A free-hanging riser is a typical riser configuration seen in the disconnected drilling riser, the water-intake riser and the deep-sea mining riser. In offshore productions, these marine risers will move back and forth in water and further generate an equivalent oscillatory current around themselves, due to the vessel motions. Both in marine operations in the field and model tests, it has been reported that such oscillatory current lead to riser vortex-induced vibration (VIV) and cause structural fatigue damage. Recently, there have been some attempts to numerically predict vessel motion-induced VIV on the compliant production risers, with emphasize on relatively large Keulegan-Carpenter (KC) numbers. In the real marine operations, the risers experience small KC number scenarios during most of their service life. Therefore, the investigation of vessel motion-induced VIV under small KC number is of great significance, especially considering its contribution to fatigue damage. In this paper, numerical investigation of VIV of a free-hanging riser attached to a floating vessel is carried out. A new response frequency model for vessel motion-induced VIV under small KC numbers is proposed and implemented in VIVANA. Validation of the proposed numerical methodology is performed against the published experimental results, where a good agreement is achieved.

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