By-pass pigging, as compared to normal pigging, reduces the pig-generated liquid slug. CFD (Computational Fluid Dynamics) was applied to understand the flow in and around the by-pass pig under both single phase and multiphase flow conditions. This can help to optimize the velocity of the pig and to define the proper operating envelop, that prevents that the pig gets stuck in the pipeline. Three levels of modeling have been investigated, namely: analytical (i.e. single point multiphase flow correlations), 1D (i.e. using the dynamic pipeline simulator OLGA) and 2D/3D CFD (using Fluent). In the CFD analysis a 2D axis-symmetric model for the single phase flow through the by-pass area is created. Furthermore, CFD simulations were carried out for the multiphase flow in a 2D channel to investigate the mechanism of the reduction in pig-generated volume. The multiphase flow results show four zones in which the flow downstream of the pig can be classified.
Offshore gas-condensate trunklines normally end in a large onshore slugcatcher. This slugcatcher can be as large as 5000 m3, and is meant to both separate the gas and condensate (and water) and to temporarily store liquid slugs. New projects (with longer pipeline lengths and with larger pipe diameters) can require even larger slugcatchers (e.g. 8000 to 10000 m3). The presence of a large slugcatcher will guarantee the continuous supply of gas to the downstream gas plant. The available literature on by-pass pigging is limited. Out [1] solved the problem of a slug of liquid between two sealing pigs in an isothermal 1-D flow field by using a standard numerical scheme with equally expanded grid intervals behind the pig. Minami and Shoham [2] used a mixed Eulerian-Lagrangian approach in the solution of the transient two-phase gas/slug system.
