VIV analysis of a riser subjected to step and multi-directional currents

A multi-strip numerical method combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a finite-element structural dynamics response has been developed previously to analyze the flow-structure interaction of a flexible riser subjected to fixed and non-uniform, two-dimensional shear currents. In this paper, we expand on the previous work using the tool to numerically compute the VIV loads and motions of a vertically tensioned riser in a stepped current. The flow conditions for this stepped current configuration were chosen to match a set of laboratory experiments carried out in the Delta Flume at Delft Hydraulics. In addition to the stepped current, the multi-strip method was extended to accommodate a three-dimensional skew current exposed to a vertically tensioned riser. Note that in this case, the skew current is non-uniform in both direction and magnitude, and the flow conditions and riser configuration were chosen to match a set of rotating-rig experiments made by Marintek. For both configurations (stepped and skewed currents), comparisons of in-line and transverse VIV displacements are presented between numerical and experimental results.