Multi-body Coupling Analysis And Improvement Of Fatigue Characteristics With Soft Yoke Mooring System

With the rapid development of social economy and the improvement of people's living standards,our demand for all kinds of energy is increasing.In the development of national economy,energy must be indispensable in transportation,social production,people's life and so on,so the energy problem has become the bottleneck of social and economic development.In the Bohai Sea of China,a number of FPSO have been put into operation,and the output of the oil fields in service is considerable,the economic benefit is high and the operation is efficient.Most of the mooring mode is soft yoke single point mooring system.The key to the problem is whether FPSO can be operated safely during service,and this is of great significance to China's offshore oil equipment.Based on the three-dimensional potential flow theory and multi-body dynamic analysis method,the hydrodynamic response of 160 000 ton FPSO soft yoke single point mooring system is studied and analyzed on the basis of limited water depth.Firstly,the multi-body coupling model of the single point mooring system of soft yoke and hull are established,and under regular wave the RAO response rule and the changing trend of added mass and radiation damping curve are calculated and analyzed,and the stiffness curve of single point mooring system of soft yoke is studied.Then the orthogonal test of the sensitivity analysis of soft yoke structural parameters and wind wave and inflow angle parameters with three factors and five levels is designed respectively.Under each test,this article studies the main horizontal factors affected,the better combination of the structural parameters of the soft yoke and the combination of dangerous conditions.Finally,the finite element model of the joint point on the Leg is established,the fatigue life is estimated,and the structure of the water tank is improved by using the TLD damping system.The main conclusions of this paper are as follows:(1)Soft yoke single point mooring system is a nonlinear multi-rigid body system.The horizontal recovery force is not proportional to the horizontal displacement and presents the characteristics of double nonlinear.(2)In the orthogonal test of sensitivity analysis of soft yoke structural parameters,when the mooring force is the smallest,the single point mooring performance is better,so the main scale parameters of soft yoke structure optimization are as follows: the length of soft yoke is 40 m,the weight of water tank is 650 t,and the tension angle of soft yoke is 44°.(3)In the orthogonal test of sensitivity analysis of environmental load wind wave and inflow angle parameters,the experimental results show that the main factors affecting FPSO pitching and pitching response are wave and wind direction angle,and the main factor affecting the Leg roll is wave angle.(4)According to the numerical simulation results,it is shown that the single point mooring system of transverse box soft yoke can better meet the FPSO mooring performance of large tonnage,and it is also suitable for the improvement of MTLD damping system,which can be divided into several small water tanks inside the ballast water tank,and it can enhance the damping effect of soft yoke single point mooring system.