The Impact Of Local Defects On The Load Capacity Of Derricks And Their Safety Assessment

Due to the severe service condition, some local defects appear often in the derrick structures, such as corrosion, perforation, incision, diagonal bracing loosing or missing, pin joint gap and so on. The unusual loading and boundary conditions may happen during the process of using derrick, too, such as the load eccentricity and foundation sinks of the derricks. The load-carrying capacity of the derricks is affected by these defects described above.In the present paper, the affection of defects on the K style laboratorial derrick is analyzed by FEA method. By comparation with the simulation results of a model without defect, the effects of above mentioned defects on stress distribution and structural stability of the derrick are analysized. The corrosion, perforation, incision on the leg of derrick and diagonal bracing missing represent only the local effects on stress distribution of the structure. The foundation sink, load eccentricity and pin joint gap have the important global influence on the structural stress distribution. The diagonal bracing missing, foundation sinks and load eccentricity prohibit the significant impacts on stability of the derrick, while the defect of pin joint gaps doesn’t affect a lot.The process of lifting derrick is a dangerous procedure for the service of derrick. In this paper, the lifting process of JJ225/41-K is simulated by ANSYS. When the lifting angle is zero, the lifting force takes it maximum value, and structure supports the most severe loads. With the increase of lifting angle, the load subjected by structure and the stress in the structure decreases. When the derrick is lifting up, the loads in fore legs of the derrick are greater than in the other parts.In this paper, the static stress test of laboratorial derrick under the five increasing load steps is carried out according the derrick test specification. The test result shows that the stresses in four legs of derrick are severely uneven. The evaluation on derrick safety and carrying capacity is done based on the experimental data.By virtue of the analysis on effect of the defects in the structure, the load eccentricity, foundation sink and pin joint gap are considered to be the main causes of the uneven loads in the legs of laboratorial derrick. Under the condition of known values for load eccentricity and fundation sink, the stiffness of gaped pin joint is used as the parameter to correct locally the FEA model. By comparison with the measurement results of the test under five increasing load steps, the errors of simulation results relative to the test ones are smaller than17.48%. It shows that the stiffness model for gaped pin joint is well feasible. The corrected FEA model is then used to evaluate the safety performance.