Experiments are described whose objective was to investigate the combined effect of current and waves by studying an elastically mounted circular cylinder in a time-varying flow. This is relevant for risers subjected to both current and wave-induced motion. The rigid cylinder section was 75 mm in diameter and 2 meters long and mounted horizontally in a towing tank. The time-varying flow consisted of a constant and a harmonic velocity component. The general conclusion from the experiments is that both the amplitude and acceleration response of a cylinder subjected to constant + harmonic current is smaller than for constant current only.
Vortex-induced vibrations (VIV) from current alone may represent a severe problem for risers in deep water both by contributing to the fatigue damage accumulation and by increasing the drag force to an extent that causes operational restrictions. Most available VIV prediction tools are developed to account for current only, and the empirical support is relatively extensive for this situation (although critical and supercritical Reynolds number data are scarce). However, the situation with both current and waves is much less known. How, and how much, will the waves affect the transverse response of a riser? The wave direction is often the same as the current direction, and both wave-induced forces and floater motion may very well induce a nearly harmonic relative velocity in addition to the constant current velocity. The main objective of the present work was to investigate the combined effect of current and waves by using an elastically mounted rigid cylinder section in a variable flow. The variable flow consisted of both a constant and a harmonic velocity component. The case where the wave-induced motion totally dominates the velocity (average current velocity equal zero) has been investigated by several researchers.
