SYSTEM IDENTIFICATION AND ADAPTIVE SELF-TUNING CONTROL...
Transcript of SYSTEM IDENTIFICATION AND ADAPTIVE SELF-TUNING CONTROL...
SYSTEM IDENTIFICATION AND ADAPTIVE SELF-TUNING CONTROL FOR
IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM
EZZ ELDIN BALLA MOHAMED HUSSEIN
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Electrical - Mechatronics & Automatic Control)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
JANUARY 2013
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Dedicated to the entire BALLA’s
And to all those that believed in me
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ACKNOWLEDGEMENT
First and foremost, my unlimited and sincere appreciation goes to the Lord of
the seven heavens and earth ALLAH (SWT) for His endless mercies, blessings and
guidance through from birth till now and forever. Alhamdullahi Rabi Alalmin.
I shall forever be grateful to my beloved parents, brothers, sisters, their
families and especially my big brother Tayallah Balla and his family for their belief
in me even when I did not and for their unending support, financially; morally,
spiritually and emotionally. To them I am highly indebted and words alone cannot
describe my gratitude. I pray ALLAH (SWT) make you reap the fruit of your labor
with me, Jazakum Allahu Khyran.
My sincere appreciation also goes to my supervisor Prof. Dr. Mohd Fua'ad
Rahmat for his continued guidance, support and encouragement to ensure this work
is a success. And also I would like to thank all my lecturers, friends and classmates in
UTM.
Finally, my earnest appreciation also goes to all my friends, colleagues and
well wishers that contributed to the success of this study. To you all I say thank you
for all support, kindness and your love.
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ABSTRACT
Pipeline infrastructure has become a very popular tool for transferring and
distributing oil, gas and raw materials. Many of these pipelines extend along
hundreds of miles and pass through remote, harsh and corrosive areas. This raises the
prospect of cracks in their walls and cause leaks. In case of pipelines fail due to
corrosion resulting in leakage, they will often lead to loss of products followed by
environmental and financial damages on a national scale. Therefore, protecting
materials and structures against corrosion is a significant issue especially in tropical
countries such as Malaysia which has high humidity climate (corrosiveness factor).
This latest, promotes the need for specialized research to be done for preventing
corrosion. Consequently, in this study we have focused on the most practical method
of cathodic protection systems which is impressed current cathodic protection
(ICCP) system. This system is widely used to prevent the external corrosion of
carbon steel structures; especially those are used in underground pipelines. Basically,
in this project the theoretical background and the concepts of ICCP systems have
been discussed. An effective laboratory scale for ICCP systems was built according
to specific concepts has been clarified. Then, parametric model of ICCP systems was
obtained using system identification approach. Furthermore, to enhance the
performance of ICCP systems, proportional-integral (PI) and direct self-tuning
generalized minimum variance (ST GMV) controllers have been designed.
Additionally, simulation and experimental works have been carried out to control
ICCP systems at different operating conditions. Finally, the ST GMV controller leads
to improve the system speed response and to decrease the integral of absolute error,
which is lower value compared to close loop using PI controller.
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ABSTRAK
Infrastruktur paip telah menjadi peralatan yang penting untuk memindah dan
mengedarkan minyak, gas dan bahan-bahan mentah. Kebanyakkan paip-paip ini
dipasang diunjurkan jauh sehingga beratus-ratus kilometer. Ada diantara paip-paip
ini melalui kawasan-kawasan pedalaman, termasuk juga kawasan yang berdepan
dengan hakisan. Ini meningkatkan kemungkinan berlaku retakan dan seterusnya
kebocoran. Sekiranya kegagalan sistem perpaipan ini adalah disebabkan oleh hakisan
yang disebabkan oleh kebocoran, ini selanjutnya akan mengakibatkan kerugian
produk, dan kerosakan alam sekitar juga termasuk peningkatan dalam kerugian kos
pada skala peringkat kebangsaan. Oleh yang demikian, bagi negara tropika seperti
Malaysia yang mana iklimnya panas dan lembap, keperluan untuk melindungi
material dan struktur daripada hakisan adalah amat penting. Dalam kajian ini, fokus
kami adalah menggunakan kaedah yang paling praktikal dalam sistem perlindungan
katod, iaitu sistem perlindungan katod arus teruja (ICCP). Sistem ini digunakan
secara meluas untuk mengelakkan hakisan luaran oleh struktur keluli karbon;
terutamanya untuk perpaipan didalam tanah. Pada asasnya, didalam projek ini, latar
belakang teori dan konsep sistem ICCP telah dibincnagkan.satu makmal efektif
untuk sistem ICCP telah dibina berdasarkan konsep tertentu telah dijelaskan.
Kemudian, model parametrik sistem ICCP telah diperolehi dengan menggunakan
sistem pengenalan identiti. Tambahan pula, untuk meningkatkan prestasi sistem
ICCP, pengawal penalaan perkadaran dan penalaan kamiran (PI) juga penalaan diri
langsung dengan varians minimum umu (ST GMV) telah direka. Akhirnya, ST GMV
pengawal mampu memperbaiki kelajuan tindak balas sistem dan mengurangkan
kamiran kesilapan mutlak, yang merupakan nilai yang lebih rendah berbanding
menggunakan pengawal PI gelung tutup.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF ABBREVIATIONS xiii
1 INTRODUCTION
1.1 Introduction 1
1.2 Background of Study 1
1.3 Significance of Study 3
1.4 Problem Statement 4
1.5 Project Objectives 4
1.6 The Project Scope 4
2 LITERATURE REVIEW
2.1 Introduction 6
2.2 Corrosion Principles and Mechanisms 7
2.2.1 Corrosion Process 7
2.2.2 The Corrosion Cell 8
2.2.2.1 Anode 8
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2.2.2.2 Cathode 9
2.2.2.3 Electrolyte 10
2.2.2.4 Metallic Path 10
2.2.3 Corrosion of Underground Pipelines 11
2.2.3.1 Uniform Corrosion 11
2.2.3.2 Pitting Corrosion 12
2.2.3.2 Corrosion Due to Dissimilar Metals 13
2.3 Cathodic Protection Principles and Technique 14
2.3.1 Sacrificial Anodes Cathodic Protection 15
2.3.2 Impressed Current Cathodic Protection 16
2.4 Criteria of Cathodic Protection 18
2.4.1 On-Potential Criteria 19
2.4.2 Off-Potential Criteria 20
2.4.3 Potential Shift Criteria 20
2.5 ICCP Systems Controllers Overview 20
2.5.1 Classical Controllers 21
2.5.2 Fuzzy Logic Controller 21
2.5.2 Neuro-Fuzzy Logic Controller 22
2.6 Adaptive and Self-tuning Control 23
2.6.1 Indirect Self-Tuning Algorithm 24
2.6.2 Direct Self-Tuning Algorithm 25
3 RESEARCH METHODOLOGY
3.1 Introduction 26
3.2 Concepts for Lab-Scale Design to Model ICCP Systems 28
3.3 Experimental Setup 29
3.4 ICCP System Identification Modeling 31
3.5 ST GMV Controller Design 32
3.4.1 Recursive Least Squares Parameters Estimation 32
3.4.2 GMV Control 33
3.4.3 ST GMV Algorithm 35
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4 RESULTS AND DISCUSSION
4.1 Introduction 36
4.2 ICCP Systems Lab-Scale 36
4.3 Modelling of ICCP Systems 39
4.3.1 Preparation to Identification and Data Acquisition 39
4.3.1.1 Finding the Sampling Time 39
4.3.1.2 Input signal for System identification 40
4.3.2 Experimental Setup and Data Acquisition 41
4.3.3 Model Structure Selection 45
4.3.4 Model Estimation and Validation 46
4.3.4.1 Choosing the Best Model 46
4.3.4.2 ICCP Model Residuals 48
4.3.4.3 ICCP Model Properties 48
4.3.4.4 ICCP Model Structure 49
4.4 ICCP System Control Design 51
4.4.1 PI Controller 51
4.4.2 Direct ST GMV Controller 53
4.4.3 ICCP System Controllers Comparison 60
4.4.3.1 Simulation Results 60
4.4.3.1 Experimental Results 61
5 CONCLUSION AND FUTURE WORKS
5.1 Conclusion 63
5.2 Future works 64
REFERENCES 65
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LIST OF TABLES
TABLE NO. TITLE PAGE 4.1 Chemical composition of the carbon steel 38
4.2 Chemical composition of the cast iron 38
4.3 Parameters in Simulink 52
4.4 The comparison of simulation and experimental PI Controller 53
4.5 Parameter values of ICCP model and GMV controller 56
4.6 The comparison of simulation and experimental GMV Controller 59
4.7 The comparison of simulation PI & ST GMV controllers 60
4.8 The comparison of experimental PI & ST GMV controllers 61
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LIST OF FIGURES
FIGURE NO TITLE PAGE
2.1 The corrosion cell 8
2.2 Uniform corrosion 12
2.3 Pitting corrosion 12
2.4 Corrosion due to dissimilar metals 13
2.5 Galvanic energy series 14
2.6 Sacrificial anodes cathodic protection 16
2.7 Sacrificial anodes cathodic protection 17
2.8 Land-based ICCP system with test post 18
2.9 Steel protection criteria 19
2.10 FLC controller for an ICCP system 22
2.11 Neuro-fuzzy controller for an ICCP system 23
2.12 General structure of STC 24
3.1 Flow chart of the project methodology 27
3.2 Schematic diagram of coating defect 28
3.3 ICCP system with coating defects 29
3.4 General block diagram of ICCP systems lab-scale 30
3.5 System identification approach 31
3.6 General structure of ST GMV control 32
4.1 Components of ICCP systems lab-scale 37
4.2 The final setup of the ICCP lab-scale 38
4.3 The step response of the ICCP lab-scale system 39
4.4 MATLAB/ SIMULINK block diagram for 63 bit PRBS 41
4.5 The experimental setup for data acquisition and control design 42
4.6 The block diagrams for ICCP system data acquisition 43
4.7 ICCP system input and output signals for system identification 44
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4.8 The general structure of ARMAX model 45
4.9 The best fit of all estimated ARMAX models 46
4.10 ICCP model residuals 48
4.11 ICCP model overview 49
4.12 ICCP model structure 50
4.13 The block diagram of the PI controller with ICCP model 51
4.14 The block diagram of the PI controller with ICCP lab-scale 52
4.15 The comparison between simulation & experimental PI Controller 53
4.16 The block diagrams of the ST GMV controller 57
4.17 The parameter estimator block diagram 58
4.18 The GMV controller block diagram 58
4.19 The comparison of simulation and experimental GMV controller 59
4.20 The comparison of simulation PI & ST GMV controllers 60
4.21 The comparison of experimental PI & ST GMV controllers 61
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LIST OF ABBREVIATIONS
CP - Cathodic Protection
ICCP - Impressed Current Cathodic Protection
PHMSA - Pipeline and Hazardous Materials Safety Administration
TR - Transformer Rectifier
NACE - National Association of Corrosion Engineers
DC - Direct Current
AC - Alternating Current
PI - Proportional–Integral
PID - Proportional–Integral-Derivative
FLC - Fuzzy Logic Controller
NN - Neural Networks
ST - Self-Tuning
STC - Self-Tuning Control
MV - Minimum Variance
MVC - Minimum Variance Control
GMV - Generalized Minimum Variance
RLS - Recursive Least Square
RE - Reference Electrode
CCSRE - Copper/Copper Sulfate Reference Electrode
DAQ - Data Acquisition Card
PC - Personal Computer
SISO - Single Input Single Output
PVC - Polyvinyl Chloride
PRBS - Pseudo Random Binary Sequence
GUI - Graphical User Interface
ARX - Auto-Regressive with Exogenous Input
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ARMAX - Auto-Regressive Moving Average with Exogenous Input
OE - Output-Error
BJ - Box-Jenkins
RTCS - Real-Time Control System
PEM - Prediction-Error Method
FPE - Final Prediction Error
TF - Transfer Functions
IAE - Integral of Absolute Error
CHAPTER 1
INTRODUCTION
1.1 Introduction
This chapter discusses about the introduction of the study which is consist of;
background of study, significance of study, problem statement, project objectives,
and the project scope.
1.2 Background of Study
The Materials Corrosion phenomenon has become a global significant
problem. Corrosion weakens strength and cause failure on material. The general
definition of corrosion is dissolution or destruction of a material as a result of
electrochemical reactions with its environment. Most of the materials which undergo
corrosion are metals; hence most of the corrosion definitions have been specified to
metals [1, 2]. However, Mars G. Fontana suggested that all materials including
ceramic, polymer and other non-metallic materials which are contributed into the
corrosion should be taken care as well [3].
Protecting materials against corrosion is a significant issue especially in
tropical countries such as Malaysia which has high humidity climate (corrosiveness
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factor). The annual cost of corrosion in many countries is very high, for example,
“In the United States the annual corrosion cost is USD$ 40 billion or RM 140
billion” [4]. Therefore, preventing corrosion need to be done onto materials in order
to reduce corrosion rate, so that less materials and money being wasted.
However, the previous mentioned definition of corrosion encompasses all
materials, in this project we have concentrated on metal-corrosion which is linked to
the carbon steel that is used in underground pipelines. In the fact that, soil
corrosiveness varies over a wide range as a result of a variety of the compositions
and solutions. Moreover, there are many factors affect the corrosiveness of the soil
such as resistivity, humidity, chlorides, sulfates, temperature and pH [2].
Several approaches have been used to protect materials against corrosion, for
an instant, coating system, cathodic and anodic protection systems. However,
cathodic protection has a great history back to around two centuries. Sir Humphry
Davy reported in 1824 that “copper could be successfully protected against corrosion
by coupling it to iron or zinc. He recommended cathodic protection of copper-
sheathed ships, employing sacrificial blocks of iron attached to the hull in the ratio of
iron to copper surface about 1:100” [5].
In general, cathodic protection (CP) is a technique has been used for a long
time to prevent metal corrosion by making it a cathode of an electrochemical cell.
This technique is universally used in protecting the external surface of pipelines,
underground storage tanks, ship hulls and sub-sea structures, etc. CP was used
mainly to prevent further corrosion after repair of damaged structures. However,
recently cathodic protection has been incorporated in new constructions in an effort
to prevent corrosion from being started by applying one of its two methods; galvanic
anode or impressed current cathodic protection [6, 7].
In this study, we have focused on the most practical method of cathodic
protection which is known by impressed current cathodic protection (ICCP) system.
This system is widely used to prevent the external corrosion of carbon steel
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structures; especially those are used in underground oil and gas pipelines. The
corrosion protection will be achieved by using ICCP methods, if the impressed
current is adequate to shift the potential of a metal to a level or protection range at
which corrosion is not occurring or negligible.
1.3 Significance of Study
Billions of dollars of crude oil or natural gas are transferred through a
widespread network of pipelines. These merchandises have to be transported
throughout the world via pipeline networks. It is obvious that modern economy relies
on these networks of pipelines that are distributed over hundreds of miles throughout
many areas around the world. Unfortunately these pipelines run through harsh
environments resulting in many problems. Among those are leakage problems.
“According to the US Department of Transportation Pipeline and Hazardous
Materials Safety Administration (PHMSA), 25% of all pipeline failure incidents
reported in the US from 2002 to 2003 were caused by corrosion (leakage main
cause)” [8].
The consequences of leakage can be more than the financial issues. It can
affect people’s lives and the environment. In the oil industry, these consequences are
more noticeable where leaks in pipelines may cause disastrous consequences that can
last for years. For oil and gas companies, pipeline leaks are one of the major causes
of failure because of their significant length in remote and harsh areas. This latest, is
prompting the need of lots research to be done in order to prevent corrosion.
Therefore, in this study we are focusing on enhancing the performance of the ICCP
in order to build strong system against corrosion to save people life as well as the
environment.
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1.4 Problem Statement
The potential required for impressed current cathodic protection depends
upon the steel being protected from its environment. Output voltage, depending on
desired reference voltage, needs to be controlled and tuned by transformer rectifier
(TR) units of the CP Stations.
Basically, TR unit with high output voltage requires high performance control
to maintain the correct output voltage. Moreover, applying the classical control or
fuzzy logic control based on trail and error and/or training data for the ICCP systems
have shown significant overshoot and oscillation in the output potential. For thes
reasons, more works regarding the tuning of the output of TR units are needed to
achieve better accuracy and performance.
1.5 Project Objectives
1. To build an effective laboratory scale for the Impressed Current Cathodic
Protection (ICCP) system.
2. To obtain a parametric model for the ICCP system.
3. To design, simulate and experiment self-tuning adaptive controller for an
ICCP systems.
1.6 The Project Scope
The scope of this project is to study the impressed current cathodic protection
(ICCP) system and its concepts in preventing the external corrosion of underground
carbon steel pipelines. Then, experimental works will be carried out to build an
effective laboratory scale for ICCP systems. The system would be used for black box
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identification so that system identification approach will be used to obtain the
mathematical model which represents the ICCP systems. After that, direct self-tuning
generalized minimum variance control would be used to design an adaptive corrosion
controller for this system as well. Finally, experimental work would be carried out to
test the performance of the designed controller.
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