Research On Burn-through Instability Criterion Of X70 Pipeline In-service Welding

Compared with traditional oil and gas pipeline repair technology,no shut-down in-service welding technology is a high efficiency and economic pipeline repair technology which has a vast application prospect and research value.Safety is the first thing to think and the key is to prevent burn-through from occurring in in-service welding.In this paper,the process of burn-through of X70 pipeline in-service welding was studied using ANSYS finite element simulation software.Research had been centered on the influence of internal wall temperature and molten pool size on burning-through.The instability criterion and burn-through database which could be used to guide the in-service welding was established.In this paper,the influence of internal wall temperature and molten pool size on the instability pressure of in-service welding were studied using finite element numerical simulation method.The mathematical relationship between molten pool size and the instability pressure under different internal surface temperature was obtained.The results showed that when the pressure was small,the underside of molten bath showed concave deformation,as the pressure increased,the deformation transformed into convex.With the increase of pipeline inside pressure,the maximum radial deformation position spread from the top to the bottom of the molten pool and finally totally below the molten pool.As burning usually occurs below the molten pool,it is more dangerous when the maximum radial deformation is transferred below the molten pool.The burn-through does not occur at the deepest point of penetration,but at the rear of the maximum position of penetration.When the internal pressure is large,the stress at the boundary of the molten pool is tensile stress,and the stress on the inner wall of the pipe is compressive stress.When the molten pool size was fixed,the higher the inner surface temperature was,the larger the radial deformation and faster the strain rate was,and the lower the instability pressure was.When the temperature of the inner wall is high,the stress at the boundary of the molten pool is tensile stress,and the stress on the inner wall of the pipe is compressive stress.When the pressure and the internal wall temperature kept unchanged and the length,depth,width of molten pool increased to a certain extent,the maximum radial deformation position would spread to the bottom of the molten pool.As the length,depth and width increased,the radial deformation and strain rate raised,while the instability pressure decreased,the influence of molten pool depth on the instability pressure was more obvious than that of length and width.The increase of the length,depth and width of molten pool enlarged the hoop stress and axial compressive stresses which would increase the likelihood of burn-through.Only the increase in molten pool depth affected the radial stresses,making the radial pressure increased.Orthogonal test was conducted on the internal wall temperature and molten pool size.The results showed that the internal wall temperature had the greatest influence on the instability pressure of in-service welding,followed by the depth and the length of the molten pool,the influence of the molten pool width was least.The instability pressure under multiple parameters was fitted using Origin software,and the instability criterion was established.The in-service welding burn through criterion database was established based on Visual Basic 6.5.This database was proved to have a certain guidance meaning for in-service welding process by experiment.