Research On The Effect Law Of Corrosion Pits On The Welding Joint Strength Of Coiled Tubing

Coiled tubing is more widely used in oil and gas field now. According to foreign data,itshows that the amount of the world’s coiled tubing have been used nearly5million meters peryear, the coiled tubing mainly used in the drilling, flushing, acidification, thermal cleaningwax, sand wash, draining, etc. However, the failure of coiled tubing has become a growingissue to concern. Coiled tubing services in very bad condition, its failure modes are varied, butbased on the domestic research, the main cause of coiled tubing can not continue to work isleak failure, such as continuous Liaohe Oilfield, Zhongyuan Oilfield, Dagang Oilfield andHenan Oilfield the ultimate failure of tubing is internal pressure leak. So coiled tubingcorrosion is the direct cause of failure, a direct consequence of the corrosive effect is to reducethe intensity of coiled tubing, lead to job failure, increased costs, so the research of corrosiveconditions under coiled tubing tensile strength, strength of bending and internal pressurestrength are very necessary.In this paper, QT900coiled tubing welded joints and base metal were tested under100℃temperature and corrosion experiments under28%HCl corrosive conditions, the results showthat the base material presented uniform corrosion morphology; welded joint area showing auniform corrosion and pitting morphology; when the corrosion time was4h, the weld capsurface appeared peeling. In order to avoid focus corrosion of the weld, welding materialsshould be optimized.Through the analysis of the coiled tubing,s working conditions, combined with the APIspecification of strength evaluation of corrosion pits,this paper recommended to use the thirdgrade assessment method of API579to evaluate the corrosion pits.The establishment of a coiled tubing corrosion pits strength analysis of finite elementmodels were analyzed strength under tensile load,under pure bending conditions, undertension when the coiled tubing through the guide frame with containing corrosion pits. Theresults shows that:when the tensile load is215.4MPa, the limit corrosion pit depth of4.386mm,the thickness may reach0.85; bending radius of curvature smaller size limit corrosion pitssmaller radius1219.2mm,1257.3mm,1447.8mm when limit the depth of corrosion pits were0.516mm,1.032mm,1.548mm; bending radius of1828mm, tensile load is under215.4MPa conditions, corrosion pits size parameters a/t=0.15, the limit corrosion pit depth of0.774mm;corrosion pit size parameters a/t=0.20, the limit corrosion pit depth is1.032mm.