Damage to flexible pipes of coiled tubing equipment due to corrosion and fatigue: methods and approaches for evaluation

Purpose. To propose an engineering approach for evaluating surface corrosion fatigue of flexible pipes of coiled tubing equipment, on the basis of the generalized data about corrosion fatigue behaviour of structural steels and ana-lysis of the existing methods and approaches for assessment of these phenomena. Methods. We applied the original method of corrosion fatigue study which takes into account the parameters of electrochemical dissolution of cyclically deformed surface in conditions of its deformation, as well as the known methods of fracture mechanics, corrosion science, electrochemistry and materials science. Findings. The stages of initial corrosion damaging and surface corrosion fatigue nucleation in structural steels were considered. The criterion ratio for evaluating the period of surface corrosion fatigue crack nucleation that relates the number of loading cycles before crack initiation, the corrosion current, the constants of electrochemical dissolution of metal surface to the cyclic loading parameters is derived. This criterion allowed to obtain and substantiate the experimental formula for predicting the number of loading cycles prior to the surface corrosion fatigue crack nucleation. Originality. The suggested new model describes the surface fatigue crack nucleation as a result of interaction between corrosion and operational cyclic loadings of the coiled tubing equipment flexible pipes. Practical implications. The proposed relationship for calculating macrocrack nucleation period is applicable for engineering assessment of serviceability and fracture risk of flexible pipes of coiled tubing equipment under assigned operating conditions.