Risk Control in Buried Polyethylene Gas Pipeline Inspection

Abstract: This paper analyzes the risks involved in inspecting buried polyethylene (PE) gas pipelines and proposes inspection items based on identified failure modes and potential hazards. The inspection items encompass document review, assessment of pipeline location, orientation, and burial depth, macroscopic inspection, direct excavation, weld joint examination, and material performance testing. Furthermore, a semi-quantitative risk assessment method tailored to polyethylene pipelines is proposed, enabling a comprehensive evaluation of the safety status of buried PE gas pipelines.

1. Introduction

Polyethylene (PE) pipelines have seen widespread adoption in urban gas distribution in recent years, owing to their superior corrosion resistance, high toughness, and economic efficiency. Nonetheless, the unique properties of PE gas pipelines and their relatively recent introduction mean that inspection technologies are still underdeveloped, which constrains the effectiveness of buried PE pipeline inspections. An in-depth analysis of the risks and failure modes of PE gas pipelines is conducted in this study, with potential safety hazards in buried pipelines identified based on actual operating conditions. A systematic inspection framework is introduced to mitigate these risks, encompassing document review, evaluation of pipeline location, alignment and burial depth, macroscopic inspection, direct excavation, weld joint testing, and material performance assessment. In addition, a semi-quantitative risk assessment method is introduced to facilitate a comprehensive and integrated assessment of the safety of buried PE gas pipelines.

2. Analysis of Polyethylene Pipeline Failure Modes

The safe and reliable operation of polyethylene (PE) pipelines depends on a complex interplay of factors across their entire lifecycle, encompassing design, manufacturing, installation, and operational phases. During the design phase, which serves as the blueprint for the entire pipeline system, a scientifically and rationally developed plan directly determines the pipeline’s operating conditions and service life. High-quality design provides accurate guidance for construction and establishes the foundation for long-term stability, making it indispensable to the success of the entire project. Numerous risk factors must be carefully addressed during the design and construction stages. The process must strictly adhere to national and industry standards, with particular attention given to site-specific environmental conditions. Materials must be precisely selected according to operating pressure and temperature, with grade and wall thickness meeting all regulatory requirements. Failures in polyethylene pipelines predominantly arise during the operational phase. In contrast to steel pipelines, PE pipelines exhibit superior resistance to electrochemical corrosion. Nevertheless, PE pipelines are particularly vulnerable to external factors, including adjacent construction work, roadway conditions, and the complexity of the surrounding environment. The failure modes are diverse and include biological interference, damage from road construction, plant root intrusion, material aging and cracking, ground pressure hazards, natural disasters, geological subsidence, and accidental or deliberate third-party damage. These failure modes can result in structural damage or functional impairment, posing significant risks to the safety of gas transmission. Crucially, these risk factors are interrelated and mutually reinforcing, with negligence at any stage potentially initiating a cascade of events that may result in severe safety incidents. Consequently, effective risk management and failure prevention throughout the pipeline lifecycle require comprehensive research and the implementation of systematic, multidimensional measures.

3. Inspection Items

3.1 Document Review

Effective risk management during the design and installation of polyethylene (PE) gas pipelines requires a comprehensive review and analysis of all relevant documentation. Design drawings, technical documentation, and strength calculations convey the pipeline’s underlying design philosophy and technical parameters. Certification documents for PE pipe components are essential, as they directly determine the reliability of component quality. Furthermore, installation supervision and inspection certificates, together with installation and final acceptance documents, confirm adherence to relevant standards, verify correct construction procedures, and ensure overall project quality. Prior comprehensive inspection reports provide an important reference for current assessments, offering insight into the historical condition of the pipeline.

3.2 Pipeline Direction and Burial Depth Detection

Accurate determination of a PE gas pipeline’s spatial position, burial depth, and orientation during inspections relies on the full utilization of design drawings, as-built documentation, and the pipeline geographic information system (GIS). These records capture key information from planning and design through completion, providing the data foundation for inspections and serving as an essential reference for subsequent inspection planning and execution. Detection methods must be selected based on the specific conditions of the pipeline. If a tracer wire maintains proper electrical continuity, external signal methods may be utilized for accurate positioning. If the metal tracer tape is damaged, missing, or intact but without a reserved connection point, a combination of acoustic, radar, and electrostatic detection methods should be employed. Ground-penetrating radar (GPR) exploits differences in the propagation characteristics of high-frequency electromagnetic waves through various media. By transmitting and receiving these waves, GPR can scan and image subsurface structures, allowing accurate determination of pipeline position and burial depth. Acoustic positioning relies on the propagation and reflection of sound waves in or around the pipeline. Analysis of the resulting acoustic signals allows inspectors to precisely locate the pipeline. Electrostatic detection leverages differences in the electrostatic field between the pipeline and its surrounding soil, identifying changes in field distribution to accurately locate the pipeline. For pipelines equipped with electronic markers, marker-based positioning is the preferred method. This technique employs specialized reading devices that communicate wirelessly with the markers to rapidly and accurately determine pipeline location and burial depth.

3.3 Macroscopic Inspection

The operating conditions surrounding polyethylene gas pipelines play a crucial role in ensuring their safe and reliable performance. Comprehensive and precise leak detection during pipeline operation is critical to maintaining safe gas transmission. Gas leaks may cause severe incidents, such as fires and explosions, representing substantial threats to both life and property. Consequently, the deployment of specialized leak detection equipment is essential for comprehensive pipeline inspections to accurately pinpoint leak locations. Pipeline connections and adjacent confined spaces are critical areas for leak detection. Where required, surface drilling or excavation can be employed to validate test results. Through ground drilling, inspectors gain direct subsurface access and obtain data on soil gas concentrations around the pipeline. Excavation permits visual assessment of the pipeline and its components, facilitating prompt detection of potential safety risks. After completing leak inspections, the requirement for further evaluation of the pipeline’s position, burial depth, and alignment should be established according to actual surface conditions. Significant changes in the surrounding environment necessitate precise verification of the pipeline’s position, depth, and orientation to detect displacement or subsidence caused by external factors, enabling timely implementation of protective or repair measures. Ground markers provide key visual cues for pipeline location and are crucial for effective maintenance, construction, and safety oversight. Inspections must verify the accurate placement, physical condition, and integrity of marker posts, test posts, mileage markers, and warning signs. In addition to supplying crucial location information for maintenance personnel, these markers serve as alerts to surrounding construction operations, thereby mitigating the risk of accidental pipeline damage. The surface environmental assessment along the pipeline route evaluates compliance with clearance standards relative to nearby structures, examines external loading on the pipeline, identifies exposed sections, and assesses disturbances in the surrounding soil. Inadequate clearance can expose the pipeline to external compression or impacts from nearby construction activities. The resulting stress accelerates material aging and compromises pipeline integrity. Exposed sections of the pipeline are vulnerable to environmental stressors such as ultraviolet radiation and temperature variations, whereas disturbances in the surrounding soil can compromise pipeline stability and operational performance. Inspection of pipeline components—including valves, flanges, and steel–plastic adapters—is equally essential. As critical system elements, their reliability directly impacts safe pipeline operation. Valves regulate gas flow, and their failure may result in flow disruption or leaks. Flange connections with inadequate sealing are susceptible to leakage. Steel–plastic transition joints, owing to differences in material properties, may experience electrochemical corrosion under varying environmental conditions, compromising their strength and sealing performance. Inspection of pipeline crossings is a critical control measure for ensuring operational safety. This includes assessing the integrity of protective structures at crossings to verify their capacity to withstand external impacts. Special attention should also be paid to changes in river channels and the condition of hydraulic protection measures. River erosion, siltation, and damage to protective structures may compromise the integrity of pipeline crossings. For example, changes in river flow can expose pipelines to currents, leading to erosion, while compromised protective structures may fail to adequately support or shield the pipeline. Valve inspections are another critical component of pipeline maintenance. Regular monitoring of valve wells for water accumulation and ensuring proper drainage is essential, as stagnant water accelerates corrosion and reduces the service life of components. It is also essential to assess the condition and integrity of protective covers and drainage devices during inspections. Missing or damaged covers can allow debris to enter the valve well, impeding valve operation, while malfunctioning drainage systems may fail to remove accumulated water, thereby elevating safety risks. In conclusion, ensuring the safe operation of polyethylene gas pipelines necessitates thorough, systematic, and comprehensive inspections. By addressing all aspects of the operating environment and component reliability, pipeline integrity can be maintained under complex service conditions, effectively preventing safety incidents.

3.4 Direct Excavation Inspection

Excavation is required during polyethylene pipeline inspections in locations exhibiting specific risk conditions to maintain pipeline safety. When ambient temperatures exceed 30 °C during pipeline installation, elevated heat may accelerate aging and degrade the performance of polyethylene materials, necessitating special attention. For pipelines older than 30 years, prolonged service can lead to material fatigue and aging, substantially increasing safety risks. Locations with a history of leaks or third-party damage can compromise the structural integrity of the pipeline, posing potential hazards. Adverse geological conditions along the route, including subsidence and landslides, can compromise the pipeline due to soil displacement and compression. Crossing sections of the pipeline are especially vulnerable to external loads owing to the intricate construction environment. Steel–plastic transition joints, due to differences in material properties, may be susceptible to electrochemical corrosion and other environment-dependent issues. Pipeline sections with deep-rooted plants present additional hazards, as root intrusion may compromise the pipeline and induce leaks. Excavation inspections should be conducted in all identified high-risk areas. Strict spot checks should be conducted at a minimum frequency of 0.3 locations per kilometer. Inspectors should exercise discretion to determine whether additional checks are necessary based on on-site conditions to comprehensively assess pipeline safety. After excavation, the pipeline surface must be meticulously inspected for mechanical defects—such as scratches, grooves, gouges, or dents—that could diminish its pressure-bearing capability. The pipeline should also be inspected for signs of aging and degradation, such as surface chalking, which indicates a deterioration of material properties.

3.5 Weld Joint Inspection

Welding of polyethylene gas pipelines is typically carried out using either butt fusion (hot-melt welding) or electrofusion (electric-melt welding). Electrofusion is typically applied to join pipe fittings to pipes, injection-molded fittings, or fittings of different specifications, whereas butt fusion is predominantly used for pipe-to-pipe connections. Weld defects in both fusion methods are frequently attributed to contaminants, including dust and debris, within the weld zone. Weld integrity can be evaluated using radiographic or ultrasonic testing, with phased array ultrasonic testing employed for additional verification when necessary.

3.6 Material Performance Testing

Assessing the material performance of polyethylene gas pipelines can be difficult and is typically necessary for pipelines suspected of aging, with uncertain material properties, or that have surpassed a designated service life. Such testing should be conducted in accordance with the relevant manufacturing standards for polyethylene pipes. Based on the test results, the remaining service life of the pipeline can be assessed, and the interval for the subsequent inspection should generally not exceed half of this evaluated remaining service life.

4. Risk Assessment

Risk assessment of buried polyethylene pipelines is essential for ensuring safe operation. The assessment should be conducted on a section-by-section basis, considering inspection results, identified issues, and the cultural, social, and natural environments along the pipeline route. The risk assessment primarily considers two aspects: the likelihood of failure and its potential consequences. For polyethylene gas pipelines, a semi-quantitative approach is typically used to assess risk. This method incorporates operational risk factors, accounts for the likelihood of accidents, and evaluates individual pipeline sections. The results of the assessment are expressed as a quantitative risk value. The failure probability score (S) for buried polyethylene gas pipelines is determined according to the scoring criteria in Table 1:

S₁: Score for third-party damage

S₂: Score for equipment and personnel operation

S₃: Score for pipeline intrinsic safety quality

For a buried polyethylene gas pipeline, the failure probability score (S) is determined according to Equation (1):

The failure consequence score (C) is determined based on the scoring items listed in Table 2:

C₁: Medium short-term hazard score

C₂: Maximum leakage score

C₃: Diffusion score

C₄: Population density score

C₅: Environmental impact score

C₆: Cause of leakage score

C₇: Impact of supply interruption on downstream users

The failure consequence score (C) is calculated using Equation (2).

The risk value (R) is calculated using Equation (3):

R=S×C

Table 1 Failure Probability Score

Table 2 Failure Consequence Score

A higher risk value indicates a greater level of pipeline risk. For sections with excessively high risk, appropriate safety measures should be implemented.

5. Conclusion

This article examines common failure modes of buried polyethylene pipelines, identifies associated risk points, and, in conjunction with pipeline operating conditions, pinpoints potential safety hazards. Through targeted inspections and risk assessments, the overall risk profile and safety status of buried polyethylene gas pipelines can be evaluated. Specific management measures are subsequently implemented for pipeline sections with elevated risk levels, ensuring safe and reliable operation for gas companies.