Riser modal identification in CFD and full-scale experiments

A systematic methodology is presented to obtain the vortex-induced vibrational modes of a riser, based on data from CFD coupled to a long beam under tension and placed in sheared cross-flow; two profiles were tested: (a) linear, and (b) exponential. The modes we estimate are in fact nonlinear equilibria between the flow-induced excitation forces and the structural dynamics and are characterized by varying amplitude and phase along the span; these are complex modes, mixtures of traveling and standing waves. Simplifying procedures to represent the VIV response in terms of a few clusters of modes have been applied successfully, reducing substantially the data needed to represent the VIV response.