Calculation of fatigue damage for tensioned risers from vortex induced vibrations
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- Institutt for marin teknikk 
Vortex induced vibration (VIV) of long flexible riser subjecting to ocean currents is ubiquitous in offshore industry. Although significant efforts have done to understand this complicated fluid structure interaction problem, the numerical modeling and predicting is still a big challenge. The primary objective of this thesis is to characterize the frequency components of VIV response measured in flexible riser model tests under shear current, and try to establish a general frequency competition system for semi-empirical model VIVANA. As the base of present work, experimental data from Norwegian Deepwater Programme (NDP) and Hanoytangen tests are studied. Wavelet analysis method is applied to reveal the frequency components included in measured signals, time-varying intensity of each active frequency will be present as wavelet result. Peak frequency and frequency range defined as two key parameter of wavelet result for further studying. In experiment, the neighbored locations on the riser often display same time-varying peak frequency, rather the whole riser length as assumed by VIVANA. In the other word, the time sharing method maybe better to perform in different zones separately. Based on the observation above, an ideal model combined time sharing and space sharing are proposed. In addition, by observations of VIVANA results, energy limit and excitation length are attempted to limit the active frequency range in VIVANA. The frequency components in measured signal are compared with eigenfrequencies in still water and eigenfrequencies calculated by Riflex, the differences are stated after discussion. Displacement histories corresponding to single participating mode are composed by modal analysis method, the frequency components of these signals are analyzed by wavelet, an unexpected observation that all decomposed signals have similar frequency components is found, Rayleigh-Ritz method and mono-frequency response are proposed as explanation. Some other study like high harmonic effects and coexistence of chaotic and stationary signals in NDP are included in this thesis, these can induced more interesting topics for future research.