Analysis On Fatigue Life Of Coiled Tubing With Defects

With the rapid development of oil industry,coiled tubing equipment,which plays a more and more important role,is widely used in oil and gas fields.During operation,the process of repeatedly winding and straightening of coiled tubing in the roller will produce low cycle fatigue.As the curvature radius of roller and the goose neck guider is far less than elastic limit radius of coiled tubing,so the plastic deformation will occur after each bending or straightening.After the plastic strain is accumulated to a certain extent,it will lead to the fatigue failure of coiled tubing.If the coiled tubing contains defect,the strain concentration will occur at the defect,which will accelerate the accumulation of fatigue damage of coiled tubing,and so its fatigue life will be shortened.If the coiled tubing which contains defect is once again down into the well,it is very possible that there will be little operation and perforation or rupture of coiled tubing.It is very common of defect of coiled tubing,in order to prevent sudden rupture that will cause safety accident during the operation,the fatigue life of coiled tubing containing defect must be well predicted.If the limit of life will be reached,the operation of coiled tubing should be immediately stopped or given up.In order to estimate the fatigue life of coiled tubing with different shapes and different sizes of defects and analyze the effects of defects on the fatigue life of coiled tubing,in this paper,finite element method is used to simulate the deformation and fatigue of coiled tubing with different sizes of the four of most common defects,including ellipsoidal,square,spherical and cylindrical,in the process of winding and straightening,and the fatigue life will be estimated by the equivalent strain method and the critical plane method,then the corresponding charts can be drawn.The sizes of the ellipsoidal defects and the square defects include different length,different depth and different width in the case of constant sizes in other direction.The fatigue life of coiled tubing is changed with the change of the size of the defect,and the influence of the change of the length direction on the fatigue life of coiled tubing is greater than that of the depth direction and the width direction.In the case of constant size in other directions,the shorter the axial length,the deeper the radial depth,and the wider the width of the ring,the shorter the fatigue life of the coiled tubing.As to spherical defects,only the radius will change.The fatigue life of the coiled tubing does not change with the size of the radius monotonously,but the fluctuations appear.The influence of the cylindrical defects on the fatigue life of coiled tubing and the influence of the square defects on the fatigue life of coiled tubing are analogous and interlinked basically.In general,the influence of the four types of defects on the fatigue life of coiled tubing is as follows: square defects,cylindrical defects,spherical defects and ellipsoidal defects.The smaller the curvature radius on the defect,the more obvious the strain concentration.For the fatigue damage of coiled tubing,the more concentrated the strain,the faster the cumulative plasticity,and the shorter the fatigue life.When the coiled tubing is winding and straightening,the stress at the defect can be seen as the axial tension or compression,shortening the defect size in the axial direction and increasing the defect size in the circumferential or radial direction will reduce the fatigue life of coiled tubing,and vice versa.