Improved Rod String Taper Design

Abstract

Abstract This study presents an improved method for designing sucker rod taper percentages. percentages. The API RP11L is the usual source for sucker rod taper percentages. When it was last revised, in 1976, it was necessary to make several simplifying assumptions because the complete calculations were too laborious for hand calculation. Introduction With a personal computer, it is now possible to design accurate tapers for possible to design accurate tapers for each specific system with minimal computational time and effort. This paper presents the logic and formulae necessary to presents the logic and formulae necessary to design optimum taper percentages, with equal service factors, for any sucker rod system. A flow chart for a computer program is included. program is included. A brief history is given of the earlier methods of rod taper calculations. This study examines each method and describes why it was inadequate. Rod designs using equal service factors will have a significant impact on operating costs and system efficiency. The calculations for rod taper percentages need to be updated to the state of the art. The Equal Service Factor method: - Eliminates the use of simplifying assumptions and permits the calculation of the optimum design, with equal service factors, for each system. - Permits accurate calculations for rod tapers which include sinker bars. - Provides a more accurate method for calculating the loads at the top of each taper. ROD STRING DESIGN FUNDAMENTALS The sucker rod string is a vital part of the pumping system, since it provides the link between the surface pumping unit and the subsurface pump. Proper design of the rod string has a great impact on the system efficiency. Rod failures always involve servicing costs and lost production. The proper design of rod strings assures good operating conditions and will reduce total production costs and increase income. To introduce the fundamentals of rod design, a discussion of the nature of rod loads and previous rod design methods will be given. Then a detailed discussion of the proper method of balanced taper rod design will be presented. Rod Loads The sucker rod string is a peculiar piece of mechanical equipment, having piece of mechanical equipment, having almost no analogies in man-made structures. Being several thousand feet long and having a maximum diameter of 1.25 inches, the solid steel rod string can be considered a perfect "slender bar". Since its weight is perfect "slender bar". Since its weight is distributed along its length, any section has to carry at least the weight of the rods below it. This results in stretch and suggests that an ideal construction would be a continuous taper from top to bottom. Since such a shape is impractical, the use of tapered strings with sections of decreasing diameters is a common design practice. practice. The most basic property of any sucker rod string is its elastic behavior.