CFD analysis of a high pressure 2” pipe test loop with water-gas flow was undertaken using three different solvers. Multiphase flow induced forces were predicted on the bends at a range of operating pressures between 10 and 80 barg and compared with forces reconstructed from vibration measurements. Overall the three different CFD solvers predicted consistent results. The fluid forces predicted on the bends of the double U-loop test rig have a good range of values compared to the test reconstructed forces. The forces predicted at low pressure were in line with the experimental reconstructed values, whilst at high pressure all three CFD solvers predicted higher forces. The trend of the forces reducing with increased operating pressure, evident in test, was matched by one of the CFD methods, but less well by the other two. At low operating pressure the forces are dominated by the momentum of the liquid in the multiphase flow, whilst at high pressure the pressure fluctuations and turbulent effects will be more important. All three CFD solvers use VOF methods and above about 40 barg it is possible that they struggle to fully resolve the flow behaviour, which will be more influenced by bubble and droplet entrainment and turbulence. Multiphase flow can induce high amplitude vibrations in piping systems, potentially leading to fatigue failures. CFD modelling offers a potentially powerful tool to provide the flow induced forces required for assessing and diagnosing multiphase flow induced pipework vibration problems in hydrocarbon production systems.
Multiphase flow through pipelines can result in severe pipe vibrations and fatigue damage. Commonly, finite element analyses are used to define the dynamic pipe stresses and calculate the lifetime of the piping. The fluid flow excitation forces can be estimated with analytical models or simulated in detail using computational fluid dynamics. If the vibration amplitudes are small, the fluid structure interaction is modelled uncoupled but additional damping due to fluid structure interaction should be added to the structural damping. When the uncoupled approach can be used, the methodology can be simplified by using random vibration theory in the frequency domain. The study, where forces and pipe acceleration of a 1″ multiple bend piping system are compared with simulation results, is presented in two papers. In this part (part-1), the focus is on the description of the experimental setup and on the statistical analysis of the data. This includes the statistical description of the measurement to determine how long does one need to measure or simulate to get a statistically converged solution. In addition, a detailed comparison is made between the damage analysis via the Rainflow counting (in the time domain) and the Dirlik damage calculation (from the frequency domain). Simulated pipe movements of fully coupled numerical simulations and simplified methods, as uncoupling fluid-structure simulations and random vibration estimation are compared with measured pipe movements in a second paper (part-2, OMAE2022-78758).
Slug flow, a flow pattern with alternating aerated liquid pockets (slugs) and large gas bubbles, is a commonly observed flow pattern in oil and gas pipelines. Due to its unsteady character, the force on a pipe bend is fluctuating which results in unacceptable motions when the piping is insufficiently supported. To investigate the risk of fatigue failure of the system, finite-element models are used to predict the dynamic stresses required to estimate the fatigue life of the system. The excitation force of the slug flow is the essential input required for accurate fatigue damage predictions. A new, simplified model of slug forces on a bend is proposed. The model is calculating the slug force by solving the momentum balance over the pipe bend using slug flow properties as liquid holdup and phase velocities. Average properties predicted by a unit slug model cannot predict the stochastic force variations caused by the slug flow. The new approach introduces the stochastic character of slug flow in the force calculations via a log-normal slug length distribution. A Lagrangian slug tracking method is used to solve the governing equations. The modelled liquid holdup, pressure and predicted forces are compared with available measurements and Computational Fluid Dynamics calculations. The measurements were done under atmospheric conditions and the fluids used were air and water. Whether these measurements are representative for high-pressure oil and gas slug flow is unknown. By using a mechanistic approach where the main equations are based on physical laws instead of fitted measured data, the model is applicable for different fluids and operational conditions. To validate the model for oil and gas flows, the results are compared with Computational Fluid Dynamics calculations done with high gas density and typical oil viscosity.
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