Research Of Corrosion And Its Prediction For Oil-Gas Steel Pipelines

Seaport oil-gas pipelines have become indispensable equipments for the exploitation of oil and gas with the fast development of ocean oil-gas industry. The service environment of oil and gas steel pipelines are complex. It is common reported that oil and gas leakage accidents were happened because of exposing to oil and gas medium or soil environment, which makes the hazard of tube corrosion more greater due to their easy flammable, explosive and toxic feature. Therefore, basing on the research of corrosion behavior and corrosion state prediction for oil and gas steel pipelines, then making out appropriate protective measures, which has become effective means for oil and gas pipeline’s safety operation.According to different service environment, the corrosion of oil and gas steel pipelines are divided into two groups:pipe inner wall corrosion and pipe exine environment corrosion. Basing on the analysis of corrosion environment for oil and gas steel pipelines, taking Q235B steel and L245 steel for research objects, the internal and exine soil corrosion behavior of steel pipelines are studied by means of electrochemical methods. Moreover, the corrosion prediction models are researched basing on the Markov stochastic process. First, Markov chain prediction models are built, which are used to predicting aging conditions for external anticorrosion and pipe wall corrosion damage. Forthermore, Markov decision is introduced in view of maintenance measures for oil and gas steel pipelines, then optimal maintenance strategy is determined. In this paper, centering on the research of corrosion, prediction and its strategic decision for oil and gas steel pipelines. The following are the main contribution of this thesis:(1) Experimental research is conducted on corrosion behaviors for steel pipelines(Q235B steel and L245 steel) inner wall in the oil-gas simulated internal environment of dalian port. The sulphur content, environment temperature and pH values of corrosion medium are considered in view of existing the present situation with sulfide corrosion in oil and gas steel pipelines inner wall. The results find that Q235B steel and L245 steel show similar corrosion behavior. In the corrosion medium, Corrosion rates of them first increase then decrease with the increase of sulphur content. The corrosion rate have the maximum value when the sulphur content is 20mg/L. Q235B steel and L245 steel occur pitting corrosion phenomenon when the sulphur content is low, while general corrosion happened in the high sulphur content, and the main corrosion product is ferriferous sulfide. The change of time constant of impedance spectroscopy in solutions with different sulphur content that is related to aseptic degeneration of impedance spectroscopy. High sulphur extent has a corrosion inhibiting effect to the steel pipelines in a certain extent. The corrosion resisting property of L245 steel is more stronger than Q235B steel in the same corrosion condition. The results of solutions with different temperatures and pH values show that corrosion processes in the corrosion medium are controlled by the cathode process.(2) According to sampling and analysis for dalian port soil near the oil-gas pipe, experimental research is conducted on corrosion behaviors for steel pipelines(Q235B steel and L245 steel) in simulated water saturated soil environment of dalian port. The results find that the thermodynamic stability of Q235B steel is higher than L245 steel, while the corrosion resisting property of L245 steel is much stronger than Q235B steel in this condition, which is related to diversity of their nature of materials. At early stage, both of their electrochemical impedance spectroscopy is shown that high-frequency region appears small capacitive reactance, medium-frequency region appears large capacitive reactance and low-frequency region appears little inductive reactance. At later stage, the low-frequency little inductive reactance has become degradation into disappearance gradually. Moreover, the corrosion velocity of Q235B steel increases first then decreases progressively, while the corrosion velocity of L245 steel decreases progressively then decreases rapidly during the measurement period.(3) The prediction method for extrrnal coating deterioration state of oil-gas pipelines is established based on Markov process stochastic theory. The prediction model of extrrnal coating deterioration state for a single pipe section and the entire pipeline are established respectively according to classification criteria quality status for extrrnal coating. Their initial state distribution are defined based on inspection data of extrrnal coating, then transition probability matrixes are obtained, and finally the trends of extrrnal coating deterioration state for a single pipe section and the entire pipeline are predicted respectively.(4) The prediction method for tube wall corrosion state of oil-gas steel pipelines is established based on Markov process stochastic theory. The prediction model of tube wall corrosion state for a single pipe section and the entire pipeline are established respectively according to classification criteria quality status for pipe body. The tube wall transition probability matrixes of a single pipe section and the entire pipeline are obtained by curve fitting and gray Markov chain model respectively. Furthermore, the trends of tube wall deterioration state for a single pipe section and the entire pipeline are predicted seperately.(5) The optimum maintenance strategy of oil-gas pipelines is studied basing on Markov decision process. Maintenance measures and their corresponding cost are analyzed after dividing the corrosion state during oil-gas pipelines’maintenance, and Markov decision process is solved by appling strategy improvement algorithm. The division of corrosion state, maintenance measures and their corresponding cost, the obtaining of transition probability matrixes and the solving of Markov decision process are discussed, which combines with a oil-gas pipeline under service during the application of Markov decision process. Finally, the optimum maintenance strategy for the pipeline is worked out.