Impact radius of light oil pipeline leakage under different operating conditions

This study investigated the impact range of pipeline leaks for light oil pipelines under different operating conditions. Three different calculation methods were employed to study the influence of various factors, including obstacle height, wind speed, leak hole diameter, and slope, on the leak impact range of flowing fires, pool fires, and overpressure waves. The results indicate that the impact range decreases as the height of obstacles increases. The most significant decrease occurs when obstacles are 2 meters high, and further increasing the obstacle height leads to a less pronounced decrease. Wind speed has a complex influence on the impact range. For flowing fires, the impact range initially increases and then decreases with higher wind speeds. For pool fires, a higher wind speed leads to a larger impact range, while for overpressure waves, higher wind speeds result in a smaller impact range. Larger leak hole diameters lead to larger impact ranges for both fire radiation and overpressure waves. Increasing the environmental slope has a slight effect on the impact range. Sensitivity analysis indicates that wind speed has the most significant impact, followed by leak hole diameter, while the environmental slope has the least influence on the impact range.


Introduction
Oil and gas media are highly flammable and prone to explosions [1].To prevent accidents, pipeline operating and management companies typically conduct regular high-consequence area identification and risk assessment.High-consequence area identification involves analyzing densely populated areas, flammable and explosive sites, natural conservation areas, roads, and other features within the potential impact radius of the pipeline.Necessary preventive and control measures are implemented for pipeline segments within high-consequence areas to minimize the human and economic damages resulting from pipeline failures [2].The potential impact radius of a pipeline is a critical factor in the analysis of highconsequence areas.Extensive research has been carried out on the impact radius of conventional oil and gas pipelines [3].However, in the case of light oil media, after a leak, the substance initially leaks in liquid form and then disperses in a gaseous state [4].The impact radius is different from that of traditional oil and gas media.Therefore, researching the impact radius of light oil under different operating conditions is crucial for accurately identifying high-consequence area segments.

Causes of light oil pipeline failures
The leakage of buried light oil pipelines results from various factors.There is a significant risk of thirdparty damage to pipelines, and natural disasters frequently occur [5].High soil moisture and oxygen content can lead to pipeline corrosion and perforation, and accelerate pipeline fatigue damage.The wide distribution and complex terrain of pipelines require high standards for pipeline design and welding.Factors such as pipeline-related issues arising from design or welds during operation, as well as human errors, can become potential triggering factors for light oil pipeline leaks.The factors affecting the spread of leakage after it occurs include: (1) Soil environmental factors: when a pipeline leaks underground, soil porosity [6], type, particle size, and temperature affect the depth, speed, and trend of oil product dispersion.Soil temperature affects the viscosity of the oil product, while the pressure environment where the leaked oil product is situated influences the resistance it encounters.The presence of hills, valleys, and underground features like concealed ditches and obstacles can all affect the extent of oil product dispersion.
(2) Pipeline-specific factors: the size and shape of the leak determine the speed of oil product leakage [7].The location of the leak hole on the upper or lower part of the pipeline and the left or right side in the direction of transport directly impacts the shape and trend of the leaking medium's dispersion.The pipeline's conveying pressure determines the leakage speed, while the leakage time determines the extent of dispersion, providing a basis for predicting the leak path and trend.The composition of light oil affects the volatility and other intrinsic properties of light oil.
(3) External environmental factors: when a buried pipeline leaks and spills onto the surface, wind speed affects the shape of dispersion [8].In mountainous regions with significant temperature differences between morning and evening, this influences the temperature of spilled oil products on the surface.The slope of the terrain affects the direction of oil product dispersion, especially in areas with considerable elevation changes.The presence of trees and stones in mountainous areas can hinder and obstruct the spread of light oil, with the size and quantity of these obstacles directly affecting dispersion.
Therefore, this paper conducts a scenario analysis based on four aspects: obstacle height, wind speed, leak hole size, and mountainous slope size.

Failure modes and damage criteria for light oil pipeline
The consequences of buried oil pipeline failures can take various forms [9], including seepage, slope surface contamination, pool fires, flowing fires, and explosions.When spilled oil flows down slopes and encounters a source of ignition, it may lead to pool fires, especially when constrained by the terrain's spatial limitations.If not restricted by terrain, it can result in flowing fires.The dispersion of leaked oil can form vapor clouds, which, when reaching explosive limits, can trigger explosion incidents.Pool fires and flowing fires are assessed by using heat flux criteria to evaluate the damage, while the impact wave damage from vapor cloud explosions is assessed by using overpressure criteria.
(1) Thermal flux criterion According to the definition in the "Guidelines for Quantitative Risk Assessment of Petrochemical Facilities" published by China Petrochemical Press in 2007, the damage threshold range for thermal radiation flux criteria is determined as shown in Table 1.
Table 1.Damage levels and thresholds as defined by the thermal radiation flux guidelines.First-degree burns within 10 seconds; 1% fatality within 1 minute.

6.0
Glass may break after 30 minutes of exposure.
Pain occurs after more than 20 seconds, blistering is not necessarily present.
Minor injury zone

1.5
No damage to buildings, equipment, etc.
No discomfort was felt with prolonged exposure.

Safety zone
(2) Blast wave injury criteria Based on the maximum overpressure generated by the explosion, the radii of the fatality zone, severe injury zone, and minor injury zone can be determined.The calculation is based on three critical pressures: 100 kPa, 44 kPa, and 17 kPa, which can be referred to in Table 2 below.

Establishment of a calculation system for the influence range of oil pipelines
This paper will determine the influence range of light oil under various scenarios using the following process: (1) Preliminary calculation of the potential impact range for different pipelines based on GB32167.
(2) Calculation of the dispersion radius of light oil pipeline leaks, as well as the thermal radiation and shockwave intensity of fires and explosions under different circumstances.This will be compared to the damage models specified in the 'Guidelines for Quantitative Risk Assessment of Petrochemical Facilities' to determine the fire and explosion damage radius for light oil pipelines.
(3) Considering the characteristics of light oil pipeline leaks, adjustments to the damage radius will be made, taking into account changes in the behavior of leak dispersion under the cover of forests and buildings.
(4) Determination of different consequence damage radii to establish a calculation system for the consequence impact range of light oil pipelines.
(5) Leakage dispersion concentration and impact radius.Light oil pipelines transport various types of oil products.When the concentration of gaseous oil vapor in the air exceeds 2.5%, it is prone to flash ignition, and when the oil vapor concentration in the air falls within the range of 5% to 15%, it is highly susceptible to explosion accidents.The corresponding concentration contour lines are 6, 700 mg/m³, 34, 900 mg/m³, and 113, 800 mg/m³.
(6) Dispersion patterns of light oil pipeline leaks.According to the definition in "Mixed Gas for Oil and Gas CJ/T341-2010", when the concentration of the mixed gas reaches the explosive limit of 1.5% to 10%, ignition and explosion occur when exposed to an open flame, leading to the formation of stable combustion.

Mathematical model (1) Flowing fire consequence calculation model
Both domestic and international flowing fire calculations are generally based on the FERC (Fire, Explosion, and Release Consequence) model, with its motion equation as shown in Equation ( 1) and the mass equation as shown in Equation (2).
where r, pool radius, m; t, time, s; g, gravity, m/s 2 ; , the proportion of the oil film thickness above the liquid level, = ( W-L ) / W, and for ground-level flowing fires, =1; W and L, the density of water and liquids, kg /m 3 ; , dimension factor, the ratio of the leading-edge oil film thickness (hf) to the average oil film thickness (h); CF, frictional resistance, m/s 2 ; , mountain slope, °; Ap = r 2 /360, liquid pool area, m 2 ; , the angle of the sector area for dispersion; Vp = Aph, the volume of oil within the liquid pool, m 3 ; Qin, m, the mass leak rate of the oil, kg/s.
(2) Pool fire consequence calculation model Assuming that the leaked liquid spreads to form a liquid pool without ground penetration and liquid evaporation, the liquid pool area S can be calculated from the total quantity of the leaked liquid M and the minimum thickness of the pool Hmin as follows: where M represents the total mass of the leaked liquid; represents the liquid density; Hmin represents the minimum thickness of the liquid pool. (

3) Explosion consequence calculation model
The equation for calculating the explosion energy of a vapor cloud is as follows: * E E V = (4) where E, the generated explosion energy of VCE, J; Ev, the equivalent fuel heating value of hydrocarbon and air mixtures under typical chemical compositions, 3.5×10 6 J/m 3 ; V, the volume of the explosion vapor cloud, m 3 .

Parameter selection
Choosing a pipe diameter of 508 mm and an operating pressure of 6.5 MPa, the simulation parameters are as shown in Table 4.

Impact of different factors on flowing fires, pool fires, and vapor cloud explosions
(1) Impact of Obstacle Height Figures 1, 2, and 3 show the thermal radiation range and different overpressure wave distances for flowing fires, pool fires, and leaking pipes, respectively.It can be observed that the influence areas of flowing fires, pool fires, and thermal radiation are significantly reduced in the presence of obstacles.When the obstacle height exceeds 2 meters, the reduction in the impact area becomes less pronounced.Therefore, it can be inferred that setting up firewalls near the leaking pipe location can effectively mitigate the damage caused by thermal radiation.As the obstacle height increases, there is also a decreasing trend in overpressure waves [10].(2) Wind speed influence Figures 4 to 6 show the thermal radiation range and different overpressure wave distances for flowing fires and pooling fires after a pipeline leak under different environmental wind speeds.It can be observed that at lower wind speeds, the thermal radiation impact area of flowing fires increases.However, as the wind speed continues to increase, the thermal radiation impact area decreases.When the leakage takes the form of a pool fire, a higher wind speed results in a larger impact area.Moreover, with higher wind speeds, the overpressure waves become smaller [11].(3) Impact of leak hole diameter Figures 7 to 9 depict the thermal radiation range and different overpressure wave distances for flowing fires and pool fires after a pipeline leak with varying leak hole diameters.It can be observed that as the leak hole diameter increases, the range of influence for both flowing fires, pool fires, and overpressure waves significantly increases [12].(4) Impact of leak slope Figures 10 to 12 represent the thermal radiation range and different overpressure wave distances for flowing fires and pool fires after a pipeline leak under various slope conditions.It can be observed that as the slope increases, both the thermal radiation and overpressure wave impact areas become larger, although the extent of change is not significant [13].

Sensitivity analysis
Sensitivity analysis is performed on the longitudinal hazard distance by using a single-factor sensitivity analysis method.The specific equation is as follows: where i a represents the sensitivity coefficient of a specific variable; L represents the change in pollutant concentration dispersion distance caused by the change in a certain influencing factor; f represents the change in the influencing factor within a certain range; L represents the pollutant concentration dispersion distance; f represents the value of a specific influencing factor.
The sensitivity coefficients for each influencing factor are calculated according to Equation (5).The relationship between the sensitivity coefficient and the rate of change in influencing factors is plotted in Figure 13.From Figure 13, it is evident that the absolute value of the sensitivity coefficient for wind speed is the highest, approximately around 9.62.Based on the average absolute sensitivity coefficient, the order of sensitivity of the four influencing factors is as follows: wind speed > leak hole diameter > obstacle height > slope.

Conclusion
Through the study of accident consequence calculation models for light oil pipeline incidents and analysis of typical fire and explosion accidents using the PHAST software, the following conclusions have been drawn: The height of obstacles can reduce the thermal radiation from flowing fires, pool fires, and the spread of overpressure from vapor cloud explosions.Increasing wind speed leads to an increase in the damage radius of flowing fires and a reduction in the duration of the pool fire's intense phase, as well as a decrease in the range of the shockwave overpressure.
Larger leak hole diameters result in increased thermal radiation damage radius for flowing fires, increased radius of radiation intensity for pool fires, and an extended range for shockwave overpressure.Flowing fires, influenced by steep slopes, tend to spread across the slope and contact flammable materials, resulting in a larger fire with significant thermal radiation that can cause severe injuries to people nearby.Vapor cloud explosions cause the second-largest damage, and the steeper the slope is, the greater the damage radius and spread are.
Sensitivity analysis of various factors indicates that wind speed has the most significant impact on the extent of failure, followed by leak hole diameter, obstacle height, and slope.After a leak occurs, it is crucial to promptly gather information about the surrounding environment and implement appropriate emergency response measures based on the specific circumstances.
production and operating equipment Within 10 seconds, a 1% fatality rate; within 1 minute, 100% fatality.Death zone 35The minimum energy required for igniting wood under no flames and Severe burns of second degree or higher within 10 Severe injury long-term radiation, with the steel structure of the equipment starting to deform.required for igniting wood under the presence of an open flame for an extended period, and for melting plastic.

Figure 11 .Figure 12 .
Figure 11.Influence distance of fire thermal radiation in pools with different leakage slopes.

Figure 13 .
Figure 13.Relationship between sensitivity coefficient and rate of change of influencing factors.

Table 2 .
Injury effect of shock wave on people.

Table 3 .
Light oil pipeline leakage parameter standards.

Table 4 .
Preliminary data of light oil spill diffusion simulation parameters.