Slug Prediction and Slug Catcher Sizing

In general, for offshore floating and/or platform-based systems, slug catchers are undesirable from a weight and space perspective. Where possible, and particularly for oil/gas systems, it may be preferable to use separator inlet chokes (possibly brought into play by the level control circuitry) to control separator liquid ingress and/or dump rate, and the resultant separator volume.

The proper size of a slug catcher (or a separator sized to avoid using a slug catcher) is determined by the normal liquid and gas production rates, the dynamic variation in those rates due to operational factors (e.g., rate change - including pigging), the dynamic variation that can be accommodated by the separator, the length of slug that must be absorbed by the catcher to protect the process, the location of the slug catcher within the system (surface or subsea), and the extent to which riser gas lift, separator inlet chokes, and separator level controls can or should be integrated. In short, the use and/or design of slug catchers are part of the overall system hardware and operational design.

For normal pigging calculations, approximate techniques can be used to predict the gas and liquid behavior in front of (and behind) the pig. For more accurate prediction of pigging and other slugs, transient multiphase simulation software is used.

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