Tag Archives: flotools

flotools, Flow Assurance, OLGA

Pigging Displacement Volume Calculation

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One of the most common practices during the operation of pipelines is “pigging”. Buildup of solids or liquids in flowlines can result in plugging, cracks, or flaws in the line, all of which could cause severe damage to the line. Pigging operations may be designed with objectives that include cleaning, inspecting of flowlines, or other maintenance operations. Thus, understanding pigging is necessary for flow assurance engineers to ensure the flow assets continue to run smoothly.

Correctly anticipating the total liquid volumes displaced during pigging operations is very important for operators, as this affects the designing and operations of receiving process equipment, such as separators and slug catchers located at the outlet of pipelines. This discussion will cover how one can run a batch of simulations and analyze the transient liquid response in terms of the pigging displacement volumes and peak liquid flowrates. This analysis can be performed with improved efficiency using evoleap’s flotools software, as compared to traditional methods. We will compare the flotools methodology with more conventional methods using the OLGA GUI and Excel.

Pre-Processing

flotools gives the ability to build a study consisting of many cases and then immediately process the results.  In this discussion we will analyze the pigging displacement volume and peak flowrates in a pipeline system by varying the following parameters:

  • Inlet Liquid Flowrate
  • Watercut
  • Gas Lift Rate
  • Inlet temperature

One of flotools most powerful features, its Parametric Studies tool, makes this process very simple and streamlined.

For a study that contains many cases, manually creating the varying cases using the OLGA GUI can be repetitive and sometimes impractical, especially if it is a particularly large case matrix, as the probability of making errors is high. If the OLGA GUI parametric tool was to be used, for every parameter to be varied, a comma separated list of each value for each individual case would need to be provided. Typically, an Excel spreadsheet would be used to organize and visualize how each induvial case’s parameters would look. The parametric studies tool in flotools simplifies this process. After creating a base case with a specific parameter set at a specific value, flotools allows you to create different cases with that specific parameter varied with different values by providing a comma-separated list of those values. For instance, if the base created for a study has the watercut for the inlet liquid rates set at 0.2 and the study requires the watercut to vary between 0.2 and 0.8 with increments of 0.2, a comma-separated list like “(0.2, 0.4, 0.6, 0.8)”, could be provided and then flotools will generate the differing cases consisting of those values.

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Figure 1 – Parametric study definition example

After defining the study variables, the next important step in generating the cases for the parametric study is defining the naming convention for the numerous cases. flotools provides an easy way to do this by providing an integer index value for every study variable specified while generating the parametric study. An example of the file naming pattern using study variable references is provided in the following figure.

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Figure 2 – Interactive naming of cases

As can be seen in the image above, each study variable has an index associated with it, for example watercut (WC) is linked to %3, the third study variable. Using these indexes to name the cases results with a systematic naming convention for the cases which makes each one identifiable. flotools also give you the option to add unlinked variables that can be referenced by other variables. These are potentially useful as they could provide a more descriptive way of naming the cases.

The total number of cases to be generated is indicated as a badge on the generate cases button. Once selected, flotools will create the indicated number of cases in a location specified in your file system. The cases are then available to be ran.

Post-Processing

Post processing the results of the study using the OLGA GUI was time consuming due to limitations with the GUI. When using the OLGA GUI to extract the results, the number of cases that can be loaded into the GUI simultaneously is limited. We found that only 47 of the completed cases could be loaded into the OLGA GUI simultaneously. Also, when extracting trend and profile data, only data at one specific simulation time could be considered; what if we wanted to obtain the maximum value during the simulation instead of the value at the last time step? Overall to get the full data set for this project, 4 separate extractions were required.

However, using flotools made this process much more streamlined. The cases just need to be loaded into a flotools workspace and then the results are available for plotting.

After obtaining the data, the next step with the traditional method is to use Excel to create plots of the pig displacement volumes. For each case, the pig exit time was extracted from the pig position in branch (ZPIG) outputs. Then using Liquid Standard Flowrate (QLST) plots, the displacement volumes were obtained for each case by integrating the QLST between the pig launch and exit time. Then the rest of the data must be formatted appropriately to generate the pivot table.

With flotools, the liquid volume rate can be plotted against the desired parameters using the parametric plots tool. The pig displacement volume, maximum liquid rate and maximum gas rate can all be calculated with flotools using its calculations tool.

A comparison of the two post processing methods can be summarized in the following figure:

Figure 3 – Comparison of workflows between OLGA/Excel and flotools/flowpad

Creating all the cases to run for a study is very fast and streamlined with flotools. Even for a case matrix with over 50 cases, the entire process was done in a few minutes (or less for expert users).

However, using the OLGA GUI and Excel took about approximately 10 times as long to create the desired plots due to the limitations of the OLGA GUI. These limitations include how many cases that could be loaded into the GUI simultaneously, and how much info could be exported at one time. Exporting the data in segments was the rate limiting part of this workflow method.

An example of a desired plot for this project can be seen below:

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Figure 4 – Peak liquid outlet rate vs. Inlet liquid rate

To summarize, a time breakdown of each method is given below:

OLGA/Excel method:

  • Open cases in GUI and export QLST and ZPIG to .csv (11 minutes)
  • Find time pig exits from ZPIG data (2 minutes)
  • Integrate QLST between pig launch and exit times (7 minutes)
  • Generate pivot tables (3 minutes)
  • Format plots (create titles, axis labels, etc.) (10 minutes)
  • Total (34 minutes)

flotools method:

  • Open flotools and load cases (5 minutes)
  • Open parametric study tool and select variables and filters/slices. (2 minutes)
  • Format plots (2 minutes)
  • Duplicate plots and change filters (2 minutes)
  • Total (11 minutes)

The difference in the processing times can be especially crucial if the case matrix needs to be modified and thus the simulations re-run. Using the parametric study tool in flotools, the existing parametric study can be copied over and then modified instead of making a new one from scratch therefore reducing the amount of time that would have potentially been spent on project. Likewise, parametric plots generated to analyze the results of a parametric study, can also be duplicated then slightly varied to cover a variety of different comparisons.

Conclusion

Using flotools for a common flow assurance task like studying pig displacement volumes can result in a much more streamlined process compared to using traditional methods like the OLGA GUI. The efficiency boost with flotools is due to several factors; namely flotools ability to process all case files in a single instance, calculations leveraged within flotools, and the ease of data handling and visualization in flotools.  To expand on these arguments, it should be noted that flotools can handle all the cases in the potential case matrix, even cases that have large data files, whereas the OLGA GUI has a limit for the number of cases it can handle at once. Using flotools also would eliminate the need for further post-processing tools like Excel and ensures repeatability of results when using calculations within flotools.  Finally, the workflows in flotools have been designed specifically for flow assurance engineering applications, thus deriving meaningful results can be quickly achieved based on a complete understanding of both input and output data inherent to the design of flotools.

Flow Assurance, OLGA

Efficient Cooldown Parametric Studies

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A common flow assurance study objective is calculating the time required to reach hydrate conditions during a shutdown (typically from steady state).  This is often referred to as ‘Cooldown’. The ability for an operator to understand the amount of time that is available to safely complete various tasks during a shutdown, or how close to the hydrate region they can operate a pipeline before running into problems is incredibly valuable.

This discussion will cover how one can run large simulation matrices for cooldown efficiently with flotools and will compare the flotools methodology with traditional methods using the OLGA GUI.

Pre-Processing

In flotools, you can setup and create large numbers of cases for a project, and post-process the results quickly. For this case, we are looking for the cooldown times to the hydrate formation temperature for a system with varying parameters: Gas Lift Rate, Flowrate, Inlet Temperature, Water cut, and different fluids. flotools makes this process very simple and easy using the Parametric Studies tool.

With a large case matrix, it can be time-consuming to create all the varying simulations in OLGA. The flotools Parametric Studies tool can create model files by replacing the text of a base file with new parameters. For example, if you have a boundary node with PRESSURE = 100 psig you can give flotools a list of comma-separated values (100, 150, 200) and flotools will create 3 separate key files that are copies of the base file, one with 100 psig, one with 150 psig, and one with 200 psig at that node. 

The only inputs needed are the variables to be varied, the parameters for each variable are defined only once. Cases can be named interactively, based on the parameters used.

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Figure 1 – Interactive Naming of Cases

The naming of cases is interactive with the inputs listed above. Each variable has an index associated with it, e.g., WC is associated with %7 (seventh study variable). The index associated with each variable can then be input back into the naming line with the %n operator to have them change with each variation of the parameters. You can even create linked variables that are not used for anything except for naming; for example, the 2nd (%2) and 6th (%6) variables above. Also, the total number of cases to be generated is indicated on the generate cases button with a small green number badge, 360 in this example. This is how many cases we will be generating with these inputs, coming from:

5 flowrates x 3 gas lift rates x 4 temperatures x 2 fluids x 3 water cuts = 360 cases

Once the matrix is set and named appropriately for the parameters of our study, flotools will create the 360 .key files in a location of our choice. flotools can output the .key/model files to any location in the directory structure as well as generate a batch file if necessary. The cases are then ready to run.  

Post-processing

Due to the large number of cases, post-processing of the results was time consuming. When using the OLGA GUI to extract simulation results for many cases you are limited by the number of cases that can be load into the GUI simultaneously. For this project, each case output file was relatively small (.tpl and .ppl file size combined to about 2 MB for each case). We were only able to load 47 of the fully completed simulations into the GUI at one time. When extracting trend and profile data you can only take data from those cases loaded into the GUI at a given time. This project required 8 separate extractions to get the full data set, which consumed a lot of extra time to load cases into the OLGA GUI.

Whereas with the flotools route, you can load the cases into a flotools workspace and be ready for plotting immediately.

Both methods are the setup for plotting and tabulating results. In Excel, to create a plot of cooldown times requires some combination of MATCH and INDEX/OFFSET formulas for each series of the MDTHYD output in the case matrix. Following this, the rest of the data must then be formatted in the right way to form a table/pivot table that you can then use to plot based on different parameters.

In flotools, there are built-in calculations for DTHYD and its (MDTHYD, MDPHYD, MAXDTHYD, MAXDPHYD), which can be used without even having to input a hydrate curve in OLGA. Hydrate curves can be input in flotools during post-processing, which flotools will then reference in comparison to the temperatures and pressures over the entire flow path during the simulation.

A comparison of the methods described above are illustrated in the following figure:

Figure 2 – Comparison of Workflows between OLGA/Excel and flowpad/flotools  

Even for large case matrices, creating all the model files to run is incredibly easy and fast with flotools. Without pre-processing the entire set of cases was created in less than 2 minutes in this instance.

Conversely, post-processing using the OLGA GUI and Excel took roughly 5 times as long to create a finished product of plots or tables. This is mainly due to limitations of the OLGA GUI as to how much info can be stored at one time, i.e., how many cases it can load simultaneously. Having to export the data in chunks was a time-consuming part of the process.

For this set of 360 cases, the goal was to create 11 plots that show the effects of each set of parameters of the parametric study. An example plot created in flotools is show below.

Figure 3 – Cooldown vs. Inlet flowrate for 0% Water cut, 0 MMscf/d Gas lift and 50/50 Mix Fluid

Time breakdowns of each method are given below (these times reflect an experienced user of each method):

OLGA/EXCEL method:

  • Open cases in GUI and export MDTHYD to .csv (22 minutes)
  • Find first MDTHYD > 0 °C (2 minutes)
  • Format all parameters and cooldown times in a table format (5 minutes)
  • Create pivot table, slicers, and pivot charts (7 minutes)
  • Format plots (create titles, axis labels, etc.) (20 minutes for 11 plots)
  • Total (56 minutes)

flotools method:

  • Open flotools and load cases (5 minutes)
  • Open parametric study tool and select variables and filters/slicers. (2 minutes)
  • Change plot formatting to liking (2 minutes)
  • Duplicate plot and change filters/slicers (2 minutes for 11 plots)
  • Total (11 minutes)

This time difference is especially impactful if the case matrix is updated or if simulations are re-run. If a similar parametric study needs to be run, you can simply copy the parametric study in flotools and modify it instead of having to make a completely new one, whereby saving time on future work.

Conclusion

flotools makes running common flow assurance tasks such as cooldowns easy to create, run and report, even if with many cases. There is a significant time savings with flotools as compared to using the OLGA GUI because flotools can handle all the cases from a project at once and can plot them instantly rather than requiring post-processing in Excel.