An Enhanced Model for the Design of Tapered Sucker-Rod Strings
- Gabor Takacs (University of Miskolc) | Mihály Gajda (University of Miskolc)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2017
- Document Type
- Journal Paper
- 1 - 6
- 2017.Society of Petroleum Engineers
- design procedure, mechanical design, optimum design, sucker rod string
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- 463 since 2007
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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 that are based on different assumptions were developed in the past. Because most sucker-rod breaks are fatigue failures, the mechanical design of a 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 use 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 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. This 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. By use of 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 do not usually have identical safety included in the different tapers. This is because of the improper calculation of rod-string loads that form the basis of calculating rod-taper percentages. On the basis of the evaluation of the different available rod-string designs, the paper introduces a novel procedure that evaluates rod loads from the predictive solution of the damped-wave equation during the design process. Because loads calculated in this manner simulate actual loads with high accuracy, the most-important limitation of previous rod-string-design procedures is eliminated. Strings designed by use of the proposed model, therefore, will have identical safeties against fatigue failures in each taper; the new design model provides a much greater safety over previous designs.
|File Size||746 KB||Number of Pages||6|
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