Analysis of fatigue crack growth and estimation of residual life of the walking beam of an oilfield pumping unit

An analysis of the fatigue crack growth and the corresponding residual life evaluation of the walking beam of an oilfield pumping unit are presented. Lifting lugs had been welded on the upper flange of the walking beam at the moment of assembling the machine. A crack nucleated at one of the weld toes and grew by fatigue up to the critical condition that led up to the instability of the waking beam after 10 years of operation, taking the equipment out of service and producing important economic losses to the operating company.As the company operates many units working in similar conditions, the estimation of fatigue residual life presents interest in order to define inspection intervals for examinations by non-destructive testing. In this way, any crack growing by fatigue will be detected and repaired, preventing the catastrophic failure of the component.The stress cycle at the failure zone was calculated and the mechanical properties of the walking beam steel were determined. With this information the final failure conditions were analyzed by using the failure assessment diagram. The fatigue residual life was then estimated by means of a model developed to consider the particular features of the crack path related to the beam geometry. Different operating situations were considered.Finally, based on the results obtained, an interval of 12 months between consecutive inspections of the examinations by non-destructive testing was proposed.