Modeling of gas transmission pipes with artificial neural networks in order to detect their defects

Dissertation for Master's Degree in Mechatronic Engineering

Persian Abstract

The purpose of this project is to introduce a new approach for troubleshooting gas transmission pipelines using mechanical waves, which is much cheaper and easier than other methods. who are currently working. These lines are usually located in harsh environmental conditions and far away and in long distances, and the use of systems that can report the defects and leaks of this pipe in real time is vital. A delay in assessing a damaged pipeline can potentially have a direct impact as a result of fire hazards from pipeline ruptures or sudden shutdowns of critical care facilities. The powerful health monitoring system of a pipeline includes the reduction of operating and maintenance costs. The presented method includes modeling a 2-inch pipe piece 50 meters long in Abaqus 6.121 software, creating 15 holes with a radius of one millimeter at three-meter intervals, capturing the vibrations (acceleration) of the pipe in a healthy state and in a defective state, transferring acceleration data to the frequency domain, using statistical techniques, creating a radial basis neural network and using this network for the presence and location of defects.

Key words: gas pipe, abacus, variance, radial basis function networks, pipe troubleshooting

Chapter 1- Introduction

Pipeline structures are sensitive to cracks and wear. The damage detection system for pipeline structures can reduce operational and maintenance costs.

This project is a new approach for diagnosing gas transmission pipelines using mechanical waves, which is much cheaper and easier than the methods of ultrasound waves and optical fibers that are currently employed. Very well and with very little cost, it is possible to diagnose the problem.

The research questions that we have to answer in this thesis are:

1) What is the maximum length of the pipeline that can be identified?

2) What is the appropriate effective bandwidth?

3) Can the location of the defect be detected using the acceleration signal?

Reference to the thesis chapters:

Chapter one: Introduction - Chapter two: Existing methods in pipeline troubleshooting - Chapter three: Steps of the project - Chapter four: Construction of the finite element model from the piece of pipe and model validation - Chapter five: Modeling of the pipeline - Chapter six: Fault finding - Chapter seven: Results - Chapter eight: Conclusion

First, the goals and specifications of each stage are briefly stated, and then the details of each stage are described in the chapters of the thesis It is presented, so that it can be used independently by researchers in the future. 1-1- 1st stage: modal analysis of three-meter pipe The modal analysis test was conducted on a piece of about three meters of a two-inch pipe used in urban lines, as a result of which the natural frequencies and mode shapes of the pipe were extracted in the range below two kHz. This experiment was conducted in Sharif University of Technology and its details are available in the thesis chapters. Although this design is mostly aimed at intercity lines, the choice of two-inch pipe has a specific reason: new modes appear in vibrations with an increase in the length-to-diameter ratio, in fact, the higher the length-to-diameter ratio in the simulation, the more natural frequencies appear in the vibration behavior and the results are more realistic. Due to limited financial resources, it was not possible for us to test pipes longer than three meters. With the maximum length fixed, the only way to get the most accurate lab results was to use a smaller diameter. It should be noted that the purpose of the first two stages of this project is to validate (a) the finite element method, (b) the proposed software for implementing this method, and (c) the type of proposed elements, and the test results are not directly used in troubleshooting, so the use of a two-inch pipe, even if the purpose is to troubleshoot larger pipes, does not cause any problem.  At the end of the first stage, the results have been compared with the results of the simulation from the second stage, which confirms the accuracy of the model built in the second stage. 1-2- 2nd stage: construction and analysis of the pipe model using the finite element method..

1-2-                                Second stage: construction and analysis of pipe model by finite element method

In this step, the pipe is modeled in Abaqus software and subjected to vibration analysis in free-free conditions, similar to the test conditions, by finite element method. This stage itself includes several steps, including pipe modeling with solid and loose (separate) elements, assigning material and mechanical characteristics (such as modulus of elasticity and Poisson's ratio) and analysis. The results of the modeling with the above-mentioned two different elements are also presented in the thesis. 1-3- 3rd step: vibration analysis: increasing the length of the pipe to 50 meters and setting the pipe to be similar to the real state without considering the effect of soil and welding in the simulation space, then determining the location of the excitation force so that its effect is visible in a significant length and obtaining the maximum excitation amplitudes in such a way that an impact function causes damage to the Create 15 holes with a diameter of 2 mm in different positions along the length of the pipe and apply force on the perforated models and the intact model for 0.1 seconds and record the determined signal. 1-4- 1-4 Step 4: Monitoring the signals Transfer the information obtained to the frequency field with windowing and fast Fourier transformation and calculate the average absolute value of the acceleration every 30 Hz and obtain the signal difference in the mode healthy and defective pipe and the statistical analysis of the data of the previous stage and finally learning the data to the radial basis neural network. Chapter 2- Methods available in pipeline troubleshooting The economy plays a role. The two main categories for fault diagnosis are: 2-1-1- First category: In this category, the entire pipe must be examined for fault diagnosis, so that either the detection device must be moved along the entire length of the pipe or this device must be installed throughout the pipe. These include the use of light or sound sensors to find leaks [1]. Other examples are the injection of flammable chemicals and the use of flame detectors along the pipeline [2], the simultaneous use of electromagnetic sources and detector sensors [3]. Another method is to use a special robot called "pig" (These pigs are actually robots that move on the pipe. These robots are generally used to detect defects on the pipe such as stress corrosion cracks, etc. In most cases, this expensive device is used to inspect gas pipelines that are not located underground.) In order to perform this method, the pipeline needs to be out of service [4]. Another example for this category is the installation of optical fiber along the entire length of the pipe [5]. All these methods are time-consuming and/or expensive.

2-1-2- Second category

In this category, to diagnose gas pipelines, some variables need to be measured at limited points of the pipeline. There are two methods in this category, the first method: fault detection based on monitoring changes in fluid properties (for example, flow rate and pressure) [6, 7]. The second method of troubleshooting is performed using ultrasound waves [8]. In the first method, it is done by using a set of solving nonlinear equations that describe the flow dynamics (for example, through linearization [9] or discretization of nonlinear equations [10]) to predict the flow velocity or pressure in the presence/absence of defects. This method still suffers from errors caused by the complex dynamics of natural gas and uncertainty in the parameters of the governing equations. In contrast, ultrasound waves have been successfully used to detect gas pipeline leaks. The main shortcoming of this method is the operating range (ten meters) and the high cost and the use of generators and ultrasonic detectors. 2-2-2- Causes of gas leaks in pipelines and leak detection methods In the following, we briefly discuss the causes of gas leaks in pipelines, leak detection methods, and the classification of leaks and gas detectors:

2-2-1-       Causes of gas leakage

Gas leakage occurs for various reasons in gas pipes and gas installations, the most important of which are:

A-Corrosion: due to defects in insulation or not implementing the correct cathodic protection of the external surfaces of the pipes, there is a possibility of corrosion.