Numerical and experimental comparisons of vortex-induced vibrations of marine risers in uniform/sheared currents

This paper presents a general theoretical reduced-order model capable of evaluating the multi-mode nonlinear dynamics of marine risers subject to uniform and sheared currents. The main objectives are to predict the vortex-induced vibration responses and parametrically compare between numerical and experimental results. The emphasis is placed on the analysis of cross-flow vibrations due to unsteady lift forces. The nonlinear equations governing riser axial/transversal motions are derived based on a top-tensioned beam model with typical pinned-pinned boundary conditions. The riser geometric nonlinearities owing to possible large dynamic displacements and multi-mode interactions are accounted for. To approximate the space-time varying lift force, the empirical hydrodynamic model, based on a nonlinear van der Pol wake oscillator with a distributed diffusive term, is used. A low-dimensional dynamic model and computationally-robust time-domain tool are then developed to evaluate the multi-mode fluid-riser interactions. These are very useful in dealing with large parametric studies involving varying system parameters. Comparisons of numerical and experimental results are performed by estimating riser response amplitudes and fatigue damage indices. Both linear and nonlinear risers are considered in the present numerical model whereas only linear riser has been considered by a referenced literature in the reconstruction of experimental displacements through measured strains. It is found that riser geometric nonlinearities play a significant role in both numerical simulations and comparisons with experiment post-processed results. In some cases, quantitative/qualitative discrepancies in riser response predictions are remarkable with linear vs. nonlinear models. These may be recognized as one of the factors why recent numerical and experimental comparisons in literature have been unsuccessful.