Undersized ball pigging

Abstract

Onshore and offshore multiphase pipelines provide a safe and effective solution for the transport of fluids from wells to production plants in the oil and gas industry. For the control of the accumulated amount of liquid present in the pipeline (the so-called liquid holdup) and for the regular maintenance of the pipeline, pigs (Pipeline Inspection Gauges) are conventionally used. To reduce the pig-generated volume (which is the amount of extra liquid that is removed from the pipeline due to the pig traverse), research in the past decades showed that a bypass pig can be used as a more attractive alternative. With the bypass pig, the size of the slugcatcher (which temporarily stores the pig-generated volume at the end of the pipeline) can be reduced. To reduce the risk of stalling of the bypass pig in the pipeline and the risk of exceeding the available liquid storage volume of the slugcatcher, undersized ball pigs can be deployed prior to launching the bypass pig. The undersized ball pig will be smaller than the pipe internal diameter (e.g., 0.9 ⋅ D) and will not experience significant friction with the pipe wall. Furthermore, the pig-generated volume will be smaller than what is found with the bypass pig as the ball pig will leak some liquid volume; this means that not all liquid is driven out of the pipe by the pig, but some liquid is moved to and left behind the pig. Although ball pigging is already used in some field operations, its use can be extended if the fluid physics are better understood and reliable models for preparing such operations are available.
In this Master Thesis Project, small scale experiments were carried out to investigate the undersized ball pig behaviour in a 52 mm diameter, 60 m long multiphase flow loop located in the Process & Energy Laboratory at the campus of Delft University of Technology. Air and water are used as the working fluids. The performed experiments were used to get a better understanding of the parameters that determine the flow around an undersized ball pig. The pig velocity and the liquid leakage were of particular interest. Two pressure sensors, a liquid hold-up sensor, three cameras, and a weighing scale were used as the measurement system. Three buoyant pigs were created using a 3Dprinter. The three ball pigs varied in size: 42 mm, 45 mm, and 48 mm. In addition, a flexible ball pig of 50 mm was used. The pigs were released in a stratified flow through a pig launcher. The presence of (wavy) stratified flow in this flow loop maintained up to a certain upper limit for the flowrates (uSG = 4 m/s, uSL = 0.0628 m/s). Higher liquid flow velocities would rapidly create intermittent slugs. 
The pigging observations showed that it is recommended for industrial applications to create buoyant pigs (e.g., 3Dprinted balls) when using a diameter ratio (the ratio between the pig diameter and the inner pipe diameter) of approximately 0.9. This reduces the risk of stalling of the undersized ball pig. From the local video recordings, it can be observed that undersized ball pigs accumulate liquid while creating one or multiple liquid slugs. The created liquid slug will propagate and separate from the pig, whereafter a new liquid accumulation at the front of the pig is created. The local pig velocity is found to be oscillatory in time, due to liquid slug accumulation and liquid slug propagation. It is found that the normalized pig velocity (being the pig velocity divided by the mixture velocity) increases with increasing superficial gas velocity according to three different regions. In the first region, almost no increase in the normalized pig velocity can be observed. In the second region, the normalized pig velocity increases significantly. In the third region, the normalized pig velocity reaches a stable level. The transition from the first to the second region was found to have a local Reynolds number of about 2000-3000 for the gas. The normalized pig velocity in the second region and in the third region can be approximated with a power law correlation for the various pigs. 
It is found that the liquid leakage increases with increasing superficial gas velocity and decreases with increasing superficial liquid velocity. The liquid leakage versus the superficial gas velocity increases linearly from approximately uSG = 2 m/s onward. A larger diameter ratio results in a lower liquid leakage. The difference in liquid leakage between the various pigs decreases for large superficial gas velocities (uSG ≈ 4 m/s). For low superficial gas velocities (uSG ≈ 0.5 m/s), a liquid leakage below zero was found. Undersized ball pigging becomes ineffective for these low velocities because the liquid flow will overtake the pig instead of leaking past the pig. The experimental leakage results have been approximated with a power law fit. At last, an analytical leakage model, provided by the OLGA commercial package for dynamic multiphase pipeline simulations, is used to compare the experimental liquid leakage. It is found that the OLGA model significantly underpredicts the values of the experimentally found liquid leakage. Improving this model is desirable so that it can be used in multiphase pipe simulations in the industry. 
The results in this study show that undersized ball pigging is associated with some distinct observed phenomena. These observations contribute to the understanding of the fundamental fluid flow structures around an undersized ball pig. The influence of the diameter ratio and the fluid flow velocities on the pig velocity and on the liquid leakage were determined. The measured influence of these parameters will help in the estimation of the pig velocity and the estimation of the leaked volume when industrial pipeline conditions are known. 
It is recommended for future research to focus on improved models to determine the pig velocity and the liquid leakage such that the use of undersized ball pigs can be reliably extended. Additional pigging experiments with local video recordings can be helpful.