Experimental Measurement of Large-Amplitude Intermittent Vibrations of an Inclined Bendable Riser Transporting Unsteady Multiphase Flows

Abstract

Abstract Marine risers transporting unsteady multiphase flows may experience internal flow-induced vibrations depending on several flow-structure parameters. This study presents an experimental investigation of large-amplitude intermittent vibrations of an inclined bendable riser transporting an air-water flow that exhibits an unsteady slug flow pattern. The riser has an initial catenary profile with a length over an internal diameter of about 320. By using a non-intrusive measurement with a high-speed motion capture camera, new insights into the riser dynamic features and slug flow-riser interaction phenomena are presented and discussed in the case of varying gas-to-liquid flow rate ratio (RGL). Pressure signals are also recorded at the riser inlet and outlet, measuring the pressure fluctuation caused by the hydrostatic change, flow-induced momentum and frictional forces. For a given liquid flow rate or superficial velocity, the inclined bendable riser experiences a modified mean drift of the equilibrium configuration and a greater vibration amplitude when increasing RGL or the superficial gas velocity. The most critical case has a spatially maximum amplitude of greater than six diameters of the riser pipe. Experimental results of the space-time varying responses and frequencies of the riser are analysed in relation to the associated slug flow features including the slug unit length, translational velocity, frequency and pressure fluctuations. Owing to a coupled slug flow-riser interaction, the liquid slugs and elongated gas bubbles vary spatially and temporally. This entails the highly modulated responses and frequencies, intermittently switching the riser multimodal response, especially when RGL is greater than two. The amplified bending stresses in the riser are reported, highlighting the critical hydrodynamic effect of unsteady multiphase flow. The characteristic slug frequencies associated with the flexible inclined riser dynamics are also evaluated and compared with those predicted by several empirical correlations in the literature based on stationary, straight and rigid pipes. Overall, experimental observations and data are useful for a numerical model development and comparison towards a practical analysis and design of flexible curved/sagged risers transporting multiphase flows.