Downhole tools fatigue resistance for different materials

There are not, at present, effective recommendations for a comparative evaluation of downhole tool fatigue resistance. Such tools usually have different stress concentrations and are produced from steels with diverse and varied mechanical characteristics. This paper presents the results of some investigations which have been performed in order to evaluate several aspects of the problem related to material and size influences on thread connection fatigue capacity. The results of experimental investigations showed that under cyclic loads the advantage of high strength thread connections may disappear for diameters bigger than 80 mm. In this case, carbon steels fatigue resistance is often higher than alloy steels fatigue capacity and the latter are more sensitive to tensile loads. It is shown that the optimum makeup stress state in Rotary Shouldered Connections (RSC) under cyclic loads, also depends strictly on material yield strength. A case history of downhole tools failures in ultradeep wells drilled in Italy and a statistical analysis of their mechanical characteristics after production are also shown.