Dynamic Analysis Of Mooring Cables For Floating Structures In The Sea

Mooring cable is one of the most important structures in floating wind turbines,and its dynamic characteristics are of great significance in practical situations.In this work,the model of the mooring cable under Morison ocean current force are established and computed with the absolute nodal coordinate formulation(ANCF),which employs the gradients to represent the local frame coordinate system.Despite that the number of generalized coordinates based on ANCF is large,it makes the computation simpler.This paper presents an approach combining Morison ocean current force with absolute nodal coordinate formulation to solve the dynamic characteristics of mooring cables under current force.The formula of the ANCF is derived,and the governing equations from the Lagrange equation is established.Moreover,the numerical discretization of the equations are provided.In the approach,the mass matrix,stiffness matrix and generalized elastic force matrix of mooring cables are all functions of generalized coordinates.The difficult part of the approach is to combine the direction of ocean current force with the frame coordinate system of ANCF.The choice of the frame coordinate system is distinct for different approaches.The frame coordinate system of the ANCF is the gradient field.The direction of flow velocity is divided into two directions in this study.The tangent direction of the cable's center line is any direction perpendicular to this direction.This decomposition method greatly improves the calculation speed.Moreover,the generalized-? method,with the advantatge of low frequency non-attenuation and high frequency attenuation,is adopted to solve the governing equations.With variable step size,the generalized-? method makes the computation faster.In this study,the displacements,velocities and accelerations of mooring cables are calculated,and the dynamic characteristics of mooring cables are analyzed.The main factors affecting the dynamic characteristics of mooring cables are also studied.The approach developed here,with good computational efficiency,contributes greatly to the development of the researches in flexible multi-body dynamics.