The increasing demand for oil and gas, currently estimated at 135 million barrels of oil equivalent per day, keeps pushing the boundaries of offshore engineering into ever deeper waters. Exploration and production activities in the Gulf of Mexico, for instance, are performed in water depths exceeding 3000 meters. For such deepwater developments, the suspended length of the marine risers adds up to several kilometers. When designing and installing risers in (ultra)deep water, the length/diameter aspect ratio of the marine riser can exceed L/D > 1000, and the features of the fluid flow in depth direction can no longer be neglected. Indeed, both the magnitude and the direction of the current change with water depth, giving rise to higher harmonics in the VIV response.
The prediction of vortex induced vibrations for deepwater risers is very challenging, owing to the fact that the incident flows are non-uniform and the associated fluid structure interaction phenomena are highly complex. These complex conditions give rise to a non linear coupled system with a large number of degrees of freedom, which depends on several physical and mechanical parameters.
In this paper, 3D CFD calculations are performed to evaluate the effect of the third dimension for risers subjected to uniform flow and sheared currents. For a uniform current velocity at the inlet boundary, it is shown that vortex shedding in the wake of long slender tubulars can give rise to the development of vortices with horizontal axis, resulting in a fluctuation of the flow in the Z-direction. These three dimensional vortices are strong enough to modulate the vortex shedding on the riser as a function of depth. The 3D simulations with uniform current velocity are then compared to marine risers subjected to sheared currents. It is shown that the presence of sheared currents invokes a shift in both phase and frequency of the vortex shedding.