One of the challenges in pipeline operational pigging in multiphase pipeline is high Pig Generated Volume (PGV) liquid slug that arrives at the end of the pipeline receiving system. This causes process upset at the receiving plant, which in turn causes production deferment each time pigging activity is performed. Introduction of bypass pig for cleaning activity is one of the best solutions to manage the arriving liquid slug at the receiving end to allow ample time for the plant to control the arriving slug and evacuate liquid without initiating a process upset. Usually to design the bypass pig would require specific hydraulic pipeline simulation with expertise and experience personnel to perform the assessment. Meanwhile, pipeline operator are also dealing with a fluctuating gas flowrate, temperature and pressure during pigging operation which effect the selection of bypass pig design. There are records of bypass pig stalled which happened as a result of not considering the variation of these input variables. This paper elaborates alternative calculation methodology using Monte Carlo probabilistic method to assess input variables via a spreadsheet, prior to determining the final bypass pig design.
Groot (2015) provide a detailed explanation on bypass pig calculation using mass balanced approach to determine the bypass pig speed based on specified bypass opening. To determine the bypass pig speed, the gas velocity through bypass opening is subtracted from gas mixture velocity. For this case study, bypass pig with disc type is used. The minimum and maximum value for input parameter is used in the calculation using RANDOM functions in the spreadsheet. The calculation is repeated for 10,000 iterations to get the different result based on random range of operating parameter that has been set earlier. The calculation boundary is determined whereby for this case is the bypass pig speed. Result which fall within the boundary will determine the success rate of the bypass pig design, without pig stall or over speed.
Using Monte Carlo probabilistic method, the optimum bypass pig opening could be determined to avoid over speed or pig stall. Probabilistic method enables the user to analyze using descriptive statistic ie. minimum, maximum or average pig speed from various conditions. In conclusion, probabilistic method has significantly help the user to assess the bypass pig speed with fluctuation of operating parameters using simple spreadsheet.