Environmentally assisted fatigue failure of drill pipe connections

Abstract

Failure of three drill pipe connections occurred relatively early in the service life and without the more gradual reduction in operating pressure or other factors that are typically associated with a pending fatigue failure. Of the three drill pipe connections, two had fractured and the third had cracked but remained intact. The drill pipe connections were investigated to determine the mode and likely cause of the failure. Failure of the drill pipe connections was the result of fatigue that had initiated on the outside surface within the first thread root adjacent to the change in section to the barrel or at the change in the section itself. Crack propagation was radially inwards towards the connection bore. Chlorine was present in the drilling mud at low concentrations and fine pitting was identified, but overall, the rods were free of corrosion. However, positive identification of corrosion pits, albeit few in number and significant cracks emanating from said pits, suggest failure of the drill pipes could be attributed to a corrosion fatigue mechanism. This would more so be the case for two of the drill pipe connections, as the stress concentration associated with a corrosion pit would be exacerbated in the threaded region. Fatigue had subsequently propagated under bending loads, likely reverse bending loads, applied to the drill string. Such loads would likely be the result of directional drilling within strata of higher than usual hardness.