Stress Analysis Of Local Defects In Welds Of Large Crude Oil Tanks

Since the large-scale strategic crude oil reserves have been carried out in various countries,large crude oil tanks have been widely used.With the continuous extension of service life,their safety has attracted more and more attention.Up to now,the research on large storage tanks mainly focuses on the static strength methods of storage tanks.Most of them are the stress calculation methods for the wall,floor and the most critical large fillet welds.These methods lay a foundation for the evaluation of tank defects.In this paper,based on the relevant research,combined with the knowledge of fracture mechanics,the finite element numerical simulation of 1×10~5 m~3storage tank was carried out,and the static strength and defect fracture performance of the storage tank were studied.The main contents of this paper are as follows.In order to establish a tank model suitable for this study,a two-dimensional tank model is established and static strength analysis is carried out based on the tank-soil contact model.According to the deformation condition and stress distribution trend of tank floor,a simplified modeling method is proposed,in which displacement boundary condition on warping section is used to replace the floor and the whole foundation which keep in contact with the foundation under hydrostatic pressure.This modeling method solves most of the complex problems such as the difficulty of mesh generation and contact analysis between the floor and the whole foundation.The comparison between the simplified model and the full tank model shows that the simplified model can completely replace the whole tank for static strength analysis.Before studying the fracture behavior of tank cracks,problems such as the establishment of crack bodies,the setting of singular elements,the meshing of crack bodies and the input of fracture command flow should be solved.Firstly,the cracks on the plate are studied,and the realization method of the above process is obtained.According to this method,the stress intensity factors of the crack front calculated by the interaction integral method are compared with those calculated by the empirical formula,and the results are in good agreement.At the same time,it is found that with the increase of the ratio of crack length to depth,the effect of three-dimensional restraint on the stress intensity factor at the deepest part of the crack decreases gradually,so the long crack can be evaluated as a two-dimensional crack.In order to evaluate the applicability of weld cracks,according to the distribution law of stress intensity factors at crack tips at different locations,cracks are divided into bottom cracks,weld cracks and weld toe cracks.The effects of relevant parameters on stress intensity factors at crack tips are studied,and the direct relationship between stress intensity factors and parameters is obtained by fitting.Based on this relationship,the stress intensity factor at the crack tip can be obtained rapidly in practical engineering according to the crack location and size obtained from the test.According to the mechanical properties of tank materials,an evaluation curve based on Choice 2 curve is established,which makes up for the excessive conservativeness of Choice 1 curve when the load is relatively large,and makes the evaluation result more accurate and economical.