**4. Experimental Validation**

Figure 9 shows the actual corrosion detection sensor and the dog-bone-shaped metals (low carbon steel), made from the same O&G pipeline material. The semicircular curved beam is made of PVC for the experiment, but long-term, it will be constructed using a 3D printer. Three corrosion detection sensors were constructed using the three dog-bone-shaped metals shown in Figure 9. The dimensions of the dog-bone-shaped metals are shown in Figure 10. The three corrosion sensors were placed in series as shown in Figure 11; Figure 12. Figure 11; Figure 12 show the experimental apparatus used for validating the performance of the proposed external corrosion detection sensor.

**Figure 9.** (**a**) Actual corrosion detection sensor. (**b**) The dog-bone-shaped metals.

**Figure 10.** Dimensions of the dog-bone-shaped metal (Lo = 29.24 mm). (**a**) Top view; (**b**) Side view.

**Figure 11.** Schematic of the experimental test setup to test the proposed corrosion detection sensor.

**Figure 12.** Schematic of details of proposed sensor.

The optical fiber used in our experiment is a single mode fiber (SMF 28) with 5 FBG sensors, with FBG sensors 1 m apart. The first FBG Sensor is located at 0.5 m from the end of the optical fiber that connects to the interrogator. Glue was used to attach the outer surface of the PVC rings to the bottom surface of the FBG sensors (see Figure 9; Figure 12). To make sure the bond between the FBG sensor and the semicircular plastic curved beam remains intact at high temperature of the pipeline (≤150 ◦C) and high humidity of the desert for prolonged periods of time, the authors of [25] recommend a compound, based on a combination of ceramic fillers with an epoxy binder that is applied with a brush technique, and this compound can withstand temperatures in the region of 260 ◦C and humidity of 75%.

The optical fiber was connected to an interrogator with wavelength range of 1270−1340 nm. The connection termination type between the optical fiber and the interrogator was FC-APC.

Most O&G pipelines are made of API 5L X42 to X70 material with carbon content from 0.16% to 0.28% (mild or low carbon steels). Other mild or low carbon steel materials are also used in the O&G industry. The dog-bone-shaped metal pieces are required to be made from the same pipeline material. For our experiment, API 5L X65 material was used. As can be seen from Figure 11, the dog-bone-shaped

metals are linked to one another using wires. Wires are soldered to the dog-bone-shaped metals and the solder joints were coated with an anticorrosion coating. Figure 13 shows the entire experimental test set-up consisting of a laptop, an interrogator, corrosion cell, and a power supply.

**Figure 13.** (**a**) Experimental test set-up. (**b**) and details of the apparatus inside acrylic tank.

A DC power supply (4 V and 1 A) was used to accelerate the corrosion reaction. Graphite rods were the cathode and the low carbon steel pieces were the anode, all of them are placed into 3.5% by weight NaCl solution which is regarded as the electrolyte. The thinnest section of the dog-bone-shaped metals is in the middle, with thickness of 1 mm. It was at this location where the corrosion failure first occurred. The experiment kept running until complete failure of one of the sensors. It took almost 14 h to corrode one of the sensors. The three corroded sensors are shown in Figure 14.

**Figure 14.** The three corrosion sensors after accelerated corrosion failure.

As was explained on page 10, the optical fiber used in our experiment is a single mode fiber (SMF 28) with five FBG sensors, with FBG sensors 1 m apart. The first FBG Sensor is located at 0.5 m from the end of the optical fiber that connects to the interrogator. Thus, the first FBG Sensor is located at 0.5 m, the 2nd FBG sensor at 1.5 m, the third at 2.5 m, and so on. In our experiment, only three FBG sensors out of five were used. Figure 15 shows the strain observed by all the FBG sensors before and after the failure of the first corrosion sensor. The yellow color trace is the strain observed by all the FBG sensors before the failure of the 1st corrosion sensor, which is about zero and the blue color trace is the strain observed by all the FBG sensors after the failure of the 1st corrosion sensor. The y-axis shows the strain (in micro strain) of all the FBG sensors and the x-axis indicates the location (in meters) of the FBG sensors. In the upper right corner of the Figure 15, the graph indicates "time domain". This implies that the strain at each FBG sensor location can change with time. If we would have continued the accelerated corrosion, we would have had the failure of the 2nd or 3rd corrosion sensors and in Figure 15, we would have seen additional peaks, as time goes by.

The peak value of the strain, at 0.5 m, where the first FBG sensor is located, is −3100 με. When the semicircular plastic curved beam is inserted to the dog-bone-shaped metal, point A (where the FBG sensor is attached to) will be strained. The strain at point A will be in tension. We tared (zeroed out) that strain in the interrogator. However, when the dog-bone-shaped metal corrodes, the semicircular plastic curved beam is released thus negative strain is observed on the interrogator display monitor.

**Figure 15.** (**a**) Yellow trace displays strain before sensor failure. (**b**) Blue trace displays strain after sensor failure due to corrosion.

It was not necessary to tare the strain. We could have done the opposite, meaning not zero out the strain and leave the strain at point A as it is and when the dog-bone-shaped metal is corroded, the strain at point A would have gone back to zero.

#### **5. Discussion**

The external corrosion rate of O&G pipelines, based on NACE Standard (NACE RP0-502: Pipeline External Corrosion Direct Assessment Methodology), is ~0.4 mm/year [26]. Due to this low corrosion rate, larger diameter O&G pipelines are inspected every 3 to 5 years. Pipeline pigging and data analysis is very costly, and due to this high cost, most O&G companies have opted for 5-year inspection interval rather than the shorter ones. The smaller diameter pipelines (flowlines) are not even pigged due to diameter of the pipeline being small and no launch and retrieval stations being available to launch and retrieve the PIG. Even if the flowlines were piggable, since pigging occurs every 5 years, if there is an aggressive corrosion occurring due to the change in the pipeline environment or surrounding, O&G industry is vulnerable to possible field accidents in between inspection interval.

As, in normal conditions, the external corrosion rate of O&G pipelines is low, real-time corrosion monitoring is not really necessary, but as long as corrosion state of the pipelines are monitored monthly or every few months, this monitoring protects the O&G industry against aggressive pipeline corrosion. Our proposed sensor can provide corrosion state of the O&G pipelines, as and when needed. Our sensor is basically monitoring the corrosivity environment near the O&G pipelines.

In this paper, we embarked upon designing a very low-cost corrosion detection sensor for O&G pipelines, requiring no power. The sensors are passive, very simple, and they are permanently deployed in the field, unlike ILIs, ultrasound probes, X-ray, or radiography tools. The sensor involves a semicircular plastic component, a sacrificial dog-bone-shaped metal made from the same pipeline material, and optical fibers. The optical fibers have up to 20 to 40 FBG sensors per optical fiber, depending on which type of interrogator used. The optical fibers are attached to the gas and oil pipelines using zip ties, straps, or large hose clamps and our sensors are attached to the FBG sensors. When corrosion is severe at any pipeline location, the dog-bone-shaped metal corrodes at that location and eventually fails and thus a signal is detected by the interrogator at the control room. Once a signal is picked up at the control room, inspection personnel will visit the pipeline at that location and conduct visual inspection first, and possibly ultrasound, X-ray or radiography inspection. If the corrosion is not severe, the dog-bone-shaped metal is replaced until the next failure. If severe corrosion is observed at any pipeline location, the pipeline is inspected using ultrasound probes, or X-rays, or eddy current probes or other inspection methods. Depending on the severity of the corrosion, pipeline may be repaired and the metal dog-bone-shaped metal is replaced till the next sensor failure. In the proposed corrosion monitoring system of this paper, all the communication between the corrosion detection sensors and control room is through the optical fibers. As only light is involved, and there are no

batteries or electricity of any kind (since interrogator sits inside the control room), this corrosion monitoring system is very safe.

The thickness of the sacrificial dog-bone-shaped metal was chosen to be 1 mm. With corrosion rate of 0.4 mm/yr, it would take 2.5 years for the sacrificial dog-bone-shaped metal to corrode completely through the 1 mm thickness. Two and a half years is the mid-span of the 5-year inspection interval. If after 2.5 years, no signals are observed on the remote interrogator for any of the corrosion sensors, then one can conclude the corrosion rate is less than 0.4 mm per year for that pipeline. When few sensors fail earlier than 2.5 years, then it implies we are having aggressive corrosion occurring at some specific pipeline locations. Our proposed sensor is very helpful to the pipeline operators as they can now go to those specific locations and find out why there is a higher rate of corrosion at those locations. Steps can be taken at those high corrosion rate locations to lower the corrosion rate back to 0.4 mm/yr or lower.

If there is a leakage of the crude oil, as crude oil is hot, the heat can create tension strain on the FBG sensors thus a signal can be picked up by the interrogator in the control room. The proposed sensor of this paper can not only detect occurrence of pipeline corrosion but also the pipeline fluid leakage.

Ground settlement is a geological phenomenon of ground elevation changing (vertical movement of the ground) caused by the compression of earth's crust surface soil due to natural and unnatural events. Caving in or sinking of the ground is one form of ground settlement. If the ground caves in, naturally, the O&G pipelines foundations will also cave in. Our fiber optic-based corrosion monitoring system proposed in this paper, most likely, does not capture ground settlement since neither the optical fiber nor the corrosion sensors are rigidly attached to the pipeline unless the ground cave-in is deep. There are few O&G companies already using optical fibers on their O&G pipelines to monitor the potential security risks, detect pipeline temperature, and detect ground settlement. For those companies, presently having optical fibers on their pipelines, it makes sense to bundle our corrosion detection system to their existing fiber optics monitoring system to also detect pipeline corrosion.

### **6. Conclusions**

The grating length of most FBG sensors is anywhere from 5 to 20 mm. If the grating length is too large, the strain experienced by the FBG sensor won't be the strain of point A but the average strain near point A (See Figure 4b). The FBG sensor, used in our experiment, had a grating length of 10 mm. The FBG sensor was picking up the average strain underneath the FBG sensor and not strain at point A only, and that is why the experimental tension strains are below the predicted ANSYS strain results.

Due to the differences in physical–mechanical properties of the matrix material and the FBG sensor, and the adhesive, the strains measured by the FBG sensor may not be equal to the actual strains experienced by the matrix material. When the FBG sensor is glued on to the PVC semicircular curved beam, the area of moment of inertia is changed and the FBG sensor thickness, material and adhesive used can have some impact on the actual measured strain.

The exact value of strain obtained at point A is not important as long as the strain is below the strain limit of the optical fiber and the strain is large enough to be detected by the interrogator; thus, the proposed sensor of this paper can be used to detect occurrence of pipeline corrosion and pipeline leakage, and provide average pipeline corrosion rate.

**Author Contributions:** N.V. and X.W. conceived the sensor design, P.R. conceived and designed the experiments; X.W. and O.S. performed the experiments; N.V., X.W. and F.F.Y. conducted the FE analysis, F.F.Y. and N.V. looked into wireless communications, X.W. and N.V. developed the closed form equations and analyzed the data; and N.V., X.W. and O.S. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** Funding for this research project came from the Khalifa University of Science and Technology.

**Conflicts of Interest:** The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.
