Surface roughness effects on a tensioned riser vortex-induced vibration in the uniform current

Abstract

Due to the long-term operation of marine riser, marine organisms will adsorb on its surface, thus affect its surface roughness. The surface roughness of the riser will affect the wake flow field of the riser, and then affect the dynamic characteristics of the vortex-induced vibration (VIV). The study of the influence of surface roughness on the characteristics of a rough riser VIV is helpful to improve our understanding of the mechanism of a rough riser VIV and ensure the safety of the riser during its service life. Based on the methods of computational fluid dynamics (CFD) and computational structure dynamics (CSD), this study presents the model considering tension variation with the structure vibration and the modified model of rough wall velocity gradient to construct the numerical calculation program of a rough riser VIV to study effects of various key parameters (surface roughness, inflow velocities) on the dynamic characteristics, vibration response, wake vortex shedding patterns and vibration trajectories of a rough riser. The results show that in the range of Reynolds number Re = 2.515 × 104∼1.258×105 and reduced velocity U* = 1.35–6.74, compared with a smooth riser, the streamwise vibration amplitude of a rough riser decreases, and with the increase of inflow velocity and surface roughness, the streamwise vibration amplitude of a rough riser decreases more and more obviously. In the range of reduced velocity U* = 2.70–5.39, the transverse vibration amplitude of a rough riser increases slightly. At a higher reduced velocity (U* = 6.74), the transverse vibration amplitude of the rough riser decreases. Larger surface roughness will enhance the multi-frequency and broadband vibration characteristics of the riser in the streamwise direction. With the increase of surface roughness, the multi-frequency vibration characteristics decrease gradually, but the broadband vibration characteristics increase gradually in the transverse direction. Surface roughness can suppress 2T and 2P wake vortex shedding pattern and enhance 2C wake vortex shedding pattern. This study will provide a profound understanding to guide the practical marine riser VIV research with the consideration of surface roughness.