Numerical and experimental assessment of the long-operated drill pipes destruction conditions

Abstract

The cause of failures in the elements of the drill string when drilling deep wells is the occurrence and development of crack-like defects in the cross section. The generalized data of defectoscopy of long-term operated drill pipes conducted at the Ukrburgaz drilling rigs from 2015 to 2017, as well as numerous literature data show that such defects are formed at the bottom of corrosion ulcers that occur during operation under the influence of workloads and drilling fluids, in particular during tripping operations, both on the inner and outer surfaces of the drill pipe. During the tripping operations, under the influence of tensile loads, they being developed go up to the surface of the drill pipe, forming scouring, which ultimately leads to the destruction of the pipe. Using known analytical solutions, the stress state in the vicinity of the semieliptic crack located on both the outer and inner surfaces of the hollow cylinder under axial load was estimated and the corresponding stress intensity coefficients at the crack tip of these defects were determined. To predict the limit state of a drill pipe with a semieliptic crack, the analysis of crack parameters reflecting the local stress-strain state near the crack tip is performed. Using the finite-element method, the stress state around the vertex of the semieliptic crack was determined and on this basis the characteristics of fracture mechanics were calculated: J – integral and stress intensity coefficients. Diagrams of fracture risk assessment in the coordinates "depth of crack-like defect - depth of drilling - stress intensity factor" are offered, which make it possible to estimate the safe depth of both internal and external semieliptic crack-like defects detected by technical diagnostics taking into account results of experimental investigations of critical crack resintance. The admissible sizes of the studied defects depending on the depth of drilling are determined.