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 (u_SG = 4 m/s, u_SL = 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 u_SG = 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 (u_SG ≈ 4 m/s). For low superficial gas velocities (u_SG ≈ 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. ... Read more

Shifting from coal and oil towards sustainable energy sources is currently an urgent and global challenge. Compared to coal and oil, natural gas is a cleaner fossil fuel and may serve as a transition fuel. To meet future demands for gas there is a desire to optimize the production from existing reservoirs with their production facilities. In case of wet natural gas wells this gas production can be hampered by the presence of liquids, namely water and condensate, thus creating a multiphase flow in the production tubing. The maximum gas well capacity is limited by the ever reducing upward gas flow velocity due to the subsurface reservoir pressure that is reducing over time. The reduced gas velocity results in the inability to transport the liquid upward from the downhole location to the surface. The accumulated liquids will then block the well bore (so-called liquification), making further gas production impossible. One solution it to decrease the cross sectional flow area by inserting an inner pipe in the existing tubing, thus creating an annulus. This results in a higher gas velocity. The multiphase flow characteristics in the annulus tubing are not yet fully understood, such as the pressure drop, the liquid hold-up and the different flow regimes. Therefore in the present study lab experiments were carried out to further investigate those characteristics to enable optimisation of predictive flow modeling. The main focus is on the behaviour of the liquid film along both pipe walls. In this study a 12 m mid-scale vertical annulus with a 124 mm diameter outer pipe and a 100 mm diameter inner pipe was designed and constructed. The experiments were conducted at atmospheric conditions using an air-water mixture as the working fluids. Multiple combinations of gas and liquid throughputs were studied. The liquid at the inlet could either be injected on the inner pipe, the outer pipe or equally divided on both pipes. The position of the inner pipe with respect to the outer pipe was adjustable to create different eccentricities. To measure the local film heights flush mounted conductance sensors were designed, built and installed in the half circumference of both pipes. The efficacy of the sensors was thoroughly studied and the sensors were subjected to multiple test cases. The concentric annulus experiments showed that the pressure drop does not depend on the method of liquid injection. However, the liquid hold-up fraction at lower liquid throughputs was observed to be lower for the single wall liquid injection as compared to injection on both pipes. The redistribution of the liquid film for the single liquid injection depends on the liquid throughput. Larger liquid throughputs gave thicker liquid films containing a wavier gas-liquid interface from which droplets are atomized and subsequently migrated to the other pipe wall. Furthermore, it was found that the critical superficial gas velocity at which liquid loading occurs is 14 m/s, which is neither dependent on the method of liquid injection nor on the liquid throughput. The designed sensors were able to provide a good indication of the film heights. Single phase experiments, using air as the working fluid, showed that the pressure drop is reduced by 30$\%$ in the fully eccentric case in comparison to the concentric case. The eccentric annulus configuration with air-water flow showed that the liquid films along both pipe walls may merge. This so-called liquid bridging mainly occurs at higher eccentricities and causes flow reversal in the narrow gap. This results in a much higher liquid hold-up and pressure drop. Eccentricity induces redistribution of the liquid film from the inner pipe to the outer pipe at higher superficial gas velocities. It was found that eccentricity promotes an unequal film height along the circumference of each pipe: the film height decreases when going from the narrow gap towards the wide gap. ... Read more