Time Domain Riser VIV Predictions Compared to Field and Laboratory Test Data

The time domain code ABAVIV has demonstrated the ability to reproduce basic features of riser vortex induced vibration (VIV). The VIV response of risers is a transient phenomenon. Consequently, time domain computer codes have a better basis in principle than frequency domain codes to model and predict the riser VIV response. Frequency domain codes have been used extensively by the offshore industry to calculate riser VIV response and to design risers for VIV fatigue. These codes have been successful in riser design by making conservative assumptions in modeling the VIV phenomenon and by using large safety factors. This paper shows comparisons of ABAVIV predictions to two sets of data — one from laboratory tests and the other from field tests. The laboratory tests were performed at the Imperial College with a smooth riser model. The field tests were performed as part of the DeepStar program and were carried out offshore Miami, Florida, with a riser model hanging from a moving boat. In the DeepStar tests, the current direction varied with depth, and wave flow was present along with the current. The tests included both smooth and straked riser models. The data from these tests demonstrated the transient character of the VIV response and the strong presence of higher harmonics. All available frequency domain codes lack the ability to model and predict the higher harmonic response. The paper will show that ABAVIV has the capability to model the higher harmonics of the VIV response with appropriate modifications in the lift force model.