In order to grasp the actual bearing capacity and residual life of the derrick of an in-service offshore workover rig,a visual finite element model of the derrick bearing capacity was established by combining ANSYS APDL command flow and the evaluation method of the derrick bearing capacity.The equivalent section method was used to establish the derrick model with defects,and the influence of corrosion,initial bending and stress concentration on the bearing capacity of the derrick was analyzed.The model was verified by the loading stress test results,and on this basis,the prediction method of derrick residual life based on annual corrosion rate was proposed.The results show that there is a big error in analyzing the actual bearing capacity of the derrick by using the defect-free derrick model,which cannot accurately reflect the actual stress and deformation of the derrick;the model analysis results of the derrick with defects are consistent with the loading stress test results,and the model is accurate;among the three defects,the corrosion defects have the greatest influence on the actual bearing capacity of the derrick.According to the analysis,the actual carrying capacity of the derrick is 865 kN,and the residual life is 13a.The analysis results provide reference for the safe operation of the derrick.