This conference paper was submitted for presentation at the NAFEMS World Congress 2025, held in Salzburg, Austria from May 19–22, 2025.

Abstract

This paper describes numerical analysis of unsteady flow phenomena in the metering system of a gas production platform. The unsteady flow had previously been identified as the root cause of a vibration issue, which can lead to fatigue failures of attached small-bore pipes, and had constrained the operating conditions of the platform for many years. The highest level of vibrations were linked to a resonance peak in the unsteady pressure between 33 and 35 Hz, with the resonance frequency varying with flow velocity and other flow conditions. Various mitigation measures and operational recommendations were applied to avoid structural damage, but to remove the flow instabilities in the inlet and outlet header of the metering system and fully resolve the problem, a substantial and expensive redesign of the pipeline was required. In order to support such extensive task, ISVR Consulting has used numerical CFD modelling to simulate the flow through the different parts of the system to identify the main excitation mechanism based on unsteady flow instability, linked to acoustic feedback and the acoustic modes of the metering trains, and correlated to the various flow regime and velocity. The key feedback mechanism in the inlet header was identified as a so-called Rossiter Tone, in which vortex shedding from a bifurcation into two metering lines caused aeroacoustic feedback from features further downstream. Surprisingly, when the flow from the two lines merged again at the outlet header this was also found to be a source of instability generating another Rossiter tone at a similar frequency. For this study, we have chosen to use OpenFOAM, a well-developed open-source software, with the potentiality to solve both unsteady CFD and acoustics, because, despite all the limitations of applying direct methods in aeroacoustics, we were mainly interested in understanding how the flow vorticity and pulsating pressure were linked in a specific low frequency tonal problem. Results have provided the basis for an in-depth understanding and localization of the aeroacoustic mechanism in the different areas of the pipeline and have supported recommendations for an efficient redesign of the entire system to control the problem at source.