Session: 08-07-01 Internal Flows & FIV

Submission Number: 156033

On Prediction of FIP in Closed Side Branches by CFD 

In single phase internal flow past closed side branches, the shear layer instability in the branch opening provides a tonal acoustic source propagating into the side branch. In the event that one of the dominant frequencies of the shear layer oscillation coincide with an acoustic mode of the fluid in the side branch, the fluid dynamics and acoustics lock-in and high amplitude resonant conditions can occur. If, in turn, the lock-in mode coincides with a structural mode of the piping system, the vibrations caused by the vibro-acoustic coupling can lead to rapid fatigue failure.

The acoustic pressure amplitude of the lock-in mode at the end of the closed side branch can be estimated via different means, such as energy balance methods or plane wave acoustic models. The required input source strength can be estimated empirically or estimated by Panel Methods. Another method, previously discussed in Macchion et al (2016) and other publications, involves the use of computational fluid dynamics (CFD) which permit a more accurate representation of the three-dimensional geometry.

This paper investigates and benchmarks the use of CFD to predict the occurrence and strength of the lock-in amplification in single and coaxial closed side branches by comparing experimental data with results from improved delayed detached eddy simulations (IDDES) and Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. The URANS results show good correlation with the experimental data, both in terms of when lock-in occurs and in terms of the associated strength of the amplified pressure wave. The IDDES results show similar correlation for some experimental points. However, for other, the strength of the amplified pressure wave is severely over predicted. This is possibly due to unphysical excitation of the lock-in mode through broadband turbulent noise caused by uncertain inlet boundary conditions.

The results indicate that for the tonal acoustic source of the shear layer instability and its acoustic lock-in amplification, URANS simulations provide equally good predictions as significantly more expensive IDDES simulations. In addition, uncertain boundary conditions to the IDDES simulations infer a risk of over predicting pressure amplitudes through unphysical turbulent excitation of the lock-in mode. The relatively low cost of URANS simulations also enables CFD analysis of a wider range of operating conditions. This, together with well and production data, allows for statistical prediction of the likelihood of lock-in within the operational envelope of the pipe system or equipment.

Keywords: Flow induced vibrations (FIV), fatigue failure, flow induced pulsations (FIP), CFD

Presenting Author: Oskar Tylén OneSubsea

Presenting Author Biography: CFD Engineer working with various analyses of subsea production and processing systems at OneSubsea in Sweden.