Analytical investigation of key factors for dynamic minimum tension in free-hanging slender structures with simplified lumped mass model

摘要

Free hanging is vulnerable to dynamic motion at the top end inducing axial compression at the touchdown point. Numerical approaches in consideration with multiple degrees of freedom and nonlinear drag force have been succeeded at evaluating dynamic characteristics of free-hanging slender structures accurately. On the other hand, analytical investigations into the mechanism determining dynamic tension and how design parameters affect dynamic tension based on sufficient reasoning are yet to be fully developed.In this paper, we aim for establishing a linear lumped mass model that gives sufficient explanations of the basic mechanism determining dynamic tension of free-hanging slender structure. The established model analytically describes how the key parameters, namely the ratio of the coefficient of the virtual mass to the coefficient of the weight in water, the maximum acceleration at the top end, and the static top angle, contribute to the dynamic tension behavior.In addition, we verify the results of the linear analytical model by OrcaFlex calculations in the time domain. We illustrate the contribution of nonlinear force on dynamic tension by introducing the ratio of the nonlinear force to the gravity force.