Pigging in pipelines basically performs operations for five reasons including cleaning the pipe interior, batching or separating dissimilar products, displacement, measurement, and internal inspection. A model has been proposed for the dynamic simulation of the pigging process after water pressure testing in a long slope pipeline. In this study, an attempt has been made to analyze two serious accidents during pigging operation in 2010 by the model which is developed by the method of characteristic (MOC) by Wylie et al. (1993) and the two-phase homogeneous equilibrium vaporous cavitation model deveoped by Shu (2003) for vaporous cavitation. Moreover, simulation results of the third operation show good agreement with field data from the previous field trial. After investigation, it was showed that the impulse pressures produced during collapse of a vapor cavity result in severe damage of tubes.
Modern hydraulic systems are widely applied to various industrial fields. Routinely, for pressure testing of segmental pigging, pigs are usually employed to remove liquids and deposits after the pressure testing. Although there are a large number of variations and special applications, pigs are basically utilized in pipelines to perform operations for five reasons including cleaning the pipe interior, batching or separating dissimilar products, displacement, measurement, and internal inspection. Pressure testing, which includes strength tests and tightness tests, is an important part of the guarantee for the safe operation of oil and gas pipelines. As, air is accessible and inexpensive, it was utilized as a medium and the pressure is increased gradually to values that the pressure testing requires. However, operation staffs have difficulty to find quality defects for small spills. For a long pipeline, once a pipeline leakage takes place, pressure would not decrease dramatically due to the compressibility of air. Moreover, there is also the risk of getting pipes burst by pressurized air. Thus, air pressure testing has been out of use since the 1970s. Fortunately, water pressure testing can cover up those problems. The pipelines of thousands of kilometers in length are divided into segments varying in length according to elevation profiles and maximum allowable differences. B 31.8 Code Committee and American Gas Association jointly made great contribution to promote water pressure testing for decades. Pressure testing gradually took advantage of water for its safety and stability. Generally, pigging process is safe under low pressure because after pressure testing, operation staffs will decrease pressure by discharging the water through the valves at the end of pipes. However, due to varying elevation profiles, there are always some segments with a long slope, especially in a mountainous area and pigging operation, that should deal with new problems. More specifically, for full-flow pipes with a long slope, valves opening and moving pigs may cause liquid transients in a pipeline. Pigging process is subject to rapid pressure transients, resulting in water-hammer events.
In a system, any change in flow velocity causes a change of pressure instantaneously. Large pressure variations and distributed cavitation (bubble flow) may be involved due to the sudden shutdown of a pump or closure of a valve. Column separation therefore may occur and may have a significant impact on subsequent transients in the system [
A pipe is divided in to two parts including the upstream air section and the downstream liquid section by a pig. Because water directly discharges into atmospheric environment, the liquid flow will be under depressurized condition. Additionally, compressed air at upstream is to be at low pressure. In order to ensure that the pig is moving forward, the pressure is just greater than the resisting force acting on the pig. The pig slowly moves and has great influence on the hydrodynamic pressure of the downstream flow. Accordingly, the pressure downstream may drop to or below vapor pressure and it would result in a large cavity of vapor usually at high points. In previous researches [
Schematic diagram of collapse of a cavity by a pig.
One-dimensional continuity and momentum equations are applied to analyze the hydraulic transients. A number of approaches have been introduced for the simulation of the pipeline transients including the method of characteristics (MOC), wave characteristics method (WCM), finite volume method (FVM) [
For a fixed cross-sectional area, the mass conservation equation can be written as follows:
The momentum conservation equation can be expressed by:
The MOC approach transforms the above partial differential equations into the ordinary differential equations along the characteristic lines and is defined as
Due to the fact that
Due to the above simplification, these equations are integrated on the characteristic lines between time steps
Characteristic lines in
These equations can be written in the simplified forms as follows:
When vaporous cavities are locally incipient, the local pressures may be less than or greater than the vapor pressure of the liquid. However, for modeling purposes in engineering, it is assumed that the local pressures are equal to the vapor pressure when vaporous cavitation is occurring. During the pigging process, vaporization occurs and vapor cavities may be physically dispersed homogeneously in the form of bubbles. Due to the factors such as the terrains, a large number of bubbles will be produced, and the flow pattern is the bubbly flow with the intense pressure oscillations along the pipeline. The behavior of the flow should be described by the two-phase flow theory. Otherwise, water hammer equations solved by the MOC can be adopted for the single-phase flow.
The basic equations for the unsteady homogeneous equilibrium flow model in a tube are
In terms of the volumetric fraction
In the above equations, the second term in (
The method of characteristics is used to transform the above equations to four ordinary differential equations:
The equations needed to solve the variables at each time step are
The two-phase homogeneous equilibrium vaporous cavitation model has no conflict between negative cavity sizes and pressures below the vapor pressure.
If
If
In either case,
Numerical results show that the volume rate range is from 0 to 0.95 m3/s. According to the present study,
Boundaries include the inlet of pipeline, the outlet of pipeline, the tail of the pig, and nose of the pig. In order to solve the flow dynamic equations, boundaries conditions must be given. Boundaries at the pipeline inlet and outlet are constant flow rate and constant pressure, respectively. In addition, it is assumed that pressure and flow rate at the tail of the pig are the same as they are in the upstream fluid, close to the pig. We assume that the pig is a moving boundary with no thickness compared with the length of the pipeline, but its weight would be considered. Based on the above assumptions, the behavior of the pig is taken into account to solve the flow dynamics equations.
The behavior of the pig in the pipeline is determined by a balance of forces acting on the pig as shown in Figure
Forces acting on the pig.
For the convenience of analysis, the main stages (labeled with roughness of back line in Figure the pig flat-segment movement stag, the pig gully-segment movement stage, the pig downhill-segment movement stage, the pig near outlet-segment movement and overpressure stage.
Four stages for pigging process.
As shown in Figure
Table
Summary of basic parameters for field operations.
Parameters | Values |
---|---|
The length of pipeline | 6.93 Km |
Pipe size | Φ1219 × 18.4 mm |
The maximum elevation difference | 178.5 m |
Wall equivalent roughness | 0.01 mm |
Mass of pig | 700.0 kg |
Frictional resistance between pig and pipe wall | 0.03 MPa |
Static friction resistance between pig and pipe wall | 0.04 MPa |
Type of compressor | XHP1070 |
Rated operating pressure | 2.2 MPa |
Air displacement | 30.0 Nm3/min |
On July 19, a couple of XHP1070 air compressors and a DN150 valve were installed for dewatering after water pressure testing. All the preparations for the pigging process were completed and the pig was put in the pipeline. At nine o’clock in the morning, the compressors started to work and the valve was simultaneously opened. At five o’clock in the next morning, an eruption of the mixture of water and gas occurred at the outlet of the pipeline, and the pigging operation had taken nearly twenty hours. Finally, a fracture of 2.6 m in length was found at the last piece of steel tube.
On September 21, an air compressor and a DN150 valve were installed for dewatering after water pressure testing. At three o’clock in the morning, a pig was put in the pipe before the compressor started to work, while the valve was opened. At half past six in the next afternoon, the second accident happened, and a fracture of 2.5 m in length was about 5 m away from the first fracture. The pigging operation had totally taken nearly thirty nine hours and half an hour. During this period of time, according to the records, the maximum of air pressure was about 1.01 MPa.
After investigation, however, the tubes have no quality default, and the rated operating pressure of the air compressor is 2.2 MPa so that the compressed air unlikely caused the damages. Additionally, bursting pressure is determined from Barlow's equation that instantaneous pressure for severe rupture of the tube should be over 20.83 MPa as shown in Table
Summary of two accidents.
The first accident | The second accident | |
---|---|---|
Number of compressor | 2 | 1 |
Total time of pigging process | 20 Hours | 39.5 Hours |
The length of drainage pipe | 150.0 m | 150.0 m |
The drainage pipe size | Φ159.0 × 8.7 mm | Φ159.0 × 8.7 mm |
The length of rupture | 2.6 m | 2.5 m |
Maximum of air pressure | 1.01 MPa | |
Bursting pressure of tube | 20.83 MPa | 20.83 MPa |
A mathematical model has been suggested for simulating the pigging process, based on our own previous work. Meanwhile, these results were obtained from simulating the process by our own program.
The simulated time of the first pigging operation was 19.6 hours. During the pig downhill-segment movement stage, some amount of water was gathered in the part of low elevation near the outlet due to the gravity. When the pig was close to the end of the pipe, some amount of gas downstream would undergo extrusion process and eventually was dispersed into water as shown in Figure
Numerical result of pressure at the end of pipe for the first accident.
There is a large slope at the end of pipe so that pressure varied with the liquid level in the pipe. In other words, pressure would decrease when the liquid level dropped, and pressure would increase when the liquid level rose. According to the results, the liquid level quickly dropped near 9 h and 28 h, and the liquid level rose near 11 h because the flow rate of outlet and top was varying during the pigging process. The flow rate at the outlet was much larger than that at the top when the liquid level quickly dropped. On the contrary, the flow rate of top was much larger than that at the outlet when the liquid level quickly rose.
The simulated time of the first pigging operation was 19.6 hours. During this period of time, the maximum pressure of compressed air was as much as 1.03 MPa as shown in Figure
Summary of numerical simulation of the two accidents.
The first accident | The second accident | |
---|---|---|
Number of compressor | 2 | 1 |
Total time of pigging process | 19.6 Hours | 36.7 Hours |
Maximum of air pressure | 1.03 MPa | |
Maximum of outlet pressure | 30.50 MPa | 37.03 MPa |
Bursting pressure | 20.83 MPa | 20.83 MPa |
Numerical result of pressure at the end of pipe for the second accident.
Numerical result of air pressure for the second accident.
To study the variation of outlet pressure due to the pigging process, a field trial was conducted in 2011 by Luo [
As Figure
Numerical result of pressure at the end of pipe for the field trial.
Numerical result of the impulse pressure for the field trial.
With comparison between model prediction and field data, it was found that the amplitudes of the impulse pressures induced by the collapse of the cavity were nearly the same, and their total time was 36 hours, and 35.5 hours (see Figure
Summary of the field trial and numerical simulation.
The field trial | Numerical simulation | |
---|---|---|
Number of compressor | 1 | 1 |
Total time of pigging process | 36 Hours | 35.5 Hours |
Value of the impulse pressure | 1.576 MPa | 1.51 MPa |
Bursting pressure | 20.83 MPa | 20.83 MPa |
Result of the impulse pressure for the field trial.
The model proposed for the pigging process was employed to understand the flow dynamics in the pipeline and to obtain transient pressures for the two accidents and the field trial. The large cavity of water vapor and air was in the down-slope pipe following the peak and slack line flow that exited for a long period of time. When the pig was close to the outlet, the cavity was compressed, and gas underwent the extrusion process and eventually was dispersed into water in the form of bubbles. Finally, the cavity collapsed by the pig, and the serious collision resulted in considerable impulse pressures. The results of the simulation illustrate that the impulse pressures caused the severe damages during the pigging process. Additionally, our model predictions for the third operation showed good agreement with field data, and the diameter of drainage had a significant effect on impulse pressures. Generally, the terrain of pipeline is a key factor for the liquid-fill flow behavior, and slack line flow may appear in a long-slope pipeline. Then, column separation would occur as a common phenomenon for a hilly pipeline, and it can cause devastating effects such as severe damages. Therefore, preventive measures are of a critical significance for practical reasons.
Friction factor
Distance along the pipeline (m)
Gravity acceleration (
Time (s)
Cross-section area of pipeline (
The index number of Darcy formula
Water head of fluid (m)
Acoustic speed of fluid (m/s)
Volume rate of fluid (m3/s)
Volume rates for given points (m3/s)
Water heads for given points (m)
Volume rate for unknown point (m3/s)
Volume rate for unknown point (m)
Velocity (m/s)
Diameter of the pipeline (mm)
Kinematic viscosity of liquid ((m2/s))
Saturated vapor pressure at liquid temperature (Pa)
Density of liquid (kg/m3)
Pig mass (kg)
The pressure on the upstream faces of the pig (Pa)
The pressure on the downstream faces of the pig (Pa)
A angle between axis and horizontal direction (rad)
The axial contact force (Pa)
The volumetric fraction of liquid
Vapor phase density (kg/m3)
Liquid phase density (kg/m3)
The mean density (kg/m3).
The method of characteristic
International Association For Hydraulic Research
Hydrodynamic cavitation.
The authors thank the National Science & Technology Specific Project (Grant no. 2011ZX05039-002), and the Key National Science and Technology Specific Project (2011ZX05026-004-03), the National Natural Science Foundation of China (51104167) for their financial support.