Dynamic Characteristics Analysis And Real-time Damage Assessment For Soft Yoke Mooring System Field Monitoring

Soft yoke mooring system(SYMS)is an important equipment for offshore oil and gas development,which has been commonly used in China Bohai Bay and Gulf of Mexico.As a compliant mooring system,SYMS reduces the impact of marine environmental load on the stability of production unit effectively by the multi-hinge connection.Due to its excellent mooring performance,SYMS has attracted attention from other marine development fields in recent years.With the increase of service time and quantity,the failure events of in-service SYMS have occurred many times.Therefore,safety evaluation of in-service mooring system is an important guarantee for production unit's operation decision and maintenance.Accurate description of mooring structures dynamic behavior and connecting structures damage status are the prerequisites for in-service SYMS safety evaluation.Due to the complexity of marine environmental load and mooring structure,numerical simulation and theoretical analysis can not reflect the real load information,the real motion state and stress state of each mooring component.Meanwhile,with the increase of service time,accurately grasp the damage status and residual life of important components become urgent scientific problem.Based on a prototype monitoring system of an in-service SYMS in China Bohai Bay,dynamic characteristics analysis and real-time structure damage assessment for SYMS field monitoring are carried out in this paper.Non-stationary random marine environment loading and mooring structure movement is difficult to accurately forecast.In order to analyze the real loading and structure movement information of in-service SYMS,a prototype monitoring system for SYMS is established,including the ocean environmental load monitoring subsystem,the structure motion monitoring subsystem and the key hinge joint monitoring subsystem.Marine environmental loads and structural responses of FPSO and SYMS during service period have been monitored.The long-term monitoring data is recorded by the prototype monitoring system,which provides a numerical basis for the mooring system multi-body dynamic characteristics analysis and connecting structure damage assessment.Considering the lack of consideration of multi-body dynamics effect in the safety analysis for in service SYMS,multi-body dynamics numerical simulation of SYMS is carried out,and a multi-body dynamic analysis model suitable for prototype monitoring is established.Combined with the monitoring limitation,the motion measurement method of in service SYMS is proposed,and the real-time internal force calculation equation is constructed by considering the friction behavior of the connecting structure.Based on the prototype monitoring data,it is proved that the proposed measurement method can accurately describe the motion state of the in service SYMS.Comparing the static method,the influence of FPSO movement on the cyclic load of hinge points is analyzed,and the problem of the in-service SYMS mooring force monitoring is solved.Based on the multi-body dynamic analysis method,the load spectrum of the key hinge joint is obtained by using long-term monitoring data,which provides effective load analysis data for the future design.In order to solve the problem of SYMS extreme conditions accurate estimation.considering that SYMS has no typical mechanical characteristics such as natural frequency and vibration mode,a Bidirectional Clustering Algorithm based on Local Density(BCALoD)suitable for prototype monitoring data is proposed.BCALoD has two processes:up process and down process,and can efficiently extract small clusters with fewer occurrences and smaller data proportion in data set.Then,based on the noise characteristics,a de-noising method suitable for BCALoD is proposed,which can automatically determine the clustering number,reasonably allocate different cutoff distance and cutoff local density for each cluster,and achieve better clustering effect in image analysis and random data analysis.Based on the long-term monitoring data of SYMS,156 working condition clusters are calculated by the bidirectional clustering algorithm,and the extreme working conditions of in service SYMS are obtained.Aiming at the damage of the connecting structure caused by the long-term service of the SYMS,combined with prototype monitoring limitation,a real-time damage identification method based on virtual moment is proposed.The virtual moment method transforms the problem of hinge structures damage identification into the problem of finding the optimal solution of dynamic identification equation.Based on the external measurement information,the Genetic Algorithm is used to evaluate the damage position and damage degree of the in service SYMS connecting structure in real time.A large scale experimental model of SYMS is established to verify the effectiveness of the virtual moment identification method.Then,combined with finite element calculation,a real-time fatigue life calculation method for SYMS is proposed.The corresponding relationship between the motion,damage state and key process areas of the mooring structure is established.Combined with the existing SYMS monitoring technology,the remaining life of hinge points can be estimated in real time.For the dangerous large lateral vibration of SYMS founded in the monitoring data clustering process,the mechanism analysis and vibration absorption design of lateral vibration are carried out.The large lateral vibration is easy to cause the collision between the ballast tank and FPSO.At the same time,the stress on hinge points increases,which will accelerate the hinge points damage.Numerical simulation and model experiment are used to analyze the lateral vibration of SYMS,and the reason of lateral vibration is determined to be the resonance behavior caused by wave excitation.On the basis of guaranteeing the longitudinal restoring force of SYMS,a vibration damping device based on TLD is constructed,and the vibration damping tests of multiple cabins and multiple working conditions are carried out.The test results show that the proposed TLD vibration absorption device has the ability to effectively reduce the lateral vibration of SYMS.