Sensitivity study of wake oscillator parameters influencing the fatigue damage of a riser undergoing VIV

A great deal of attention has recently been paid to the semiempirical modelling, nonlinear response prediction and estimation of stress and fatigue damage of marine risers undergoing vortex-induced vibrations (VIV). This is because when as the offshore industry move into deeper waters, the impact of VIV caused by ocean currents becomes detrimental being one of great concerns for offshore operators and riser engineers. For computational efficiency in time domain, the van der Pol wake oscillator, coupled with the structural equation of motion, has been an attractive approach. However, wake oscillator models rely on empirical coefficients, which have uncertainties whose effect on the fatigue reliability is not well understood. This study focusses on the wake oscillator proposed by Skop and Balasubramanian (1997). Using a toptensioned riser model, a sensitivity study is performed on several random variables to determine their influence on the uncertainty of the fatigue damage. These variables include the maximum vibration amplitude, frequency ratio, damping ratio and lift coefficient. In addition, Monte Carlo simulations are conducted to predict the probability of failure