Multi-body Dynamic Characteristics Analysis And Fatigue Damage Assessment Of Soft Yoke Mooring System

The development of offshore oil and gas resources mainly depends on various forms of offshore platforms.Among them,the Floating Production Storage and Offloading unit(FPSO)is widely used in Bohai and the South China Sea.There is no power device in the hull of FPSO,so the performance of mooring system will directly affect the working state of FPSO.The Soft Yoke single point Mooring System(SYMS)is one of the most important mooring modes of FPSO.In recent years,the safety accidents caused by mooring failure have occurred,and the mooring system's movement behavior and failure mode have become the focus of the research.At present,hydrodynamic simulation analysis and model experiment are the standard design method of Marine engineering structure.At the same time,the multi-body dynamics analysis of the multi-hinged joint characteristics is also needed for SYMS and other structures.In this paper,the multi-body dynamics equation of SYMS is established,and the differential-algebraic equations describing the motion states of each body are obtained.In order to solve the violation of constraints in the process of solving differential-algebraic equations,a numerical integration method with simple solution format and self-satisfied symplectic property of Hamiltonian system is constructed based on the idea of Zu-type method.This method has a high accuracy for solving nonlinear systems.Two groups of data of FPSO's six degrees of freedom under different sea conditions are selected to calculate the dynamic response and mooring restoring force of each body of SYMS.The results show that,compared with the traditional statics method,the real force state of the structure with more obvious dynamic characteristics can be obtained by this method.At the same time,the motion response of the structure is consistent with the actual monitoring information,which can better restore the real motion state of SYMS.The SYMS is composed of multiple hinge joints.The vulnerability failure of hinge joints is one of the main failure modes of SYMS.The analysis of monitoring data shows that,the fatigue damage of upper hinge joints of SYMS is mainly caused by a large number of horizontal pendulum behavior of mooring legs.Based on the prototype monitoring data,the friction and wear tests of hinged joints were carried out,and the variation law of friction coefficient of hinge joints was obtained.Then the fatigue life of hinge joints under long-term wear condition is studied with finite element analysis,and the maximum damage degree and the average damage degree of hot spot stress area are calculated.The calculation results show that there is potential fatigue damage during the design service life of hinge joints,and the fatigue life design is biased towards danger.