Strain Response Estimation Of Jacket Platform Based On Vibration Monitoring

The safe operation of the jacket platform in service is the basis to ensure the normal production of offshore oil and gas.Due to the harsh environment,cracks will inevitably occur in the structure,and the continuous accumulation of fatigue will eventually lead to structural failure,resulting in safety risks.Therefore,various technical means have been adopted at home and abroad to carry out structural health monitoring of jacket platform.However,it is difficult to place sensors in the most dangerous position of jacket platform,so virtual sensing technology is produced.The virtual sensing technique uses indirect measurements to estimate the strain time history of other unmeasured locations on the jacket platform.In this paper,the structure of indoor jacket platform is taken as the research object,the vibration test and the strain response estimation of jacket platform based on the mode expansion method are carried out.(1)Carry out vibration test and modal parameter identification research of indoor jacket platform structure.Firstly,based on the platform finite element model,the Block Lanzcos method was used for modal analysis and solution,and the modal parameters of each order of the finite element model were obtained.Based on the mode shapes,the effective independent method(EI)was used to optimize the layout of acceleration sensors.Secondly,the acceleration vibration test system is used to obtain the platform response time history data under multiple environmental excitation.Then,the random subspace method driven by covariance was used to identify the modal parameters,and the natural frequency,damping ratio,modal modes and other modal parameters of the model of the conduit platform were obtained.The frequency and modal modes correlation analysis were carried out between the modal parameters of the finite element model and the experimental model.(2)Finite element model modification of jacket platform structure based on modal parameter sensitivity analysis.Firstly,elastic modulus and stiffness were determined as global correction parameters,and wall thickness of pile leg was selected as local parameters to be modified,and the sensitivity matrix of each parameter to be modified with different characteristic quantities(modal frequency,modal frequency +MAC value)was analyzed by using the normalization method.Then,CCABS were selected as the convergence criterion,and the modified finite element model was obtained after several iterations.The results show that the error between the modified finite element model and the measured model is greatly reduced compared with the pre-modified finite element model,which provides a reference model for the subsequent strain response estimation.(3)Global strain response estimation of jacket platform structures based on modal expansion.Firstly,acceleration time history data under random excitation is processed to obtain displacement time history data of measuring points.Secondly,the modal decomposition of the displacement time history data is carried out to obtain the displacement mode shapes and related modal coordinates.The truncation error in the process of mode decomposition is analyzed,which shows that the truncation error has little influence on the system response.According to the local correspondence principle,the truncated displacement mode shapes were matched with the modified finite element mode shapes to realize the mode expansion.According to the displacements and strains transformation matrix,the strain responses of all nodes of the jacket platform were obtained,and the locations of the most dangerous nodes of the jacket platform were determined.The accuracy of the proposed method is verified by comparing the strain data of the measuring point with the strain data obtained from the mode expansion,and based on the value of the time response assurance criterion coefficient(TRAC).