Elastic-plastic collapse limit analysis of coiled tubing under complex stress state

Abstract

Collapse failure of coiled tubing (CT) due to complex stresses is an important issue during the operations in the oil and gas wells. This paper presents new analytical models to define the collapse limit of CT and solve the problem of CT failures. Elastic-plastic collapse limit models of CT are established based on the twin shear unified strength theory. The effects of axial force, buckling, and internal pressure on the elastic-plastic collapse limit are analyzed. Moreover, the comparisons among the new model, the existing model and API specification are further discussed. The results indicate that the elastic-plastic collapse limit models determined using the twin shear unified strength theory can describe the mechanical behaviors of CT under complex stress state. Elastic-plastic collapse limits of CT decrease with increasing axial tensile force and increase with increasing axial compressive force. The effect of buckling and bending stress on the elastic-plastic collapse limit cannot be ignored in the analysis of CT collapse. With increasing external pressure, the plastic region keeps extending to the outer surface of CT. Plastic collapse limit is the value of external pressure for which the entire wall thickness of CT becomes plastic. The proposed approach can be used to set the elastic-plastic collapse limits for other oil tubulars.