause accidents and severe disruption to production. With this in mind, the present paper provides a thorough review of progress in bypass pigging techniques, as well as introducing a self-regulated bypass pig with anti-blocking capability that has the potential for wide application in natural gas pipeline systems. The main contributions of the paper are as follows. First, the three key structural factors with significant effects on pig motion, namely, the bypass fraction, pressure drop coefficient, and friction force, are comprehensively summarized, and their influence on overall pigging performance is quantified. Second, the quantitative benefits of bypass pigging in reducing pig velocity and slug volume are analyzed, and the results of this analysis provide valuable insights into the potential effectiveness of bypass pigging from the perspectives of both laboratory experiments and engineering practice. Finally, prospects for future research on bypass pigging are discussed, and a self-regulated bypass pig is introduced that possesses an anti-blocking capability sufficient to ensure safe and efficient pigging operations.

Figure 2 shows the forces acting on a simplified bypass pig traveling through an inclined pipeline section with velocity Vpig, driven by gas with velocity Vg. During its motion, the pig is subjected to the driving force exerted by the gas at its rear, the friction force between the pig disk and the inner surface of the pipeline, the force due to gravity, and the shearing force of the driving gas exerted on the bypass port. To obtain the key factors with a significant influence on the overall pigging performance, it is necessary to establish an equation describing the pig velocity.

FIG. 2.

Schematic of bypass pig motion.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

FIG. 2.

Schematic of bypass pig motion.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

Close modal
From momentum balance during pig motion,4,
mdVpigdt=P1P2ApipeAh+FsmgsinαFfric,
(1)
where m is the mass of the pig (kg) and Vpig is its velocity (m/s), Apipe and Ah are the sectional areas of the pipeline and the bypass port, respectively (m2), P1 and P2 are the pressures on the rear and front ends of the pig (Pa), Fs is the shearing force of the driving gas acting on the bypass port (N), and Ffric is the friction force between the pig disks and the inner wall of the pipe (N). The shearing force Fs is given by
Fs=τsπdL=AhP1P2,
(2)
where τs is the shearing force of the driving gas on unit area of the surface of the bypass port (N/m2), and L and d are the length and diameter, respectively, of the bypass port (m). The pressure drop of the driving gas bypassing through the pig is given by32,
P1P2=12KρbpVbp2,
(3)
where K is the pressure drop coefficient (which depends on the port structure of the given bypass pig), ρbp is the driving gas density in the bypass port (kg/m3), and Vbp is the driving gas velocity passing through the bypass port relative to that of the pig (m/s). Vbp can be calculated by using the gas continuity equation:22 
Vbp=VgVpigφ,
(4)
where φ is the bypass fraction parameter, which is equivalent to Ah/Apipe, and Vg is the rear gas velocity (m/s).
On combining the above equations, it is found that the pig velocity can be described by the following equation:
mdVpigdt=12KρbpVgVpig2Apipeφ2mgsinαFfric.
(5)
If we assume that the pig is in a steady state in a horizontal pipe system, then this equation can be simplified to
Vpig=Vgφ2FfricApipeρbpK,
(6)
from which it can be seen that the pig velocity Vpig is determined collectively by the rear gas velocity Vg and the pig structural parameters φ, Ffric, and K, as well as by the pipeline size Apipe. Conventional and bypass pigs differ in the introduction of the bypass structure in the latter. If no bypass port is present in the pig body, the velocity of the pig will be equivalent to that of the rear driving gas. Hence, the key to controlling the pig velocity is through regulation of the bypass fraction. It should be noted that the bypass pig can encounter some increased resistance during its motion, owing, for example to pipe irregularities such as diameter deformation and to accumulation of debris, which can lead to an increased friction force Ffric and thus an increased risk of the pig being blocked in the pipeline (Vpig = 0). In such a scenario, as shown by Eq. (6), if the bypass fraction φ is decreased by an appropriate amount depending on the pig’s state of motion, its velocity can be recovered to the normal range for efficient pigging performance.

For pigging in a given gas pipeline, the pigging performance is determined primarily by the pig velocity, and, as can be seen from Eq. (6), there are three key factors related to the bypass structure that have a significant influence on the pig velocity, namely, the bypass fraction, the pressure drop coefficient, and the friction force. Accordingly, in this section, the current understanding of these three indicators will be presented, with a particular focus on their quantitative impact on pigging performance.

The bypass fraction φ = Ah/Apipe, defined as the ratio of the minimum sectional area Ah of the pig’s bypass port to the sectional area Apipe of the pipeline, is one of the most important structural parameters that differentiates the pigging performances of conventional and bypass pigs. The presence of a bypass port in a pig, and thus a nonzero bypass fraction, leads to an effective decrease in pig speed, thereby improving flow assurance and pigging efficiency. Specifically, as the bypass fraction increases, the amount of driving gas that is able to bypass the pig body increases, and so the driving force on the pig body decreases, leading to a lower pig velocity. By contrast, as the bypass fraction decreases, less driving gas can bypass the pig body, which is consequently subjected to a greater driving force, leading to an increase in its velocity. In this context, to quantify the influence of the bypass fraction in reducing pig velocity, Chen et al.4 designed the experimental rig shown in Fig. 3(a), using a transparent horizontal pipeline to visualize the whole pigging process in a single-phase gas system. The pigging section had total length of just over 15.8 m, with a diameter of 64 mm. To implement pigging, a bypass pig prototype was designed as shown in Fig. 3(b). By changing the bypass nozzle in the front of the pig body, bypass fractions could be adjusted from 0% to 5%. The results for the average pig velocity at different gas rates and bypass fractions are shown in Fig. 3(c). It is evident that for a given bypass fraction, the average pig velocity changes linearly with the flow rate of the driving gas. At a given gas flow rate, as the bypass fraction increases, the pig velocity is noticeably reduced. It should be noted that the extrapolation of each curve in Fig. 3(a) to its intersection with the horizontal axis indicates the minimum driving gas flow rate required to start the pig. The velocity difference between the driving gas and the pig is shown in Fig. 3(d). It is evident that for a bypass pig with a given bypass fraction, the velocity difference remains constant regardless of the driving gas flow rate. In particular, for a conventional pig without a bypass port (i.e., with a bypass fraction of zero), the difference between gas and pig velocities is zero, indicating that the pig moves with the same velocity as the rear driving gas. With increasing bypass fraction, the velocity difference increases.

FIG. 3.

Bypass pigging system to characterize pig velocity. (a) Experimental pigging system. (b) Prototype of bypass pig. (c) Pig velocity vs driving gas flow rates for different bypass fractions. (d) Difference between the gas and pig velocities vs driving gas flow rates for different bypass fractions.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

FIG. 3.

Bypass pigging system to characterize pig velocity. (a) Experimental pigging system. (b) Prototype of bypass pig. (c) Pig velocity vs driving gas flow rates for different bypass fractions. (d) Difference between the gas and pig velocities vs driving gas flow rates for different bypass fractions.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

Close modal

Practical engineering pigging operations have also demonstrated successful application of pigs incorporating bypass ports. For instance, an optimal bypass fraction of 12% has been adopted for pigs in a Malaysia liquefied natural gas (LNG) trunkline system with a length of 121 km and pipeline diameter of 36 in. and has resulted in considerable benefits, with substantial reductions in production deferment.41 A bypass fraction of 5% has been adopted for pigging in a pipeline in China11 and has greatly reduced the production deferment from 3 days to 20 h. Therefore, from both laboratory experiments and practical experience, it is clear that the use of pigs incorporating a bypass port enables a significant improvement in pigging performance. The appropriate bypass fraction should be determined from dynamic pigging simulations conducted before practical pigging operations commence.

As can be seen from Eq. (6), the pressure drop coefficient K is an important factor influencing the pigging process. From Eq. (3), the pressure drop coefficient can be expressed as
K=P1P212ρbpVbp2,
(7)
which represents the ratio of the pressure drop of the driving gas across the pig body to the average dynamic pressure of the gas in the bypass port. The pressure drop coefficient is a crucial parameter providing a quantitative distinction between bypass pigging and conventional pigging.

According to the results of previous studies,8,32,33 the value of K depends primarily on the parameter of the bypass structure in a bypass pig. Hence, a given bypass pig structure corresponds to a certain pressure drop coefficient. A higher value of K corresponds to a greater pressure drop of the driving gas across the bypass pig and thus a higher pig velocity. The pig velocity is sensitive to variations in the value of K, which can thus have a noticeable influence on the motion of the pig.32 Usually, for practical pigging operations, it is necessary to provide in advance the pressure drop coefficient of the utilized bypass pig, obtained through the use of numerical simulation tools such as LedaFlow.35 In this context, determination of the values of K for different bypass pig structures in different bypass pig prototypes is able to provide significant guidance for pigging applications in engineering practice.

Previous research has found that the value of K depends primarily on the structure of the bypass port, although it is also affected by the driving gas flow rate in the bypass port.32,33 Azpiroz et al.33 investigated the values of K for a typical bypass pig with or without a deflector disk. In some bypass pig configurations, a deflector disk is installed at the exit of the bypass hole and can be used to increase the force exerted by the driving gas on the pig, thus starting the pig in its launcher. A deflector disk can also be used to distribute corrosion inhibitors onto the inner surface of the pipeline. In view of the advantages of equipping bypass pigs with deflector disks, it is important to investigate the corresponding pressure drop coefficients. Using a computational fluid dynamics (CFD)-based method, Azpiroz et al.33 found that the value of K of a bypass pig with a deflector disk can be increased to as high as 4. By contrast, the values of K for commonly used bypass pigs without a deflector disk range from 1 to 1.5. This result has significant practical implications for the application of bypass pigs equipped with a deflector plate. Hendrix et al.32 proposed a building block approach to more extensively investigate the values of K for different bypass pig structures. Such an approach can also be employed to simulate the pressure loss coefficients for complicated flow geometries.

Chen et al.8 investigated the values of K for a novel self-regulated bypass pig with a spring-controlled bypass regulating valve. They employed a CFD-based method with the pig modeled using the mesh shown in Fig. 4(a) and examined the effects of driving gas flow rate, bypass fraction, and the presence of the internal regulating module on the values of K. As shown in Fig. 4(b), K grows rapidly as the bypass fraction increases, but remains nearly unchanged under variations in the relative velocity between the driving gas and the pig, which indicates that K is influenced primarily by the bypass fraction and hardly affected by driving gas flow conditions. Accordingly, in preliminary feasibility analyses of pigging operations, the value of K should be determined according to the selected optimal bypass fraction. Notably, as shown in Fig. 4(c), by comparing the values of K for bypass pigs with or without the regulating module, it was found that the value of K for a bypass pig with a built-in bypass regulating module is noticeably higher than that in the absence of a bypass valve. This difference between these two types of pigs increases considerably with increasing bypass fraction. In comparison with a bypass pig with a simple bypass structure, the presence of an internal regulating module induces an additional drop in gas pressure, thereby greatly increasing the value of K. Specifically, as the bypass fraction increases to 7%, the values of K for bypass pigs without and with a regulating module are 1.03 and 1.97, respectively, representing a difference of more than 91%.

FIG. 4.

Investigation of pressure drop coefficient K for a self-regulated bypass pig equipped with an internal regulating valve. (a) Model mesh generation. (b) Variations in K with bypass fraction and with relative velocity of driving gas and pig. (c) Comparison of values of K between pigs with and without an internal valve.8 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 107, 104776 (2022). Copyright 2024 Elsevier.

FIG. 4.

Investigation of pressure drop coefficient K for a self-regulated bypass pig equipped with an internal regulating valve. (a) Model mesh generation. (b) Variations in K with bypass fraction and with relative velocity of driving gas and pig. (c) Comparison of values of K between pigs with and without an internal valve.8 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 107, 104776 (2022). Copyright 2024 Elsevier.

Close modal

The bypass fraction and pressure drop coefficient are correlated and both have a significant influence on pigging performance, and it is therefore necessary to conduct a sensitivity analysis to further study their influence on pig velocity. Chen et al.8 considered the basic operational pigging parameters and established a relationship between pig velocity, pressure drop coefficient, and the bypass fraction, as presented in Fig. 5. It can be seen that for a bypass pig under given pipeline operational conditions, the pig velocity is influenced collectively by the K value and the bypass fraction. The pig velocity decreases rapidly with increasing bypass fraction. By contrast, however, there is relatively little change in the pig velocity as the value of K increases. Each point in Fig. 5 represents a balance of the forces exerted on the pig body, corresponding to steady-state motion of the pig. It is obvious that large bypass fraction as well as a small value of K can result in a pig velocity of zero, causing an accident in which the pig becomes stuck. Hence, correct determination of bypass size and structure before pigging operations are carried out is essential to provide efficient and safe pigging with guaranteed flow assurance.

FIG. 5.

Relationship between pig velocity, pressure drop coefficient, and bypass fraction.8 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 107, 104776 (2022). Copyright 2024 Elsevier.

FIG. 5.

Relationship between pig velocity, pressure drop coefficient, and bypass fraction.8 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 107, 104776 (2022). Copyright 2024 Elsevier.

Close modal

As one of the key factors determining the pig velocity, the friction force between the pig disks and the pipe walls should also be determined for a given bypass pig. The friction force, which is a required input parameter for pigging simulation tools such as OLGA34 and LedaFlow,35 also needs to be determined in advance of dynamic pigging simulations to evaluate the effectiveness of pigging performance. With regard to the effect of friction force on pig velocity, Chen et al.4 evaluated the variations in pig velocity for various bypass fractions and pig friction forces, with the results shown in Fig. 6. As is evident, with increasing friction force, the pig velocity is notably reduced. Each point on the surfaces in Fig. 6 represents a balanced state of all the forces exerted on the pig. As shown in Fig. 6(a), if the bypass fraction and friction force are kept constant, the gas driving force cannot get the pig into motion if the gas flow rate induces a relative velocity that corresponds to a point lying below the surface. Therefore, the points on the surface in Fig. 6(a) show the minimum driving gas velocities required to start the pig. For a gas velocity of 15 m/s, the relationship between pig velocity, friction force, and bypass fraction is demonstrated in Fig. 6(b). Again, each point on the surface corresponds to steady-state pig motion. At high values of the bypass fraction and friction force, the pig velocity can be reduced to zero, indicating that the pig is blocked in the pipeline. Hence, for a given pipeline system, these surface maps can be used to elucidate the correlation of pig velocity with friction force and bypass fraction.

FIG. 6.

Effects of friction force and bypass fraction on pig velocity. (a) Relative velocity between driving gas and bypass pig for different bypass fractions and friction forces. (b) Pig velocity for different bypass fractions and friction forces.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

FIG. 6.

Effects of friction force and bypass fraction on pig velocity. (a) Relative velocity between driving gas and bypass pig for different bypass fractions and friction forces. (b) Pig velocity for different bypass fractions and friction forces.4 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 73, 103059 (2020). Copyright 2024 Elsevier.

Close modal

Figure 7 shows a schematic of a pig dragging experiment that was performed to measure the friction force on the pig’s sealing disks as they were deformed by the restricted geometry of the pipeline.37 To provide a drag force, the pig was pulled through the pipeline from the inlet by a cable that was attached to its front and rolled onto a drawworks, whose speed was monitored electronically. Once the pig had reached and maintained a constant velocity, the drag force was taken to be equivalent to the friction force and was recorded by a data collection unit.

FIG. 7.

Pig dragging experiment to determine the friction force.37 Reprinted with permission from Zhu et al., J. Nat. Gas Sci. Eng. 23, 127–138 (2015). Copyright 2024 Elsevier.

FIG. 7.

Pig dragging experiment to determine the friction force.37 Reprinted with permission from Zhu et al., J. Nat. Gas Sci. Eng. 23, 127–138 (2015). Copyright 2024 Elsevier.

Close modal

In addition to experimental studies, numerical methods have also been proposed to evaluate the friction force on a pig. Zhu et al.37 proposed a 2D nonlinear model to simulate the pig contact force and compared the results of this model with those of a linear model in the case of complex motion of a pig in a pipeline.38 The influence of the sealing disk’s parameters (thickness, interference, clamping rate, and chamfer dimension) on the pig contact force has been studied, and the results have provided valuable information on the basis of which theoretical models for predicting the friction force between pig disks and pipe walls can be formulated.

In summary, the motion of a pig in a pipeline is influenced by three main factors, all of which depend on the structural parameters of the pig. Specifically, the bypass fraction is determined by the minimum size of the bypass port in the pig body, and it has a significant impact on the pig velocity and overall pigging performance. To ensure smooth pigging operations, one of the primary objectives of dynamic bypass pigging simulation is optimization of the bypass fraction for a specific oil or gas pipeline system. The pressure drop coefficient is determined primarily by the geometry of the bypass port. To calculate this coefficient, a CFD-based numerical approach is often employed, especially for pipelines with complex bypass structures. The pressure drop coefficient is a crucial parameter that needs to be specified as an input variable for dynamic pigging simulations. Furthermore, the friction force between the pig disks and the inner walls of the pipeline plays a crucial role in determining the effectiveness of bypass pigging operations. Although preliminary determination of the friction force can be achieved through dragging experiments or numerical models that relate this force to the pig’s structural parameters, it should be noted that the actual friction force can vary in response to the motion of the pig within the pipeline system. Thus, accurate prediction of the variations of friction forces during the dynamic pigging process remains a challenge. Optimizing the bypass fraction, determining the pressure drop coefficient, and accurately predicting the variations of friction forces are therefore essential tasks for ensuring efficient and effective pigging operations in oil and gas pipelines. Further research and development are required to address these challenges and enhance our understanding of the dynamics of pig motion.

The potential benefits of bypass pigging include reduction in pig velocity, elimination of production deferment, and reduction of the pigging-generated slug volume. The advantages derived from bypass pigging can significantly enhance flow assurance for oil and gas pipelines. In this section, both the benefits arising from successful engineering applications and the results from laboratory experiments will be described to reveal the full potentials for the use of bypass pigging as a strategy to improve pigging safety and efficiency.

There have been a number of engineering applications of bypass pigging that have achieved satisfactory pigging performance in a production context. A case history of successful bypass pigging operations is presented in Ref. 41, which describes an application of bypass pigging in a trunkline system of Malaysia LNG (MLNG), where significant production benefits have been achieved at near-zero cost. In this application, after dynamic pigging simulations to evaluate the overall pigging performance in a trunkline system of length 121 km and diameter 36 in., the optimal bypass fraction was determined to be 12%, which enabled a significant reduction in production deferment. In one year, a total of nine pigging operations were carried out on this pipeline with remarkable results. Notably, bypass pigging has now become the standard pigging strategy for this pipeline. Successful field experience with bypass pigging in China in 2012 has also been reported.11 This pipeline had a diameter of 24 in. and a length of 23.5 km. Owing to the fact that the terminal central treatment plant had no slug catcher, conventional pigging operation without bypass was originally adopted, which resulted in pigging-induced slug volumes of up to 1000 m3 and production deferments of three days. After a detailed analysis, a bypass pig equipped with a deflector plate and an optimal bypass fraction of 5% was adopted and used to carry out pigging operations. This enabled a 60% reduction in pigging-generated slug volume to 420 m3, and the production delay was reduced to 20 h, thereby significantly improving production efficiency. Bypass pigging has also been applied in a wet gas/condensate subsea pipeline of diameter 20.6 in. and length 10 km, running from a wellhead platform to a central processing platform.20 A typical bypass pig without a deflector plate and with an optimal bypass fraction of 8% was used for the pigging operation. A total of three bypass pigging operations were carried out, with very good results that enhance the flow assurance of the pipeline.

With regard to amelioration of pigging-induced liquid accumulation though the use of the bypass pigging technique, Chen et al.30 conducted an experimental study in the two-phase gas–liquid bypass pigging system shown in Fig. 8(a), in which a terminal liquid receiver was installed to receive the pigging-generated liquid volume from the pipeline. The results showed that owing to the gas–liquid carrying effects occurring during the bypass pigging process, a more manageable liquid arrival profile could be achieved. For instance, as shown in Fig. 8(b) for bypass pigging under a liquid loading volume of 14 l, with bypass fractions increasing from 0% to 2%, the liquid outflow rate was decreased by 75%, from 4.8 to 1.2 cm/s. Hence, by increasing the bypass fraction, it is possible to greatly extend the liquid outflow time and thus reduce the liquid outflow rate. Furthermore, as demonstrated in Fig. 8(c), for different volumes of liquid loading in the pipeline, bypass pigging is able to reduce the pigging-generated slug volume. For volumes of liquid loading in the pipeline ranging from 6.3 to 20.9 liters, increasing the bypass fraction from 0% to 1% reduces the liquid outflow rate by amounts ranging from 38.4% to 47.6%. These results confirm the benefits of bypass pigging in reducing the large pigging-generated slug volume to keep it within the capacity of terminal slug catchers and thus avoid overflow accidents, thereby greatly enhancing pipeline flow assurance.

FIG. 8.

Effects of bypass pigging in reducing pigging-induced slug flow. (a) Schematic of experimental system. (b) Effect of bypass fraction on liquid outflow under a liquid loading of 14 l. (c) Effect of liquid loading on liquid outflow under a gas flow rate of 7 m/s.30 Reprinted with permission from Chen et al., Ocean Eng. 198, 106974 (2020). Copyright 2024 Elsevier.

FIG. 8.

Effects of bypass pigging in reducing pigging-induced slug flow. (a) Schematic of experimental system. (b) Effect of bypass fraction on liquid outflow under a liquid loading of 14 l. (c) Effect of liquid loading on liquid outflow under a gas flow rate of 7 m/s.30 Reprinted with permission from Chen et al., Ocean Eng. 198, 106974 (2020). Copyright 2024 Elsevier.

Close modal

Although bypass pigging has shown promising benefits in improving pipeline flow assurance, pigging safety, and efficiency, there are still challenges that need to be addressed to enable its widespread implementation in natural gas pipeline systems. On the basis of existing research, the future prospects for bypass pigging can be detailed as follows.

First and foremost, the security concerns associated with bypass pigging operations require further attention. One of the primary concerns is the risk of blockage accidents. Although bypass pigging effectively reduces pig traveling velocity, resulting in significant advantages such as elimination of production deferment and reduction of pigging-generated slug volume, the challenge lies in preventing pigs from getting stuck in the pipeline. During the pigging process, it is inevitable that increased resistance will be encountered due to pipeline deformation, irregularities, or debris accumulation. If a bypass pig maintains the same bypass fraction, the rear gas force acting on the pig body may prove insufficient to overcome this increased resistance, resulting in blockage. This can lead to severe production problems and economic losses for the entire oil and gas field. A potential solution to this issue involves designing a regulating module in the bypass port to adjust the bypass fraction based on the pig’s state of motion. Recently, Chen et al.9 proposed a self-regulated bypass pig incorporating a regulating valve to provide enhanced anti-blocking capability during pigging processes (see Fig. 9). The working principle of this bypass pig design is as follows.9 As the pig travels through the pipeline under specified conditions with the optimal bypass fraction, the bypass module remains stationary at a pre-set position. However, if the pig encounters increased resistance that reduces its velocity, the bypass regulating module automatically moves forward driven by the increased rear gas force acting on the bypass valve. This adjustment continues until the pig’s velocity returns to its normal pre-set range. As a result, this novel bypass pig possesses anti-blocking capabilities that enhance pigging safety. It is worth noting that this self-regulation relies primarily on the design of the bypass module, as shown in Fig. 9. The effectiveness of this design is dependent on the compatibility of the compression spring and the bypass regulating valve. The spring stiffness must be appropriately determined to respond to the gas forces acting on the bypass valve during normal pig motion and when the pig is stuck in the pipeline. Thus, the controlling spring exerts an adjustable force on the regulating valve in accordance with the pig’s state of motion.

FIG. 9.

Schematic of self-regulated bypass pig: 1, bypass nozzle; 2, guiding disk; 3, spacing disk; 4, sealing disk; 5, steel skeleton; 6, metal pressing plate; 7, bypass regulating valve; 8, fastening bolt; 9, compression spring; 10, 11, supporting plates.9 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 108, 104850 (2022). Copyright 2024 Elsevier.

FIG. 9.

Schematic of self-regulated bypass pig: 1, bypass nozzle; 2, guiding disk; 3, spacing disk; 4, sealing disk; 5, steel skeleton; 6, metal pressing plate; 7, bypass regulating valve; 8, fastening bolt; 9, compression spring; 10, 11, supporting plates.9 Reprinted with permission from Chen et al., J. Nat. Gas Sci. Eng. 108, 104850 (2022). Copyright 2024 Elsevier.

Close modal

Second, it is necessary to improve the accuracy of predicting pigging-induced slug size and duration in bypass pigging operations. It is important to recognize that bypass pigging involves complex fluid dynamics. To address this, upgrading pigging simulation tools to incorporate bypass pigging components can significantly improve the accuracy of predicting pigging parameters, including the pig velocity, duration, and volume of pigging-generated slugs generated. Additionally, further research on the pressure drop coefficient and pig friction force is crucial, since these are essential predetermined parameters for dynamic pigging simulation. By studying these factors in relation to specific bypass pigs, it is possible to further improve the accuracy of pigging simulations.

Third, given that the pig velocity plays a crucial role in determining pigging performance, it is important to direct further research toward pig velocity control, encompassing both active and passive control strategies. An active control strategy will involve utilizing the power input to artificially regulate the opening of the bypass valve. On the other hand, a passive control strategy will entail the incorporation of an internal bypass controlling module to adjust the bypass fraction according to the pig’s state of motion.

It is important to highlight the significance of optimizing the pig’s structural parameters, such as the bypass fraction and the internal regulating module, for practical bypass pigging operations with this self-regulated bypass pig. This optimization is crucial to ensure pigging safety and achieve optimal effectiveness in specific oil and gas pipeline systems. To accomplish this, dynamic pigging simulations will be conducted based on the production parameters of the targeted gas fields and pipeline systems, as well as the physical parameters of the pig. These simulations will thoroughly evaluate the effectiveness of the selected bypass fraction in reducing pigging-induced slug volume and controlling the pig velocity within appropriate ranges. Through iterative simulations, the bypass fraction can be optimized. Once the optimal bypass fraction has been determined, the controlling spring for the internal module can be designed on the basis of the valve force analysis approach described in Ref. 9. Such a design will ensure compatibility with the regulating valve, enabling automatic adjustment of the bypass fraction in response to the pig’s state of motion. By integrating these optimization processes into practical bypass pigging operations, the safety and effectiveness of pigging can be enhanced, leading to improved performance in oil and gas pipeline systems.

This study has presented comprehensive analyses of the development of bypass pigging technology, including its working principles, recent progress, and potential benefits in improving pigging safety and efficiency. An equation describing the pig velocity has been established based on the momentum balance of bypass pigging motion, incorporating the key factors affecting the pig velocity, namely, the bypass fraction, pressure drop coefficient, and pig friction force. The introduction of a bypass structure distinguishes bypass pigging from conventional pigging, and the control of pig velocity relies on regulation of the bypass fraction. Three key factors influencing bypass pigging performance have been detailed, and laboratory experiments have revealed that the pig velocity depends linearly on the rear driving gas velocity at a given bypass fraction. Increasing the bypass fraction significantly reduces the pig velocity, enhancing pigging safety and production efficiency. The pressure drop coefficient, determined primarily by the structural parameters of the bypass port, can reach a value of 4 for a bypass pig with a deflector disk, while it ranges from 1 to 1.5 for commonly used bypass pigs without a deflector plate. The inclusion of an internal regulating module in a self-regulated bypass pig significantly increases the pressure drop coefficient by over 91% compared with a bypass pig equipped only with a bypass structure. The friction force, a dominant factor in determining the pig velocity, can be determined through pig dragging experiments or numerical approaches for a given bypass pig.

The potential benefits of bypass pigging include reduced pig velocity, elimination of production deferment, and reduced pigging-generated slug volume. Successful engineering applications have demonstrated its efficacy in enhancing pigging safety and efficiency, as well as preventing production deferment and overflow accidents in terminal slug catchers. In fact, one trunkline system has adopted bypass pigging as the standard pigging strategy for its pipelines. However, it is important to note that the possibility of blockage accidents caused by pigs getting stuck in pipelines requires further attention. A novel self-regulated bypass pig with anti-blocking potential represents a promising solution, although further optimization of its design and tests of its practical application in pipeline systems are necessary. Future research should focus on accurate prediction of pigging-induced slug size, pig control strategies, optimizing the bypass module, and conducting pigging tests and performance evaluations using the bypass pig prototype in real pipelines. Addressing security concerns and developing self-regulated bypass pigging techniques are crucial steps toward enhancing the safety and effectiveness of bypass pigging operations in natural gas pipeline systems.

This work contributes to enriching fundamental understanding of bypass pigging and provides a comprehensive overview of the state of the art of this technique. It can serve as a valuable resource for further development and application of this novel strategy in oil and gas fields.

The authors acknowledge financial support provided by the National Natural Science Foundation of China (Grant Nos. 52074342 and 51874341).

The authors have no conflicts to disclose.

Qingping Li: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Haiyuan Yao: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing – review & editing (equal). Jianheng Chen: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Xiaoming Luo: Formal analysis (equal); Investigation (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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