Nonlinear Finite Element Modeling and Experimental Validation of Advanced High Fatigue Strength Threaded Connections for Sucker Rod Pumping Applications

Abstract

Abstract Rod pump systems can be the most commonly used artificial lift technology for oil production. A rod pump system that brings underground oil to the earth’s surface consists of a prime mover, a surface pump, a sucker rod string, and a downhole pump. A pin at each end of the rod and a box coupling are threaded together to connect the sucker rod and form the rod string. The sucker rod string acts as the connecting link between the surface pumping unit and the subsurface rod pump. The loads generated from the reciprocating upstroke and downstroke motions could impose a risk of fatigue failure in the sucker rod connections with cyclic tensile stresses. The increasingly harsh field operational conditions that require deeper wells and higher production are pushing the connections to their limits. As a result, failures of the standard API (the American Petroleum Institute) sucker rod connections are becoming increasingly more frequent. The resulting rod string failures are expensive to repair because the whole string must be disassembled and removed so that the failed rod can be accessed, and the rod string must then be reassembled. Such high repair cost is unaffordable for the wells with low production rates. Therefore, there is a strong demand for having a threaded connection design with high fatigue strength that is suitable for sucker rod pumping applications. To enable a long service life and to reduce cost of service delivery caused by frequent repairs, an advanced rod pump threaded connection design with significantly improved fatigue resistance over the standard API design has been developed and experimentally qualified. Modeling and simulation techniques had been extensively used to drive the design and qualification process. The newly developed connection design is featured by an optimized thread form and stress relief groove, and an advanced manufacturing process. Nonlinear finite element analysis (FEA) was extensively utilized to predict the fatigue resistance of the threaded connection with makeup torque and service stroke loads, and to explore and optimize the designs of the threads and stress relief feature prior to physical prototyping and testing. The FEA models had favorably predicted the performance of the new design, which was later experimentally validated through full-scale axial and rotary bending fatigue tests, respectively. The work presented in the paper sets a successful example of model-driven design, which can significantly shorten the development time and cost. The newly developed high fatigue strength sucker rod connections have great potential for mitigating the risk of rod string failures during pumping operations and therefore improving operational reliability for both beam and progressing cavity pumping systems.