Numerical investigations on the VIV response of a deep-water drilling riser with buoyancy modules

Abstract

To reduce the effective submerged weight of deep-water drilling risers, buoyancy modules (BMs) are strategically installed at periodic intervals, predominantly near the upper section of the riser. The inclusion of BMs significantly influences the riser's vortex-induced vibration (VIV) response under challenging offshore environments. The present study comprehensively investigates the coupled crossflow (CF) and in-line (IL) VIV responses of a deep-water drilling riser with buoyancy modules subjected to linear shear flow. The governing equations of the coupled structural and wake oscillator are discretised and solved using the finite difference method of the second order, in both temporal and spatial domains. Both constant axial tension and depth-dependent variable tension models are evaluated, with results indicating that the constant tension assumption systematically overpredicts VIV amplitudes by approximately 5 % to 40 %, depending on flow and structural conditions. Furthermore, a parameter sensitivity analysis is conducted by varying key environmental factors, riser parameters and operational parameters. The primary VIV response metrics are assessed and compared, including root-mean-square (RMS) displacement amplitudes in both IL and CF directions, vibration mode envelopes, and temporal displacement histories. The results reveal that increased flow velocities intensify modal excitation, leading to enhanced IL and CF vibration amplitudes. Conversely, elevated top tension ratios and increased riser wall thickness are shown to suppress vibration amplitudes by approximately 9 % to 22 %. Additionally, specific combinations of internal fluid density and riser diameter contribute to damping the structural response, resulting in reduced dynamic amplification and more stable vibration patterns. The results reveal that configuring the riser with ten buoyancy modules uniformly spaced at 15 m intervals results in an approximate attenuation of 10 % in CF and 20 % in IL peak vibration amplitudes, under operating conditions at a water depth of 1000 m.