Evaluation of the effectiveness of API-modified Goodman diagram in sucker rod fatigue analysis

Date
2001-08
Journal Title
Journal ISSN
Volume Title
Publisher
Texas Tech University
Abstract

In petroleum engineering applications an understanding of the fatigue behavior of metals is of great importance primarily in the selection of sucker rods. Of all the components of the oil-well pumping mechanism, sucker rods have the greatest effect on the action and performance of the whole pumping system. The behavior of the sucker rods in the transfer of forces and loads determines the action of the pump. Their inherent characteristics affect the loads imposed on the surface equipment. For years, the Goodman diagram is the basis of analyzing the endurance (fatigue) life of ferrous material. Goodman published the diagram during the year 1908. An American Petroleum Institute (API) Committee suggested several design factors for Goodman Diagram in order to adapt it specifically to specifically sucker-rod pumping installations. The diagram is known as the API Modified Goodman diagram. The committee had suggested the following revisions to original Goodman Diagram. They are: (1) The apex of the diagram should be set at the tensile strength of the material. (2) The >^-intercept should have a factor of safety of two. (3) The safety factor on the material tensile strength apex should be 1.75.

The result was the creation of API Modified Goodman Diagram. The design formula for the allowable stress line is given as: S,={T/4+MSmin)SF…………………………………………………………………………………(1) where Sa = Allowable maximum stress, psi, T= Minimum tensile strength, psi, M= Slope of the allowable stress line = 0.5625, Smin = Minimum stress, measured or calculated, psi, SF = Design Safety Factor.

Since this diagram is used for the design of permissible stresses for sucker rod installations, it is imperative that the design engineer understands the loads imposed on the sucker rods; however, conventional polish rod dynamometer analysis does not provide sufficient information regarding down-hole load conditions.

Since the rods available today is superior due to the improved manufacturing processes and testing facilities, this thesis is an experimental study the effectiveness and accuracy of the "API Modified Goodman Diagram." Tests conducted at 160% of the stress range as specified in the API Modified Goodman Diagram on 5/8 inch. Grade 'D' sucker rod specimens resulted in the following findings:

  1. The Y-intercept of the diagram can be changed to r/2.5 as compared to T/4 given by the API Modified diagram.
  2. The slope of the allowable stress line is found to be 0.3 as compared to the 0.5625 given by the Modified Goodman diagram.

In effect, the test results indicate that the present API Modified Goodman diagram is conservative and the formula for the allowable stress line could be revised to Sa =(T/2.5+ MSmin)SF………………………………………………………………………(2) where, Sa = Allowable stress, psi, T = Minimum Tensile Strength, psi, M= Slope of the allowable stress line = 0.3, Smin= Minimum Stress, psi, SF= Safety Factor.

Description
Keywords
Structural analysis (Engineering), Sucker rods, Metals -- Fatigue
Citation