Evaluation Method Of Derrick Steel Structures Based On Damage Identification And Updated Model And Its Application

To correctly analyze the static and dynamic mechanical behavior and quantitatively evaluate the load-capacity of derrick steel structures with damage and deficiencies can improve the security and production efficiency in the process of extraction of oil and gas, and make available resources being full use of. In the background, this paper has reseached the methods of damage identification and the updated theory of finite element model. The sensitive parameters and quantitative relations between physical parameters and damage have been determined. The direct updating and patial model updating mathematical model of the finite element model for derrick steel structures are established. Then, a novel approach is proposed to quantitatively evaluate the safety and ultimate load-carrying capacity.The linear elastic mechanics, geometric nonlinearity, material nonlinearity, double nonlinearity, dynamic characteristics and stability of derrick steel structures are analyzed using finite element method. The mechanical characteristics as displacement, stress, natural frequency, modal shapes and dynamic response are acquired. The results offers foundation for damage identification, model updating and forecast of safety and ultimate loading-capacity.Damage identification for derrick steel structures has been researched. According to the feature and character of the structure, equivalent damage coefficient is taken as the unite index to identify the existence and degree of damage. Based on the index and static and dynamic test technology, four approaches of damage identification methods are proposed. The first is named as direct identification method, and the other three methods are presented respectively combined with strain or stress, frequency variation ratio or the square of frequency variation ratio, deformation difference curvature in unit load. The four theoretical methods are proved to be correct and effective by simulation analysis.Finite element updating theories of derrick steel structures have been studied. Two kinds of updating methods are proposed. One is direct updating method, and the other is partial model updating method. Applying the former method, the revising mathematical models of corresponding damage are constructed according to the general existing damage and deficiencies such as corrosion of section, initial flexural deformation of member bars, initial flexural deformation of integral structure, eccentric acting load, the decrease of rigidity of knock-off joints, and base sinkage. Based on sensitivity analysis, partial model updating mathematical models are constructed, in which, strain, stress, frequency, mode shapes obtained from static and dynamic measurement are taken as the key referenced indicators, while the relevant design parameters as the input updating objects. The updating parameters are acquired by direct analytic or iterative approach. The problem of incomplete testing data is solved by using the latter method.Testing theory and model experiment of derrick steel structures have been studied. According to the shock exciation mode, the formulas are deduced for identifying modal parameters in the condition of measuring input and unknown input. The similar scale models of derrick steel structures were made, and dynamic grading load test, safety load test and detruction test were performed on them. The experimental results are compared with the simulative values and an agreement between both is very satisfactory, which proved the correctness and feasibility of patial model updating theory and direct updating theory. From the above analysis, it can be concluded that the the ultimate bearing capacity can be obtained by analyzing the updating simulation model from safety load test. In addition, the relations between load and dynamic characteristics of simple model are achieved from dynamic test with load fluctuation. The experimental results also offer the reference for the evaluation theory based on dynamic characteristics.On the foundation of previous research, a novel evaluation system for forcasting safety and ultimate load-carrying capacity of derrick steel structures is established on the basis of static test data and dynamic parameters. The former is constructed on the reliability theory and the criterion of API Spec 4F, while the latter is based on natural frequency to forecast directly, or relies on the updated finite element model which is obtained according to dynamic measurement parameters to evaluate synthetically. The novel evaluation system is applied successfully in forcasting the safety load-capacity and ultimate bearing capacity of in-service derrick steel structures.