Simulating fundamental and higher harmonic VIV of slender structures

Abstract

Abstract Vortex-induced vibrations (VIV) of slender marine structures are complicated response processes, where a number of distinct frequency components might be simultaneously active. These are categorized as fundamental and higher harmonics respectively, where the latter can have a tremendous impact on the fatigue life. The present work proposes a method for time domain simulation of such multi-frequency response, by introducing a higher harmonic load term to a pre-existing semi-empirical hydrodynamic force model. Forced motion tests of a circular cylinder were simulated to experimentally and qualitatively validate the fluid-structure energy transfer. Next, the model was used to predict the response of a tension dominated riser in uniform current, for 22 velocities in the range 0.3\u202fm/s to 2.4\u202fm/s. The empirical input was chosen to give an average best fit with respect to the cross-flow strain measurements, which allowed dominating frequencies, fatigue damage, higher harmonics and response variability to be predicted with a high level of realism. Same set of empirical coefficients was subsequently used to predict VIV of three additional flexible pipe experiments in uniform flow, with significant differences in structural properties. The results were satisfactory for all cases, but could be improved by moderate changes to the empirical input.