Definition of limit state of flexible pipe material in coiled tubing technology

Currently, the use of coiled tubing technologies has become widespread in drilling and oil production. The flexible pipes (FP) used in this technology undergo a complex of several impacts that accelerate the process of damage accumulation and thereby cause an accelerated process of seats of destruction formation, which often leads to accidents. Statistics show that the main reason for the destruction of the FP is the fatigue of the material loading and basically it is a cyclic bending according to the repeating stress cycle scheme. This loading scheme has a feature that is associated with the fact that tensile loads occur mainly in the convex zone of the pipe and compressive loads in the concave zone. Since the physical and mechanical properties of steels under stretch and compression are different, the process of damage and destruction accumulation during cyclic stretch and cyclic compression will have a different character. Information about the patterns of changes in the properties of the FP material allows to estimate the period of reaching the limit state and, thereby, prevent accidental destruction. In this regard, fatigue tests were carried out with a step-by-step measurement of magnetic characteristics that are structurally sensitive to the accumulation of fatigue damage. The determination of the stress amplitude for the FP fragment was determined by preliminary calculation in the ABAQUS PC. The test results showed that during cyclic stretch of the samples, the magnetic characteristics for assessing the limit state in the FP material were not informative. Notably in samples loaded with cyclic compression, the results of magnetic characteristics measuring (the resulting intensity and the gradient of the intensity of the constant magnetic field) showed sensitivity to the onset of the limit state. Based on the conducted studies, it is proposed to control the limit state of FP in the concave zone, where compressive loads mainly occur. These measurements allow us to estimate the value of the residual life of the FP material and to put this object out of operation in a timely manner, preventing accidental destruction.