Studies On Model Updating For Offshore Platform Based On Vibration Response

Large-scale civil engineering structures (for example, offshore platform structures) are prone to be damaged during their service lives, caused by various environmental loads. Therefore the occurrence of damage during the life of an offshore structure is unavoidable. Thus, techniques for nondestructive damage detection of large structures based on vibration response are essential to ensure safety. Offshore structures continually accumulate damages as a result of the action of various environmental forces during their service life. Clearly, the development of robust techniques for damage detection is crucial to avoid the possible occurrence of a catastrophic structural failure. And model updating is of great importance since it provides accurate Finite Element Model (FEM)for dynamic analysis, modal testing and damage detection, etc.Structural damage identification is the core of structural health monitoring. And it needs accurate finite element model as a foundation. However, A lot of factors make a great influence on the result between finite element model of the structure and the real structure, such as the active complex environment where the offshore platforms locate, many degrees of freedom, sensor layout problems and the various theories of artificial assumptions, the approximate boundary conditions, the idealized stiffness of the node connections and so on. Based on this issue, model updating is raised. And the finite element modal is corrected by it used the measured data to be consistent with the actual structure. In addition, model updating technology can optimize the modeling process, reduce human and material resources and increase economic efficiency. Therefore, it is a great significance that how to update the structure of the finite element model.According to the problem of the finite element model that due to limitations of existing tools and equipment, the measured data is often incomplete, or even just a one-thousandth or one-millionth of the finite element model data, it can't fully reflect the dynamic characteristics.So this paper mainly focuses on the following two aspects:(1) While rigid or pinned assumptions are adopted for steel frames in traditional modeling via finite element method, the actual behavior of the connections is usually neither. Semi-rigid joints enable connections to be modeled as partially restrained, which improves the quality of the model. To identify the connection stiffness and update the FE model, a newly-developed cross modal strain energy (CMSE) method is extended to incorporate the connection stiffness estimation. To illustrate the capability of the proposed parameter estimation and model updating algorithm, a four-story frame structure is demonstrated in the numerical studies. Numerical results indicate that an excellent updating is achievable and the connection stiffness can be estimated by CMSE method.(2) In dealing with Spatial Incompleteness (SI), CMCM in conjunction with four iterative correction method based transformation matrix iterative updating method is proposed. In a numerical analysis of offshore platform structures, seven injured are assumed respectively. In order to reflect the randomness of damage, both seven injured units and the degree of damage of each unit are different. And then update the modals by the iterative correction method based on CMCM and the above four iterative correction method, compare the stiffness coefficient and the frequency difference and modified data, finally conclude that the method based on CMCM and IOR method can accurately update mass and stiffness of jacket structures, the updated model keeps its physical connection. And the frequency and stiffness before and after correction coefficient can be better matched, reflecting the realistic dynamic characteristics of the target model. These model updating methods can be better applied to the structural design; timely detection and reinforcement, prior to the effective operation of offshore platforms to improve the safety factor to ensure work safety, reduce risk and improve the economy benefits. They provide a strong basis for the safety detection of offshore platforms, health assessment and service life figures.