Pigging is one of the most common methods for operating pipelines. Pigging is a common practice in the operation of pipelines. It can lead to plugging, cracks or other damage to the lines. Some pigging operations have cleaning or inspection objectives. To ensure that flow assets run smoothly, flow assurance engineers must understand pigging.
Operators must accurately predict the liquid volume displaced during pigging operations. This affects the design and operation of receiving equipment such as separators or slug catchers at the pipeline outlet. This article will discuss how to run several simulations and analyze the transient fluid response in terms the pigging displacement volumes as well as peak liquid flowrates. This analysis can be done with flotools software at a higher efficiency than traditional methods. The flotools method will be compared with other methods, such as Excel and OLGA GUI.
Pre-Processing
flotools allows you to create a study containing multiple cases and then process the results immediately. We will be discussing the pigging displacement volume (and peak flowrates) in a pipeline system.
Inlet Liquid Flowrate
Watercut
Gas Lift Rate
Temperature inlet
Parametric Studies is one of flotool’s most powerful features. This makes the process simple and efficient.
If you have a large study with many cases, it can be tedious and difficult to manually create the cases using the OLGA simulation software. A comma separated list would be required for each case if the OLGA GUI parametric tool is to be used. An Excel spreadsheet is used to organize and visualize the parameters of each case. This is made easier by flotools’ parametric studies tool. By providing a comma separated list of the values, flotools makes it easy to create multiple cases from the base case. If the study base has a watercut for inlet liquid rates of 0.2 and the study demands that the watercut vary between 0.2 to 0.8 with increments 0.2, flotools can be used to create different cases using a comma separated list such as “(0.2), 0.4, 60.6, and 0.8” and then flotools will generate those cases.
The next step in creating the cases for the parametric research is to define the naming convention. This is easy with flotools, which provides an integer index value to each study variable that has been specified during the generation of the parametric study. The following figure shows an example of file naming patterns using study variable reference.
Each study variable is associated with an index. Watercut (WC), for example, is linked to the %3 study variable. These indexes are used to name cases. This results in a consistent naming convention that makes each case identifiable. You can also add unlinked variables to flotools that can be referenced with other variables. These can be very useful because they provide a descriptive way to name the cases.
The badge that appears on the button to generate cases indicates how many cases are being generated. Once you have selected flotools, it will generate the specified number of cases at the location in your file system. These cases can then be run.
Post-Processing
The limitations of the GUI made it difficult to process the results of the study. The number of cases that can simultaneously be loaded into the GUI using the OLGA GUI for extracting the results is limited. Only 47 cases were able to be loaded simultaneously into the OLGA GUI. When extracting trend or profile data, data can only be used at one time. What if we want to get the maximum value in the simulation, instead of the value at each step? To get the complete data set needed for this project, four separate extractions were necessary.
This process was made easier by flotools. All that is required to plot the cases is to load them into a flotools workspace.
Once you have the data, it is time to use Excel to plot the pig displacement volumes. The pig exit time for each case was calculated from the ZPIG outputs. Next, using Liquid Standard Flowrate plots (QLST), the displacement volumes for each case were calculated by integrating QLST between pig launch time and exit time. To generate the pivot table, the rest of data must be formatted in a suitable manner.
The parametric plots tool can be used to plot the liquid volume rate against desired parameters with flotools. With flotools, the maximum liquid rate, maximum gas rate, and pig displacement volumes can all be calculated using the calculations tool.
With flotools, creating all cases for a study can be done quickly and efficiently. The entire process of creating a case matrix of over 50 cases took only a few minutes, or less for experts.
Due to limitations in the OLGA GUI, it took approximately 10x longer to create the desired plots using Excel and the OLGAGUI. Limitations include the number of cases that can be loaded simultaneously into the GUI and the maximum amount of information that can be exported at once. This workflow method was limited in terms of the speed.
Below is a summary of the time taken by each method.
OLGA/Excel method:
Open cases in GUI, export QLST or ZPIG to.csv (11 min).
ZPIG data 2 minutes to find time pig exits
Incorporate QLST (7 minutes) between pig launch time and exit time
Generate pivot tables (3 minutes)
Format plots (create titles, axis labels, etc.) (10 minutes)
Total (34 minutes).
flotools method
Load cases and open flotools (5 minutes).
Use the parametric study tool to select variables and filters/slices. (2 minutes)
Format plots (2 minutes).
Do not duplicate plots.
Total (11 minutes).
If the case matrix has to be changed and the simulations need to be re-run, the difference in processing times can be critical. The flotools parametric study tool allows you to copy and modify an existing parametric study, thereby reducing time spent on the project. Parametric plots that are generated from parametric studies can be copied and modified to allow for a wide range of comparisons.
Conclusion
Using flotools to perform a common flow assurance task such as studying pig displacement volumes, can make the process much simpler than using traditional methods like OLGA GUI. The flotools efficiency boost is due to several factors, including flotools ability process all case files at once, calculations that can be done within flotools and flotools’ ease of data manipulation and visualization. These arguments can be expanded upon by noting that flotools can handle all cases in the possible case matrix, even large data files. The OLGA GUI limits the number of cases it can handle at one time. Additionally, flotools eliminates the need to use additional post-processing tools such as Excel. This allows for repeatability in calculations and helps ensure that results are consistent. The workflows within flotools were specifically designed for flow assurance engineering applications. Meaningful results can be quickly obtained if you have a thorough understanding of both the input and output data.
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