Fiberglass rods are mainly used to overcome the design limitations of rod pump equipment when additional lift capacity is required. Economic analyses for new installations or repair jobs must consider the life span of fiberglass rods. This study evaluates the cost-effectiveness of fiberglass rod applications by reviewing their life spans in the Permian Basin.
The objective of this project was to build a stress fatigue diagram to help minimize expenses by maximizing the effective life of fiberglass rods. In theory, this diagram would define the allowable stress range that could be applied to fiberglass rods without causing excessive failures. Each data point would consist of one fiberglass rod failure, and all calculations would be performed over the age of the taper, from initial installation to failure. These operating limits would then be applied to field applications.
The industry's rule of thumb for the "end of life" for fiberglass rods is 30-40 million rod reversals. However, most failures occurred before the rods reached one-third of their life expectancy, even though the fiberglass rods were operated well within the recommended stress ranges provided by the manufacturers. There was a positive relationship between average fiberglass taper failure frequency and average peak polished rod stress. Failures mainly occurred in the steel connection in the pin. Therefore, failures were not due to tensile stress fatigue in the fiberglass body. Failure frequency was so high in some fields that upsizing the pumping unit made more economic sense than installing a fiberglass taper.
The recommendations from this project were to: (1) understand connection failures better through improved root cause failure analysis (RCFA) data collection and manufacturer involvement; (2) reassess and improve operational conditions at failure, such as rod pump design, pump off setpoints, and pump fillage; (3) evaluate switching to a 100% metal string with an upsized pumping unit or installing a different artificial lift method if failure frequency is not reduced by operational changes; and (4) re-evaluate rod string design criteria to maximize value, as current designs are based on tensile loading in the body of the top rod, but actual failures were not due to tensile stress fatigue in the body.
Significant cost savings can be achieved if the average life span can be increased to the industry standard of 30-40 million rod reversals. More work needs to be done to understand connection failures.