| Sucker rod is an important device in pumping wells, which is the main parts transfer loadbetween pump and units. Sucker rod fatigue strength and service life determine the depth andthe maximum pump displacement of the entire pumping system.As the continuous exploitation of oil, most of the large oilfields in our country presnetly are in medium and high water cut stage, and many of them adopt the mechanical recovery method oil production.About 80% of production of oil wells in oil fields are using the pumping with sucker rod technology,75% of those produced using sucker rod. And every year it needs millions of meters’s rod for pumping, which means that the rod occupies an important position in oil production in the country.In addition, according to the theoretical calculation and experimental results, the current service life of sucker rod disigned by oil production workers is normal for ten millions times, whcich translated into conversion time is close to 40 months. There are a lot of sucker rod reach their service deadline or recycle after the oil well pump’s maintenance.But a lot of sucker rod without significant injury could be recycled.Due to the total amount is large, the recycling of sucker rod can significantly reduce the production cost. So the research on failure of sucker rod has important engineering significance.According to oil field data, the fatigue fracture is the main cause of sucker rod failure.Therefore,this article has studied the rod body elliptical surface crack on sucker rod by using the finite element software ansys workbench made simulation analysis for its surface crack. The analysis model of rod is 25 mm in diameter,and made by 20CrMo steel grade D sucker rod from engineering application background.After referencd some related literatures,this paper calculated the maximum and minimum stress intensity factorin two calculation methods-the limited boundary influence coefficient method and the fatigue crack propagation rate method.The result of the calculation result shows that the former one is larger than the latter one,and it stays the same after comparing multiple sets of calculation the conclusion.This paper put forward that the reference limited boundary influence coefficient method of safety coefficient is higher during in engineering calculation.After the finite element analysis by using the software of sucker rod body loaded crack,it concluded that the stress concentration coefficient on rod body with a crack is about 7.5.the simulation from the deepest part of the Ki value and the corresponding calculation basic keep within 10% deviation.the paper put forward.When doing engineering analysis,the simulation results can be assumed to be estimated on the basis of the deepest crack stress intensity factor values.And the point’s value on the surface of the crack is opposite bigger.The deviation rate betreew K.i value and simulation value is relatively stable, but according to the engineering actual situation to decide whether the value of reference.It is concluded that the crack front of Ki value distribution and maximum/minimum value.It has compared simulation with the theoretical calculation value deviation. It shows this way would be helpful to the theoretical calculation conclusion, and calculated the multiple sets of data to verify its correctness.After getting the calculation that the simulation results can be reference, this paper analyzed the elliptical crack on the surface of a rod body in the simulation.To the same depth under different crack size has carried on the simulation analysis and calculation, it is concluded that the crack shape ratio and the relationship between the crack size after simulated thec racks which has different crack size under same depth.It puts forward the relationship between the crack size and shape of crack, and points out that the maximum stress factor at the crack tip is obtained when the a/c tends to zero, which is known as one of the most dangerous cross section shape. Based on former conclusion,it has simplified formula of K1, under static load for the engineering practice.After the formula is simplified, the crack depth calculation become more conveni-ent the because of independent variable remains one-a.Of course, the applicability of the formula and the determination of modification coefficient needs be summarized through the production practice experience.Based on the function relationship in paris formula which was caculated by Newmen-Raju,Fleck-Smith,Jolles,etc,this paper puts forward a new calculation model for calculating the residual life of sucker rod surface crack.Refered to the related literature, this paper had used the experimental data to verify the model.It turns out that:when the initial crack is deep, the devition between experimental results and the new model calculation results are around 6%. It is concluded that model is credible.The model can be applied to caculate the crack in the residual life prediction of the component in service, which has great importance to the economic and social significance in reasonable arrangement of sucker rod and prevention of rod string’s fracture failure in downhole. |
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