Operational pigging in a 36", 120 km subsea export trunkline has been extremely challenging for a client due to large liquid hold-up and the limited onshore liquid handling capacity. Production turndown along with bypass pigging is part of the standard operating procedure in order to slow down the pig and minimize excess liquid surges. In spite of these measures, the pigging operation has resulted in slugcatcher flooding and production deferment which initiated a detailed investigation using the OLGA multiphase simulator. The hydraulic model and the pig bypass model in OLGA will be calibrated using historic field data, and pig tool operational experience to mimic the process parameters and pig tool behavior in the field. Flow assurance analysis will be undertaken primarily to understand the impact of liquid hold-up in the pipeline, estimate the ideal bypass opening and operating limits. OLGA modelling predictions will be compared against historic field pigging runs for model validation. For this purpose, the basic pipeline model was extended to include a detailed onshore slug catcher with slug suppression control system to closely mimic the actual system in the field.
One of the main challenge is this analysis is the estimation of the wall frictional parameters considered in the flow models which affect the pig motion behavior. Hence, special considerations were given to the parameters affecting the bypass pig velocity, such as bypass pig-wall frictional forces and pressure loss coefficients. These parameters were modified based on detailed 3D CFD modelling of the pig tool, desktop calculations and tool operational experience. The bypass pig model and the overall hydraulic properties in the OLGA model were consequently tuned, showing a good match against the real field data. Finally, with the tuned OLGA model, selected sensitivity studies were carried out considering the current production flowrates, receiver pressures, bypass opening etc. to recommend a desired procedure for a successful pigging campaign. Ideal pig bypass opening was suggested for the trunkline where the client could maintain normal production flowrates, whilst slowing down the pig and resulting in manageable liquid slugs. This paper discusses the challenges and the process of determining the right bypass opening for a pigging operation, along with flow assurance analysis in order to develop an effective and a safe pigging solution.