In the oil and gas industry, pipeline networks are used to transport the production fluids from wells to production plants. During normal operation, the pipelines need regular cleaning and inspection. Typically, the pipeline maintenance is performed by pigging, which refers to using devices known as “pigs ”(Pipeline Inspection Gauges) to perform various maintenance operations of the pipeline. In order to describe the motion of the pig in the pipeline, detailed understanding of the flow around the pig is required. In this research, a CFD (computational fluid dynamics) approach was applied to model fully turbulent flow (Re ¼ 107) around various types of bypass pigs. We especially focused on the relation between the overall pressure drop, which was represented by a dimensionless pressure loss coefficient, and various dimensionless parameters describing the flow and the configuration. The pressure loss coefficient is caused by the fluid that passes through the bypass area. If the pressure loss coefficient is known, together with the friction between the moving pig and the pipe wall, the motion of the pig can be described. Moreover, often the flow in the pipeline is in multiphase (stratified flow) condition. Therefore, in this research the effect of multiphase flow around a bypass pig was also investigated. For the single phase study, two types of bypass pigs were investigated: the disk pig and the complex bypass pig. The disk pig has a fixed and relatively simple geometry, and it is based on the conventional bypass pig, with a deflector plate in front of the pig body. The complex bypass pig geometry is based on the disk pig, though now the bypass area is created by holes which can be adjusted. In reality, for these complex bypass pigs, the bypass pig velocity is controlled by adjusting the bypass area. For the conventional bypass pig, previous studies have shown that the Idelchik’s correlation for thick orifices can predict the pressure loss coefficient accurately. Thus a similar approach was applied in the disk pig study in order to obtain a theoretical correlation to predict the pressure loss coefficient for the disk pig. Indeed such a correlation was found which gives an accurate prediction for a certain parameter regime. In the complex bypass pig study, we mainly focused on the influence of the bypass area fraction on the pressure loss coefficient. Two correlations based on two approaches were suggested. It was found that these correlations can predict the overall pressure drop across the complex bypass pig accurately, especially when the opening fraction of the bypass adjusting holes was relatively large. Furthermore, for the multiphase study, the simpler pig models were investigated. First of all, the flow in front of a pig without bypass region was investigated. One of the practical purposes of this study is that we want to investigate under which condition the full pipeline perimeter gets wetted with liquid. This is important for the distribution of corrosion inhibitors. Moreover, the multiphase flow around a (conventional) bypass pig was investigated, to obtain a better understanding of the multiphase flow behaviour for bypass pigs.