Analysis of Drillstring Failures

Abstract The drilling industry is currently exploring various ways of reducing drilling costs through a multitude of initiatives; for example C.R.I.N.E. (Cost Reduction In the New Era). One of the major areas where there is potential for cost reduction is downtime due to hole problems and drillstring failures. Over the last five; years, British Gas (B.G.) has experienced a number of drillstring failures despite following the industry accepted practices of drillstring design. As a result of this, a substantial research programme was initiated to investigate the fundamental mechanics of drilling vibrations, and in particular the prediction of the onset of a resonance that contributes to a fatigue failure. The programme concentrated on four major areas: A full metallurgical analysis of the failed components;Development of mathematical models to predict natural frequencies;A collection of field data; andBuilding a laboratory based scale model drillstring rig to generate vibration data and validate the mathematical models in conjunction with field data. This paper presents details of the above research programme that results in a substantial reduction of drillstring failures. It will present field examples from both onshore and offshore wells. The paper also details British Gas's approach to the reduction of harmful resonance through the strategic placement of a shock sub.