Analysis On Motion Characteristics Of Deep-water Mooring Lines In Slack-taut Process

The structural style of chain-fiber-chain has always been used in deep water taut mooring system.The taut mooring system may cause slack-taut transformation with the motion of floating body,which could results in significant increase of dynamic tension amplitude in mooring line.The instantaneous increase of dynamic tension can be defined as impact tension,which could lead to breakage of mooring line and failure of mooring system,resulting in serious problem of floating structure.Extensive research has been carried out on taut mooring systems,but most of the research focused on the dynamic stiffness characteristics of the fiber rope and the coupling motion between floating platform and mooring system,while the motion of mooring line in slack-taut process and the influence factors on the changes of impact tension in the mooring line are not yet very clear.Therefore,it is very important to investigate the motion characteristics of the slack-taut process of deep water mooring lines.This thesis mainly focuses on the motion response of deep water taut mooring line during the slack-taut process.A sinusoidal displacement excitation is applied to the top end of mooring line,the change law of dynamic tension in mooring line is calculated and analyzed by adjusting the frequency and amplitude of excitation.The motion of slack-taut process in mooring line is studied,the impact tension in mooring line is extracted,and the influence of pretension and other factors on the impact tension is analyzed and compared.Firstly,the model of the taut mooring line based on ANSYS 18.1 finite element analysis software is built.It is ensured that the force and constraint conditions of the finite element numerical model are consistent with the prototype mooring line.The slack-taut process of mooring line by using the function of "birth and death" of the element is simulated.The birth and death options can be used to deactivate elements when its stiffness drops to zero or reactivate elements when its stiffness greater than zero.The reliability and accuracy of numerical model are verified by an example.Then,the parameters of pretension,the mooring modulus,the cross section diameter and the length of the mooring line are changed.A sinusoidal displacement excitation is applied to the top end of the mooring line to calculate the motion response of the mooring line based on the established numerical model.The change process of mooring line into slack-taut state is studied by changing the excitation frequency and amplitude.Finally,the data of motion track of node and the dynamic tension time history of mooring line are extracted and analyzed.The relationship of mooring line slack-taut station between the critical excitation amplitude and the frequency are summarized,the influence of the four factors on the impact tension are analyzed and compared.The results show that the displacement excitation along the axis of mooring line is the main reason for slack-taut of mooring lines.Increasing the pretension of mooring line,selecting the cable material with large elastic modulus and small diameter,and making the mooring line as short as possible,could keep the mooring line in taut station and avoid the impact tension.The results of this paper will be useful for further exploring the mechanism of impact tension and the design and safety assessment in mooring systems.