CFD Investigation for Vortex-Induced Vibration of Flexible Risers

Steel flexible risers and electrical cables are critical components in floating offshore production platforms and offshore renewable energy systems such as floating wind turbines and solar farms. These long, slender, flexible structures are often subject to vortex induced vibrations (VIV) due to the complex interaction of the flexible structure with the varying hydrodynamic loading in current due to vortex shedding.

In the design and fatigue assessment of these flexible structures it is essential to be able to accurately predict the motions and deformations of the structures. Computational Fluid Dynamics (CFD) has the potential to provide these answers due to its full, non-linear modelling of the complex physics involved. However, CFD is still in its early stages for predicting Fluid-Structure Interaction (FSI) due to the complex nature of the flow around cylinders, the complexity of the coupling of the structural model with the hydrodynamic flow solvers and the calculation times involved, the latter leading to large HPC requirements.

In this paper a VIV study is presented for a long, flexible riser with L/D = 380. The CFD code ReFRESCO is utilized using its recently implemented FSI capabilities. A comparison is done to high quality model test results, where the riser was towed through a basin laboratory under different current velocities. The main objective of this paper is to investigate whether the response of the flexible riser can be assessed in the CFD calculations.

For the in-line flow direction the calculated CFD results match reasonably well with the experiments for the local deformations and response modes of the riser. However, for the cross-flow direction, it can be observed that higher modes seem to be more dominant in the CFD results than in the measurements. This could originate from the complexity in modeling the boundary conditions in the structural model and/or in the complexity of the flow around a cylindrical structure for these Reynolds numbers, and/or a difference in internal mechanical damping between CFD and model test. Therefore, further research is required for accurate VIV assessment with CFD.

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