CONICET | Buscador de Institutos y Recursos Humanos

A drill bit with polycrystalline diamond (PDC)inserts lost one of its three blades when operating in an oil well, leading toa costly failure. Operating conditions and associated stresses were analyzed,bit core material in the failure and adjacent areas was analyzed and tested,and fracture surfaces identified. Base material is a Ni-Cu-Mn matrix with tungsten carbide precipitates. Fracture surfaces showedcleavage planes and loss of particles, indicating a brittle fracture. Microstructuresand hardness were similar in all analyzed regions, and according to specifications.The symmetry and characteristics of the fracturesurfaces allow defining that the loads that caused the failure were not appliedduring the drilling operation. The blade broke apart due to a downward forceapplied at its base. Finite elements numerical modeling allowed pinpointing aspecific moment in the pulling operation, in which a 28 ton overpull force wasrecorded, as the immediate operational event that caused for the failure. Operating procedures that reduce the likelihoodand amplitude of impact loads, are difficult to implement; more promising isthe alternative for a redesign of the drill bit. Commercial designs focusmostly on the efficiency of the cutting cycle; blade geometry can be alsooptimized to take into account the pull out conditions. The most efficientredesign for this specific drill bit model relies in a re-machining of theblade base, so that a large overpull load would crack a small sector, on whicha PDC insert is located. In this gecko-tail type solution, only one insertwould be lost, preserving the integrity of the rest of the drill-bit.Subsequent repair would involve standard thermal spray base metal techniques,including reconstitution and brazing of a new insert.

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