Research On The Effect Of Weld Type Between Stiffening Ring And Pipe Wall On Stability Of Buried Penstock Against External Pressure

Along with the development of the pumped-storage power station construction,the high head and large diameter of penstock underground use more and more widely.When the pipe is emptied,groundwater acts as the main external pressure load on the wall of the pipe,which is easy to cause buckling and instability of the thin shell structure of the pressure steel pipe,which brings huge losses to the project.Therefore,stability analysis of steel pipe against external pressure is an important content of underground pipe design,which also puts forward higher requirements.Usually stiffening rings are set to improve the steel pipe against external pressure.The welding forms of stiffening ring and pipe wall mainly include butt weld and fillet weld,among which fillet weld is more widely used in practical engineering because of its low difficulty in welding operation.However,in the theoretical calculation and numerical simulation of the stability of the underground pipe with stiffening ring,most scholars simplify or directly adopt the form of butt weld,that is,the stiffening ring is directly connected with the steel liner,ignoring the possible influence of the fillet weld connection,that is,the fillet weld is connected with the steel liner and the stiffening ring respectively,and the steel liner and the stiffening ring are in a contact state.This is different from engineering practice.Based on the above problems,based on the contact nonlinear theory and two buckling analysis methods,this paper establishes the finite element model of connecting the stiffener ring with the pipe wall of a pumped storage power station by using butt weld and fillet weld respectively by using the large threedimensional finite element software ANSYS.The point-to-point contact element is used to simulate the force transfer mechanism of fillet weld.The stress-strain state of the two weld forms under the water pressure outside the design was analyzed,and the sensitivity of fillet weld foot height to stress-strain was explored.Based on the buckling mode obtained by eigenvalue buckling analysis method and used as the initial defect introduction,the critical external pressure value of steel pipe with different height of welded foot was calculated by using nonlinear buckling analysis method.The sensitivity of welded foot height of fillet weld to the stability ability of buried pipe against external pressure was analyzed,and the stability ability of the two weld forms was compared.The effects of initial gap,stiffener ring spacing and stiffener ring size on the stability of the buried pipe under external pressure when fillet weld joints are used are further investigated.In order to provide some reference and basis for the actual construction of the project.The results show that:(1)For the pressure steel pipe,compared with the butt weld,the stress level and radial displacement at the welding foot are slightly larger when the fillet weld is used,but increasing the height of the fillet weld can reduce the local stress concentration,improve the overall stiffness of the steel pipe,and also reduce the stress of the pipe wall between the rings.The thinner the thickness of the steel pipe,the more obvious the correlation between the stress and displacement of the weld and the height of the welding foot.(2)The increase of fillet weld height can improve the critical external pressure value of steel pipe to some extent,but the sensitivity of fillet weld height against external pressure decreases with the thickening of pipe wall and the increase of stiffening ring spacing.Compared with butt weld,selecting the proper height of fillet weld can make the stability of pressure steel pipe better.(3)By comparing the finite element simulation results with the normative formula,it is found that the calculation results of the theoretical values are conservative and have some limitations,and the finite element simulation results are more suitable for the engineering practice.(4)Reducing the initial gap,shortening the spacing of stiffening rings and increasing the size of stiffening rings can enhance the stability of the underground pipe against external pressure when fillet weld joints are used to a certain extent.(5)The comprehensive analysis results suggest that when the underground pipe uses fillet welds to connect the stiffening ring to the pipe wall,the fillet welds with higher height of welding feet should be selected within a reasonable range under the premise of considering that the welding seams are small and the cooling too fast will lead to the decrease of the hardened structure and the influence of "overburning" on the base metal caused by excessive welding seams.For the pumped storage power station studied in this paper,The height of the welding foot shall be no less than 12mm and no more than 28mm.