Abstract
This paper presents a method of analyzing the loading capacity of drilling derrick steel structures containing defects and defacements in keeping with the current demand in the oil industry for evaluating safety and the loading capacity of derricks during their service lives. Adopting the three dimensional beam element as the basic element for FEM analysis, the problem of large deformation analysis is treated using the incremental tangent stiffness matrix method incorporating both the geometric and deformation stiffness matrices of the element, and the material nonlinearity is simplified as a concentrated inelasticity approach. Thus allowing for here two types of non-linearity analysis of the loading capacity of a perfect derrick structure has been studied. Applying a new Referential Uniform Reduction Method, special defects and defacements could then be introduced into the mathematical model of the structure, and a group of load-displacement curves representing different states of imperfections are obtained. The ultimate strength of the derrick in service can be determined finally by comparing the computed curves and the full-scale displacement test results. Two examples of different types of derrick structure are analyzed.
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