Risk assessment of girth welds of high-grade steel pipelines based on fuzzy theory and index method

Abstract

Among the failure modes of high-grade steel pipelines, cracking or leakage failures of girth welds account for nearly 1/3 in total. Thus, different assessment methods are adopted for various types of indicators, and a risk assessment method based on comprehensive fuzzy theory and index method was proposed. Specifically, the non-quantifiable assessment indicators were obtained by the fault tree method and the maximum membership degree was calculated by combining the fuzzy assessment and the entropy weight method. On this basis, the quantifiable assessment indicators were determined according to the requirements of relevant standards, and their risk values were calculated by the index method. Finally, a risk assessment matrix was established according to the maximum membership degree and risk values obtained with the index method, so as to determine the risk level of girth welds. Herein, risk assessment was performed for the girth welds in a section of Myanmar-China Natural Gas Pipeline with the proposed method. According to the results, the failure risk of girth welds in the pipeline section is mainly determined by the non-quantifiable indicators and the pipeline section is at high risk. Definitely, poor implementation of pipeline maintenance plan, unacceptable qualification of welders, unclear welding procedure specification and other objective factors are the main causes of girth weld failure, and the assessment results are consistent with the field accident reports. Generally, the proposed method supplements and improves the risk assessment system for girth welds of high-grade steel pipelines, and could provide theoretical reference for solving the problem of girth weld failure in high-grade steel pipelines.

Keywords

• high-grade steel pipelines
• girth welds
• fault tree
• fuzzy evaluation method
• index method
• risk assessment