Numerical Simulation of a Submersible Buoy Using a Wake Oscillator Model Calibrated for VIM

Abstract

New discoveries of petroleum reservoirs in ultra deep-water depths, like Pre-salt fields in Santos Basin, are demanding new riser systems concepts. In this scenario, the Free-Standing Hybrid Riser (FSHR) system is a viable choice. A submersible buoy connected by rigid and flexible risers constitutes this riser system. The sea current can cause the Vortex-Induced Motion (VIM) of the buoy, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. In this context, wake oscillator models used for VIV prediction in actual form is not adequate for the VIM prediction of the buoys. In this paper, a VIV wake oscillator model is calibrated for VIM, through hydrodynamic coefficients found in the technical literature. In order to verify accuracy, the VIM calibrated wake oscillator model is used to reproduce some FSHR reduced model tests. The results of amplitude and frequency of oscillation against the reduced velocity obtained from the numerical simulation are compared with the experimental results. The numerical results presented the same trend with some differences in amplitude. The amplitude deviation could be related to the hydrodynamics coefficients used in the calibration of the wake oscillator model.