A flow assurance engineer is familiar with the routine of computing surge volumes. Large volumes of liquids can be swept from the pipeline into the vessel at the receiving facility by operational scenarios such as slugging, pigging, and production ramp-up in multiphase manufacturing systems. These liquid surges often arrive at rates far beyond the processing capacity of the receiving facility. The vessel, usually a slug catcher, acts as a buffer, where liquid surges can be collected and processed over time. To properly size the slugcatcher, flow assurance studies aim to determine the maximum liquid surge that can be observed across different operations. Surge volume is the maximum volume a slugcatcher can hold for any given operation.

OLGA simulation software allows you to calculate surge volumes when at least one of ACCLIQ or ACCOIQ is listed in the list. This calculation assumes that the slugcatcher is located downstream of the trended location and that the vessel can be drained at an agreed upon maximum drain rate.

Accumulation Variables vs. Instantaneous Rate Variables

OLGA uses the accumulated variables to calculate surge volume instead of using the instantaneous liquid rate variables (QLTHL, QLTWT). Let’s examine the surge volume equation’s instantaneous rate form to understand why.

The average instantaneous rate for a given time window is roughly equal to its average accumulation rate. This assumption is often false because instantaneous rates capture rate spikes of very short duration that would not be indicative the average rate for the time window.

As you can see, the average QLT (from AcCLIQ) doesn’t show the flowrate spikes as the QLT variable does. These spikes, although they are likely to occur in a flowing system in short time windows, usually occur in simulations with a shorter output interval. The worse the assumption, the larger the output interval.

OLGA, in our opinion, has used the accumulated variables to calculate the surge volume.

Handling Negative Terms

Equation has a maximum operation. This ensures that the calculated volume of the slugcatcher is never below zero

A numerical simulator can correctly predict negative rates at the outlet boundary. This is normal and valid. The ACC variable can decrease in value when OLGA predicts negative pipeline rates. This will cause equation to reduce the slug catcher volume at an rate that is faster than the drain rate. The calculation doesn’t exclude the possibility of liquid leaving via the liquid drain or the inlet of a slug catcher. This is not a sound assumption if you take a look at the schematic below of a typical Slug Catcher. The inlet nozzles at the top of slug catchers are for gravity separation of phases. The liquids quickly settle to the bottom once they have gotten in. Negative flow will likely be mostly gas, with very few liquid droplets in the gas phase.

Depending on the facts of your case, you may encounter significant errors when using the OLGA method for calculation. We found a 10% error in one case at a particular drain rate. Problem is, the error isn’t on the side conservatism.

It is clear that filtering out negative values leads to higher surge volumes and lower drain rates. The differences will eventually disappear if there are high enough drain rates. We believe this equation is too conservative, as surge volume calculations are used to determine the size of the slugcatcher. Our modified equation should be used instead, which provides a more conservative estimate for surge volume.