Elastic–Plastic Limit Load of Coiled Tubing Under Complex Stress State

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Coiled tubing (CT) endures complex stresses during the operations. The loading imposed on the CT can push it close to its performance limits. This paper presents new analytical models to define the limit load of CT and solve the problem of CT failures. Elastic–plastic limit load models of CT are established based on the twin shear unified strength theory. Moreover, the effects of residual bending stress, buckling, axial force, and external pressure on the elastic–plastic limit load are discussed. The results indicate that the elastic–plastic limit load models determined using the twin shear unified strength theory can describe the mechanical behaviors of CT under complex stress state. The initial yield elastic limit load of CT decreases with increasing axial compressive force. The effect of buckling and residual bending stress on the initial yield elastic limit load cannot be ignored in the analysis of CT strength. With increasing internal pressure, the plastic region keeps extending to the outer surface of CT. Plastic limit load is the value of internal pressure for which the entire wall thickness of CT becomes plastic. The proposed approach can be used to set operation limits for other oil tubulars.