CFD modelling of flow-induced vibration under multiphase flow regimes

Internal multiphase flow-induced vibration (MFIV) in pipe bends poses serious problems in oil and gas, nuclear and chemical flow systems. The problems include: high amplitude displacement of the pipe structure due to resonance; fatigue failure due to excessive cyclic stress, induced by fluctuating forces; and structural wear, due to the relative motion of the pipe and its support. Current industry guidelines are based on single phase flows, while the few existing MFIV models in literature are based on small scale laboratory experiments, which do not completely address the complexities in multiphase flows, or the differing multiphase flow mechanisms between small and large pipes. Therefore, numerical simulations of two-phase flow induced fluctuating forces, stresses, displacements and natural frequencies at 900 bends have been carried out, in order to investigate the characteristics of MFIV in pipes of 0.0525m, 0.1016m and 0.2032m internal diameters (I.D.). An integrated high-fidelity CFD and FEA-based numerical-analytical modelling framework was applied, to predict the defining characteristics of MFIV in the pipes. The CFD simulations of thirty-five cases of slug, cap bubbly and churn turbulent flow-induced fluctuations at the bends were carried out using the volume of fluid (VOF) model for the two-phase flows, and the κ - ε model for turbulence modelling. A one-way fluid-structure interaction was carried out to evaluate stress and displacement. Simulations results based on 0.0525m I.D. show good agreement of the volume fraction fluctuation frequencies of slug and churn flows with the reported experiment. The behaviours of the flow induced void fraction, forces and stress as functions of gas superficial velocities in the 0.0525m I.D. pipe showed a good correlation to the observed behaviours in the 0.2032m I.D. pipe. The same correlation was not prominent in the 0.1016m I.D. pipe and was attributed to the transition behaviour of gas-liquid two-phase flows caused by Taylor instability in pipes of non-dimensional hydraulic diameter of 18.5 < DH * < 40. Also, based on the present study, modification of Riverin correlation which was based on small scale laboratory experiment to predict RMS of flow induced forces was carried out by adjusting the constant parameter C to 20. This modification, improved the predictive capability of the model for a wider range of pipe sizes and gas volumetric fractions between 40% and 80%. The significant findings in this study would be useful input in developing a comprehensive industry guideline for MFIV.