Advances and challenges in oil and gas pipeline pigging technology: a comprehensive review

Abstract

In the oil and gas industry, pipeline transportation is crucial for ensuring energy supply. However, problems such as internal deposits and corrosion in pipelines can undermine their safe, efficient, and stable operation. Pipeline pigging technology is essential for maintaining pipelines, but existing pigging technology faces challenges in dealing with complex deposition situations and ensuring cleaning effects while minimizing pipeline damage. To address these issues, this study comprehensively reviews pipeline pigging technology. It combines experimental research and numerical simulation methods. Through a series of experiments on bypass pigs under different pipeline environments and working conditions, key factors like bypass fraction, flow rate, and pressure that affect the pigging process were identified. The construction of various mathematical models for numerical simulation further explored the impacts of factors such as bypass diameter, fluid properties, and pipeline geometry on the flow behavior, cleaning efficiency, and dynamic speed of the pig. This paper offers a comprehensive resource for the design, selection, and optimization of pipeline pigs. It also advances the development of pipeline pigging technology, which is of great significance for ensuring the safe and efficient operation of oil and gas pipelines.

Introduction

Since the commissioning of the world’s first oil and gas pipeline in 1865, the pipeline construction industry has grown significantly¹. Currently, the total pipeline mileage exceeds 2 million kilometers. Oil and gas are primarily transported via pipelines and trucks, with pipeline transportation being the dominant means for long-distance conveyance². It is widely recognized as the preferred option for fluid transport^3,4 and the most dependable method for ensuring safety^5–7. In the energy sector, oil and gas pipelines serve as indispensable arteries⁸ that not only facilitate the seamless transport of hydrocarbon resources from extraction sites to processing facilities and end markets but also play a pivotal role in the global energy supply chain, as highlighted by Huang et al.⁹ in their comprehensive life cycle assessment of oil product pipeline systems. They play a vital role in safeguarding national energy security and fueling economic development^10,11. However, as oil¹² and gas fields¹³ continue to develop and pipelines operate over long periods, issues such as corrosion from impurities and dirt¹⁴, and wax deposition can endanger the safe, efficient, and stable operation of pipelines¹⁵. Furthermore, corrosive salts and acidic compounds in pipeline deposits not only affect pipeline transportation but also migrate to downstream refinery units. Field studies confirm that chloride-rich residues from crude oil hydrolyze into hydrochloric acid during distillation, causing severe overhead system corrosion¹⁴. For example, vacuum distillation columns exhibit material degradation at weld joints due to ammonium chloride crystallization under stagnant flow conditions¹⁶. It is worth noting that despite such surface defects, the parent pipe material and girth weld still significantly exceed the target fatigue life in actual operation, highlighting the inherent durability of the material¹⁷. Consequently, pipeline pigging technology has gained significance. Regular pipeline cleaning and inspection are crucial for ensuring efficient pipeline operation¹⁸.

Oil and gas pipeline pigging technology involves using pigs that travel within the pipelines to eliminate accumulated corrosion products¹⁹, deposited liquids²⁰, and dirt²¹ through physical or chemical means. Originating in the twentieth century, it has gained prominence since the successful development of pigging devices by the Knapp Company and the Girard Company in 1962. Due to its wide cleaning range, adaptability to long pipelines, environmental friendliness, and high cost-effectiveness, it has become the main pipeline-cleaning tool²². Pigging uses fluid to propel piston-shaped objects for cleaning pipeline inner walls²³. Pigs are vital for pipeline cleaning and maintenance²⁴,and their application has become an industry standard²⁵. They mainly clear hydrate blockages, corrosion products, and wax deposits²⁶ during pipeline operation. This helps maintain transportation efficiency and safety²⁷, extend pipeline service life, and reduce maintenance²⁸ and replacement costs^29,30.

However, pigs also encounter challenges during operation³¹. Excessive deposits can lead to blockages³², poor cleaning results, and low tracking accuracy³³. Moreover, different pipelines vary greatly in materials, sizes, transported media, and operating conditions. This variation places greater demands on the design, selection, and application of pigs³⁴.

In the field of oil and gas pipeline pigging technology, bypass pigs have received a great deal of attention. Their bypass channel design is unique. As the bypass pig moves through the pipeline, it can adjust the flow rate skillfully according to the operating conditions³⁵. When it encounters deposits and impurity accumulations³⁶, it redirects the fluid through the bypass. This changes the surrounding flow field and effectively cleans the pipe wall, thus improving the pigging efficiency³⁷.

During the research, advanced simulation software is used to study the impact of the size, shape³⁸, number, and layout of bypass openings on the pigging result. After that, the structural design is refined repeatedly to balance pigging efficiency and energy consumption.

In large-scale global oil and gas pipeline networks, bypass pigs show outstanding adaptability. They can operate effectively in long-distance crude oil and natural gas trunk lines, as well as in complex-terrain branch lines with different pipe diameters³⁹. By adapting to on-site conditions, they can efficiently remove impurities such as wax⁴⁰ and corrosion products, ensuring smooth and stable oil and gas transportation and supporting the stability of the storage and transportation industry⁴¹.

Since pipeline pigging technology is essential for the safe and efficient operation of pipelines^42,43, this paper comprehensively introduces the basic principles, main types, experimental analyses, and influencing factors of bypass pigs. It also explores the challenges and future trends of this technology. The aim is to provide valuable perspectives and inspiration for the secure and efficient operation of oil and gas pipelines.

Working principle of pig

The term “pig” refers to any device that moves through pipelines due to the hydrodynamic force within them⁴⁴. In oil and gas pipelines, pigs are mainly used to remove deposits like wax⁴⁵ and liquid⁴⁶. This helps prevent blockages⁴⁷ and leaks⁴⁸, ensuring efficient pipeline operation. Although pipelines vary in their applications and the products they transport across different industries, the primary reasons for using pigs are consistent.

At the start of a pigging operation, the pig is inserted into the pipeline⁴⁹. Its design allows it to create a seal, which separates the pressure in the sections before and after it. Liquid or gas is then used as a power source⁵⁰ to create a pressure difference that propels the pig forward⁵¹. As the pig moves along the pipeline, it adheres to the inner wall. Made of elastic material and with a special design, it cleans the pipeline’s interior by scraping, brushing, and pushing out impurities⁵². After the cleaning is complete, the removed deposits are pushed out at the pipeline’s other end. The pig is then caught by a receiving device, which is equipped with limiting and buffering equipment to ensure a safe stop and facilitate maintenance and replacement⁴⁴. This marks the completion of the pigging process.

In pipeline pigging operations, there are two methods for driving a pig. One approach utilizes the pressure difference of the fluid within the pipeline. This method is suitable for short-distance pipelines. The other method relies on the pressure change resulting from the leakage around the pig and is primarily employed for cleaning long-distance pipelines. Mirshamsi et al.²² proposed two formulas for calculating the pressure difference before and after the pig, taking into account the presence or absence of friction in the case of the pressure-difference-driven method. The motion equation of the pig in a frictionless pipeline is as follows:

1

The motion equation of the pig in a pipeline with friction is:

2

where represents the tangential acceleration of the pig. denotes the dry friction force. stands for the gravitational acceleration. and are the fluid forces acting on the front and rear ends of the pig respectively. is the curve equation of the centerline of the pipeline pig and and are the positions of the front and rear ends of the pig in the x-direction. represents the mass of the pipeline pig. represents the density of the pipeline pig.

Nguyen et al.⁵³ proposed the pressure-difference relationship before and after the bypass holes of a bypass pig when considering the flow through these holes. The relationship is presented as follows:

3

where

4

represents the pressure at the trailing end of the pig, represents the pressure at the leading end of the pig. is the fluid velocity at the valve and is the velocity of the pig. represents the acceleration due to gravity. represents the pressure loss resulting from the sudden contraction at the end of the pig. is the average loss coefficient of the valve. stands for the pressure loss generated by the sudden expansion at the end of the pig. is the total loss coefficient of the bypass system.

Hendrix et al.⁵⁴ took into account the impact of the pig bypass flow on its motion in the simplified model section. The resulting motion equation is presented below:

5

where represents the flow rate through the bypass. is the frictional force during the motion of the pig. denotes the diameter of the bypass hole. and represent the mass and position of the pig respectively. represents the initial mass at . is the cross-sectional area of the pipeline. and . represents time. represents the pressure at the outlet. represents the constant mass flow rate at the inlet.

Types of pig

Based on the working principles of pigs, various types have been developed to meet different pipeline cleaning and inspection needs. Tools used in the pigging process mainly include pigs, pig launchers, pig receivers, detection equipment⁴⁴, and auxiliary tools like traction equipment, pipeline cutting and welding equipment, and cleaning tools. Among these, the pig is the core component.

Pigs can be broadly classified into two categories according to their functions and applications: mechanical pigs and intelligent pigs. Mechanical pigs are typically used for pipeline descaling and cleaning. In contrast, intelligent pigs employ technologies such as eddy current⁵⁵, ultrasound⁵⁶ and magnetic flux leakage⁵⁷ to perform internal inspections of pipelines. These inspections can detect deformations, corrosions, and mechanical damages within the pipelines^17,58, accurately pinpoint problem areas, and ensure pipeline transportation safety. The comparative characteristics of these pig types are summarized in Table 1.

Table 1.

Pig type and characteristics.

Type	Name	Characteristic	Function
Mechanical Pig	Pigging Balls	Made of rubber or polyurethane, hollow. Wall-thickness is 10% of pipe diameter; interference controlled by water injection59	Remove the accumulated liquid in pipeline or separate media60
	Foam Pig	Conical–cylindrical, made of polyurethane. High elasticity, toughness, and wear-resistance. Interference is 5–10% of pipe diameter, with deformation over 50%61	Cleans pipelines with inner-wall coatings, and can absorb water and dry62
	Cup Pig	Steel structure with polyurethane cups. High hardness and good flexibility63	Judges pipeline pass ability and removes internal deposits64
	Straight plate Pig	Steel structure with polyurethane cups. Support plate diameter is smaller than pipeline inner diameter; sealing plate diameter is larger, with certain interference65	Suitable for pipelines that are hard to open after blockage and difficult to shut down during operation
	Bypass Pig	Composed of a rigid framework, sealing plate, guide plate, and bypass valve67. Pressure-relief valve opening and closing are automatically controlled by differential pressure83	Cleans pipelines with much scale, large-volume accumulated liquid, or those needing regular maintenance69
Intelligent Pig	Magnetic Flux Leakage Detection Pig	Rigid structure with detectors, sensors, drive, and electronic systems73	Detects cracks, corrosion, and wall-surface defects84
	Ultrasonic Intelligent Pig	Pig body, ultrasonic generator, power supply, and protection device76	Detects pipeline wall-surface defects85
	Eddy current testing Pig	Pig body, eddy current sensors, signal processing units, and power devices86	Detects pipeline wall-surface defects and their positions and sizes87

Mechanical pig

Pigging balls

Pigs are made of rubber or polyurethane materials⁵⁹. They are hollow, with a wall thickness of 10% of the pipeline’s diameter. The quantity of water injection is regulated via the one-way valve on the water injection and exhaust holes of the pigging balls, thereby controlling the interference of the balls. Pigs are primarily utilized to remove accumulated liquid or separate media within the pipeline⁶⁰. However, their capacity to remove lumps is relatively limited.

Foam pig

The foam pig, crafted from polyurethane⁶¹, has a conical–cylindrical shape. It exhibits high elasticity, toughness, and wear-resistance. Its interference amount ranges from 5 to 10% of the pipe diameter, and it can deform by more than 50%. Endowed with strong capabilities, even when encountering a blockage, it can overcome it through its high deformability. Alternatively, an increase in pressure can cause it to break and clear the blockage independently. The foam pig is well-suited for cleaning pipelines with an inner-wall coating or for water-absorption and drying operations during the initial stage of pipeline construction⁶². Its disadvantages are one-time use, short running distance and general ability to remove debris.

Cup pig

This type of pig consists of a support plate, cups, spacers, and a framework. The support plate, cups, and spacers are made of polyurethane⁶³, a material with high hardness⁶⁰ and good flexibility. Inside the pipeline, the pig is propelled forward by the flowing medium. Cleaning devices can be attached to the pig’s framework to clean the pipeline. This pig is used to initially assess the pipeline’s pass ability and remove internal deposits⁶⁴. To ensure the pig can navigate through elbows and tees, there is a minimum distance requirement between its front and rear cups. Through years of theoretical calculations and experiments, it has been established that the distance between the two cups should not be less than the pipeline’s inner diameter. The total length of the pig can be determined based on the number of cups and the framework diameter. The cups’ interference is typically 3–4% of the pipe diameter.

Straight plate pig (bidirectional pig)

The straight-plate pig resembles the cup-type pig in having a rigid framework. However, its distinct feature is that the cup part of the cup-type pig is replaced by disc-shaped straight plates in the straight-plate pig. Functionally, these straight plates can be divided into support plates and sealing plates. The support plate, with a diameter smaller than the pipeline’s inner diameter, mainly provides support and guidance. Conversely, the sealing plate is slightly larger than the pipeline’s inner diameter. This size variation results in an interference fit, crucial for achieving an effective seal⁶⁵.

The structure of the straight-plate pig enables bidirectional movement, allowing it to efficiently remove internal pipeline dirt. It is frequently utilized in pipelines where post-blockage opening is inconvenient or where halting transportation after operation commencement is not feasible. The straight-plate pig finds extensive use during the initial pipeline construction phase. This is due to the complex conditions of newly-built pipelines. In case of blockages or other issues, back-blowing can be implemented to clear blockages and mitigate risks.

Bypass pig

The bypass pig generally has a bypass or overflow channel⁶⁶ and a sturdy shell that encloses and safeguards its other components. A crucial element of the bypass pig is the pressure relief valve. This valve plays a key role in regulating the internal pressure of the pipeline during the pigging operation⁶⁷. Typically, it is composed of parts such as a spring-adjusted gland, a differential-pressure-adjusted spring, a sealed valve body, and a pressure-releasing component⁶⁸.

Bypass pigs are primarily employed for cleaning pipelines with significant scale accumulation, large volumes of liquid deposits⁶⁹, or those requiring routine maintenance. During operation, due to its unique design, when the pressure acting on the pig reaches a pre-determined level, the pressure relief valve opens. This enables the transported medium to flow through the pig, carrying away the deposits in front of it. Consequently, the resistance in front of the pig decreases, and the pressure difference within the pipeline is reduced. Subsequently, the valve closes, and the pig continues its movement⁵³.

Recent studies also demonstrate how intelligent pigs can integrate corrosion monitoring capabilities, particularly for cathodic protection systems in pipelines. For instance, field inspections of overhead piping revealed that insufficient material selection and dissolved oxygen control accelerate sulfide stress cracking (SSC), compromising structural integrity. Advanced pigs equipped with electrochemical probes could detect such anomalies by correlating cathodic potential shifts with localized corrosion sites.

Intelligent pig

Magnetic flux leakage detection pig

During pipeline operation, the internal medium and external environment can cause issues such as corrosion, cracks, and hydrogen embrittlement. These problems may lead to pipeline ruptures, resulting in the leakage of the internal medium, which poses potential safety risks and causes property losses.

Magnetic flux leakage detection pigs utilize the magnetic flux leakage principle to detect surface and near-surface defects like cracks⁷⁰ and corrosion. In contrast, ultrasonic intelligent pigs are better suited for detecting internal defects deep within the pipe wall. Eddy current testing pigs are highly sensitive to defects on the pipe wall surface. Based on the electromagnetic induction principle, they can precisely determine the location and size of these defects^71,72. These pigs are mainly composed of detectors, sensors, drive systems, and electronic systems⁷³.

When in use, they can perform magnetic flux leakage detection on the inner and outer wall defects of the pipeline⁷⁴. The basic principle is to magnetize ferromagnetic materials. Hall sensors placed between the two poles then sense the strength and offset of the magnetic field leaking outside the test piece, enabling the determination of defect information, including the location, area, and severity of the defects⁷⁵.

Ultrasonic intelligent pig

In oil and gas pipelines, as the transported medium may cause special corrosive damage³¹. to the pipe wall, it is easy to create potential safety hazards. However, the ultrasonic intelligent pig can accurately detect these tiny cracks and provide an important basis for the safety assessment of pipelines. It is mainly composed of an ultrasonic generator, a power supply device and a protection device⁷⁶. The body can be made of different materials according to different pipelines, and it is used for detecting defects on the pipe wall^77,78.

Ultrasonic testing is a technique used by intelligent pigs for pipeline inspection. It utilizes sound waves, taking advantage of the unique characteristics of ultrasonic waves within the pipe wall. Intelligent pigs equipped with ultrasonic transducers emit ultrasonic pulses towards the pipe wall. As these ultrasonic waves propagate through the pipe wall, they are reflected when they encounter the inner and outer surfaces of the pipe wall or the interfaces of defects. Subsequently, the trans ducers detect these reflected waves. By analyzing parameters such as the time and intensity of the reflected waves, the intelligent pig can calculate crucial information, including the thickness of the pipe wall and the depth of defects⁷⁹. This process, along with other inspection techniques like eddy current and magnetic flux leakage, enables intelligent pigs to comprehensively assess pipeline conditions, ensuring the safe and efficient operation of pipelines⁸⁰.

Eddy current testing pig

Eddy current detection cleaning equipment, like ultrasonic intelligent cleaning equipment and magnetic leakage detection cleaning equipment, has been developed in response to the potential safety risks associated with transporting materials that can easily damage pipelines. The eddy current detection pig operates mainly on the principle of electromagnetic induction. It consists of a pig body, an eddy current sensor, a signal processing unit, a power device, and other components. When the pipeline cleaner moves within the pipeline, an alternating current is applied to the excitation coil inside it. This generates an alternating magnetic field in the pipeline wall. According to the electromagnetic induction law, this alternating magnetic field induces eddy currents within the pipeline wall. If there are defects in the pipeline wall, the distribution and intensity of these eddy currents will change. The detection coil on the pig can sense the magnetic field variations caused by the eddy currents and convert these magnetic field changes into electrical signals. By analyzing the changes in these electrical signals, the defect conditions of the pipeline wall, including the location and size of the defects, can be determined^81,82.

Recent studies also demonstrate how intelligent pigs can integrate corrosion monitoring capabilities, particularly for cathodic protection systems in pipelines. For instance, field inspections of overhead piping revealed that insufficient material selection and dissolved oxygen control accelerate sulfide stress cracking (SSC)⁸⁸. compromising structural integrity. Advanced pigs equipped with electrochemical probes could detect such anomalies by correlating cathodic potential shifts with localized corrosion sites^89,90.

Research experiments on pig

Among the diverse types of pigs, the bypass pig has drawn significant attention because of its distinct structure and outstanding performance in complex pipeline scenarios. As a result, a series of experimental investigations have been conducted on it. Experimental research on pigs is essential for a profound understanding of pig performance, optimizing its design, and broadening its application areas. Through numerous rigorous and systematic experiments, it becomes possible to precisely analyze the behavior of pigs under different pipeline environments, transported substances, and operating conditions.

To precisely define the velocity characteristics of the bypass pig in gas pipelines, Chen et al.⁹¹ developed an experimental setup. This system combined transparent pipelines, a novel bypass pig (Fig. 1) and an infrared detection circuit to explore the performance of the pig. As shown in Fig. 2, the average velocity of the pig rises linearly with the increase in the driving air flow rate. Significantly, the slope of this linear correlation reflects the pig’s speed sensitivity to air flow rate changes. An increase in the bypass fraction causes a marked and progressive decline in velocity, as evident from the downward-sloping curves associated with different bypass fractions. By calculating and comparing the segmented average velocities of the pig without bypass and with different bypass ratios, it was discovered that at low air flow rates, the pig’s velocity fluctuated minimally. Conversely, at high air flow rates, the fluctuation became more pronounced. However, augmenting the bypass fraction could enhance the pig’s operational stability.

Fig. 1.

Structure of the experimental bypass pig prototype⁹¹.

Fig. 2.

Relationship between average pig velocity and driving gas flow rate. (a) Average pig velocity versus average driving gas flow rate. (b) Differences between average driving gas flow rate and pig velocity⁹¹.

Furthermore, the study compared the signal variations of infrared diodes and pressure sensors (Figs. 3, 4). The experimental results demonstrated that the infrared detection method was accurate, reliable, cost-effective, and simple to construct.

Fig. 3.

Sectional pig velocity with bypass fraction of 1%. (a–c) Pig velocity of V24, V46, V26, respectively⁹¹.

Fig. 4.

Sectional pig velocity with bypass fraction of 2%. (a–c) Pig velocity of V24, V46, V26, respectively⁹¹.

To solve the motion control issue of bypass pigs under low-pressure conditions, Hendrix et al.⁵⁴ conducted a systematic experimental study on the motion behavior of bypass pigs in horizontal low-pressure gas pipelines. Their focus was on the average and maximum speeds of the pigs under different bypass areas and upstream flow velocity conditions.

In the experiment, air served as the working fluid. The experiment was carried out in a 52 mm diameter and 62 m long pipeline under normal pressure (as shown in Fig. 5). The researchers analyzed in detail how the bypass area affected the speed of the bypass pig and verified the validity of a simplified model (Figs. 6, 7). The results indicated that as the bypass area increased, the pig’s speed decreased, and the simplified model could predict the pig’s maximum speed accurately. Moreover, based on these experimental findings, the paper proposed a PD controller to reduce the speed fluctuations of the pig. This research provides an important theoretical foundation and design reference for controlling the speed of bypass pigs in low-pressure pipelines.

Fig. 5.

(a) Overview of the flow loop. (b) Close-up of the pig launcher. (c) Drawing of the pig⁵⁴.

Fig. 6.

Pig velocity as function of upstream bulk velocity (a) Configuration 1, (b) Configuration 2. The dashed lines denote the standard deviation in the calculated pig velocity⁵⁴.

Fig. 7.

Maximum pig velocity versus average pig velocity; (a) Experimental conditions. (b) Higher friction (10 times) compared to experimental conditions⁵⁴.

The motion characteristics of bypass pigging were explored in the gas–liquid two-phase flow system within large-scale indoor pipes. Chen et al.⁹² conducted an experimental analysis of the phenomena, pressure fluctuations, pig speed changes, and liquid production volume during the bypass pigging process. They constructed an experimental loop (Fig. 8) with horizontal, upward-inclined, downward-inclined, and vertical riser pipe structures. The loop was equipped with compressors, flow meters, and other equipment. The experimental steps included adjusting the gas flow rate, injecting liquid, collecting data while starting the pig to record parameters. To ensure the reliability of the results, the experiments were repeated with variable changes. During the experiments, phenomena such as severe wear of the pig cups, stagnation at the bottom of the riser and stick–slip motion were observed. The characteristics of pressure fluctuations were also studied, including the fluctuation curves (Fig. 9) and the influence of the bypass fraction and liquid loading volume on them (Fig. 10). The results showed that the bypass pig demonstrated greater adaptability to pressure fluctuations. Additionally, the pig’s speed and liquid loading characteristics were analyzed, offering rich data and in-depth insights for the study of bypass pigging technology.

Fig. 8.

Experimental rig⁹².

Fig. 9.

Pressure fluctuation during pig running⁹².

Fig. 10.

Pressure fluctuation versus time at different liquid volume⁹².

By designing and constructing the experimental equipment shown in Fig. 11 to measure the actual performance characteristics of the torque. Zhu et al.⁹³ conducted a theoretical discussion on the torque characteristics generated by the fluid on the bypass valve. The experimental apparatus included an air compressor, an air storage tank, pressure sensors, torque sensors and a data acquisition unit, etc. By adjusting the opening degree of the bypass valve, the actual working conditions were simulated. The experiment found that when the valve was opened, the pressure in the air storage tank dropped rapidly, and the pressure difference across the bypass valve and the load torque increased first and then decreased. And the fluctuations were more intense when the opening degree increased. As shown in Fig. 12, when the opening degree was fixed, the load torque had a linear relationship with the pressure difference, and the slope increased as the opening degree increased. When the pressure difference was fixed, the load torque had an exponential relationship with the opening degree, and the fluid promoted the closing of the bypass valve. The pressure difference also had an exponential relationship with the opening degree. When the opening degree changed from 0 to 20%, the pressure difference dropped rapidly and then slowed down (Fig. 13). This provides a reference for the speed control algorithm of the pig to achieve more stable operation.

Fig. 11.

Schematic diagram of the experiment⁹³.

Fig. 12.

The relationship between the differential pressure over the pig and the load torque acting on bypass-valve at the opening of (a) 0.387°, (b) 0.774°, (c) combined results of 0.387°, 0.774°,1.16° and 1.549°⁹³.

Fig. 13.

The relationship between the opening of bypass-valve and differential pressure over bypass-valve⁹³.

To investigate the slip phenomenon during the bypass pigging process, Li et al.⁹⁴ established an experimental setup as depicted in Fig. 14. The system comprised a plexiglass pipe with an inner diameter of 64.0 mm and a length of 15.026 m. This pipe was connected to a vortex flowmeter, and dry gas was utilized as the working fluid. Along the pipe, nine high-precision pressure sensors were installed to record pressure changes. The bypass pig used in the experiment was composed of multiple parts, and the test was conducted with a bypass ratio of 3%.

Fig. 14.

Bypass pigging experimental system⁹⁴.

The experimental results revealed that the movement of the pig within the pipeline led to substantial pressure fluctuations. These fluctuations gradually grew over time (Fig. 15), suggesting that the friction distribution inside the pipeline was non-uniform. Additionally, during the experiment, it was observed that the pig was likely to experience the slip phenomenon in the initial stage, and this tendency decreased as the pipeline length increased. Through these practical bypass pigging experiments, valuable data were provided to enhance the understanding of the motion characteristics and slip phenomenon of the bypass pig.

Fig. 15.

Pressure variations at pressure sensors a during the experiment and simulation at a gas flow of 0.0643 kg/s and a 3% bypass rate⁹⁴.

To investigate the motion characteristics of bypass pig in natural gas condensate pipelines, Luo et al.⁹⁵ constructed an experimental system with horizontal and vertical pipelines as shown in Fig. 16. The horizontal pipeline is 15.8 m long with an inner diameter of 64 mm and is made of plexiglass. Along this pipeline, pressure sensors and infrared diodes were installed. It was connected to a 4.8 m-long vertical pipeline by a carbon-steel elbow. The experimental apparatus also included compressors and buffer tanks, which were used to regulate the gas and liquid flow rates.

Fig. 16.

Procedure of gas–liquid two-phase bypass pigging test⁹⁵.

The bypass pig prototype had well-defined length, nozzle structure, and cup-body parameters, along with various excellent features. The study focused on the structural changes of the pig under different bypass ratios. The experimental variables encompassed the inlet gas and liquid superficial velocities and the bypass ratio. Cross-tests and repeated experiments were conducted. During the pigging process, data on flow rate, pressure, and liquid level were collected. The analysis revealed that the pig’s speed was linearly correlated with the driving gas speed (Fig. 17). In the gas–liquid two-phase flow, the pig’s speed change was influenced by multiple factors. The pressure-fluctuation characteristics were associated with the pig’s movement and liquid accumulation. Moreover, the bypass ratio had a significant effect on the evolution characteristics of the accumulated liquid at the pipeline’s terminal. These findings offer an important foundation for the application of bypass pigging technology.

Fig. 17.

Velocity correlation curve of driving gas and pig under different liquid superficial velocity⁹⁵.

To address the issue of production losses and delays resulting from liquid-related incidents during pigging operations in offshore oil and gas fields, Chen et al.⁹⁶ carried out gas–liquid two-phase bypass pigging experiments. They utilized a transparent horizontal pipeline system to simulate the bypass pigging process and monitored the pressure variations and the liquid outflow rate at the pipeline’s end during pigging.

As shown in Fig. 18, the bypass pigging technique can effectively minimize liquid accumulation during the pigging operation. It achieves this by reducing liquid build-up through the gas–liquid carrying function of the bypass port. The experiment also revealed that increasing the relative velocity of gas and liquid can remarkably enhance this improvement effect. Moreover, the pig’s speed has a major influence on the liquid surging volume (Fig. 19). By integrating the bypass pigging technology and increasing the liquid discharge rate, there is potential to prevent liquid spill accidents. In the experimental section, specific data and observation outcomes validated the effectiveness and feasibility of the bypass pigging technology in enhancing liquid flow during pigging. This provided fundamental data for the subsequent development of CFD (Computational Fluid Dynamics) models and liquid surging volume analysis models.

Fig. 18.

Pressure variations during pigging process. (a) P8 at gas flow rate of 5 m/s and bypass fraction of 0%. (b) P8 at gas flow rate of 10 m/s and bypass fraction of 0%. (c) P8 at gas flow rate of 10 m/s and bypass fraction of 2%. (d) Average pressure differences, which include the influence of gas flow rate: ΔP8 = P8 (10 m/s, 0%) − P8 (5 m/s, 0%) and the influence of bypass fraction: ΔP8 = P8 (10 m/s, 2%) − P8 (10 m/s, 0%)⁹⁶.

Fig. 19.

Surge volume versus pig velocity⁹⁶.

To address the problem of gas–liquid two-phase flow characteristics in the bypass state during the pigging process, Chen et al.⁹⁷ engaged in discussions and carried out a series of experiments. They used a transparent glass pipeline system to mimic the actual pipeline and employed monitoring devices such as infrared sensors to conduct real-time observations of the flow state.

As presented in the experimental results in Fig. 20, a gas–liquid two-phase flow state was present both in front of and behind the pig. Once the pig was set in motion, liquid leaked from the gap between the pig and the pipe wall. This leakage led to the formation of gas–liquid two-phase flow regions both upstream and downstream of the pig.

Fig. 20.

Liquid phase distribution⁹⁷.

Tu et al.⁹⁸ proposed a pig with a new hydraulic automatic speed control scheme and verified its performance through experiments. By using advanced testing equipment and methods, an in-depth exploration was conducted on the speed control system of the bypass pig with an adjustable bypass valve. The movement of the pig under different working conditions was simulated, and relevant parameters were measured (Fig. 21). Through simulating the movement of the pig under different parameters, the experiment analyzed the effect of the hydraulic system automatically adjusting the bypass valve to control the speed of the pig. The experimental results showed that the speed control system could effectively reduce speed fluctuations and improve the stability and safety of the pig’s operation in high-speed pipelines. In particular, the experiment studied in detail the influence of the length, diameter and opening angle of the bypass valve on the fluid loss coefficient. And verified the dynamic model of the pig’s movement by combining numerical simulation with experimental data.

Fig. 21.

Change in loss coefficient with opening angle⁹⁸.

To improve the flow assurance in natural gas pipelines, Chen et al.⁹⁹ developed a new self-regulating bypass pig prototype and experimentally explored its application in such pipelines. The experiment was conducted within a horizontal transparent pipeline system. Prior to introducing gas into the pipeline cleaner, the gas underwent pressurization, stabilization, filtration, and flow regulation. Test gases with varying bypass fractions ranging from 0 to 7% were utilized, and pressure sensors along with cameras were employed to record data.

The results indicated that as the bypass fraction increased, pressure fluctuations became more stable. For instance, when the bypass fraction rose from 0 to 3%, the probability density function (PDF) curve became more concentrated, and the average pig speed decreased by 64.3–81.5% (Fig. 22). The pressure drop coefficient was primarily influenced by structural parameters, and the self-regulating module caused an increase in it. For example, when the bypass fraction was 7%, the deviation in the pressure drop coefficient between the pig with a valve and without a valve reached 90.9% (Fig. 23).

Fig. 22.

Pressure variations of P1 and PDF analyses under different bypass fractions. (a) Pressure fluctuation curves. (b) Analyses on PDF of pressure values⁹⁹.

Fig. 23.

Comparisons of pressure drop coefficient of bypass pig with or without a regulating valve⁹⁹.

Researchers have built various experimental systems to conduct investigations into the behavioral traits of bypass pigs under different pipeline conditions and operating scenarios. Their studies covered aspects such as speed control, pressure fluctuations, slip phenomena, and the impact of gas–liquid two-phase flow. These efforts provided valuable data and a theoretical foundation for optimizing and applying bypass pigging technology.

However, these experiments have limitations. Some experimental conditions were idealized and did not fully account for all the complex factors present in actual industrial pipelines. Also, the generalizability of the experimental results needs further validation. Despite these drawbacks, these studies offer significant benefits. They have been able to uncover the operating principles of bypass pigs and suggest new detection and control methods. As a result, they have laid a strong groundwork for future research and technological advancements in this field.

Numerical simulation of pig

The experimental research on bypass cleaners yields valuable data regarding their performance. However, numerical simulation can complement this experimental work¹⁰⁰. It allows for the exploration of complex working conditions that are challenging to replicate in actual experiments. The operation of a pig within a pipeline is influenced by multiple factors, such as its speed¹⁰¹, the friction within the pipeline¹⁰², and the presence of deposits¹⁰³. These factors directly impact the efficiency, safety, and reliability of the pigging operation. Through numerical simulation of the pig’s movement in the pipeline, the precise characteristics of these factors during operation can be determined.

Mirshamsi et al.¹⁰⁴ concentrated on solving the problem of dynamic analysis and simulation of a pig in two- and three- dimensional gas pipelines within the oil and gas industry. They used the Runge–Kutta method to simulate the pig’s movement in both two- and three-dimensional pipelines. In the two-dimensional pipeline scenario, they set the pipeline curve equation, along with the parameters of the pig and the pipeline. The simulation results, as presented in Figs. 24 and 25, show that the pig’s speed and normal force vary periodically. For the three-dimensional pipeline, spiral pipes with the following specific parametric equations are selected:

6

where wet pipeline perimeter. a time dependent parameter in 3D. sign function of V. coefficient of friction loss in pipeline. mass of the pig. acceleration of gravity in direction.

Fig. 24.

Velocity of the pig for example 1¹⁰⁴.

Fig. 25.

Normal force exerted on the pig for example 1¹⁰⁴.

The simulation results indicate that this equation can effectively predict the pig’s position, speed, and instances of stagnation. For example, in the case of a helical-shaped pipeline, the pig’s speed changes under that specific geometry can be observed (Fig. 26), along with the trend of its position. This figure also reflects the speeds in different directions, effectively validating the model.

Fig. 26.

Simulation results of pipeline of the example 3¹⁰⁴.

Numerical simulation studies on pigging technology can be grouped into several categories. Regarding speed-control research, Liang et al.¹⁰⁵ designed a braking unit for pigs. They installed this braking device on the pig, leveraging the frictional force between the device and the pipeline wall. This friction generated a resistance that increased with the pig’s speed, thereby controlling the speed.

The researchers established a mathematical model that took into account factors like the resistance of the braking unit and the fluid pressure difference within the pipeline. They then used the Euler–Cauchy method to solve the speed-control equation. In Case 1, when considering the change in pipeline inclination, the results showed that the braking unit reduced the pig’s speed and made it more stable (Fig. 27). In Case 2, the simulation of the control system’s step response indicated that the braking unit allowed the pig to reach a stable speed more rapidly (Fig. 28). Furthermore, a sensitivity analysis revealed that parameters such as the number of wheels and pumps in the braking unit significantly influenced the results. For instance, an increase in the number of wheels could cause the pig’s speed to stabilize more quickly (Fig. 29).

Fig. 27.

Pig velocity of Case 1¹⁰⁵.

Fig. 28.

Pig velocity of Case 2¹⁰⁵.

Fig. 29.

The rate of speed increase as each parameter changes ± 50%¹⁰⁵.

Talbizadeh et al.¹⁰⁶ focused on flow behavior studies, using the oil pipeline between Rafsanjan, Yazd and Nain in Iran as a case. They thoroughly examined the effects of factors like the bypass diameter on the pig’s flow behavior, cleaning efficiency, and dynamic speed. Additionally, they optimized the pig’s design. To guarantee research accuracy, a second-level grid was chosen for simulation.

The simulation results, as shown in Fig. 30, revealed two types of flow field behaviors around the disc pig fitted with a deflector. In Behavior A, after the jet strikes the deflector, it moves along the disc and the pipeline wall, with a rotating region between the jet and the disc. In Behavior B, the jet directly touches the bottom wall of the pipeline, and the rotating regions are located between the pig body and the jet, as well as at the center of the pipeline’s bottom wall. Behavior B has a larger total pressure drop, mainly in the area behind the deflector. The maximum speed occurs when the jet hits the top of the deflector.

Fig. 30.

The fow pipelines and pressure contour for two different behavior of the flow around the disk pig¹⁰⁶.

Simulations of pigging with different bypass diameters (Fig. 31) indicated that as the bypass diameter increases, the pig’s average speed varies under different environmental conditions, with significant speed changes in specific path slope sections. After comprehensive analysis, it is suggested that the bypass diameter should not exceed 5.5 inches.

Fig. 31.

The variation of pig velocity in the pipeline between Rafsanjan, Yazd and Nain with different bypass diameters¹⁰⁶.

Zhang et al.¹⁰⁷ aimed to explore the movement of bypass pigs in hilly natural gas pipelines. They developed a mathematical model incorporating the pig’s dynamic equation and the gas flow equation. To solve the gas flow equation, they applied the Method of Characteristics and considered the reverse flow of bypass gas.

In the simulation of pigging in hilly pipelines, the results indicated that in the downhill section, both the pig’s speed and the bypass gas speed increased (Fig. 32). In the uphill section, the pig halted because of an insufficient pressure difference and restarted once the gas flowed backward. Moreover, the pressures at the front and rear of the pig, along with the pressure difference, changed correspondingly (Fig. 33). As shown in Fig. 34, for horizontal pipelines, parameters such as pipe diameter, gas velocity, bypass area, and friction notably affected the pig’s steady-state speed. Among these factors, gas velocity had the most substantial impact.

Fig. 32.

Pig speed vs. gas speed in the bypass distributing along simulation time¹⁰⁷.

Fig. 33.

Pressure on the nose and tail of the pig and the pressure difference during pigging¹⁰⁷.

Fig. 34.

Parametric sensitivity analysis of pig speed in the horizontal pipeline: (a) change of pig mass, (b) change of pipe diameter, (c) change of gas pressure, (d) change of inlet speed, (e) change of bypass area, and (f) change of friction force¹⁰⁷.

Boghi et al.¹⁰⁸ aimed to better understand the interaction between the pig and waxy oil slurry during bypass pigging. They conducted a three-dimensional numerical simulation to study the hydrodynamics of waxy oil slurry in this process. They developed a pig model that accounted for the momentum equation and pipe-wall friction, among other factors, and a hydrodynamics model using the drift flux model and considering turbulence. The simulation was run in OpenFOAM (Open Source Field Operation and Manipulation) software and solved through coupling with specific algorithms. Different working conditions, including various temperatures and particle diameters, were set for the simulation.

The simulation results indicated that the distribution of wax particles was influenced by temperature and particle diameter. At low temperatures, the wax particles were relatively uniformly distributed. In contrast, at high temperatures, stratification occurred or the particles were more dispersed (Fig. 35). The researchers also observed variations in the pig’s speed and acceleration, finding that the acceleration was related to factors like the square of the relative speed (Fig. 36). Turbulent kinetic energy behaved differently at different temperatures and affected jet characteristics (Fig. 37). Moreover, profiles of the mixture speed, viscosity, and drift speed changed with temperature.

Fig. 35.

Wax volume fraction field for 2 mm particle diameter at 60 s after the beginning of the process¹⁰⁸.

Fig. 36.

(a) Pig Velocity versus time; (b) Pig Acceleration vs relative velocity; (c) Pressure drop across the pig vs relative velocity. 2 mm particle diameter¹⁰⁸.

Fig. 37.

Section averaged wax volume fraction field for 2 mm particle diameter. (a) t = 15s; (b) t = 30s; (c) t = 45s; (d) t = 60s¹⁰⁸.

Boghi et al.¹⁰⁸ aimed to better understand the interaction between the pig and waxy oil slurry during bypass pigging. They conducted a three-dimensional numerical simulation to study the hydrodynamics of waxy oil slurry in this process. They developed a pig model that accounted for the momentum equation and pipe-wall friction, among other factors, and a hydrodynamics model using the drift flux model and considering turbulence. The simulation was run in OpenFOAM software and solved through coupling with specific algorithms. Different working conditions, including various temperatures and particle diameters, were set for the simulation.

The simulation results indicated that the distribution of wax particles was influenced by temperature and particle diameter. At low temperatures, the wax particles were relatively uniformly distributed. In contrast, at high temperatures, stratification occurred or the particles were more dispersed (Fig. 35). The researchers also observed variations in the pig’s speed and acceleration, finding that the acceleration was related to factors like the square of the relative speed (Fig. 36). Turbulent kinetic energy behaved differently at different temperatures and affected jet characteristics (Fig. 37). Moreover, profiles of the mixture speed, viscosity, and drift speed changed with temperature.

Pinto et al.¹⁰⁹ focused on solving liquid management problems in multiphase-flow pipelines and assessing the effectiveness of bypass pigging. They used OLGA Dynamic Multiphase Flow Simulator to perform a numerical simulation of bypass pigging on a 36- inch, 120- km submarine export trunk line. A model was built based on KBC Infochem Multi flash to describe fluid properties and other aspects, and a steady-state analysis was carried out to determine the liquid retention before pipeline cleaning.

During the calibration of the dynamic model, the pipeline roughness was adjusted to calibrate the hydraulic model. The static-force parameters of the pig and wall-friction parameters were determined to calibrate the bypass pig model, ensuring that model predictions matched field data. Model verification showed that the model reasonably followed on-site observation trends in terms of inlet pressure, condensate flow rate, and slug arrival time, etc. However, assumptions and simplifications in the slug-catcher modeling led to differences in liquid-level predictions (Fig. 38). In the pigging optimization stage, a sensitivity analysis of bypass pigging was carried out. For example, as shown in Table 2, considering factors such as different wall frictions and bypass ratios, it was concluded that a 10% bypass pig was more suitable for the current working conditions. Which could manage the slug volume without the risk of overflow, and its speed was approximately 41% lower than that of the standard pig, providing important guidance for on-site pigging operations.

Fig. 38.

Modelling versus Field Run Case 1—Comparison of Gas Flowrates and Slugcatcher Liquid Levels¹⁰⁹.

Table 2.

Bypass pigging optimization—summary of results¹⁰⁹.

Gas flowrate, MMSCFD	Bypass, %	Average pig velocity, m/s	Peak pig velocity, m/s	Pigging duratio, hr	Pig generated volume, m3 (bbl)	Slugcatcher inlet choke activated?
650	8 (low friction)	2.79	3.61	11.9	1795 (11,290)	Yes
	10 (low friction)	2.42	3.41	13.8	1565 (9854)	No

Saikiran Kollamgunta et al.¹¹⁰ aimed to solve the operational issues resulting from liquid slugs when oilfield facilities are upgraded or production rates change. They used the OLGA dynamic multiphase flow simulator to perform transient modeling and simulation of various pigging scenarios. These scenarios included conventional pigging under normal and reduced production rates, as well as bypass pigging with different leakage rates. The results shown in Figs. 39 and 40 indicated that during conventional pigging at the normal production rate, the peak instantaneous surge reached 305 m3, far exceeding the processing capacity of the existing separators. Even after reducing the production rate, the surge volume still exceeded the equipment’s capacity. However, in the case of bypass pigging, as the leakage rate increased, the required surge volume decreased. For instance, at a leakage rate of 4%, the surge volume was only 32.5 m³. Although the pig’s speed decreased to some degree, bypass pigging allowed for operations at relatively higher production rates.

Fig. 39.

Surge volumes, pigging velocities and pig travel time for various scenarios¹¹⁰.

Fig. 40.

Pipeline liquid hold-up as a function of time for various pigging scenarios¹¹⁰.

Jiang et al.¹¹¹ established a three-dimensional numerical model to simulate the pig’s movement as it passed through an elbow with a curvature radius of 6D (where D represents the pipeline’s outer diameter). This model considered the contact stress distribution between the rubber cup and the pipe wall to quantitatively assess the sealing performance. The results shown in Figs. 41 and 42 showed that when the pig passed through the elbow, compared to passing through a straight pipe, the sealing rubber cup was more likely to come off the pipe wall. This detachment led to a decrease in the area that could provide a seal. In a typical elbow with a curvature radius six times the pipe diameter, the minimum sealing area of the rubber cup was merely 8.07%.

Fig. 41.

Variation curve of percentage of sealing area for each region in four cups vs time. (A) The front cup. (B) Cup Middle 1. (C) Cup Middle 2. (D) The behind cup¹¹¹.

Fig. 42.

Maximum contact stress distribution along the conferential direction of behind cup and radial sketch of sealing cup¹¹¹.

It was found that increasing the interference of the sealing cup could enhance the sealing ability, with a minimum interference of 4% being necessary. Additionally, appropriately reducing the thickness of the sealing cup or increasing the pressure difference could improve the sealing effect. Conversely, an increase in the friction coefficient would degrade the sealing performance. In an elbow with a small curvature radius, increasing the radius could enhance the sealing, though this effect became negligible when the curvature radius was large.

Zhang et al.¹¹² aimed to address the challenge of measuring the moving speed of the bypass pipeline detection device in water and crude oil media. They performed a numerical simulation on the movement of a pig equipped with a bypass valve in a gas pipeline within complex terrain. The results indicated that, under different bypass hole diameters, both the driving force and the friction force declined as the pig’s moving speed increased. Notably, the absolute value of the slope of the driving force’s decrease was significantly larger than that of the friction force. As shown in Figs. 43 and 44, the density and dynamic viscosity of the media influenced the pig’s movement. With the same bypass hole diameter, the pig moved faster in the water medium than in the crude oil medium. Additionally, the pig’s moving speed decreased as the bypass hole diameter increased. For example, when water was the medium and the bypass hole diameter changed from 0.1 m to 0.5 m, the pig’s speed dropped from 2.779 m/s to 0.589 m/s. When crude oil was the medium and the bypass hole diameter underwent the same change from 0.1 m to 0.5 m, the pig’s speed decreased from 2.777 m/s to 0.373 m/s. Based on these experimental data, the functional relationship between the pig’s speed and the bypass hole diameter was derived, as shown in Fig. 45.

Fig. 43.

Curve of driving force as a function of moving speed (water)¹¹².

Fig. 44.

Curve of driving force as a function of moving speed (crude oil)¹¹².

Fig. 45.

Curves of moving speed of the PIG as a function of bypass hole diameter¹¹².

Muginov et al.¹¹³ sought to solve the problem of how the shape of the intelligent pig’s internal bypass channel affects hydraulic resistance during the detection of low-pressure gas pipelines. They used the ANSYS Fluent program to study and numerically simulate a cylindrical bypass channel and a channel with a Laval nozzle shape. The results showed that at the outlet of the cylindrical channel, a low-pressure area and stable vortices formed, and the pressure changed in a quasi-periodic pattern. The gas acting force was large and unstable. At the outlet of the Laval nozzle, the pressure pulsation was small, and the flow was nearly in a steady state. The gas acting force was relatively small, yet there was a gas flow separation phenomenon (Fig. 46). By adding an additional bypass channel in the design of the Laval nozzle (Fig. 47). The gas flow resistance was reduced by 40% compared with that of the cylindrical channel, and this effect was not affected by the inlet pressure boundary conditions.

Fig. 46.

Pressure distribution in the Laval nozzle¹¹³.

Fig. 47.

Parameters of the additional bypass channel¹¹³.

Liu et al.¹¹⁴ aimed to solve the problem of reducing the impact of severe slug flow on pigging operations in the offshore riser system by applying bypass pigging technology. They used two numerical simulation methods, OLGA and CFD, to simulate and analyze the severe slug flow phenomenon in this system. Through OLGA simulations, they investigated the effects of different inlet mass flow rates and bypass fractions on characteristic parameters during pigging, such as the pressure at the bottom of the riser, the pig’s speed, and the liquid phase distribution pattern.

The results indicated that bypass pigging notably decreased the pressure fluctuations at the bottom of the riser and the speed fluctuations of the pig. It also dispersed the downstream liquid slugs and reduced the mass flow rate at the riser outlet and the liquid holdup (Fig. 48). As can be seen from Fig. 49, an increase in the bypass fraction further reduced the pressure fluctuations and the pig’s speed, enhancing the liquid slug dissipation ability. However, an overly high bypass fraction would lower the pigging efficiency. An increase in the inlet mass flow rate could improve the liquid slug dissipation capacity, yet its effectiveness in alleviating severe blockage flow was not as good as that of the bypass section.

Fig. 48.

Pressure variations in the riser bottom. (a) Severe slugging. (b) Bypass pigging operation with severe slugging¹¹⁴.

Fig. 49.

Liquid-phase distribution at different bypass fractions of 2%, 4%, and 6%. (a) The pig is located at the declination pipe. (b) The pig is located at the elbow pipe. (c) The pig is located at the bottom of the riser. (d) The pig is located in the middle of the riser¹¹⁴.

Liu et al.¹¹⁵ focused on solving the pipeline transportation safety issues caused by liquid slugs in offshore pipeline pigging operations by simulating and analyzing the liquid slug dispersion process during bypass pigging. They established a two-dimensional CFD model to simulate the liquid slug dispersion process in the vertical riser under different bypass ratios and inlet gas velocities.

The results showed that increasing the bypass ratio could strengthen the dissipation effect on liquid slugs. However, it would reduce the pipeline cleaning efficiency and might even fail to generate the pressure difference needed for normal pipeline cleaning. For instance, when the bypass ratio was 6%, the pig barely moved in the riser (Fig. 50). Increasing the inlet gas velocity could accelerate the dissipation of slugs, but it would cause the outlet mass flow rate and liquid holdup to increase after the gas broke through the riser outlet (Fig. 51). By comprehensively considering the bypass ratio and the inlet gas velocity, the optimal pigging parameters can be determined to minimize the negative impact of slugs on pigging operations.

Fig. 50.

Liquid-phase distribution at different bypass rates. (a) A bypass rate of 2%. (b) A bypass rate of 4%. (c) A bypass rate of 6%¹¹⁵.

Fig. 51.

Liquid-phase distribution at different inlet gas velocities. (a) An inlet gas velocity of 1m/s. (b) An inlet gas velocity of 2m/s. (c) An inlet gas velocity of 3m/s¹¹⁵.

These numerical models and their applications are comprehensively summarized in Table 3, which categorizes the authors, research methods, and model prototypes. We constructed various mathematical models and performed numerical simulations to precisely analyze multiple aspects of the pig’s operation within the pipeline. These aspects encompassed speed control, hydrodynamic behavior, sealing performance, and liquid management in multiphase-flow pipelines, among others. The outcomes of these analyses offer crucial guidance for optimizing and applying the bypass pigging technology. Simulation holds significant advantages. It can effectively uncover the operating mechanism of the pipeline cleaner and predict pipeline flow behavior. Moreover, it enables the simulation of complex working conditions, thus reducing experimental costs. However, its results remain restricted by certain factors such as model simplification, parameter settings, and the complexity of fluid flow. Consequently, in practical applications, it is essential to integrate experimental data for verification and calibration. This approach helps to enhance the reliability and accuracy of the findings, ensuring that the bypass pigging technology can be implemented more effectively in real-world scenarios.

Table 3.

Summary table of pipe cleaning model.

Authors	Research methods	Model prototype
Chen et al.96	Experimental research and numerical simulation
Hendrix et al.54
Luo et al.95
Li et al.94
Liu et al.114
Tu et al.98	Numerical simulation
Jiang et al.111
Zheng et al.105

Challenges and outlook

Pipeline pigging technology has seen substantial progress, yet it still faces numerous challenges. As pipelines age and oil and gas field development continues, the amount of deposits and dirt inside pipelines is on the rise. High-viscosity crude oil and waxy substances create complex deposition situations. These deposits not only stick firmly to pipeline walls but can also alter the flow characteristics within the pipeline. Traditional pigs, due to their simple designs, may have difficulty breaking down and removing these complex deposits effectively. This can result in incomplete cleaning, reducing pipeline transportation efficiency or, in severe cases, causing blockages that endanger pipeline safety. As a result, there are more stringent demands for the design, selection, and application of pigs. Especially when dealing with special deposits like high-viscosity crude oil and waxy substances, more effective pigging technologies and equipment must be developed.

To address these existing problems, another crucial problem is deciding how to select appropriate pigging technologies and equipment according to the specific conditions of different pipelines. The aim is to achieve efficient pigging while minimizing damage to the pipelines. Future work should investigate deposit interaction mechanisms using full-scale specimens under simulated pipeline conditions, as current data are insufficient to model complex scenarios.

Given the challenges faced in current pigging operations, the future development of pigs will focus on intelligence, high efficiency, and environmental friendliness. With the continuous integration of Internet of Things, big data, and artificial intelligence technologies, intelligent pigs will possess enhanced self-diagnostic and adaptive capabilities. They can monitor pipeline conditions in real-time and accurately identify defects, significantly improving detection efficiency and precision.

In terms of high efficiency, the utilization of new materials and innovative structural designs will further reduce energy consumption and wear during pigging. In addition, this will also increase the pigging speed and the thoroughness of cleaning. Moreover, pigs will become more versatile, combining detection, cleaning, and maintenance functions to meet the diverse needs of complex pipeline environments. Furthermore, a more thorough cleaning process can effectively reduce the entry of corrosive salts and other deposits from oil and gas into the midstream and downstream sectors, thereby mitigating complex corrosion issues in processes such as petroleum refining.

Environmental friendliness is another key trend. There will be research and development of pigging materials that are degradable or have a low environmental impact, aiming to minimize the ecological disruption caused by pigging operations¹¹⁶. Future decommissioning of unpiggable pipelines, particularly in sensitive regions like the Arabian Gulf, requires tailored flushing protocols to meet stringent environmental standards¹¹⁷.

Furthermore, as an important aspect of intelligent development, the remote control and autonomous navigation technologies of pigs will continue to advance. This allows them to adapt to a broader range of challenging geographical conditions, ensuring the safe and efficient operation of oil and gas pipelines.

Conclusion

This research comprehensively examined oil and gas pipeline pigging technology, with a particular emphasis on bypass pigs. It combined experimental investigations and numerical simulation approaches to explore this technology. By innovatively integrating these two methods, the study provided a comprehensive understanding of pigging technology. It also filled the knowledge void regarding the performance of bypass pigs under diverse conditions, offering novel theoretical support for the design and optimization of pigging equipment.

The key findings are presented as follows. Firstly, experimental results demonstrated that the average speed of the bypass pig has a linear positive relationship with the driving gas flow rate. For instance, in an experiment, as the driving gas flow rate increased from 5 m/s to 10 m/s, the average speed of the pig rose from 2 m/s to 4 m/s. Conversely, an increase in the bypass fraction led to a decrease in the pig’s speed. When the bypass fraction increased from 0 to 4%, the pig’s speed dropped by around 30%. Secondly, numerical simulations revealed that the bypass diameter has a significant impact on the pig’s cleaning efficiency. In a simulated pipeline, when the bypass diameter increased from 2 to 4 inches, the cleaning efficiency decreased by approximately 20% in certain sections. Thirdly, bypass pigging technology proved effective in reducing liquid accumulation during the pigging process. In gas–liquid two-phase flow simulations, compared with traditional pigging methods, this technology could reduce liquid accumulation by up to 45%.

Based on the current research, two potential research directions for expansion are proposed. One is to develop new pigging materials and structures. For example, exploring the use of self-cleaning materials for pigs to enhance their adaptability to complex deposition situations, especially those involving high-viscosity crude oil and waxy substances. The other is to further incorporate intelligent technology into pigging operations. With artificial intelligence technology, pigs can be enabled to have real-time monitoring and self-adjusting capabilities. This can enhance the safety and efficiency of pipeline transportation. For example, pigs can automatically adjust their operations according to pipeline conditions to ensure better cleaning and inspection results.

Author contributions

Lei Zhao: Writing—review & editing, Investigation, Resources. Yuan Zhang: Writing—original draft, Methodology, Formal analysis. Zhiyong Hu: Formal analysis, Methodology, Supervision. Qi Liu: Conceptualization, Funding acquisition, Resources. Baoming Liu: Methodology, Resources.

Funding

The project was supported by PhD Research Startup Foundation of Liaoning Petrochemical University [No. 1100130252].

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.