Failure analysis of drillstring and bottom hole assembly components

Abstract

Drillstring and bottom hole assembly component failures continue to afflict the offshore oil and gas industry, annually involving direct and consequential costs extending to millions of pounds. This wide ranging problem has been exacerbated by recent industry trends towards the drilling of deep, deviated wellbores. Further intensification of the problem may occur if extended reach, horizontal drilling and multiple lateral completion programmes become more prevalent. Analysis of a wide range of recently failed drillstring components has been carried out using both quantitative and qualitative techniques. Although the failures studied were primarily threaded connections, failures from a number of other design details have also been apparent. The general nature of drilling operations combine to incur post-failure damaged to the failed components—particularly to the fracture surfaces—presenting significant problems with regard to metallographic and fractographic failure analysis. Both direct and inference based analysis approaches have been developed to overcome these problems. Visual examination was found to be the most useful ivestigative method, provided it was supported where necessary by metallography, chemical analysis and mechanical testing. Fractographic examination was generally unproductive, due in the main to post-failure damage incurred in the harsh downhole environment. The investigations highlighted some design shortcomings, but have primarily demonstrated that fatigue, corrosion fatigue and simple overloading are the principal causes of failure, promoted by alternating axial, torsional and bending stresses and aggressive media.