The power criterion of the resistance to failure of the metal of coiled tubing (CT) was experimentally evaluated. The conditions are defined under which failure is possible during the operation of CT containing an external semi-elliptical crack. The interconnection of the critical depths of external semi-elliptical cracks from the ratio of their semiaxes is established. It is shown that, during the course of operation, the external transverse semi-elliptical crack with the ratio of semiaxes $(a/c)_{i} = 1/2$ is the most dangerous for coiled tubing. The experimental and calculation procedures are proposed that give us an opportunity to interpret the results of their technical diagnostics in evaluating the conditions under which the failure of flexible pipes containing outer transverse semi-elliptical cracks is possible during tripping operations.
