Vortex induced vibrations (VIV) are generally less critical in wave dominated flow conditions than in pure current flows. A steady state response needs time to build up and continuous variation in flow velocity generally reduces the vibration amplitudes. For low Keulegan-Carpenter (KC) flows in-line VIV is generally removed entirely, replaced by forced response at the wave frequency, while cross-flow vibration amplitudes are reduced. For large KC numbers, the wave induced flow behaves similarly to a current and the oscillatory nature of the flow no longer influences the VIV response as much.
Empirical models to predict the influence of waves in VIV design of offshore cylindrical structures are formulated in offshore design codes. For most flow regimes the models are sound and supported by a solid base of empirical test data. There are, however, exceptions — particularly for the low KC number regime, defined here as LKCR. For such flows the oscillating pressure differentials which cause vortex induced vibrations are no longer governed by traditional dimensionless parameters such as the reduced velocity, but instead the oscillating lift is governed by the ratio between the eigen-frequency of the structure and the wave frequency. Particularly, the frequency of the lift force is twice the wave frequency in regular waves. In irregular wave conditions there are necessarily also spectral peaks at both the actual wave frequency and at three times the wave frequency, but the governing spectral density is concentrated at two times the wave frequency.
The present study introduces a novel response model to conservatively assess cross-flow VIV in LKCR.
