Structural finite element model correlation of large-scale structures by large admissible perturbations

Offshore and ship structures have large manufacturing tolerances and structural imperfections which make Finite Element (FE) modeling and numerical predictions of response highly inaccurate. Differences between predicted and measured response may be large. Model correlation is the process of finding corrected values of correlation variables in the FE model so that predictions by a FE model match the response of the corresponding physical structure. Two structural FE models are involved in this correlation problem: An Initial FE Model (IFEM) which fails to predict accurately the static and dynamic response of the modeled structure, and an Updated FE Model (UFEM) which must satisfy all measured response data. A large admissible perturbation method in cognate space is developed to solve the problem of correlating a FE model to the corresponding real structure for which some natural frequencies and some complete or incomplete mode shapes, and/or static deflections have been measured. This algorithm should be used after all possible modifications of the FE model--related to the manufacturing tolerances and structural imperfections--have been made. Measurements are assumed to be exact. The solution algorithm can handle differences between IFEM and UFEM, in correlation variables and correlation measures, as large as 100% to 300% depending on the scale of the structure and correlation measures. The problem of model correlation when the force vector depends on the structural geometry is also solved by developing an appropriate large admissible perturbation algorithm in cognate space. Guidelines for selecting elements or groups of elements allowed to be changed in the model correlation process are developed based on energy considerations. Thus, the correlation process becomes faster and more accurate. A 5-element 9-dof beam model, a 10-element 20-dof offshore pile, a 104-element of 192-dof offshore tower model, and the 605-element 810-dof Ambrose Tower are used in the numerical applications to test the developed correlation algorithms.