| The applications of coiled tubing in oil industry are broader and broader because of its efficiency and its capability of operation under pressure. Coiled tubing needs to be spooled on the reel. In the process of operation, the coiled tubing winded on the reel goes through the gooseneck and then straightened by injector unit and injected into oil well. When operating is complete the tubing is retrieved from the well and spooled back onto its reel by the opposite process. Coiled tubing needs to be wound on the reel whose diameter cannot be too large due to transportation. Since the bending radius is much smaller than the radius of the elastic limit of coiled tubing, the plastic strain due to wrapping and unwrapping of coiled tubing on the reel will produce residual deformation. The cyclic plastic strain caused by repeat wrapping and unwrapping of coiled tubing on the reel will result in low cycle fatigue of coiled tubing, and ultimately will result in fatigue failure of coiled tubing.In the process of the use of coiled tubing, main failure forms of fatigue can be summarized to following kinds:cracking or piercing; permanent deformation such as too large ovality, bulge due to diameter growth. The large ovality of coiled tubing will affect its injection capacity. The coiled tubing cannot inject into the well when the ovality is too large and affect the sealing and clamping of the equipment at the same times. What’s more, it has great impact on the collapse resistance of coiled tubing. If the ovalization of coiled tubing is serious, the collapsing strength will be greatly reduced; in the process of repeatedly bending of coiled tubing, the plastic strain will accumulate. When the cumulative plastic strain reaches a certain extent, tiny cracks in coiled tubing due to the low cycle fatigue will appear and propagate with the winding process. When the crack grows to a certain extent, cracking or piercing will appear which will directly result in fracturing of coiled tubing; diameter growth of coiled tubing is ascribable to its axial stress, circular stress and radial stress caused by bending and inner pressure. With the increase of the number of cycles, the diameter of coiled tubing will become larger and larger, and the wall thickness of coiled tubing is getting thinner and thinner at the same time. Too large diameter growth will cause incompatible of coiled tubing with the equipment.Researching the variation of the parameters such as ovality, diameter growth, wall thickness, accumulative plastic strain and their influence on the fatigue life of coiled tubing while winding and unwinding on the reel, can give advices to the operators during their operating of coiled tubing, and can find out suitable type of coiled tubing in different working condition and the best working condition of coiled tubing. So the research can give suggestions to the rational use of coiled tubing in the field, which has important significance to the application of coiled tubing.To the author’s knowledge, there are still some incomplete in the test and the theory of the fatigue deformation and failure of the coiled tubing, which needs to be improved. This thesis will be studied from the following aspects:(1) Analysis of the force situation of coiled tubing when it is bending.Forces and deformations of coiled tubing during its testing in fatigue test unit is analyzed in this thesis. At the same time, the result is also verified by the finite element method.(2) Analysis of the material model of coiled tubingThe repeated bending and straightening of coiled tubing result in its material to be loaded and unloaded, and the material hardening rules have an important influence on the research results. Find out material model for the analysis of coiled tubing by researching the relationship of binding times, inner pressure and the diameter affect by different model such as ideal elastic-plastic model, bilinear elastic-plastic hardening model and Ramberg-Osgood model and different hardening methods as isotropic hardening and kinematic hardening.(3) Research on the relationship of binding times and parameters such as ovality, accumulated plastic strain and diameter growth rate with different internal pressure and different bending radius of coiled tubing, analysis of the influence of different operating conditions on fatigue life of coiled tubing.In the process of operation, coiled tubing is subject to both bending load and internal pressure. When coiled tubing bending and straightening repeatedly, it will appear geometric deformation such as the cross section becomes ovality, diameter growth and thinning of wall thickness. All these geometric deformations are caused by plastic deformation due to the complicated loads. Variations of parameters with cycles such as ovality, accumulated plastic strain and diameter growth rate of bending coiled tubing are investigated by 3D and 2D finite element models, and some influences of different working conditions on fatigue life of coiled tubing are found.(4) Analysis of secondary loads as coiled tubing wrapping and unwrapping on reel, and calculate the local hoop stress caused by secondary loads.There is a large curvature when the coiled tubing is bending, which will generate secondary loads on coiled tubing. Secondary loads induce local hoop stress of coiled tubing, and produce also the similar damage effect as inner pressure coiled tubing does.(5) Calculation of the fatigue life of coiled tubing.By calculating the hoop stress caused by secondary loads, correct Tipton’s calculation method of fatigue life of coiled tubing and calculate the fatigue life of coiled tubing.Through the above analysis, we can get the following results or conclusions:(1) It is mainly bending moment that makes coiled tubing to bend. This gives a suggestion to study on the low cycle fatigue life of coiled tubing.(2) By means of three-dimensional finite element model, it is proved that the bending of coiled tubing is mainly the result of bending moment load, which provides a basis for the establishment of two-dimensional model. The influences of different operating conditions on geometric deformation parameters are studied by using two-dimensional finite element model, and the phenomenon that the number of times can be bent of coiled tubing subjected to higher inner pressure will reduce seriously is found. The appropriate type of coiled tubing needs to be chosen in coiled tubing operating field and makes it worked under low hoop stress.(3) Secondary loads as coiled tubing wrapping and unwrapping on reel is derived, and the local hoop stress caused by secondary loads have calculated. This gives a suggestion to accurately calculate hoop stress of coiled tubing in zero inner pressure.(4) The problem of difference between fatigue life of coiled tubing without internal pressure or at low internal pressure calculated by theory and experiment is solved. A method for calculating the fatigue life of coiled tubing under all internal pressure is found. |
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