Research On Internal-Pipeline Corrosion Monitoring Technique

Pipeline transportation occupies a very important position in modern industry, and high attention has been paid to the safety reliability of the pipeline operation. The detection technique and the monitoring technique of external corrosion are maturing, but there is no completed real-time monitoring system for the internal corrosion of pipeline. To ensure the security of pipeline operation, Dalian University of technology and CNOOC applied for the project of monitoring technology of the internal corrosion in submarine pipeline. In this paper, the relevant researches were carried out, and the internal-pipeline real-time monitoring system was developed; moreover a new safety evaluation method of internal-pipeline corrosion was proposed. The major contents and conclusions are surmmarised as follows:(1) An indirect monitoring method was put forward. It is indicated that the corrosion rate is closely related to the corrosion capability of the fluid in the pipeline. Corrosion capability is decided by the content level of the corrosive conponents. There is a corrosion equivalent relationship between the consumption of corrosion capability and the consumption of metal. The internal-pipeline corrosion monitoring system was developed to evaluate the consumption of metal based on monitoring the changes of the corrosion capability of the fluid.(2) Real-time monitoring system, centering on the sensors and corrosion monitoring instrument, is developed by VB and SQL Server 2000. The connection based on TCP/IP between client and the web database server, which has more flexibility and is available for long-distance safeguarding.(3) Galvanic sensor and electric resistance sensor were respectively developed to the electrolyte environment and the non-electrolyte environment, and fundamental researches on the sensors were carried out in the laboratory. Artificial seawater was applied to the test as the corrosion medium, and consequently concentration of dissolved oxygen was the scale of corrosion capability. After long-term testing in a 50-metre-long pipeline and analysis, the effective working period of the sensors were found, and the influence caused by the oxidative film on the electrode working face was eliminated by an amendatory coefficient; a complex and nonlinear relationship, among the monitoring signals and the temperature, velocity and the concentration of dissolved oxygen of the fluid in pipeline, is acquired. Finally, BP artificial neural network is applied to estimate the concentration of dissolved oxygen of the fluid through other factors. In this way, the consumption of the dissolved oxygen can be estimated by the galvanic sensors setting at the entrance and the exit of the pipeline. Furthermore, the corrosion condition of the internal pipeline, which was evaluated by the average corrosion thickness, can be evaluated in the monitoring system.(4) A new safety assessment method by the max pitting depth was put forword. Based on the topography characteristics and optimal distribution of pitting corrosion, a statistical model was established by assuming the diameter and the depth of pitting subordinated to log-normal distribution. Thereby, the total volume consumed by corrosion can be calculated by the model. Quadratic polynomial curve-fit was used to build the non-linear relationship among the total corrosion volume, pitting number, diameter-depth ratio and shape factor and the max pitting depth. Consequently, the max pitting depth can be calculated by the average corrosion thickness, and the internal-pipeline corrosion safety assessment can be given according to SY/T6151.