Lock-In, Transient and Chaotic Response in Riser VIV

We show using experimental data on a model riser that lock-in of long flexible risers placed in sheared or uniform cross-flows is a much richer phenomenon than lock-in of flexibly-mounted rigid cylinders under similar conditions. In particular, we find that the frequency content of the riser response may be either narrow-banded around a single dominant frequency (Type I response) or distributed along a relatively broad range of frequencies (Type II response). Distinct transition from Type I to Type II response, and vice versa, can occur several times within a single experimental record. Type I responses reveal features of a quasi-periodic oscillation, often accompanied by large 3^rd harmonic components in the acceleration and strain signals, increased correlation length, stable riser trajectories, and monochromatic traveling or standing waves. Type II responses, on the other hand, are characterized by features of chaotic oscillation with small or negligible 3^rd harmonic components in the acceleration and strain signals, reduced correlation length, and a continuous spectrum. We study how the fatigue damage differs in the two types of riser response.