An examination of the forces acting on a sucker rod string and the relation of those loads to the tendency of a rod string to buckle is presented. Emphasis is placed on the fact that effective, as opposed to true force is the governing parameter for rod buckling. Sample calculations indicate the importance of effective tension in both uphole rod analysis and near pump rod and sinker bar design.
The analysis of working loads in a sucker rod string during a pumping cycle has received substantial coverage in the petroleum literature. These load predictions have tended to focus on mechanical design considerations such as excess load and fatigue prediction. In contrast, the current study addresses the issue of buckling associated with working axial/pressure loads in an attempt to clarify the means of both predicting buckling and minimizing its effects.
The study begins with a review of the static loads acting near the pump, and proceeds to a discussion of how these loads relate to the tendency of a rod string to buckle on the downstroke. Critical to this discussion is the concept of effective tension. Definition of the effective tension leads to the application of this concept to sinker bar design as a means of mitigating the buckling tendency of a rod string. Key points are reinforced by illustrative examples.