Variability effect of the drag and inertia coefficients on the Morison wave force acting on a fixed vertical cylinder in irregular waves

This paper examines the effect of the drag and inertia coefficients used in Morison's equation as predicted by some calibration approaches on the short-term wave-induced force acting on a fixed vertical cylinder. The four published irregular wave tests considered in this study are all highly inertia-dominated. The average peak and troughs approach is implemented to extract the significant wave height and the peak period from the wave elevation records. Linear random wave theory is adopted since it is the most appropriate theory to numerically estimate the water kinematics in deep water. The calibration approaches considered to investigate the constant drag and inertia coefficients are the least squares fit to the force time domain, the least squares fit to the force spectrum, the conventional method of moments and the lower method of moments. The variability of the coefficients is then examined by adopting the wave by wave method. Thereafter, Morison's wave force is estimated and compared to the experiments in the time and frequency domain. The probability of exceedance of the extreme wave force values is estimated for each test individually in order to acquire a more comprehensive overview on the problem. Most results show a good agreement between the predicted and experimental wave-induced force. However, the probability of exceedance for the extreme wave force obtained by the wave by wave method converge well with the experiments extreme force.