An Assessment of Anti-Flip Technology for Rough Bore Flexible Piping Using CFD Modelling

Abstract Flow Induced Pulsation (FLIP) can occur under specific conditions when gas flows through rough bore flexible pipe systems and produces distinct tonal noise (singing riser) caused by the feedback between pressure waves and vortex shedding. This phenomenon can lead to environmental concerns and structural vibration in connected pipework or equipment. Baker Hughes have proposed a low impact solution for FLIP mitigation in rough bore flexible pipe which, unlike any other solution, does not require the use of an insert covering the inward facing carcass gaps to provide FLIP protection. By controlling surface roughness on the carcass bore, the onset velocity for FLIP can be delayed or eliminated. This study explores a computational approach to evaluating this solution. Computational Fluid Dynamics (CFD) modelling is employed to predict strength and frequency of the pressure oscillations produced by the vortex shedding, which is the source of FLIP. Modelling is performed initially on a known carcass geometry to demonstrate that the CFD methodology can predict FLIP to a suitable degree of accuracy. This methodology is then used on the same carcass geometry with the anti-FLIP technology applied to assess if the onset of FLIP is delayed or eliminated. The strength of pulsating vortices created by the flow interaction with the corrugation is evaluated by monitoring static pressure at the entrance of each cavity. A Fast Fourier Transform (FFT) is applied to the time varying pressure to assess the frequency content of the oscillations. If the CFD simulations of the carcass geometry indicates a well-defined vortex within the cavities, with distinct frequency, this could suggest the presence of FLIP. The phenomenon appears when vortex shedding has the capacity to excite the natural frequency of system components and create a lock-in frequency between the pressure flow wave and vortex shedding. Hence, FLIP is predicted when the FFT frequency analysis shows well-defined peaks whose frequency and magnitude increase with flow speed. Strouhal number analysis indicated that the onset velocity of FLIP as predicted by CFD matched that calculated by an industry standard tool developed based on experimental and field data. The anti-FLIP technology disturbs the vortex shedding process by creating a thinner, but more turbulent boundary layer, which increases the shear stress. Simulations predict a reduction of the peak frequency and amplitude of the pressure oscillations. Furthermore, additional noise was observed in the frequency spectrum, resulting in less well-defined peaks and which may help to mitigate FLIP within normal operating parameters. This technology forms part of a suite of Baker Hughes’ anti-FLIP solutions. Experimental validation of the observed phenomena will be a primary focus of future research effort.