Vortex induced vibrations (VIV) of long flexible risers subjected to ocean currents are of vital interest to the offshore industry. Although significant efforts have been seen during the last decades, reliable prediction of this complicated fluid structure interaction phenomenon is still a challenge.

The primary objective of this paper is to characterize the frequency components of VIV measured in flexible beams subjected to sheared current, and try to establish a general model for frequency participation for use in semi-empirical models for calculation of fatigue damage from VIV. Experimental data from the well known Hanøytangen tests and the Norwegian Deep-water Programme (NDP) high mode experiments have been used in this study. The present paper is mainly based on results from Ziguang Zhao (2011).

Wavelet analyses are applied to reveal the frequency components in the measured signals. These analyses give information on the time-varying intensity of each active frequency at a specific position on the beam. The dominating frequency and range of other active frequencies are two key parameters from the wavelet analyses that are further used in this work.

By comparing synchronic measurements from various positions along the beam, we can see that neighbor locations often will display the same time-varying peak frequency. However, the response at two positions apart form each other may be dominated by different frequencies. Hence, the time sharing concept needs to be reformulated from describing a frequency variation valid for the total length of a riser, to consider different zones separately. Based on the observation above, a combined time sharing and space sharing model is proposed. The controlling parameters in this model are an energy based parameter that ranks the participating frequencies, a threshold for a frequency candidate to become active, and the length of the excitation zone for each frequency. All parameters can easily be found for cases of practical interest.

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