In view of the vegetation reduction caused by the continuous construction of oil and gas pipelines, the pipelines have been designed to be laid in one ditch to reduce land occupation. However, owing to the small spacing between the pipelines, the fault correlation between pipelines has been proven to increase the potential hazard of adjacent pipelines and routing environments. The neglect of failure correlation in existing risk assessment methods leads to inaccurate results, which will lead to errors in maintenance decisions. Therefore, this paper proposed a risk assessment system for pipelines using this laying method. In the risk assessment, pipelines laid in one ditch (PLOD) were regarded as a series system relative to the routing environment. Therefore, the functional relationship between the total risk of the pipeline system and the risk of each pipeline was obtained by combining the engineering system reliability theory with the mathematical induction method. In addition, fuzzy set theory combined with fault tree analysis was used to calculate the failure probability of each pipeline in the system. Event tree analysis was used to sort out all the possible consequences of pipeline failure, and then the consequences were unified into monetary units to evaluate the severity of failure consequences. Finally, the two parts were merged into a bow-tie diagram to realize the risk management and control of the pipeline. Meanwhile, risk acceptance criteria were formulated to analyze risks and to guide pipeline maintenance. This system provides a complete risk assessment system for the pipeline system laid in one ditch, including the methods of risk identification, risk assessment, and risk analysis, which are of great significance to ensure the safety of the pipeline and the surrounding environment using this laying method.

In the transportation process of urban gas pipelines, there are various uncontrollable risks and uncertainties possibly leading to the failure of gas pipelines and thereby serious consequences, such as city gas shutdown, nearby casualties, and environmental pollution. To avoid these hazards, numerous studies have been performed in identifying and evaluating the occurrence of risks and uncertainties to pipelines. However, discussions on risk reduction and other maintenance work are scarce; therefore, a scientific method to guide decision making is non-existent, thereby resulting in excessive investment in maintenance and reduced maintenance cost of other infrastructures. Therefore, the as low as reasonably practicable (ALARP) principle combined with optimization theory is used to discuss pipeline maintenance decision-making methods in unacceptable regions and ALARP regions. This paper focuses on the analysis of pipeline risk reduction in the ALARP region and proposes three optimization decision models. The case study shows that maintenance decision making should consider the comprehensive impact of maintenance cost to reduce risk and loss cost caused by pipeline failure, and that the further cost–benefit analysis of measures should be performed. The proposed pipeline maintenance decision-making method is an economical method for pipeline operators to make risk decisions under the premise of pipeline safety, which can improve the effectiveness of the use of maintenance resources.