Vulnerability analysis method based on risk assessment for gas transmission capabilities of natural gas pipeline networks

Highlights

• Improved vulnerability analysis method based on risk assessment.

• The severity of risk consequences is calculated by utility theory.

• Identifying critical components of the pipeline network by combining risk assessment and vulnerability analysis.

Abstract

The crucial part of vulnerability analysis is identifying the critical components of a pipeline network. In this study, we proposed a novel analysis method- “Risk-Vulnerability,” which combines the characteristics of risk assessments and vulnerability analyses methods. Risk-Vulnerability identifies the critical components of a pipeline network from three perspectives: pipeline operating status, transmission performance, and network characteristics. The formulas of the importance value of each component were established. Then the component risk indicators were established, and the component risk values were calculated. And a utility theory was introduced to calculate the severity of the consequences. Finally, the component importance and risk values were multiplied to obtain the vulnerability of the component. The feasibility and effectiveness of the method were verified by comparing the identification results of this method to those from the weighted flow capacity rate (WFCR). The Risk-Vulnerability method provides an improved insight into the pipeline criticality compared to the WFCR and improves the calculation formulae based on the identification content, which can not only be used to identify critical components, but also can be used to formulate research on risk reduction measures for pipeline network systems.

Introduction

Recently, the natural gas pipeline network in China has developed rapidly. Trunk pipeline constructions have been accelerated, and a series of pipeline network system reform measures have been issued to promote the construction of natural gas pipeline network interconnections. The topological structure of the pipeline network has become increasingly complex, which has created safety hazards in the network. Consequently, it is very important to accurately assess the safety of the pipeline network. At present, pipeline network security assessments mainly include three methods: risk assessments, reliability analyses, and vulnerability analyses [1,2].

Risk refers to the potential impact of uncertain factors on the system based on a specific environment and time [3,4]. Risk assessments are associated with the risks that could be encountered by the system, the likeliness of the encounter, and their outcome(s). Risk assessment methods can be divided into qualitative, semi-quantitative, and quantitative risk assessments [5]. A quantitative risk analysis is an effective tool for formulating safety strategies to prevent accidents [6] and is usually conducted during routine reliability and risk assessments [7]. A reliability analysis probabilistically describes the ability of a gas pipeline network to deliver a predetermined transmission capacity. A reliability analysis can be regarded as part of a quantitative risk assessment [8]. It provides probabilistic inputs for risk assessment to estimate the probabilities of various failure scenarios [9,10].

The research on the reliability of a pipeline network mainly focuses on the following two aspects. First, the structural reliability of the pipeline network: It focuses on the physical structure of the pipeline network system [11], analyzes the reliability of the complete structures of equipment units such as pipelines, compressors, and valve chambers within a specified time, and uses the reliability of these subsystems to attain the overall reliability of the pipeline network system. Second, the gas supply reliability based on a gas transmission capacity: The reliability of the gas supply focuses on the user market, and on the probability that the pipeline network will eventually meet the gas supply tasks demanded by the users and market during a specified delivery task period [12], [13], [14].

In the “Society for Risk Analysis” (SRA) glossary, the vulnerability of a system is defined as the risk and degree to which a system can be affected by a risk source or agent [15]. The vulnerability comprises the negative impacts and loss of function when the network suffers sensitive damage. Many scholars have extensively studied the vulnerabilities for improving the safety management of pipeline networks [16], [17], [18], [19]. A vulnerability analysis is often divided into two important parts: a global vulnerability analysis of the system and a critical component analysis [20,21]. The global vulnerability analysis considers how adversely an accident affects the system, such as whether the changes affect the system performance. The analysis of these changes can be static or dynamic [22].

The aforementioned methods assess the pipeline network security from different perspectives, with different focus areas. Risk assessments analyze the safety of pipeline networks by calculating failure probabilities, whereas vulnerability analyses focus on inherent attributes, rather than the environment and various probabilities. The most popular method for a vulnerability analysis is based on using deterministic analysis technology [23] to identify the impacts of accidents or failures on the system, in contrast to using probabilistic methods. Fig. 1 is the risk curve that shows the relationships among risk, reliability, and vulnerability. The accident scenarios are placed on the horizontal axis according to the severity of the accident consequence x. The vertical axis represents the cumulative probability of scenarios with consequences greater than or equal to x within a certain period of time [24]. Reliability studies the possibility of the system performing the required functions in the specified time and under the specified conditions. It is related to the upper left section of the risk curve in Fig. 1. Vulnerability is about events that are infrequent and have considerable adverse consequences. It is thus related to the lower right section of the risk curve [24]. It can be seen from Fig. 1 that vulnerability can be used to analyze events that occur infrequently but have serious adverse consequences; that is, the vulnerability focuses on low-probability and high-consequence events, which makes up for the negligence of such events through reliability and risk assessments. The specific vulnerability function (Vulnerability curves) can be commonly implemented in general for the assessment of natural events triggering technological disasters [25], [26], [27]. Owing to the analysis characteristics of risk assessments and vulnerability analyses, the safety analysis results for the same pipeline network can be different. Each analysis method has its own advantages and disadvantages. A risk assessment is based on data statistics; the accuracy of the probability is crucial for ensuring accurate analysis results. The vulnerability analysis avoids probability calculation methods and focuses on the degree and severity of system accident consequences. Thus, it would be useful to identify which result is more effective, and/or whether the probability analyses used in the risk assessment and reliability analysis can be combined with the vulnerability analysis to reduce the impacts of the shortcomings in the respective assessments. Therefore, in this study, a method based on introducing a risk assessment probability analysis into the vulnerability analysis of a pipeline network was proposed. The key advantage of this method is that it considers both the probabilities of the occurrence frequencies of hazardous events and the impacts of components that have low failure probabilities with serious consequences on the performance of the pipeline network. Therefore, it can analyze the impacts of component failures on the performance of the pipeline network more accurately, and identify critical components more effectively.