Propose a New Model for Improve Accuracy of Stress Corrosion Cracking Direct Assessment

摘要

Pipeline integrity management system (PIMS) as a subset of asset integrity management have an influential role to improve pipeline safety during its operational life; For this purpose, various methods can be used such as In Line Inspection (ILI), Hydrostatic Test (HT) and Direct Assessment (DA) Which DA contains several techniques including External Corrosion Direct Assessment (ECDA), Internal Corrosion Direct Assessment (ICDA)and Stress corrosion cracking direct assessment (SCCDA).The objective of SCCDA is improve pipeline safety by assessing and reducing the impact of SCC in high-pressure transmission pipelines by using of NACE SP0204 as a reference standard. The first and the most important purpose of this standard is to priority potentially susceptible segments of pipelines and selects specific sites for excavation by systematically gathering and analyzing data for pipe having similar operational characteristics. In this paper a 16-inch pipeline containing sour gas downstream of the compressor station that was suspected to SCC was evaluated. But due to the lack of sufficient data regarding the history of pipeline, coating flaws that were detected by DCVG, as mentioned in standard, identified as susceptible areas to SCC; Afterward all the coating defects that were prioritized depending on the severity of defects were excavated. Direct inspection results indicated that just a few percent of these points was faulty and excavation over all coating flaws, is time-consuming and costly; then for improve the performance of this method the achieved results were reviewed again. By comparing the results and their relative compliance with areas where SCC defects were found, it was revealed that the probability of SCC could be a function of some important features of pipeline such as soil characteristics, Type of Coating, depth of cover and stress level in pipeline; accordingly a proposed model for detection SCC in gas pipeline was developed. This model is able to increase accuracy of SCCDA and thereby reduce time and cost of DA.