Investigating the Factors That Reduce the Urban Gas Pipeline Vulnerability to Explosion Threats
 
Mohammad reza Fallah Ghanbari*¹ , Mohammad Eskandari¹, Ali Alidoosti^1
 
1 Department of Emergency Management, Polytechnic University of Malek Ashtar, Tehran, Iran
 

ARTICLE INFO	ABSTRACT
ORIGINAL ARTICLE	Introduction: Buried pipelines used to distribute water, gas, oil, and etc. are considered as one of the vital arteries. The experiences of the past wars have confirmed that the invading country focuses on bombing and destroying vital centers, and that gas pipelines can be a source of serious personal and financial losses as an important transmission arteries during war in the event of damage Methods: The vulnerability of buried urban gas pipelines to explosion was determined and the methods for reducing the vulnerability of pipelines were investigated. To this end, the three-dimensional model of the soil-pipe system in ABAQUS software was used to study the effect of factors affecting the pipe behavior, including pipe diameters, diameter to pipe thickness, internal friction angle of soil, soil type, amount of explosives, depth of buried, the distance of explosion site to the pipe burial site, has been investigated on the pipe deformation capacity according to the ALA regulation. The soil was modeled using Solid three elements and shell element. For parametric studies, analyses were performed by the finite element method using ABAQUS software 6.10.1. Results: Studies were conducted for 4 and 12 inch diameter, diameter/thickness ratio of 26, 21 and 35, burial depth of 1, 2, 3 and 4 meters, the explosive charge of 15, 30, 45, 60 and 200 kg TNT and for soil material, hard, soft and clay sands. The results showed that proper burial depth had the most effect in reducing the vulnerability of pipelines against explosive threats. By increasing the pipe thickness and increasing the diameter and applying soft soil around the pipe, a better behavior of the pipe was observed during the explosion Conclusion: To reduce the vulnerability of gas pipelines against explosive threats, the use of buried pipelines has a greater effect on reducing damage due to explosion compared to other parameters, and it is recommended to use this method to increase the resilience of highly important gas pipelines. Keywords: Buried Pipelines, Non-active Defense, Soil-pipe System, Pipe Strengthening
Article history: Received: 16 Sep. 2018 Revised: 20Dec. 2018 Accepted: 10 Jan. 2019
*Corresponding author: Mohammad reza Fallah Ghanbari Address: Department of Emergency Management, Polytechnic University of Malek Ashtar, Tehran, Iran Email: Mrfg1806@yahoo.com Tel: +98-9196380621
Abbreviations: ALA: American Lifelines Alliance API-5L: American-Petroleum Institute

 
Introduction

G

iven the threats that Iran faces at various times with varying intensity and severity, the need for the construction of resistant structures and facilities is considered a national necessity. Gas transmission lines are one of the most important vital arteries for energy in today's societies. The experience of security threats shows that the enemy is using all its power to destroy refineries and energy sources. Therefore, maintaining the function of this facility against the explosion is a major factor because gas pipes contain flammable materials, and gas leaks and subsequent fires can cause injuries and financial damage (1). In addition, the destruction of gas pipelines due to its arterial effects can put adverse effects on other vital arteries, such as power generation, and cause problems and delay in their continued operation.
Aghasi et al. investigated the effect of soil characteristics on buried steel facilities using the ABAQUS software (2).  In this study, a three-phase soil model was considered as a Drauger- Pruger, and pipes of high diameter m1 with depth of burial of more than m3 were investigated. Their studies showed that by increasing the modulus of elasticity of the soil around the pipe, less displacement occurred in the pipe and the stresses created in the pipe are reduced. Olarewaju et al. (3) conducted studies to predict the effect of explosions on pipelines using the ABAQUS software. Akbardoost et al. (4) examined the impact of the explosion wave on steel pipes on the surface of the earth using the Autodyne software as a 2-D model of the pipe and examined the extent of the pipe deformation due to the explosions at different distances. Their studies showed that as the distance decreased, a large deformation in the pipe was created, so it is imperative that the distance to explosion site be observed.
Asakereh et al., in their study of the effect of the burial depth of the buried pipes in the sandy soil under explosive loading using the finite element method, investigated the effect of different parameters on the vulnerability of the pipelines. In this regard, the soil and slag repair, or the use of mobilizers and the pipes of suitable diameter, thickness, and materials and their placement in optimal depth can be suitable for reducing damages caused by explosions or other seismic waves. This study was conducted using numerical modeling of buried pipes in explosive loading soil using Explicit/ABAQUS software. By investigating the role of pipe depth in the effect of explosion on buried pipes and performing comparative work, the depth of installation could be up to 3 m (5).
Parviz et al., in a study on the numerical modeling of the effect of explosion on buried pipelines of oil and gas transmission in different soils by Eulerian-Lagrangian method, to investigate stresses and explosions under buried pipes in soil using finite element software Ls Dyna has been dealt with. In this study, five models of fluid, air, soil, pipe and TNT were used. In the following, a comparison was made between the stresses and the pressures obtained from the fluids, and the results showed that with decreasing the fluid density, the pressure on the pipe increased and higher stress and pressure were transferred to the pipe, and the volume of the fluid increases, lower stress and pressure are introduced into the pipe. By increasing the density of soil used in modeling in the explosion, more stress and more pressure are transferred to the pipe. By decreasing the density of the soil, soil behavior acts like a damper, and less stress and pressure are introduced into the pipe, resulting in less damage to the pipe.
By gaining knowledge about the soil type's function in the transfer of stress and pressure in buried pipes under explosion, it can be found that in high-density soils, due to the high stress and pressure transmission, high-strength pipes can be used, and in lower density soils due to better and more suitable soil (such as damper) and lower stress and pressure transfer, low-strength pipes can be used,which could affect the price of oil and gas transmission projects, and economically significant savings in the implementation of oil and gas pipeline projects (6).
Janalizadeh et al. in the study of the function of buried pipes under the influence of explosive load, conducted software modeling using the ABAQUS software, and studied the effect of burial depth, geometric characteristics of the pipe, pipe materials, modulus of elasticity and soil density. Comparison of variations in tension and displacement on buried pipelines indicated that with increasing depth of burial, tensions and displacements were reduced. The results showed that with increasing the depth of pipe placement in the soil, the number of displacements would decrease. Also, by increasing the amount of explosives, the amount of energy absorbed by the module increases, and if the modulus of elasticity of the steel is increased, the amount of absorbed energy will be lower (7).
In agreement with previous studies, the purpose of this study was to investigate the effects of explosions on urban gas pipelines in order to find out the link between various parameters affecting the vulnerability of gas pipelines and the introduction of factors that have the greatest impact on increasing the resistance of these lines to explosive threats.
To this end, various factors such as pipe diameter, pipe depth, burial depth, soil material, explosive mass, distance from the site of explosion, etc. are required, and the effect of each of them in reducing the vulnerability of the gas pipelines should be specified.
Materials and methods
The primary factor on which the remaining parameters should be modeled based is the issue of explosive threats and various scenarios associated with it (8).
For this reason, two scenarios are assumed in this study: a) the terrorist groups and the domestic and foreign enemies throw explosives using mortars or rocket launchers toward around the pipelines burial. In this scenario, the vulnerability and the resistance level of the pipes are examined b) in the second scenario; it is assumed that terrorists blow up a car carrying explosives on the street and near the pipe burial site.
In this scenario, vulnerability and resistance of pipelines are investigated. Figure­1 shows schematics of these scenarios.

Figure 1: Direct hit (right) and indirect explosion (left) and their effects on gas pipelines
 

Rockets and mortars weigh about 5 to 70­kg (9, 10). In our modeling weights 15, 30, 45 and 60 kg were considered. The weight of explosives in a car bomb was considered to be 200kg.
The ABAQUS software was used for modeling the effect of explosion on buried pipes in soil. This software is one of the most widely used tools for performing finite element analysis. This software provides a wide range of capabilities for simulation in linear and nonlinear applications. Problems with multiple components and different materials can be simulated by defining the geometry of each component and assigning its constituent material and then defining the interaction between components.
The analysis of the results can also be done after the completion of the processing stage, when tensions, displacements and other basic variables have been calculated (1).
The variables studied in this study included the diameter of the pipe, the diameter to pipe thickness, the internal friction angle, the soil type, the amount of explosives spent, the depth of the pipe burial, the explosion distance to the burial ground, all of which are independent variables in this study.
The deformation rate of the pipe is considered dependent variable. Among these variables, the pipe diameter and the diameter-to-thickness ratio are interrelated, and the internal friction angle of the soil and soil type are also related to each other.
In this paper, the explosion loading was defined in Dynamic Explicit Analysis and defined by Amp load definition.  
Soil elements were selected as C3D8R. Soil in this study was modeled using Drauger-Pruger criterion and considering hardening and modeling was performed for four soil types with different soil mechanical properties. The pipe was modeled using S4R elements (5).
The mechanical properties of the pipe were considered to be a those of API-5L steel pipes. The Fon Mises model has been used for modeling the steel pipe elastic behavior with regard to isotropic hardness. For composite modeling, the S4R elements of the ABAQUS software were used.
The contact between the pipe and the soil is also called hard contact with a friction coefficient of 0.3. The soil dimensions were determined by sensitivity analysis to be 100 × 50 × 25 m.
The validity of the analytical method adopted has already been investigated in previous studies in this field, which is referred to by ABAQUS software to model the impact of the explosion on the soil and pipelines (11).
In addition, in future studies, using empirical relationships and manual calculations, the validity of the results can be tested.
Theory and calculations
One of the important points in explosive loading in the soil is the calculation of the maximum value of pressure at a certain distance from the explosion center, which is known as the explosive base pressure, represented by Ps.
Usually, in the authoritative references, the equation for Z is expressed in terms of the value of the equation 1 in order to calculate the maximum base pressure (8, 13).
Equation [1]:          
X (m): Maximum displacement of soil particles
W (kg): TNT equivalent mass
R (m): Distance to the target point
C (m/s): Seismic speed
n: Wavelength coefficient is selected which is  proportional to the soil type and is a criterion for the energy absorption of waves energy in the soil. The value of this coefficient is obtained through an unidirectional test on an unconfined sample.
f_c: The coupling coefficient obtained by determining the depth of the explosion center from the surface of the earth is shown in Figure2.
By increasing the explosion of mate and land, the impact of the weapon increases. In the case of buried explosive charges and the soil around, a smaller number than one is considered (13). Table 1
Shows Decreasing coefficient and k factor for all types of soils

Figure 2: The coefficient of f_c (force) based on the scale (Z)
Table 1: Decreasing coefficient and k for all types of soils (14)

Decreasing coefficient (n)	k coefficient	Soil type
1.5	30000	Saturated clay
2.5	20000	Saturated clay and layer
2.5	10000	Highly dense, dry and moist sand
2.75	5000	Dense, dry and moist sand
3	2000	Loose, dry and moist sand
3.25	1000	Very loose, dry and moist sand

 

The amount of explosion pressure is calculated by Equation (2).
Equation [2]:
The duration of the explosion or the positive time of the explosion is the time at which the explosion pressure is greater than the peripheral pressure.
This time depends on the velocity of the shock wave, in addition to duration of maintaining the maximum pressure and the amount of pressure reflected in the explosion.
 In the TM5-1300 instruction, a graph is presented for calculating the positive phase duration that was summed up by Izadifard and Maheri inEquation 3 (16) .
Equation [3]:
 

Changes in time pressure are expressed in terms of Friedland's relationship.
The relation [4]:

In this equation, b is a waveform that is a function of the maximum pressure Ps and its value is determined in accordance with the characteristics of the explosion, which is presented below. This is the phase of continuity of the positive phase. The negative phase of the wave is often much weaker and much more gradual than the positive phase, and therefore its effect in the bombing is ignored (15).
Regarding the necessity of strengthening the urban gas pipelines in non-active defense, effort was made to use diameter of 4-inch for urban pipes, 12-inch diameter for transmission pipes and 8-inch and 10-inch diameter for pipe networks in calculations. Specifications of pipes used in Table 2 are given. Due to the variety of pipes used, several pipe samples of various thicknesses are used for modeling. Modeling for a pipe of 12 inches pressure psi250 and for a pipe of 6 and 4 inches, a pressure of 60 psi is considered. Due to the pipes used in Iran, the mechanical properties of ST-37(Standard 37) were considered for steel pipe. The steel pipe's density is 7850 kg/m³, the Young's modulus is 210 Gpa and the Poisson coefficient is 0.3. In order to study the effect of soil on the behavior of pipes under the explosion of three types of soils, hard soils, soft and clayey sand were investigated and the depth of burial was considered­1, ­2,­3 and 4 m. The amount of explosive charges was also 15, 20, 30, 45, and 200 kg.

 
Table 2: Specifications of pipes used in modeling

Number	Diameter (in)	Thickness (mm)	Diameter (in)	Thickness  (mm)	Diameter (in)	Thickness  (mm)
1	4	6.8	12	8.7	8	11
2	4	8.7	12	11	8	14.3
3	4	10	12	14.3	10	11
4	4	14	12	21	10	14.3

 

Functional level of pipe breakdown: The optimal function of pipes is selected in accordance with the ALA instruction (15,­11). This level of function is the failure of the pipe and the lack of loss of the ability to maintain the contents of the pipe during the explosion. In order to achieve the desired level of function in this definition, the following should generally be true:

• Tensile strain: limiting the tensile strain of maximum 4% for steel pipes.
• Pressure strain: limiting the pressure strain to the value obtained in relation (5).

Equation [5]:  
Results
In this project, we tried to define the appropriate scenario for the threat of pipelines in the first step, and in the second step, in accordance with the scenario, the type and means of threat should be identified and determined and the amount of spent explosives should be determined.
In step 3, the vulnerability that the threat tool of interest can cause to the pipeline, and, in step­4, existing solutions to reduce the vulnerability of the pipelines were investigated.
In order to investigate the reduction of pipeline vulnerability in the two studied scenarios, the effect of increasing pipe thickness, increasing diameter, increasing burial depth, increasing the distance to explosion site and changing the soil material were separately investigated and analyzed.
Pipeline Vulnerability Results in Threat Scenario 1
In Figure 3, a schematic analysis of the explosion model software in the scenario of the direct hit of rocket (20­kg) to the burial site of pipelines is illustrated.