Paper: Analysis of Sucker Rod Sinkerbar Failures

This paper presents results from a study to analyze the performance and failures of the sucker rod/sinkerbar string used in beam-pumping operations through metallography, structural finite element analysis, and detailed failure data collection. Metallography has demonstrated that the microstructure of the steel bar stock needs to be considered to improve the fatigue resistance of the sucker rod strings. The current specification based on tensile strength, or yield strength, may not be appropriate since failure occurs because of fatigue and not yielding, and tensile strength is not always a good measure of fatigue resistance. Finite element analysis of the threaded connection identifies stress and fatigue concentrations and quantitatively assesses the performance and failure of coupling designs under a variety of loading. conditions. Subcritical fractures observed in the metallography are also suggested by the calculated stress distribution in the threaded coupling. Failure data illustrates both the magnitude and frequency of the failures, as well as categorizing the suspected cause of failure. This detailed failure information alone can reduce failures by indicating specific problem areas where focused action can yield immediate results. Application of the results in each of these project areas is expected to yield improved choice of metal bar stock, thread design, and make-up practices which can significantly reduce the frequency of sucker rod failures. Sucker rod failures at the rates observed today are not inherent in the process, but can be minimized through the application of new technology and observation of common-sense practices.