Research On Repair Welding Principle Of Pipeline Girth Weld Based On Structural Safety

The main pipe is the key component of the nuclear reactor and also the important component of the nuclear cooling system.In general,the main pipe has a large wall thickness and is connected by welding process.In the process of pipe’s service,cracks will appear due to the combined reaction of cooling water pressure,residual stress and service environment.It is of great significance to determine whether the defective pipelines can continue to serve and to provide the corresponding repair principles for extending the service life of the main nuclear power pipelines.In this paper,the welding residual stress of thick-wall pipe is studied systematically by the method of finite element analysis.By simplifying the number of welding passes,the influence of the characteristic geometrical dimension r/t and the welding heat input Q?on the residual stress after welding is studied,and the prediction formulas of the membrane stress and bending stress including the characteristic geometrical dimension r/t and the welding heat input Q?are given,which provide the data support for the structural safety evaluation of the pipeline.Secondly,through theoretical calculation and finite element analysis,the working load of the pipeline in service is calculated,and the axial force of the pipeline under internal pressure is 42.36 MPa,and the circumferential force is93.824 MPa.Then,the calculation formula of the stress intensity factor at the crack tip caused by the residual stress is given.According to BS 7910,the structural safety of the pipeline containing the cracks with a/c= 0.5 and a/c= 0.2is evaluated.Finally,the critical crack sizes is 14 mm for a/c= 0.5 and 15.5mm for a/c= 0.2 respectively.Finally,the more proper repair principles of thick-wall pipe are explored by the method of finite element analysis.By changing the length,width and depth ofrepairs,the finite element analysis is carried out to compare the axial stress and circumferential stress of weld center line(WLC)and weld toe(WT)under different repair models,as well as the membrane stress and bending stress after decomposition,and finally the best repair welding principle is given.The results show that the length,width and depth of repair welds have a significant influence on the residual stress after repair,which is mainly manifested in the expansion of the high stress area in the direction of wall thickness with the repair depth increases,and in the smaller axial bending stress and the larger circumferential membrane stress in the weld center line and weld toe.So,the more ideal residual stress distribution can be obtained by selecting a larger repair welding length.With the increase of repair width,the compressive stress of axial stress on the inner wall of the pipe changes into tensile stress.It is more advantageous to choose a smaller repair width.The increase of repair length will lead to the decrease of axial bending stress and circumferential membrane stress in the direction of pipe wall thickness,and the increase of circumferential bending stress,but the decrease of axial bending stress and circumferential membrane stress is larger than the increase of circumferential bending stress.Choosing a larger repair welding length can obtain a more ideal residual stress distribution after repair.The structural safety assessment of the pipeline with the best repair welding principle is carried out,which shows that the critical crack size of the pipe decreased to about 13.5mm after repair.