Pumping Units Well’s Pump Inspection Period And Reliability Prediction Method Of Fatigue Life For Rod String

Sucker rod is the most important mechanical component in the pumping unit well,which is loaded alternating loads in use. Because of corrosion, up and down rod string inoperation and service life, fatigue failure will occur. The fatigue fracture is the maininvalidation mode, which not only increases the cost of checking pump, but also affectsthe oil production. It is an increasingly outstanding problem to the oil production. Thefatigue life and reliability study has been made for rod string to some facts that rod string,tubing and pump system are usually damaged.This research provides theoretical basis forbreakdown repair. It’s very important to decrease oil cost and improve the oilfielddevelopment macroeconomic benefits. This paper mainly studies the aspects as follows:A fault tree model for sucker rod, tubing and pump system is established byconsidering the fault form of rod string, tubing and pump system. The probability densitydistribution model and reliability predication model are established based on statisticalregulation of pump inspection period in oil field.Based on the wave equation of longitudinal vibration, the simulation mode of rodstring axial load distribution is proposed. The maximum equivalent stress and stressconcentration factor for crack and corrosion spots pit-containing sucker rod are analyzedby the ANSYS finite element analysis (FEA) software. Meanwhile, regression model ofstress concentration factor is put forward. Considering the randomicity for the calculatingerror of stress, material mechanical properties and flaw sizes, reliability predicationmodels of the fatigue life for arbitrary section and the whole rod string are established.Taking the effects of rod string investment, operation cost and output loss caused bychecking pump into account, a reliability evaluation methodology of economic lifetime forsucker rod is presented with the objective of minimizing yearly total cost in the rod’sservice life.Based on the theory of fracture mechanism, the calculating model of crackpropagation residual life for sucker rod with crack in the surface is established. Meanwhile,reliability predication models of fatigue life for arbitrary section and the whole rod stringare represented by crack expansion. The corresponding software for life and reliability simulation analysis of rod string isprogrammed by VB 6.0. The effects of actual using factor of stress, fatigue strengthinfluencing coefficient at transition section, service life, crack and corrosion pit on thepump detection period and reliability are also analyzed. At the same time, the effects ofthe unit price of sucker rod and the crude oil production on the economic lifetime ofsucker rod are simulated.