جستجوی مقالات مرتبط با کلیدواژه "gas pipeline" در نشریات گروه "مکانیک"

Thermomechanical steels are widely used in oil and gas pipelines due to their high toughness and high resitance against crak growth. A large part of the steel pipelines used in the oil and gas industry in Iran is made of API X65 steel. The fluctuations of internal gas pressure in steel pipes can cause fatigue failure and lead to gas leakage and explosion. So, the control of damage initiation and structural integrity of gas pipelines is of great importance. In this study, the S-N curve and the fatigue strength of the base metal of the API X65 steel were estimated by performing fatigue tests. For this purpose, 24 and 25 test specimens along the seam weld in the coil transverse direction, and perpendicular to the seam weld along the coil rolling direction were prepared according to ISO 1143 standard, respectively. All test samples were cut from an spirally welded pipe with 1219mm outside diameter and 14.3mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. The mean curve, characteristic curve, and confidence interval of the results were obtained both in the finite fatigue life range and in the fatigue resistance. The mean endurance limit of the base metal perpendicular to and parallel to the seam seam were 305 and 291 MPa, respectively which were in the range of 0.4 to 0.6 of material tensile strength and above the seam weld endurance limit (258 MPa).

In this paper, the position of the leakage in a pressurized gas pipe is localized. As an experimental example, the leakage in a steel pipe is simulated by different orifices. To decrease the risk of explosion and environmental contamination during experiments, pressurized air was used instead of natural gas. Most of the published papers are based on using high sampling rate sensors placed on both sides of the leakage. In this study, a new method is extended due to leakage localization in a gas pipe. To achieve this goal, attenuation analysis is implementing by installing two sensors on one side of the pipe. In the proposed technique, the dominant frequency of recorded signals is used, to decrease the effect of background noises on final results. Experimental results reveal by increasing the pressure the leakage localization error is decreased. Finally, by implementing signal processing into the data the average value of leakage localization error is reported 13.77 cm.

Shield metal arc welding on the high strength low alloy steels in pipelines to transport natural gas from Iran is of great importance. In this paper, mechanical and metallurgical properties of multi-pass weld in weld zone, heat affected zone (HAZ), and base metal (with 36 inch outside diameter) for determination of critical area is investigated. Also Chemical analysis, metallography, tensile, impact and hardness testes in different positions of pipe (hours 12-1:30, 1:30-3, 3-4:30 and 4:30-6) were carried out. Tensile test results showed the lowest ultimate strength (Vertical to weld and in position 3-4:30) equal to 463mpa, and the lowest energy impact (Vertical to weld and in position 1:30-3) equal to 96J that determine 61 percent reduction compared to the base metal. The amount of hardness variation in different areas and positions is negligible (less than 5 percent). Images of metallographic test made by light and electron microscopes demonstrated that the amount of perlite cap pass weld and heat affected zone near the base metal were decreased compared to main metal to respectively 12.5 and 25 percent. The amount of Nb reduction in position (4:30-6) relative to the base metal is 0.025.

Analysis of natural gas transient flow in transmission pipelines is one of the most important issues in the gas industry. Despite the previous studies, the accuracy and the computational time have yet considered as two important challenges in this field. In this paper, a parallel algorithm for numerical simulation of isothermal and non-isothermal gas flows is presented. Numerical analysis of the flow is performed using the implicit Steger-Warming flux vector splitting method. For parallelization, the computer program has been parallelized using Message Passing Interface library. In order to demonstrate the capabilities of the developed computer program, the flow inside two pipelines with different conditions is solved, and the results are validated. Then, some factors such as the computational time, reduction of the time, and the speed up criteria are obtained to demonstrate the computational efficiency of the proposed method. The results show that parallel processing method can significantly reduce computational time of natural gas flow in long transmission pipelines. Moreover, it is shown that application of this approach on the fine computational grids is more efficient than on the coarse grids.

The weld residual stresses decrease the design stress in gas transportation pipelines. In this paper, two X70 steel pipes of 56 inch outside diameter were firstly girth welded. Experimental hole drilling test was conducted to evaluate the residual stress distribution in this joint. Then, the finite element simulation of the welding process was performed to evaluate the residual stress distribution precisely. The numerical results were verified by comparison with the obtained experimental measurements. The qualitative results achieved match properly with the experimental results. Simulation results (with a difference about 15% compared to experimental results) evaluated the maximum residual stress in hoop direction of pipe’s external weld metal. The experimental data showed that the maximum tensile residual stress was located on the center line of the weld gap on the pipe outer surface alongside with the pipe hoop direction. Moreover, the maximum compressive (hoop and axial) residual stresses occurred on the pipe inner surface in heat affected zone. The variations of the hoop residual stresses on the inner and outer surfaces of the pipe had similar trend with tensile distribution at the center line of the weld gap. However, these stresses showed different trends (tensile stress on the outer surface, and compressive stress on the inner surface) with distancing from the weld center line.

Plain strain fracture toughness is extremely important for failure assessment in high strength steels used in gas transmission pipelines. In the current research an experimental method based on three point bend test specimens was used to calculate fracture toughness of steel pipes of grade API X65 (with outer diameter of 1219 mm and wall thickness of 14.3 mm). A value of 308 MPa√m was found for fracture toughness of test material. Then a finite element solution for the test specimens was conducted using modified Gurson’s damage theory. In this model, the critical crack tip opening displacement was calculated from damaged elements in the early stage of crack growth. This model resulted in 297 MPa√m of fracture toughness. A comparison between the experimental and numerical results illustrated the fitness of common methods for determining fracture toughness of tested steel in ambient temperature. Due to the lack of experimental data for this steel, the obtained results can be used for safe performance of domestic gas pipelines made from API X65 steel.

Plain strain fracture toughness is extremely important for failure assessment in high strength steels used in gas transmission pipelines. In the current research an experimental method based on three point bend test specimens was used to calculate fracture toughness of steel pipes of grade API X65 (with outer diameter of 1219 mm and wall thickness of 14.3 mm). A value of 308 MPa√m was found for fracture toughness of test material. Then a finite element solution for the test specimens was conducted using modified Gurson’s damage theory. In this model¡ the critical crack tip opening displacement was calculated from damaged elements in the early stage of crack growth. This model resulted in 297 MPa√m of fracture toughness. A comparison between the experimental and numerical results illustrated the fitness of common methods for determining fracture toughness of tested steel in ambient temperature. Due to the lack of experimental data for this steel¡ the obtained results can be used for safe performance of domestic gas pipelines made from API X65 steel.

Numerical simulation of non-isothermal transient gas flow is performed using implicit Steger-Warming finite difference method. Because of nonlinearity of the governing equations، they are linearized at each time step. The linearization either reduces computational effort or analyzes the flowfield more conveniently. In order to validate and evaluate the accuracy of current numerical method، Fanno and shock tube flows are investigated first. Then، transient flow in a gas pipeline that its inlet pressure changes with time is simulated. The results of present study show that Steger-Warming finite difference scheme can well captured the sudden changes in the flowfield. Moreover، the present method is able to analyze transient gas flows as nearly accurate as the nonlinear one with less computational effort.

In case of studying and modeling gas flow in the pipeline network, there are two types of flow: steady flow and transient flow. The natural gas pipeline network consists of pipelines, pressure compressor stations, pressure reduction stations, line break valves and storage fields. The pipeline is the key element in determining the overall network dynamic behavior. Dynamic behavior of a gas pipeline due to changes of gas flow rate for isothermal transient status has been modeled and investigated in this study. The modeling of isothermal transient flow is resulted in differential equations using mass conservation equations, momentum and an equation of state. By solving these equations according to time and place, the behavior of gas pipeline will be obtained. Three cases of the operating conditions for gas transmission pipelines in transient status have been studied. The modeling results of each case have been shown and interpreted in curves of flow rate and pressure variations along pipeline in different time spots.