Application of SCADA System by Using (Fuzzy Logic Controller) on the Cathodic Protection System for Oil Pipelines

Jasim A. Harbi Ministry of Oil, Baghdad, Iraq. Email: jasimaadi@gmail.com Abstract This study is dedicated to design and execute Supervisory Control and Data Acquisition system (SCADA) to monitor and control the corrosion of a pipeline buried under ground. A microcontroller equipped with many sensors and communication system used to control and monitor the process of an impressed current cathodic protection ICCP process for pipeline. The combination of the hardware, was built with LabVIEW and personal computer (PC) interface presents a “SCADA” system for two methods: Fuzzy Logic Controller (FLC) presents closed loop, and a conventional for open loop system. Hence, under temperature of 30°C, an assessment and comparison were carried out for two methods of controls examined at low moisture of soil (50%) and high moisture of soil (80 %) for measuring the potential between pipeline and anode, current, and power. It was found a decrease of (39.7%) in consumed power when the moisture of soil changing from the low to the high level. It was attained that the close loop (FLC) was the best method of performance, and consumption of the power.


Introduction
"SCADA system is an abbreviation of supervisory control and data acquisition. A SCADA system is a type of industrial control system used to collect and control data from remote location that be considered as the backbone of modern control systems in different fields of the industry for the current century. It is employed in many control application in different branches of industry such as power plants, oil industries, transportation, water treatment plants [1]. Furthermore. SCADA systems can be used to control the corrosion in pipes, equipment's and structures through collecting data from cathodic protection system sensors in real time and display the obtained data to the observers. Also, monitoring and controlling the remote system parameters (such as current and voltage) [2].
More of engineering and economic problems come from corrosion of pipelines in oil industry.
(ICCP) is an important method in industry of oil to minimize the corrosion of pipelines. Iraq has a great number of pipelines in industry of oil and transportation crude oil, natural gas and hydrocarbon products [3]. When theappropriate protection methods are used, these pipelines are exposed to damage by corrosion. Many ways were applied to minimize the corrosion of pipeline made of carbon steel such as coating, cathodic protection and painting. [4]. CP is a method applied throughout the whole world to prevent and control the electrochemical corrosion for pipelines, [2]. Corrosion happens if four elements exist: cathode, anode, metallic path and electrolyte as shown in the Figure (  Cathodic protection has been studied by many researches for the different processes.  This study is concerned to design of a SCADA system for ICCP system to control and monitor. This CP system was built in two methods namely; the conventional system (open loop method) and (FLC) that support closed loop system. The control process was designed by controlling the applied voltage for anode to keep the protection voltage within the protection limits.
The inputs in open loop systems are applied to drive the outputs with unknown value of the output of system through feedback signal as shown in the Figure (3).

Fig. (3) Open loop System [11]
Thus, this method of control is not effective and is mostly influenced by the disturbances [12]. variable y (t) and the reference input r (t) [13].

Fig. (4) FLC Block Diagram [14]
FLC is a closed loop control system that based on a mathematical input values in terms of logical variables that take the values between 0 and 1. That used by expert systems in process control. It uses to control processes symbolized by subjective, linguistic variable input, output and descriptions boundaries of fuzzy sets. It is an active choice when precise is necessary. A (FLC) use known rules to control the fuzzy process system that based on the present of input  [14]. The multi inputs is the error (difference between the desired value and the process variable the output value of system) and the rate change of error (difference between the error and the new error). laboratory; the analysis result shows that the chemical composition as illustrated in Table (1).    Fig. (7) Anode.

Experimental work
A suitable Copper wire brazed to the outside surface of pipeline as (cathode) to achieve negative electrical voltage for system as shown in Figure     It is supported by National Instruments, and used to design and implement the experimental work of the systems for both methods as shown in Figure (11). Shows the design of HMI for the conventional system where data manual controlling processes and visually monitoring.

Results and analysis
The strategy of research is based on applying an ICCP process on pipeline with two control methods, the conventional and automatic system to achieve the protected pipeline within the lower and upper protection limits. If the protection voltage PSP changes under the lower protection limit the oxidation occurs. so, when the PSP raised up the upper limit, the surface of coating for pipeline will suffer from degradation.
The aim is to obtain the optimum measured parameters and test the performance of ICCP system and effectiveness of both methods. The results of two methodes are taken at temperature environment (30 ○ C) and two soil moisture levels: the medium level of 50% and the high level of 80%.

Conventional Open Loop Results
In this method, the protection potential PSP reach to (-0.9 V) for both moisture of soil levels requires 50 s which is too small. As shown in Figure (

The Smart (FLC) Closed Loop Results
The (FLC) provides the ICCP system with the desired anode voltage to make the  the undesired components. The purpose of using Kalman filter is to remove the noise from data and provide accurate measurements for the ICCP system parameters. Hence, using Kalman filter in the ICCP system taking more time than the no filter case. The purpose of (FLC) to keep the PSP in range of the protection limits (-0.85 V to -0.125 V).
When the moisture of soil changed from the medium level (50%) to high level (80%) after an amount of water adding along the pipeline as shown in the magnified area of Figure (17) a, b.
The (FLC) monitors the output signal and compares it with the desired set point value. The signal of feedback form PSP sensor is analyzing by the rules of (FLC). The signal of returned feedback sensor PSP is computed by the (FLC). Error is the difference between desired value and measured value is applied with (error change) as feedback to the input of (FLC) to make the output near to the set point desired value) of (-0.9 V). An effectiveness of the smart system to keep the protected potential PSP at the specified protection limits at -0.9 V. The accuracy of the smart system to preserve the PSP value at (-0.9 V) under changing moisture of soil. So, the important advantage is a fast system response time (40 s) of the FLC, as shown in the Figure (17), the desired value was adjusted at the -0.9 V value for the protection voltage PSP. During using Kalman filter, the undesired components were canceled, in the smart ICCP system.

Conclusions
The concluding remarks are drawn from the experimental and computational investigations.
1. The closed loop controller is more stable and higher efficiency for the ICCP system.
Where, no rust formation was observed through the FLC.
2. Open loop system must be continuous monitoring and manual control according to the value of PSP visually observed.
3. Kalman filter is important filter to minimize the noise of PSP signal and improve the process of control.
4. The measured parameters: anode voltage, current and consumed power is increasing with increase the moisture of soil for both the conventional and the intelligent systems.