(435b) Simulation, Analysis, and System Reliability Assessment of an Air-Water Experimental Flow System

In this work, we present a comprehensive simulation study of two‑phase air–water flows in pipes with four different sizes —originally investigated experimentally by Waltrich et al. ^[1]—using Aspen HYSYS simulations, both with and without the incorporation of Aspen Hydraulics. The experiments spanned 2‑, 4‑, 8‑, and 12‑inch vertical pipes, covering liquid flow at velocities up to 2.4 ft/s and gas flow at velocities up to 30 ft/s, with measured liquid holdup and pressure‑gradient data. In the simulations, we modelled each pipe segment and used the Aziz, Govier, and Fogarasi correlation for vertical pipe flow.

The case studies we conducted were used to investigate the effect of fluid flow rate on liquid holdup and pressure drop across vertical pipes—two critical parameters in the design and safety assessment of oil and gas wells. Liquid holdup significantly influences the estimation of in-situ fluid volumes and affects the accuracy of worst-case discharge (WCD) scenarios, which are essential for regulatory compliance and emergency response planning ^[1]. Pressure drop on the other hand, directly affects the sizing of surface and downhole equipment and impacts production safety ^[2]. The simulation results showed that liquid holdup is relatively higher at lower gas and liquid velocities and in smaller-diameter pipes, reaching a maximum of around 0.4. As flow velocities and pipe diameters increase, holdup values decrease. Furthermore, pressure drop decreases with increasing gas velocity, emphasizing the complex relationship between flow regime, frictional forces and pipe geometry in multiphase transport systems.

Overall results show that HYSYS simulations incorporating the Aspen Hydraulics unit reproduce experimental liquid holdup and pressure‑drop trends significantly more accurately (within ±10–15 %) than standard pipe simulation model (which exhibit errors up to 30 %). Discrepancies are attributed to certain assumptions made, uncertainties related to conditions of inlet flow rates to the pipes in experimental study, and correlation limits at high gas–liquid ratios. This study validates ASPEN HYSYS Hydraulics for multiphase flow design in large‑bore wellbores and highlights the importance of detailed hydraulics modeling for reliable worst‑case discharge assessments. Furthermore, we use the oil and gas simulator (OLGA) modelling tool to further validate the multiphase flow results obtained from the Aspen HYSYS Hydraulics runs. This comparison provides additional insights that lead to more accurate predictions and assessment of functionality of different tools under mixed flow conditions.

In addition to validating the multiphase flow results, we will present a high-level system reliability analysis with the goal of estimating overall system reliability and availability. We plan to do so using reliability block diagrams (RBDs) to visually represent how the air-water mixture flow system’s components interact with each other from a reliability perspective. This analysis allows for the prediction of overall system failure rate and the identification of components that are critical to system reliability. We plan to use failure and repair data for the system components from the following state-of-the-art references: the Offshore and Onshore Reliability Database (OREDA), Non-electronic Parts Reliability Data (NPRD) Handbook, and the Center for Chemical Process Safety (CCPS) Process Equipment Reliability Database (PERD). The insights from this analysis can be used to provide recommendations for enhancing the reliability and performance of existing air-water mixture flow systems as well as informing future system designs.

References

1. Waltrich, P. J. (2016). Experimental Investigation of Two-Phase Flows in Large-Diameter Pipes and Evaluation of Flow Models Applied to Worst-Case-Discharge Calculations. US Department of the Interior, Bureau of Ocean Energy Management (BOEM), Gulf of Mexico OCS Region.’
2. Sun, Y., He, K., Sun, J., Zhang, J., & Huang, N. (2025). Pressure drop characteristics and model construction of the actual rough wellbore wall in gas drilling horizontal well. Physics of Fluids, 37(3).