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REAL TIME ESTIMATION OF LEAK LOCATION IN A NATURAL GAS PIPELINE – A FLUID FLOW MODELING APPROACH
ABSTRACT
This thesis investigates into the real time modeling of leak estimation in natural gas pipeline using fluid flow approach. The best bet to leak management is to provide a robust system that will provide a quick information on tracking the leak location, the actual time this leak occurred and the volume of fluid loss. This thesis employed a mathematical model of analyzing the fluid flow in the pipeline to provide these solutions of leak estimation in a natural gas pipeline which is in a steady state, isothermal condition. The leak estimation model was developed from first principle by modifying the Weymouth‘s equation. The modified equation were used to establish the fluid flow rate equation for pipeline with no leak case and a pipeline with leak case. The pipeline with leak case were partitioned into three sections. The first section depicts the point before the leak occurred, the second section depicts the point of leak occurrence and the third section depicts the point after the leak has occurred. Resolving the fluid flow equations for these pipeline cases yielded two results (models); the first result determined the fluid pressure in the pipeline at the point of leak occurrence, while the second result determined the point of leak location. Further analyses were carried out with the aid of the two results obtained to determine the fluid loss pressure at the point of leak and also to determine the pressure gradient profile of the pipeline with leak case. The pressure gradient profile of pipeline with the leak case were then compared with that of no leak case. The results of the analyses led to the actual time of leak (which is quite different from the time the leak were detected by a pressure transducers). The actual time of leak were then used to develop a model that quantify the volume of fluid loss in the pipeline. All the results were validated experimentally using a field data. The maximum error percent recorded for the point of leak location is 0.437% and the minimum error percent recorded for the point of leak location is 0.113%. The study further reveals that the output pressure of a pipeline experiencing leak can be estimated.
Keywords: Leak, Natural Gas, Isothermal, Steady State, Weymouth, Fluid Flow, Pressure Gradient, Pressure Transducer
TABLE OF CONTENTS
Title Page i
Certification ii
Dedication iii
Acknowledgements iv
Table of contents vi
List of Figures xi
List of Tables xii
Abstract xiii
CHAPTER ONE
Introduction 1
1.1 Background of the Study 1
1.2 Statement of Problem 3
1.3Objectives of Study 4
1.4 Scope and Limitations of Study 5
1.6 Justification of Study 6
CHAPTER TWO
Literature Review 8
2.1 Causes of Pipeline Leak 8
2.2 Structural Factors 8
2.3 Operational Factors 9
2.4 Unintended Third Party Damage 10
2.5 Intended Damages 10
2.6 Implications of Failure to Detect Leaks 12
2.7 Leak Detection Systems 13
2.8 Classification of Leak Detection Systems 17
2.9 Non-Continuous Systems 18
2.10 Inspection by Helicopters 18
2.11 Use of Trained Dogs 19
2.12 Smart Pigging 20
2.13 Continuous Systems 21
2.14 Externally Based Systems 21
2.15 Acoustic Emission Leak Detection 21
2.16 Fibre Optic Sensing Method 25
2.17 Vapour Sensing Tube Method 27
2.18 Liquid Sensing Cable Method 28
2.19 Soil Monitoring 28
2.20 Internally Based Systems 29
2.21 Mass-Volume Balance 29
2.22 Real Time Transient Modeling 30
2.23 Negative Pressure Wave (NPW) 33
2.24 Pressure Point Analysis 36
2.25 Statistical Leak Detection 37
2.26 Digital Signal Processing 39
2.27 Leak Localization 40
2.28 Externally-Based Leak Localization Methods 40
2.29 Internally-Based Leak Localization Methods 40
2.30 Gradient Intersection Method 41
2.31 Wave Propagation Method 44
2.32 Advantages of Wave Propagation 47
2.33 Disadvantages of Wave Propagation 48
2.34 Expanded Wave Propagation Method 48
2.35 Available Technologies for Leak Detection 49
2.36 Artificial Neural Network 49
2.37 Frequency Response Method 50
2.38 Well Logging 50
2.39 Air Surveillance 51
2.40 Satellite High Resolution Reconnaissance Photography 51
2.41 Intelligent Pigs 51
2.42 Electrical Resistance Tomography 52
2.43 Computer-Based LDS and Its Components 52
2.44 Operational Data 54
2.45 SCADA Systems/Communications 55
2.46 Hardware 57
2.47 Level of Instrumentation 58
2.48 Flow Meters 60
2.49 Pressure Transducers 63
2.50 Pressure Available For Fluid Flow in Pipeline 65
2.51 Frictional Effects 66
2.52 Effects of Pipeline Elevation 66
2.53 Effect of Changing Pipe Delivery Pressure 67
2.54 Hydraulic Pressure Gradient of Gas Flow in Pipeline 67
2.55 Drag Reduction Effect 71
2.56 Pressure Drop in A Pipeline 72
2.57 Calculating the Pressure Drop in A Pipe 72
2.58 Pipe Fitting Loss Calculations (Minor Losses) 78
2.59 Pressure Loss Due To Change in Elevation 81
2.60 Overall Pipe Pressure Drop Calculation 82
2.61 Flowrate 84
CHAPTER THREE
Methodology 87
3.1 Natural Gas Modelling Principles 87
3.2 Natural Gas Modeling 88
3.3 Basic Equations of Compressible Fluid Flow 88
3.4 Continuity Equations 89
3.5 Gas Leak Detection Modelling 91
3.6 Leak Localization or Location 92
3.7 Leak Equation Modeling 93
3.8 Determination of Pressure At The Point Of Leak 103
3.9 Time of Leak 105
3.10 Proposed Model for Detection of leak Period 110
3.11 Equation for Estimation of Fluid Loss Volume 110
3.12 Model Simulation 112
3.13 Simulation One – Leak Location 112
3.14 Simulation Two: Time of Leak 112
3.15 Simulation Three – Determination of Volume of Fluid Spilled 113
CHAPTER FOUR
Analyses of Results 114
4.1 Results for Natural Gas Modeling Simulation 114
4.2 Simulation Results For Leak Location 114
4.3 Calculated Results for Leak Location 115
4.4 Comparison of Model with Actual Field Results 118
4.5 Sensitivity Analyses for Leak Location 120
4.6 Interpretation of the Leak Pressure Profile graph 134
4.7 Simulation for Time of Leak 135
4.8 Results of Time of Leak using Mass balance Method 136
4.9 Results for Time of Leak using Proposed Pressure Wave Method 137
4.10 Simulation Results for Volume of Leak 140
CHAPTER FIVE
5.0 Conclusions and Recommendations 143
5.1 Conclusions 143
5.2 Recommendations 144
5.3 Contribution to Knowledge 145
Nomenclature 147
APPENDIX 148
Appendix A: Estimation of After Detection of Leak 148
Appendix B: Merging PR AND PF 150
Appendix C: Pressure Gradient Equation 152
References 154
CHAPTER ONE INTRODUCTION
1.1 Background of the Study
The need for transportation of petroleum becomes necessary from the onset of its production. The transportation of natural gas (and other petroleum products) are most commonly accomplished through pipeline systems and networks. Pipelines provide the most efficient means of transporting petroleum fluids (Payne, 2007). Often times the integrity of pipelines suffer as a result of factors ranging from human, mechanical and environmental occurrences which causes disturbances to the pipelines leading the effluence of pipeline fluids to the environment known as leak.
Aside the huge financial loss to the government and operating companies, leak poses severe environmental implications. The effluent fluids being hydrocarbons are toxic and causes severe degradation of the underlying ecosystem. Humans, animals and plants are grossly affected both on land and in water bodies. In some regions this has caused youth restiveness which further complicates the issue.
As is clearly pointed above, leak is undesirable. Even though the complete avoidance of it may be viewed as wishful but its occurrence could be greatly curbed and managed that the adverse effect would be made less potent.
Because of the enormous cost associated with leak, it is a major concern to the public (Mpesha, 2001). With increasing awareness and concern for the environment recent pipeline leak incidents have shown that the cost is much more than the associated down-time and clean up expenses. An effective and appropriately implemented leak detection program will easily pay itself through reduced spill volumes and increased public confidence. Leak detection technology has developed to a sophisticated level of automation for onshore gas and liquid transmission pipelines and this technology is routinely applied for shallow water offshore pipelines (Siebert, 1981). However, deep-water and arctic flow-lines operate under conditions rarely found in previous development schemes. Leak detection technologies can be categorized based on a variety of criteria. One such criterion used in the past was to classify the methods based on where the measurement was made.
The need for effective pipeline leak detection system is as important as the energy itself. Several leak detection systems exist for pipeline leak detection (PLD). There are internally (observing hydraulic behavior) and externally (released fluid detection) based PLD technologies. Computational pipeline monitoring (CPM) is an internally based PLD technology, it uses pressure, flow, temperature, and/or acoustic instruments to measure single or multi-phase fluid parameters within a segment of pipeline. Typical external devices include systems employing optical fibers, acoustic sensors, chemical sensors, and electrical sensors. External methods can result in an accurate detection of the location and the size of the leak, but comes with the expense of production shutdown. The high cost/long time to run the physical detection, which is very crucial in a long-distance gas pipeline is also a challenge (Ekwueme, 2014). Internal methods detect a gas leak by solving the governing equations, thus leading to quick evaluation at much lower costs, but with higher uncertainties.
Due to the long mileage of pipelines, it is important that dependable leak detection systems be used to promptly identify when a leak has occurred so that appropriate response actions are initiated quickly. The swiftness of these actions can help reduce the consequences of accidents or incidents to the public, environment and
facilities.
Accordingly, leak detection of a gas pipeline plays an important role in the overall pipeline integrity system. And to achieve this, it is more enhanced through careful pipeline supervision, early detection of the location followed by quick responses to the incidence. This work seek to develop a model for leak parameter identification.
1.2 Statement of Problem
Pipeline leak is undesirable and can only be curbed. Remediation procedures relies on the leak detection system employed.Authors have worked towardsdeveloping softwares that will increase on the reliability and accurate estimation of leak parameters which includes estimation of leak location, the time this leak occurred and the volume of fluid spilled. Available literature on leak determination was unable to determine the pressure at the point of leak to justify the calculation of the leak location. Also, the effect of negative pressure wave at the upstream sections of pipelines were not considered by earlier models developed by researchers for leak detection. Estimating the volume of fluid loss has been problematic to researchers because they were unable to determine the actual time this leak occurred which is quite different from the time this leak was dictated. Another major challenge of accurately estimating leak is in considering the back pressure of the environment to the leak opening during leak. All these becomes the focus of this research study.
1.3 Objectives of Study
The main objective of study is to investigate into the real time modeling of leak estimation in natural gas pipeline using a fluid flow approach.
The specific objectives this study includes the following:
- To develop models governing fluid flow in natural gas pipeline both in the absence of leak and during leak occurrence.
- To compare the fluid pressure profile in the pipeline before and during leak occurrence.
- To determine the point of leak along the pipeline.
- To model the actual time the leak occurred in the pipeline
- To estimate the volume fluid loss due to leak from the pipeline considering the actual time the leak occurred from the pipeline.
1.4Scope and Limitations of Study
This work borders on the development of mathematical model for steady state single phase isothermal fluid for a horizontal pipeline. The fluid employed here is Natural gas. The leak hole here considered is a fracture or rupture without full breakage of the pipeline. Thus in the case of leakage fluids are still delivered to the outlet. It is to be noted that this model is not applicable for total pipeline rupture where the fluid is totally discontinued from passing to the downstream end of the pipeline. This study however integrates the concept of flow through an orifice, single phase liquid relief through a relief valve and line depressurization to develop a time-dependent model that quickly and accurately determine the location and volume of spill when a leak is detected along the line.
1.5Justification of Study
This work will be of huge significance in the area of leak detection in natural gas pipeline. The model gave wide applicability in pipeline leak systems. The model considered jointly, the pressure at the point of leak, the negative pressure wave and the back pressure of the environment which were the mixing link in literatures during leak location determinations. These factors considered jointly enabled the model to give more accurate prediction of leak location.
Concerning the time of leak, other time of leak models have assumed erroneously that a leak actually occurred when it was detected. Our time of leak model counters this assumption by bringing in the notion that a leak may actually occur for periods even before it was detected by the available pipeline leak detection as provided by SCADA. Using the this time of leak model developed in this work in design of leak alarms system will help in determining leak occurrences earlier than that gotten from existing leak alarm systems. This will lead to earlier pipeline remediation action, thereby reducing fluid wastages from pipeline due to leak and thus reduce financial expenses that will accrue from leak occurrences. The models are highly useful in pipeline monitoring systems in real time pipeline surveillance to ensure quick and reliable investigation of pipeline flow systems. The model when implemented in the Niger Delta pipeline monitoring will ensure proper pipeline supervision and management and help reduce losses and incidences in natural gas transportation through pipelines.
REAL TIME ESTIMATION OF LEAK LOCATION IN A NATURAL GAS PIPELINE – A FLUID FLOW MODELING APPROACH
