Predictions and Model Tests of an SCR Undergoing VIV in Flow at Oblique Angles

The paper describes how an existing computer code, VICoMo, see Moe & Arntsen (2002), has been extended to analyse Vortex Induced Vibrations (VIV) of SCR’s for situations in which the flow is neither in the plane of the riser, nor perpendicular to it. Various strategies were attempted in order to make the solution deal with flows at oblique angles. It was found that the best approach was to assume that the riser motions occurred in plane perpendicular to the flow, while motions in the flow direction were disregarded. One source of uncertainty is associated with the empirical database in VICoMo which is based on tests in which the flow was perpendicular to the riser elements. The timeseries from the experiment showed large temporal variations. Actually the average motions amplitudes at given points may be as low half of the maximum value. Also the largest amplitudes at different points on the experimental envelope on the riser will usually have been reached at quite different times. These features were not included in the theoretical model, and made direct comparisons between predictions and measurements difficult. We ended up comparing the envelopes of motion response. In several of the cases the predictions matched the experiments closely, for frequency, modeshape as well as amplitudes. In other cases the match was somewhat less satisfactory, however.