Antiresonant Frequency And Its Application In Finite Element Model Updating Of Jack-up Platform

Offshore platforms are the basic facilities for the development of offshore oil and gas resources,which are of great significance to the implementation of national Marine strategy.Structural damage detection and safety evaluation are important guarantees for their safe operation.The structural damage identification method based on finite element model modification is widely concerned because it can determine the damage location and evaluate the damage degree,but how to establish an accurate finite element model is the key problem.Traditional methods generally use modal frequency and modal mode as the objective function,but for the large structure of the offshore platform structure,the modified finite element model is not accurate because it can only measure the low-order frequency,the vibration mode error is large,and the equation is not positive when many parameters are modified.The anti-resonance frequency proposed in recent years has been used in finite element model modification due to its advantages of being easy to measure,accurate and reflecting local information.Taking the jack-up platform physical model as the research object,this paper studies the objective function in the finite element model modification,the establishment of an accurate finite element model,the modification of parameters and the optimization algorithm.(1)In this paper,the undamped five-degree-of-freedom system will be taken as an example,and the anti-resonance frequency will be obtained through numerical calculation and case simulation for comparative analysis.The anti-resonance frequency and its properties will be studied and verified by the cantilever beam experiment,so as to provide theoretical basis for its use as the characteristic quantity of the objective function.(2)Due to the limitation that the traditional EMA method needs to measure the input signal and OMA needs to excite white noise,it is difficult to apply it to large and complex structures such as offshore platforms.On the basis of studying the transfer rate function and identifying the anti-resonance frequency,the method of identifying the resonance and anti-resonance frequency through case simulation is still studied with the aforementioned five-degree-of-freedom system as an example,and the conclusion that it has nothing to do with the excitation signal property is verified by the cantilever beam experiment with pink noise.(3)In order to make the finite element model of jack-up platform consistent with the prototype,the physical model of jack-up platform with truss pile legs was taken as the object.After the reasonable simplification of each component structure of the platform,the grid independence analysis was carried out respectively to eliminate its influence on the dynamiccharacteristics of the structure.By studying the sensitivity of the connection stiffness between the hull and the pile leg to the dynamic characteristics of the platform in the platform model,and considering the convenience of calculation when correction is taken into account,the connection between each pile leg and the hull is simplified into a horizontal spring and a corresponding torsional spring,with a total of four correction parameters.(4)in this paper,using RBF neural network to optimize the algorithm,the uniform design method is adopted to sample group,according to the study,combined with anti-resonance frequency and the resonant frequency as the objective function to modify jack-up finite element model,the results show that modified parameters and modal frequency of the revised error less than 5%,between the target and effectiveness of the method is validated for damage detection and practical platform provides the theory basis for safety assessment.