Slug flow is the prevailing flow pattern in oil and gas flowlines. Slugging can cause several flow assurance issues, such as the force on fittings, pressure cycling, operational control instability, and inadequate phase separation. Slug frequency is one of the practical parameters used to design flowlines and facilities. This paper proposes a new unified model for the prediction of slug frequency. The effort includes data collection, correlation development and validation, and comparison with other models. A database of 1507 data points was created covering a pipe diameter range of 51 – 156 mm, liquid viscosity range of 1 – 600 mPa-s, and inclination range of 0 – 90°. Originally, the amount of low-viscosity oil data was small in the collected data from the literature. Therefore, additional slug frequency measurements in horizontal flow using low-viscosity oil were obtained. The measured data are used to test the performance of the proposed model.

The new correlation was developed based on the methodology proposed by Schulkes (2011). This approach separates liquid fraction, liquid viscosity, and pipe inclination effects. Improvements in the prediction of these effects have been incorporated. The unified correlation was adjusted by including the influence of the Froude number on the low-viscosity horizontal-flow multiplier function, changing the Reynolds number limit for the flow regime specification, and simplifying the multiplier function for pipe inclination. The performance of various models was evaluated in the range of pipe inclinations from horizontal to vertical upwards. The proposed model performed well, especially for high slug frequency conditions, which is important for flow-induced vibration predictions.

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