The paper presents the basic principles and the main features of a new model for Vortex Induced Vibrations analysis in vertical tensioned risers. Such a model is based on the Finite Element Method to account for riser pipe's structural behaviour and on a distribution of Equivalent Oscillators to reproduce the forces that water vortexes induce on riser's external surface. The equivalent oscillator formulation and calibration are based on series of simple experiments that are described in the paper. The model is implemented in a software called MOTIVA (Motion Time Domain Induced Vibration Analysis) that performs VIV analyses in the time domain. Time histories of displacements and stresses are produced for any location along the riser so that fatigue damage due to VIV can be evaluated. The performance of the MOTIVA code was verified by means of a series of benchmarking exercises performed against experimental data derived from both laboratory and full scale tests. Results of such comparisons are also presented in the paper.

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