A routine part of the work of a flow assurance engineer is to calculate surge volumes. Operational scenarios, such as slugging or pigging, or production ramp-up for multiphase production system, can result in large volumes liquids being swept out from the pipeline to the first vessel on the receiving plant. These liquid surges often exceed the capacity of the receiving facility to process liquids. The vessel, which is often called a “slug catcher”, acts as a buffer that allows liquid surges to be collected and processed in a controlled manner. The purpose of flow assurance studies is to determine the maximum surge liquid that can exist across different operations. This will allow the slug catcher to be sized appropriately. The surge volume refers to the maximum volume a snail catcher must hold for a particular operation.
OLGA can be used to calculate surge volumes if at least one ACCLIQ/ACCOIQ or ACCWAQ is in the list. The calculation assumes that a slugcatcher is present at the location where these variables are trended, and that the vessel has a fixed maximum drain speed during the operation.
Accumulation Variables vs. Instantaneous Rate Variables
OLGA’s calculation for surge volume uses the accumulated variable variables rather than the instantaneous volume rate variables. Let’s first look at the surge quantity equation’s instantaneous rates form.
The average of the instantaneous and average accumulation rates for a specific time window is about equal. This is usually a false assumption, as the instantaneous rates capture rate surges that are extremely short-lived and could not be used to determine the average rate in the corresponding window.
It is clear that the average QLT from ACCLIQ does not display the flowrate spikes that QLT variables show. These spikes may occur in flowing systems but are usually shorter than the simulation’s output interval. The more severe the assumption, the greater the output interval.
OLGA simulation‘s approach was correct, and the accumulated variables were used as the basis to calculate the surge volumes.
Handling Negative Terms
Equation includes a max operation. This ensures that no volume is ever calculated in the slug catcher.
It is quite normal and acceptable for a numerical sim to predict negative rates near an outlet boundary. When OLGA predicts negative rate at the pipeline’s outlet, the ACC variable might decrease in value. If this occurs, equation will result a reduction of the calculated slugcatcher volume at a faster rate than the assumed drain rate. However, this calculation doesn’t prevent liquids from leaving the slug catcher via the liquid drain. The schematic of a typical, slug-catcher is shown below. This may not be a valid assumption. The inlet nozzles for slug traps are placed at or near top of the vessel and are intended to allow gravity separation of phases. Once liquids have been poured in, they quickly settle at the bottom. A negative flow of liquids is more likely to be a lot of gas and very little liquid.
Depending upon your case, significant errors may occur when you use the OLGA basis to calculate. In one instance, there was a 10% error at the drain rate. The problem with this is that it is not on the side conservative.
You can see that filtering the negative values results is larger surge volumes at lower rate of drain. These differences are eventually eliminated if the drain rate is high enough. This equation, which is used to calculate surge volume in order for the slug catcher to be sized, does not seem conservative. Instead, we recommend using our modified version in equation which provides a more conservative estimate to surge volume.
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