The Ultimate Sucker-Rod String Design Procedure

Abstract

Abstract A properly designed sucker-rod string should provide failure-free pumping operations for an extended period. Improper design of rod tapers can lead to early mechanical failures (rod breaks) with a complete termination of pumping action and an inevitable loss of production. Because of its prime importance in sucker-rod pumping technology several rod string design procedures based on different assumptions were developed in the past. Since most sucker rod breaks are fatigue failures the mechanical design of sucker-rod string must consider the cyclic nature of rod loading and the appropriate fatigue endurance limit of the rod material. This is why most string designs utilize the modified Goodman diagram for calculating the lengths of rod tapers. Available sucker-rod string design models calculate rod taper lengths that ensure proper operation without premature fatigue failures. Their common design problems are (a) defining the principle of taper length determination, and (b) calculating the true mechanical stresses along the string. The universally accepted principle of taper length calculations is to provide the same level of safety against fatigue failures in each taper section. Mechanical loads and stresses, on the other hand, are usually found in most of the design procedures from very approximate formulas. These loads, therefore, can greatly deviate from the true mechanical loads that would be measured in the rod string run in the well. The paper details the main features of available rod string designs and discusses their main characteristics; it provides a thorough comparison of designs involving the calculation of loads and stresses predicted from the solution of the damped wave equation. Using a predictive analysis program rod stresses are calculated that, plotted on the modified Goodman diagram, provide a proper comparison of the merits of the different rod string design methods. Rod strings designed by available design procedures usually do not have identical safety included in the different tapers. This is due to the improper calculation of rod string loads that form the basis of calculating rod taper percentages. Based on the evaluation of the different available rod string designs the paper introduces a novel procedure that estimates rod loads from the predictive solution of the damped wave equation during the design process. Since loads calculated that way simulate actual loads very accurately, the most important limitation of previous rod string design procedures is eliminated. Strings designed using the proposed model, therefore, will have identical safeties against fatigue failures in each taper; the new design model provides the ultimate safety over previous designs.