NON-DIMENSIONAL DATA-DRIVEN MODELS OF COMPUTATIONAL FLUID DYNAMICS (CFD) OF MULTI-PHASE FLOW IN ANNULUS

In the offshore drilling process of an oil and gas well, the blowout preventer (BOP) serves as a critical safety device to monitor and control wells to prevent blowouts. However, it is usually subject to the high-temperature and high-pressure (HTHP) conditions. These extreme working conditions challenge the BOP’s robustness and threaten its liability. For example, the cavitation damage the elastomer sealing surface and lead to BOP malfunction or failure. To understand the flow characteristics under extreme working conditions and ultimately enable the estimation of the BOP lifetime, experimental approaches and multi-physics numerical simulations can be employed. However, the full-scale experiments are very costly, and the coupled full-scale simulations are time-consuming. To overcome these challenges, a scalable model is required to connect the cavitation criterion, pressure conditions, and geometric parameters of the BOP sealing. In the present work, the BOP sealing was simplified as a converging-diverging annulus. Various annulus geometries necessary to quantify annulus flow were identified and investigated using computational fluid dynamics (CFD) tools. Based on the simulation results, the criterion of thin or thick annulus was established. In addition, two non-dimensional data-driven models were developed to describe the cavitation conditions in terms of downstream/upstream pressure ratio and non-dimensional geometric parameters. The first model evaluated the critical downstream/upstream pressure ratio to cause cavitation, and the corresponding discharge coefficient of the annulus system under two-phase (liquid, vapor) flow condition. The second model extended to the three-phase (liquid, vapor, air) situation, closer to the actual offshore BOP working conditions. Using the obtained non-dimensional models, the annulus flow rate of either two-phase or three-phase flow can be estimated under both choked and unchoked flow conditions across multi-scales. The study results can provide numerical guidance for small-scale annulus flow experiments, build connections between small-scale experiment results and full-scale applications, and ultimately enhance future governing standards to evaluate flows in blowout preventer (BOP) and its potential failure in the oil and gas industry.