Abstract
During drilling operations the drillstring is subjected to a variety of loading conditions. Cyclical mechanical loads in combination with the corrosive effect of the used drilling fluid are particularly challenging for metallic materials used in load-bearing drillstring components. Drilling fluids often include large amounts of chloride ions. In addition, the temperature inside the drillstring approximates to the formation temperature, which can be significantly high depending upon the type of well. Field experience shows that when exposed to these harsh environments during service, structural materials used in drillstring components might undergo pitting corrosion and/or environmentally assisted cracking (EAC) e.g., stress corrosion cracking (SCC) and corrosion fatigue (CF). Therefore, metallic materials used in load-bearing drillstring components should be properly assessed with regard to their CF strength. For this purpose a test methodology including a test rig capable to mimic service conditions and appropriate testing environments was developed. The test rig consists of a rotating bending test machine equipped with a heatable corrosion test chamber enclosing the test sample. The temperature can be monitored directly in the test chamber and can be controlled up to 200 °C. Test solutions were also developed to mimic typical service conditions in terms of chloride content and pH-value present in drilling operations. This paper discusses how the selection of testing parameters affects the CF strength of structural materials typically used in drilling equipment including austenitic CrMn stainless steels and Ni-based alloys. In addition, results obtained using the developed test methodology are compared with those collected using traditional testing environments such as high-chloride acidified solutions.
