The routine work of flow assurance engineers includes the calculation of surge volumes. Slugging, Pigging, or production ramp up in multiphase production systems could all lead to large quantities of liquids being swept from the pipeline and into the receiving vessel. These liquid surges can often reach levels that exceed the facility’s processing capabilities. The vessel, often a slug catcher, acts as a buffer so that the liquid surge can be captured and processed over time. In order to properly size the slug catcher, flow assurance studies are used to measure the maximum surge of liquid across operations. The surge volume is the maximum volume that a Slug Catcher can hold during a given operation.
OLGA is a method to calculate surge volumes when ACCLIQ, ACCOIQ and ACCWAQ are included in the trended outputs. The calculation assumes the slug catcher exists just downstream from the location where these variables trend and that the vessel is capable of being drained at a set maximum drain rate during operation.
Accumulation Variables vs. Instantaneous Rate Variables
OLGA’s surge volume calculation uses the accumulated variables instead of the instantaneous fluid volume rate variables (QLTHL/QLTWT). Let’s take a look at the surge volume equation in its instantaneous form.
The average sum of all the instantaneous rates within a time window is approximately equal the average accumulation rate within that window. This is a common mistake because the instantaneous rates capture spikes in rate that are short-term and do not reflect the average rate over the same time window.
The average QLT (from ACCLIQ), does not show spikes in flowrate, as shown by the QLT variable. These spikes are possible in flowing systems, but they usually happen in shorter time windows than the output interval. The assumption is more dangerous the longer the output interval.
OLGA took the right approach, using the accumulated variables for the surge volume calculation.
Handling Negative Terms
Equation uses a max operation. This ensures that there is no drop in the volume calculated by the slug catcher.
It is possible for a numerical simulator, and it is valid, to predict negative rates at an outlet border. If OLGA simulation predicts that there will be negative rates at the pipeline outlet, the value of the ACC variable could decrease from one step to another. The equation will show a decrease in the calculated volume of the slugcatcher at a rate greater than the presumed drain rate. This calculation doesn’t rule out the possibility that liquid could escape via the liquid drain and the inlet. It is possible to see a schematic of a typical snail catcher like the one below. The inletnozzles of slug catcher are located at or near the top. They are designed to gravity separate phases. Once liquids are in, they settle quickly to the bottom. The negative flow is most likely to be mostly of gas with very little liquid being carried in the gas phase.
Depending on your situation, the OLGA base for calculation could lead to significant errors. One case showed a 10% error for a certain drain rate. The problem is that this error is not in favor of conservatism.
As you can see, filtering out the negatives results in higher surge volumes at lower rates of drain. These differences disappear when there is enough drain rate. Because surge volume calculations are done to size the slugcatcher, this equation should not be used. Our modified version of equation, which gives a more conservative estimate on surge volume, should be used.
