The compressible module allows the motion of a pig in a gas pipeline to be simulated. Factors such as the system pressure, flow rates and pig friction are considered in the analysis. In this respect, Piglab Compressible can be used to simulate pigging operations such a waterfill and dewatering during pre-commissioning, or motion of pigs in gas pipelines during operations. When used in conjunction with other modules, aspects of the pig performance such as seal wear, and placement in the pipeline can be taken into account. Output includes pressure transience along the pipeline and pig velocity and position.

The program works by dividing the pipeline up into distance increments and solving the continuity and momentum equations for the gas around the pig. The pig motion is then considered by solving the force balance across it. A moving grid is used, which is refined near the pig to allow a stable solution to be achieved.

Analysis of subsea pig launching using hydrocarbon gas and effect of various hose sizes.

The following sample output shows the motion of a pig in a 24-inch pipeline at 50bar with velocity excursions at various line features. The velocity signature from the pig is due to the gas flow and the effects of pig friction that introduces instability.

In single diameter lines, it is sometimes argued that it is not possible to simulate the motion of pigs using this type of technique as the frictional relationship is not always known, or the pig stops at a pipeline features such as a weld, which are not built into the program. However, this is not the case. The program can be used to determine the worst velocity conditions for a given operating scenario. It is then possible to redesign the pig or influence the operation to help avoid problem conditions.

In summary, Piglab-Compressible is an ideal tool for investigations into the motion of pipeline pigs in gas pipelines. In conjunction with the other models, full simulation of pigging operations can be undertaken. Knowledge of pig motion can be used to: –

  1. increase pig efficiency;
  2. reduce dangerous pigging practices;
  3. reduce the chance of getting pigs stuck in a pipeline;
  4. monitor the progress of pigging operation (comparison of actual and predicted);
  5. design away from situations which may cause damage to the pig or disrupt the operation;
  6. keep inspection pig trains within velocity tolerance.
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