Numerical Study of the Flow Around an Undersized Ball Pig in a Horizontal Pipe

Abstract

Pipeline networks are extensively used in the oil and gas industry to transport fluids under multiphase flow conditions from the wells to the production platforms or plants. Over time, the flow rate of the production fluid decreases as the reservoir pressure decreases. When the flow rate decreases to below a certain value, unstable flow with liquid slugs occurs and the transportation of the fluids becomes more difficult. This is because of the high amount of liquid accumulation (holdup) in the pipeline. To extend the operation life of the production system, it is important to maintain the continuous production at low flow rates. Regular pigging, which refers to transporting a “pig” (Pipeline Inspection Gauge) through the pipeline, has been a conventional way of managing the liquid holdup in the oil and gas industry. Also "by-pass" pigs have been actively used to manage the liquid holdup since they generate smaller slug volumes compared to the traditional non-bypass pigs. However, since the pipeline can be as long as a hundred (or more) kilometers, the volume of the pig-generated slug at low flow rates can be very large. Even this reduced slug volume produced by using a by-pass pig may exceed the capacity of the downstream separator or slug catcher. Moreover, by-pass pigs with a large by-pass area have a high chance to get stuck in the pipeline at low flow rates. To enable pigging operation at those conditions, the use of an undersized ball pig was proposed. Here the ball pig is transversed through the pipeline prior to the by-pass pigging operation. A few test runs with undersized ball pigs were carried out in the F14 multiphase pipeline in Sarawak. To improve its performance, it is of much interest to study the detailed flow around an undersized ball pig. Although in actual operation the ball pig will be used under multiphase conditions (i.e. gas and liquid flow), in this study, as a starting point, single-phase flow was considered.
This study investigated the detailed flow around an undersized ball pig in a horizontal pipe with the help of Ansys Fluent version 18.2. First, benchmark data from experiments are used to find the best numerical model which can be used to simulate the flow around a sphere. The comparison between experimental data and numerical results shows that the 2D laminar simulation can accurately capture the flow when Re < 300. The 2D simulation with the SST k-ω turbulence model (low-Reynolds number correction) gives an accurate prediction of the drag coefficient when 300 < Re < 5E5.
The selected numerical models are then used to study the flow around an undersized ball pig in a pipe. For each set of conditions, this requires to carry out a number of simulations at different ball pig velocities, and find the velocity that gives zero drag force on the ball pig. The undersized ball pig is first moved to the centre, and 2D simulations were performed. The force analysis around the undersized ball pig at low Reynolds number shows that at equilibrium state, the undersized ball pig experiences a positive pressure force and a negative viscous shear force. Moreover, the computed profile of the normalized terminal velocity of the pig with a diameter ratio of 0.5 at various Reynolds numbers shows that the normalized terminal pig velocity profile experiences three stages when the Reynolds number is increased. In the first stage where Re < 365, the motion of the pig is dominated by the viscous shear force and the normalized terminal velocity of the pig is constant. When Re increases, the flow enters the second stage where the inertia force starts to affect the motion of the pig and the normalized terminal velocity of the pig decreases dramatically. When Re is further increased to above 5340, the motion of the pig is completely affected by the inertia force and the normalized terminal velocity of the pig stays in a stable range when Re increases. After this detailed study of the flow around an undersized ball pig with a fixed diameter ratio, simulations of the flow around undersized ball pigs with various diameter ratios are studied to find the influence of the pig diameter on the terminal velocity of the pig. The results show that the obtained profiles of the normalized pig terminal velocity against Re at various diameter ratios have a similar shape as when the diameter ratio was 0.5; the terminal pig velocity decreases when the pig diameter increases at a given Re. A further data regression analysis shows that the normalized pig terminal velocity in the region when Re < 300 and Re > 10000 has a second-order polynomial relationship with the diameter ratio.
After the simulations for the flow around an undersized ball pig that moves along the axis of the pipe, the pig is moved to the bottom of the pipe and the influence of this position change is studied at a diameter ratio of 0.9. Due to this position change, a 3D simulation is required. First a 3D simulation for the previous configuration with the pig at the centre is performed. This simulation shows that the flow is actually unsteady and asymmetric. The terminal pig velocity at a given Reynolds number obtained from the 3D simulation is slightly higher than the one obtained from the 2D simulation. As the 2D simulation cannot capture the asymmetry of the flow, it provides less accurate results. The 3D simulation results of the flow past an undersized ball pig moving along the axis of the pipe is then compared with the one moving along the bottom of the pipe. The comparison shows that the ball pig terminal velocity is decreased when the pig is moved from the centre to the bottom of the pipe.