Bending Stresses In Otherwise Straight Sucker Rods

Abstract

Abstract In many cases sucker rods fail in the upset portion near the pin end. This may be due to bending stresses resulting from lack of straightness in the rod, even though it was manufactured to be within the API Specification 11B. A number of new, unused rod coupling pairs were analyzed for straightness and a model was developed to predict the bending stresses. It was found, that for some rod coupling configurations, there existed bending stresses as high as 68% of the axial stress. This suggests that the API 11B Specification may not be stringent enough. Introduction Research into sucker-rod problems has been ongoing for 50 years or more, resulting in several significant improvements in rod-coupling performance. Nevertheless, failures of rod strings continue to be a problem for the oil producer. As down-hole components get older and well conditions become more corrosive this problem will become more severe. However, there are three avenues which can be followed to alleviate the problem:Reduce the loads on rods appropriate to the specific well conditions (dog-leg, etc.) and environment (i.e. apply more stringent design service factors).Replace current rods using materials with improved properties.Improve the current design and manufacturing techniques to eliminate the weaknesses inherent in existing rod strings. While (1) above may not be practical except under a total replacement program and (2) requires long-term research and careful examination, (3) may provide the most benefit In the short term but, like (1), would only apply to replacement rods and strings. The purpose of this paper is to describe a problem related to the third category involving the lack of straightness in suckerrods which are made within API 11B Specification(l). It will be shown that non-straight rod ends can lead to undesirable bending stresses near the upset pin-end. Statement of Problem For the purpose of this study, a rod end is defined as the length from the pin end to a point 860 mm (34 in.) back towards the rod body. The rod end is defined as non-straight if its centre line deviates from a theoretical centre line with extends through the pin. To examine bending stresses in non-straight rod ends, a dimensionless bending stress ratio (DB) is dermed as: Equation (Available In Full Paper) The dimensionless bending ratio (DB) can be defined in terms of the axial load P, second moment of area I, cross-sectional area A, and y the distance from the theoretical straight line to the rod centre. Thus, Equation (Available In Full Paper) In an earlier work(2) it was shown that bending stresses can be determined in rods of uniform cross section with sharp bends in them by considering elementary beam theory where the bending moment M is given by Equation (Available In Full Paper) and yo-y = deflection of bar from the unloaded position, EI is the rod stiffness and z the distance along the rod as measured from the pin end.