Fatigue Failure Analysis And Proto-type Monitoring Scheme Design Of The Upper Hinge Joint Of FPSO

FPSO is widely used in China with accounting for 75% oil production of total offshore oil production.The fatigue failure of offshore structures during the service period brings great risks for the safety of offshore platforms.The inaccuracy of marine structural design makes the actual in-service life of marine structures lower than their design criteria.Non-stationary and time variant characteristics of ocean environmental loads bring the great challenges to design of marine engineering structures.In this paper,a real-time fatigue evaluation research based on prototype monitoring technique is developed for the upper hinge joints of tower soft yoke mooring system.In the traditional fatigue life design,the load and boundary conditions of hinged joint are obtained by hydrodynamic modeling.The loading based on the prototype monitoring data analysis is more consistent with the actual load condition.In addition,the variation of mechanical properties with time in the process of structural service is considered,and the influence of different friction coefficients on the fatigue life of structures was analyzed,it is proved that the deformation in the traditional design is dangerous when the structure is not considered.Moreover,the real-time prediction method of hinge joint fatigue life is based on the prototype monitoring data to count the number of reciprocating movements of the hinge structure,and the prediction of the fatigue life of the structure is more accurate.In this paper,the fatigue failure of hinge joint of FPSO is calculated by using finite element software(FEM),and the fatigue life of hinge joints under specified angles is obtained.According to the experiment,the variation range of friction coefficient under the condition of structural wear is obtained,and the fatigue life of hinge joint under different friction coefficients is analyzed,it is proved that the change of friction coefficient of hinge joint has a great influence on fatigue life of structure.Based on the actual situation of the joint,the hot spot stress monitoring method of hinge joints is established according to the distribution law of the structural stress in the reciprocating behavior of hinge joint,and the stresses in the joints are monitored,and the structural contact surface properties are evaluated.Based on the prototype monitoring data and the fatigue analysis results of joints with multi angle and variable friction coefficients,a real-time prediction method for fatigue life of hinge joints is established,the true fatigue damage of hinge joint is calculated and the residual fatigue life of structure is predicted.According to the characteristics of structural motions,the essential parts of hinge joint and thrust bearing are designed,and the integrated friction and wear test machine of joint node thrust bearing is manufactured.The friction characteristics of the joints and the thrust bearings are analyzed by friction and wear test machines;the accuracy and feasibility of the hot spot stress-monitoring program for hinge joints are verified by model experiments.