Finite Element Model Updating And Survivability Evaluation Of The Aged Offshore Platforms

Differences between the prototype structure and the corresponding numerical model are inevitable.In order to provide an accurate finite element model for the safety assessment of in-service offshore platforms,it is necessary to update the finite element model according to the experimental data,and based on the corrected finite element model of the offshore platform,the viability was evaluated.The main contents of the article are summarized as follows:1.Modal experimental study on the scaled model of jacket structure.A 1:25 scaled experimental model of the jacket platform is designed and manufactured,which is based on the criteria of geometric similarity,stiffness similarity and mass similarity,to carry out the modal test.In the experiment,a multi-group time domain signals are obtained.Then,using thedenoised acceleration signals,the complex index method and the ARMA time series analysis method are used to identify the modal parameters,modal frequency is selected by damping ratio deletion method and stabilization diagram method,the first six order natural frequencies of the structure are obtained under the X direction strike and Y direction strike.2.Finite element model updating of offshore jacket platforms based on design parameters.The modal results of the finite element(FE)model are calculated by ANSYS.According to the modal confidence assurance criterion(MAC),the correlation of the modal data obtained by the experimental and FE analysis is assessed.In order to identify the key parameters to be corrected effectively,the sensitivity analysis method is used to classify the structural design variables.As to the FE model updating of the intact jacket platform,the design parameters at all levels are corrected by the trial calculation method.As to the FE model updating of the jacket platform with unknown damages,the optimization method is applied for damage location.Taking the experimental model containing two components missing as an example,the FE model is updated through optimization.Finally,both the FE models before correction and after correction are analyzed by ANSYS,and the response results are compared so as to verify the validity and feasibility of the design parameter modification method.3.Finite element model updating of offshore jacket platforms based on modal simulation.According to the additional mass and additional stiffness theory,the additional stiffness correction model and the additional mass correction model based on additional stiffness are established.Firstly,through changing the additional mass 3 times,the modified model is analyzed by using ANSYS,and then the natural frequencies and mode shapes are obtained.Secondly,after changing the additional mass,through changing the additional mass of the model 6 times,the modified model is analyzed and the modal parameters are extracted.Finally,according to the modal confidence assurance criterion(MAC),the correlation of the modal data obtained by the experimental and FE analysis is assessed.4.Assessment of survivability of offshore platform based on modified results.Firstly,the load similarity ratio between the scaled model and the prototype structure is studied.Environmental loads are applied to the modified and unmodified scaled model,and then the improved static pushover analysis(IPOA)method is proposed to determine the limit state of the structure.For the platform under typhoon and wave loads,the IPOA is carried out by using ANSYS software.According to the elastic-plastic static pushover capability curve of the platform structure,the corresponding environmental loads levels to different structural states are judged,inclueding structural integrity,first level damage,second level damage,second level damage and structural collapse.Finally,the performance of the platform structure is evaluated according to the curve.