The Force Exerted by Surface Wave on Cylinder and Its Parameterization: Morison Equation Revisited

The Morison equation is widely used to estimate the loads by surface waves on cylinders. The formulation and coefficients determination method of the original work by Morison et al. are revisited, it is found that there exist some issues yet to be explained, e.g., the larger uncertainties in drag coefficient and the underestimated maximum moments. Numerical simulation with a similar configuration is used to reproduce these issues and the results help discover the reason and mechanism for these phenomena. The analysis shows that the larger uncertainties in drag coefficient are caused by the introduction of linear wave theory, which is used to derive the velocity and acceleration at cylinder location as direct measurements are not available. The results also show that the underestimation of maximum moments is induced by the wave run-up process, which is neglected in the Morison equation. The scale of wave run-up is approximately the length of cylinder diameter. The results indicate although most recent studies are focusing on the high-frequency loads on cylinders by nonlinear waves, there still exist some issues to be resolved in the linear wave regime. Further studies are required to parameterize the additional loads by wave run-up to strengthen the robustness of the Morison equation.