Study On Fatigue Failure Mechanism Of Defective Coiled Tubing

Coiled tubing equipment has been widely used in the field of petroleum development because of its many advantages,but in the process of transportation and operation,corrosion,mechanical damage,manufacturing defects and human error will lead to its scratches,indentation and other defects.It is easy to cause premature fatigue failure of coiled tubing under high pressure and bending load.How to accurately evaluate the fatigue life of coiled tubing with defects and ensure the safe service of coiled tubing with defects are the key scientific and engineering problems that need to be solved urgently.In order to solve the above problems,CT110 coiled tubing was taken as the research object,and the fatigue failure of grooved and spherical coiled tubing was studied by means of numerical simulation,experimental research and theoretical research.First of all,through the finite element method on fatigue life of coiled tubing containing grooves scratch defects calculation analysis,the axial Angle of defects,defects,defect depth and defect length width,defect to the distribution and the analysis of the axial distribution of six parameters,select more influence to the coiled tubing fatigue life of four sensitive parameters(defect along the axial Angle,length of defect,defect depth and defect width);At the same time,the influence of spherical flaw diameter and spherical flaw depth on fatigue life was analyzed by finite element method.Secondly,the influence of four sensitive parameters of groove defects on the fatigue life of coiled tubing is studied by using fatigue experiment.The influence of four sensitive parameters of groove defects on the fatigue life of coiled tubing is finally obtained by orthogonal experiment as follows: the depth of groove > the width of groove > the Angle of groove > the length of groove.The fatigue life of coiled tubing with spherical indentation defects was studied by fatigue experiment.The experimental results lay a foundation for the establishment of the theoretical model.Thirdly,the failure of defective coiled tubing is caused by the initial crack.In order to obtain the critical criterion for evaluating the initial fatigue failure of defective coiled tubing,CTOD(crack tip opening displacement)is introduced in this paper.The criterion of evaluating whether the coiled tubing with groove defects and spherical defects is fatigue failure is presented in this paper.Finally,based on the results of previous numerical and experimental studies and the existing classical fatigue life models,the theoretical models of the fatigue life of coiled tubing with groove defects,the theoretical models of the fatigue life of coiled tubing with spherical scratch defects and spherical indentation defects were established,considering the influence of defect parameters on its fatigue life.Experimental results show that the above model is feasible.On the basis of the above,a cross-platform C++ graphical user interface application program development framework is used to compile the fatigue life calculation software of coiled tubing containing defects.Finally,the feasibility of the software is verified.The current research work to some extent complements the deficiency of the fatigue failure theory of the coiled tubing with defects,but the accurate assessment of the fatigue life of the coiled tubing with defects still needs a lot of laboratory and field experiments,and the theoretical model and methods need to be constantly improved.