Evaluation of corrosion defects in oil pipelines based on the approaches of fracture mechanics
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EVALUATION OF CORROSION DEFECTS IN OIL PIPELINES BASED ON THE APPROACHES OF FRACTURE MECHANICS A. Žiliukas,1 J. Janutnien,2,3 H. Nykyforchyn,4 and M. Bereisa2
UDC 539.375:620.178
We study the development of corrosion defects in an industrial oil pipeline. Defects appearing in the pipeline after 5 and 10 yr of operation are measured by the ultrasonic method. It is shown that, after 10 yr of operation, the pipe wall becomes thinner by 22% (as compared with its initial thickness). To evaluate the defectiveness of state of the pipeline, we use the concepts of J -integral and crack-opening displacement . The experimental data on corrosion defects are used to find the values of J-integral. Keywords: corrosion, defects, fracture, J -integral, crack opening displacement.
In the course of operation of oil pipelines, despite their anticorrosion protection, the manifestation of the influence of corrosive media is inevitable, and the defects formed as a result decrease the reliability of the systems. Their negative influence becomes stronger with the period of operation of pipelines because the material loses its initial mechanical properties and becomes less plastic [1–3]. Much attention is given to the problems of failures of oil pipelines (see, e.g., [4–7]), including the investigations of the influence of corrosion on their strength and integrity [8–11]. The most dangerous corrosion defects are most often located along the axis of the pipe, and the event of fracture itself depends on the properties of the material [12], geometry of the pipe, and action of the media [13]. The cracks formed in pipelines are characterized by significant plastic strains at their tips [14–16] and, hence, it is reasonable to apply fracture criteria taking into account the possibility of plastic fracture of the materials, in particular, the J -integral and the crack opening displacement . The aim of the present work is to analyze and apply these criteria. Defects in Oil Pipelines Defects most often encountered in the pipes have the form of dents, lamination, and corrosion defects. Dents begin to appear in the pipes even in the process of mounting. Lamination is most often revealed in the course of ultrasonic testing. By using the contemporary diagnostic equipment, we performed the experimental investigation of the defectiveness of operating industrial oil pipelines. The available statistical data show that the defects appear in the pipes of API5L X60 steel as early as after five years of operation of the pipeline (Fig. 1a): The external defects constitute about 16% of the total number of defects, and the other defects are connected with lamination and dents. We did not discover any defects caused by internal corrosion. 1
Kaunas University of Technology, Kaunas, Lithuania. Klaipeda University, Klaipeda, Lithuania. 3 Corresponding author; e-mail: [email protected]. 4 Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. 2
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 46, No.
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