Failure analysis of drillstrings

Abstract The cost of drilling a well is measured in tens of millions of dollars. The incidence of downhole failure of the drillstring can increase this figure dramatically. The focus placed on cost reduction in the early 1990’s – when the oil price was much lower than today’s levels – resulted in some scrutiny of drilling operations, amongst other areas. Drillstring failure was a natural part of this. Despite the earlier attention, failure of drillstrings remains to this day an undesirable feature of oilwell drilling. The costs associated with lost time (to recover the drillstring from the well or to sidetrack; and recommence drilling) and the material cost of the damaged drillstring elements can be very high. This is especially true where the failing drillstring is not detected at the wash-out stage and complete separation subsequently takes place downhole. Premature failures commonly fall into two general groups: at the threaded connections or in the body of the drillpipe at the internal taper. This paper presents a number of case studies from the authors’ own work that collects the results of investigations on failed drillstrings and components spanning over a decade of activity in the North Sea. Improvements in relation to design practice, manufacture, use and inspection are also discussed.