Review of Structural Health Monitoring Techniques in Pipeline and Wind Turbine Industries

: There has been enormous growth in the energy sector in the new millennium, and it has enhanced energy demand, creating an exponential rise in the capital investment in the energy industry in the last few years. Regular monitoring of the health of industrial equipment is necessary, and thus, the concept of structural health monitoring (SHM) comes into play. In this paper, the purpose is to highlight the importance of SHM systems and various techniques primarily used in pipelining industries. There have been several advancements in SHM systems over the years such as Point OFS (optical ﬁber sensor) for Corrosion, Distributed OFS for physical and chemical sensing, etc. However, these advanced SHM technologies are at their nascent stages of development, and thus, there are several challenges that exist in the industries. The techniques based on acoustic, UAVs (Unmanned Aerial Vehicles), etc. bring in various challenges, as it becomes daunting to monitor the deformations from both sides by employing only one technique. In order to determine the damages well in advance, it is necessary that the sensor is positioned inside the pipes and gives the operators enough time to carry out the troubleshooting. However, the mentioned technologies have been unable to indicate the errors, and thus, there is the requirement for a newer technology to be developed. The purpose of this review manuscript is to enlighten the readers about the importance of structural health monitoring in pipeline and wind turbine industries.


Introduction
For a long time, corrosion has been a major cause of concern in the oil and gas industry, proving to be fatal to production in a plethora of ways, including storage, transportation, infrastructure, production, and exploration [1][2][3], thereby causing a significant amount of loss to the industry. Hydrogen cracking also exhibits its catastrophic capabilities in oil and gas plants as it develops in various forms, leading to a loss of structural integrity of the plants. There are more than 528,000 km (328,000 miles) of natural gas transmission and gathering pipelines, and 119,000 km (74,000 miles) of crude oil transmission and gathering pipelines. The energy sector uses several infrastructures that are metallic and undergo Scheme 1. Damages and their types.

In Pipeline Industry
Corrosion is a common phenomenon in places that contain oil. Recently, concentrations of corrosive substances have increased dramatically in crude oil. Materials such as sulfur and chloride and the increasing acidity are some of the common reasons. Several types of degradations inside refineries can take place with the varying environmental ef- The key function of SHM is to maintain track of changes in the structural system's dynamic properties for detecting and locating damage, as well as to automatically determine whether the damage is harmful to the structure. Damage detection usually entails data processing to look for changes in structural dynamic characteristics (such as modal frequency, damping ratio, and mode shape) as well as inter-story drifts. Because structural damage results in a loss of stiffness, and the dynamic characteristics of a structure are directly connected to stiffness, it is reasonable to utilise natural frequency variations as a damage indicator. Changes in natural frequency are not always a reliable sign of damage, as the reaction of the damaged structure is nonlinear, and in most cases hysteretic, according to examinations of recorded data from structures. Furthermore, different environmental variables (for example, temperature) can alter the natural frequency of buildings without causing any structural harm (Scheme 1).

In Pipeline Industry
Corrosion is a common phenomenon in places that contain oil. Recently, concentrations of corrosive substances have increased dramatically in crude oil. Materials such as sulfur and chloride and the increasing acidity are some of the common reasons. Several types of degradations inside refineries can take place with the varying environmental effects on naphthenic acids present in oils within the temperature range of 150-400 • C. These types include non-uniform, uniform, or localized pitting. This corrosion needs to be correctly and efficiently measured with the application of improvised techniques, and thereafter, the risk is to be calculated and minimized [11][12][13][14].
The production of fuel is also becoming increasingly difficult to this enhanced corrosive nature. The presence of corrosive elements in the oil is somewhat difficult to predict as it depends on a number of factors especially when it is caused by the naphthenic acids [15][16][17][18][19][20]. Some parameters that affect naphthenic acids corrosion are given in Table 1. Hydrogen cracking is another common problem in the pipeline industry. It could also be termed as cold cracking or delayed cracking. In carbon or low-alloy steels, hydrogen cracking occurs as atomic hydrogen diffuses and forms molecular hydrogen in it. Molecular hydrogen formation can be facilitated by inclusions or trap sites. Hydrogen cracking may also occur in the absence of any tensile stress. Molecular hydrogen formation pressurizes the material internally and starts cracking. A hard brittle structure, hydrogen produced by welding, and the tensile stress that acts on the welding joint are the three major factors that give rise to cracking. Hydrogen cracking is also caused by hydrogen strain (change in dimension of the pipeline due to tensile stress). Hydrogen cracking is a form of embrittlement of hydrogen common to carbon steels that originates in a step-like way from the propagation and linkage of small laminar cracks resulting from hydrogen pressurization trap sites. The strain of the internal hydrogen pressure inside laminar cracks increases as the hydrogen concentrations increase, causing adjacent cracks to be connected. The crack generally initiates in the heat-affected zone in the C-Mn steel, but it could also propagate into the weld metal. These cracks are linked with the coarse-grained region and could be intergranular, transgranular, or a mixture. Five types of hydrogen-assisted impairment for metals and alloys are being considered by the ASM materials handbook; these are: (1) hydrogen attack, (2) hydrogen-induced blistering, (3) hydride formation, (4) cracking from precipitation of internal hydrogen, and (5) hydrogen embrittlement [21]. Internal hydrogen embrittlement (IHE) is a part of hydrogen-assisted cracking (HIC), which is defined as the accretion of hydrogen, which already exists near large stress concentration sites and hydron environment embrittlement (HEE), elucidating cracking because of hydrogen sulfide or the exposure of hydrogen [22]. In the case of high-strength steels, hydrogen embrittlement is indeed a challenge, as the saline water along with other corrosive chemicals come into contact with it, which promotes the absorption of atomic hydrogen [23]. Several methods could be implemented to reduce the loss that occurs as a result of hydrogen cracking such as (1) minimum stress on joints, (2) proper preheating and post-weld heating, (3) appropriate selection of filler material, and (4) careful handle and storage of filler metals [24].
Another prominent issue in the pipeline industry is welding. Friction stir welding is a solid-state joining process invented at the welding institute in the United Kingdom. The friction stir welding was found to be efficient for joining hard to weld metals and joining plates of different thicknesses or different materials. The typical way to connect two natural pipes is shielded metal arc welding. When welding, we first need to tack the two pipes together to keep them in place. Then, we apply the method of butt welding. At present, to produce a flame hot enough to melt steel in the energy industry, this process involves the use of two gases: acetylene and oxygen. Through applying this flame to the steel being welded, a permanent weld is made; if required, an additional filler rod can also be attached to the welding area. Oxygen acetylene welding does not require the formation of electric welding. There are several disadvantages of this method, such as a visible hole being created in the welding plates. It is also less flexible as compared to the arc welding process. FSW cannot weld non-forgeable materials; also, filler joints cannot be created by it. Gas welding, on the other hand, showcases some advantages. This technique has the advantage of balancing the temperature at low temperatures. In the case of I-joint, it could weld the metal of thickness up to 6 mm. Generally, the pipes and tubes are welded in confined spaces, so gas welding is preferable here over other methods, as it requires equipment that is not bulky in nature. Moreover, this equipment does not require any electric supply. The light generated by its flame could be used to spot joints before welding begins. A major disadvantage of gas metal arc welding is the creation of a spatter, which are droplets of molten matter near the welding arc. It occurs when the welding currents are extremely high or in the case when gas shielding is not sufficient. For avoiding spatter, the welding current should be reduced, and the welder should correct the polarity. Moreover, a check over shielding gas type and flow rate should also be done [25]. Other common problems associated with gas welding include porosity, slag inclusions, incomplete penetration and fusion, incorrect wire delivery, deformation, cracks, and undercut.

Optical Endoscopy
Endoscopy: A boroscope and the use of CCTV are the means of visualizing the damage qualitatively. This image can be analyzed with advanced equipment to scan the contours in a topographic analysis and single out the defects. Laser ring triangulations and the visual odometry function with the deployment of crawlers mounted with equipment inside the pipeline is done [26,27].
Fiber Bragg Grating: These can be created using the property of variations of refractive indices of the core of an optical fiber, periodically. Utilizing predefined parameters, it is possible to predict the refractive indices at different places in the fiber with the variation of conditions of the environment. The light will result in constructive interference only at certain expected positions. There is a general relation for the wavelength variation [28,29]. where t = average wall thickness, r = average pipe radius, P = relative internal pressure, σ hoop stress, ε = longitudinal strain, ξ = thermo-optic coefficient, λ B = Bragg wavelength, α = thermal expansion, T = temperature, ρ e = photoelastic constant, and E = Young's modulus.

Electromagnetic Inspection
Inspecting through electromagnetic means would entail the utilization of fields after generating them. Electromagnetic health monitoring allows for the detection of burning, impact lamination, fiber breaking, and liquid ingress. The electromagnetic method essentially sends an electromagnetic image to the receiver to identify possible damage in the structure [30]. Their measurements and interactions yield the conductivity of the material and the test component's permeability. The different types of eddy currents generated, namely, Conventional Field, Remote Field, and Pulsed Field, provide the information as to the time or extent or decaying through measurements of the variation of these currents. Impedance is decreased with the permeability and conductance, and the phase lag is related to thickness of the wall.

Radiographic Inspection
The energy is directed through a source to a detector with a number of modifications performed on the rays that then produce a measurable damage quantity. The basic elements of all radiographic measurements remain the same. The radiographic method is particularly used in industry when isotropic material is encountered. They are not very precise and may even be damaging to the structure in certain cases, but they still are widely popular. The use of traditional X-radiography in the checking of carbon fiber/epoxy composites is limited, since they are low absorbers of X-rays. Thus, techniques that involve the use of penetrants (i.e., zinc iodide solution) have been introduced for enhancing the sensitivity of the method for these materials.
where I = intensity of a beam of radiation, I = radiation beam intensity entering the material, µ = linear attenuation co-efficient, x = material thickness, σ = total atomic attenuation co-efficient, N = Avogadro's constant, ρ = material density, A = atomic mass, σ pe = photoelectric affect, σ s = Compton scattering, σ pp = pair production, σ pd = photodisintegration, E = film exposure, I f = radiation intensity on the Film, T = time of exposure, G D = contrast measured at density D, D = density, I t = intensity observed on the film. These techniques are applied in various ways. Computed radiography involves digital scanning of the plates that are similar to the conventional films, and this allows reuse. Digital radiography nullifies the time lag between image production and ray penetration. This is tried in different orientations to produce different but predictable results.

Acoustic Method
Acoustic emission originally occurred naturally within materials, and the term AE is used to define the transient elastic waves that resulted from a sudden strain energy release within a material due to the occurrence of microstructural changes. When enough energy was released, audible sounds were produced. The cracking in timber subjected to loads near the yield point produces audible noises, which are indicators of impending failure of wooden structures, in the same way that the crying of tin, the cracking of rocks, and the breaking of bones are familiar to nearly everyone's ear. With a predefined threshold of noise, any extra generated vibrations that are produced by deformations of the pipelines will provide a location of the source of such a phenomenon. The magnitude of changes in frequency and intensity can be compared to different types and extents of the deformation and damage to the parts in question. Acoustic emission in the proper sense covers the audible frequencies up into the high ultrasonic range. Measurement today is carried out in the range between 50 kHz and 2 MHz. At higher frequencies, the acoustic emission is not intense enough in most cases, and the material absorbs large parts of the signal. In general, at lower frequencies, background noise disturbs the measurement e.g., vibrations from vehicles and noise from pumps or from flowing medium. Acoustic emission is especially used in places where leak size estimation is not as necessary as the accuracy [20,31,32].

Ultrasonic Technique
In the ultrasonic technique, we exploit the propagation of ultrasonic waves to obtain information about a mechanical component. For example, we can determine the mechanical properties of a material (the wave velocity is dependent on the material properties) or detect defects in a component (the presence of a defect will modify the characteristics of the propagating wave). Through combinations of various types of wave modes, coverage, transduction methods, transducer motions, and transducer configurations, a large number of possible configurations of detection of damage can be brought to light. The measurements of the voltages, time, and various other factors are listed in the following equations [31,[33][34][35][36]. The main advantages of this technique are the low power consumption of the sensors and the capability to cover large areas using few sensors. The ultrasound technique for SHM consists of a system that is composed of several modules: signal generators that provide waves at specific amplitude and frequency with enough power to drive a series of sensors; several parallel acquisition systems for the signals received at the transducers; a digital control system to synchronize the modules, perform calculations on the signals, and determine the state of the structure; a communications system with a central node, which is critical when multiple structures are monitored; and, obviously, the piezoelectric transducers. All these modules must be embedded in a low volume system and show large capacities: high reliability, signal integrity, accuracy, speed of response, etc.
The waves that need the boundaries for their existence are known as guided waves. When the longitudinal and shear bulk waves undergo constructive interference, ultrasonic guided waves are generated. There are several advantages of using ultrasonic guided waves in structural health monitoring, which include (1) long-range inspection, (2) full exposure of waveguide cross-section, and (3) high sensitivity to small defects compared to overall vibrations [37]. Due to their capability of inspecting large formations from a single probe position, ultrasonic guided waves are considered a great tool for structural health monitoring. Furthermore, it also possesses excellent flaw detection, and it could also establish wave resonance by tuning its frequency and setting up its mode. Different methods comprising magnetostrictive-type sensors, angle beam transducers, and comb-type transducers can be utilized to produce ultrasonic guided waves in a structure. The advantage of ultrasonic guided waves includes their assessment ability of concealed structures-which are present underwater as well as in concrete, coatings, and insulations-with magnificent sensitivity. Due to its simplicity and speed, it is considered a cost-effective solution for structural health monitoring [38]. The application of ultrasonic guided waves includes various structures such as tear straps, landing gears, lap splice joints, and so on. Its aircraft applications are also presented in the references [38][39][40][41][42]. Ultrasonic guided waves can also be used to perform surgery by the harmonic scalpel. In this case, the energy used for activation is sent to the tip of the rod, which is deployed for surgical use [43]. These waves are also used in inspecting rails in the railroad industry [44][45][46]. Moreover, guided waves could also show their impact in some adhesive bonding and joining applications [47][48][49]. At the interface between materials, these waves can generate both longitudinal and shear waves energy, which is especially useful in the adhesive bond inspection. The result of the dispersion of a sample guided wave is illustrated in Figure  5. Fourier transform was applied to obtain this profile, and then, the result obtained is compared with the theoretical value in which the dominant modes of the structure are highlighted [50].

Health Monitoring in Corrosion
The estimated corrosion-related cost is about $5.8 billion annually to monitor, replace, and maintain these assets. It has been estimated that corrosion costs a sum of $1.4 billion annually to the United States in production and exploration alone, with $589 million used in surface pipeline and facility costs, $463 million in downhole tubing expenses, and $320 million in capital expenditures related to corrosion [51]. One out of every four natural gas transmissions and gathering incidents has been the consequence of corrosion in the last three decades according to the Pipeline and Hazardous Materials (PHMSA) database, and more than half of these were due to internal corrosion [52,53]. However, it is even more difficult to inspect the insides of a pipe, and more so when the pipe is thousands of kilometers long and the problem could be anywhere. Hence, steps have been taken to achieve easier detection in these scenarios. Corrosion is an electrochemical process and usually requires an anode, a cathode, and an electrolyte to occur. These components are fulfilled by the fluids found in the reservoirs, thereby promoting corrosion [54][55][56][57][58][59]. Cathode: This reaction causes a decrement in mass and material and also unexpected cracking and catastrophic failures in oil and gas industries. Corrosion is thermodynamically favorable, so it occurs, but its kinetics can be brought under control. Localized corrosion or pitting occurs due to hydrogen sulfide and chloride ions, which provide structural weak points, resulting in cracking without the force being enough to cross the mechanical threshold. Microbes are also another causative agent of corrosion, especially the sulfate-reducing ones [60][61][62]. More than one-quarter of the damages done by corrosion could have been prevented in the United States. Widely used carbon steel is prone to corrosion; monitoring it to plan maintenance can be helpful in corrosion prevention [51]. The carbon steel is used in many places including drill pipes, transmission pipes, and casing tubing [63] owing to its excellent mechanical properties and reasonable prices. SHM is required to detect signs of risk that could cause catastrophic events. Monitoring allows for the identification of corrosive elements, and this especially comes in handy when thousands of miles of pipes are being discussed. Corrosion sensors are broadly classified as direct or indirect based on detection through environments or consequences. The optimum type of sensors needs to be selected in this scenario ( Figure 1). This reaction causes a decrement in mass and material and also unexpected cracki and catastrophic failures in oil and gas industries. Corrosion is thermodynamically fav able, so it occurs, but its kinetics can be brought under control. Localized corrosion pitting occurs due to hydrogen sulfide and chloride ions, which provide structural we points, resulting in cracking without the force being enough to cross the mechani threshold. Microbes are also another causative agent of corrosion, especially the sulfa reducing ones [60][61][62]. More than one-quarter of the damages done by corrosion cou have been prevented in the United States. Widely used carbon steel is prone to corrosio monitoring it to plan maintenance can be helpful in corrosion prevention [51]. The carb steel is used in many places including drill pipes, transmission pipes, and casing tubi [63] owing to its excellent mechanical properties and reasonable prices. SHM is requir to detect signs of risk that could cause catastrophic events. Monitoring allows for the ide tification of corrosive elements, and this especially comes in handy when thousands miles of pipes are being discussed. Corrosion sensors are broadly classified as direct indirect based on detection through environments or consequences. The optimum type sensors needs to be selected in this scenario ( Figure 1).

Conventional Corrosion Sensors
Conventional and commonly used corrosion sensors for SHM techniques in the O& industry are discussed in this section. A review or summary on corrosion monitoring tec niques in general or other areas is also available in references [64][65][66][67].

Corrosion
A corrosion coupon involves placing a certain patch of a material for a certain cor sive environment. The coupon is designed to be able to satisfy our needs to realize t rates of corrosion. It has a known weight and shape. The weight it loses is measured

Conventional Corrosion Sensors
Conventional and commonly used corrosion sensors for SHM techniques in the O&G industry are discussed in this section. A review or summary on corrosion monitoring techniques in general or other areas is also available in references [64][65][66][67].

Corrosion
A corrosion coupon involves placing a certain patch of a material for a certain corrosive environment. The coupon is designed to be able to satisfy our needs to realize the rates of corrosion. It has a known weight and shape. The weight it loses is measured to find out the actual damage done by corrosion [68]. This type of technique is unable to give an analysis of corrosion when it actually happens and just gives a mean rate of the corrosion that would take place. The coupon is normally placed within the working material, and hence, another one of its drawbacks includes the fact that it is difficult to remove and then to be placed again ( Figure 2). Its versatility lies in the fact that any shape or weight can be used to find out the corrosion rate.

Electrical Resistance Probe
Owing to corrosion, there is a reduction in the material's mass, and this fact can be advantageous, since the resistance is increased due to a lowered material mass, and this resistance can be measured to get to the proportional relationship between mass loss and resistance in the material. Since resistance has the ability to fathom the amount of corrosion in the material, it can be termed as a real-time corrosion coupon. With the help of this technique, it is possible to remotely log in data. Since it is possible for them to operate in conducting and non-conducting surroundings, it can be considered to be an additional advantage. Water-based corrosion is responsible for a conducting environment, while atmospheric corrosion is responsible for non-conducting. Electrical resistance-based investigations are plagued with primarily one disadvantage, which is that they are not equipped to identify variable corrosion, which means that if the amount of corrosion occurring at various places is not uniform, then an electrical resistance probe will give a result that is averaged throughout. If there is a requirement for an increased sensitivity toward variable corrosion, then multiple probes will have to be installed, and thus, it would complicate the system and increase the monetary investment needed.

Electrochemical Sensors
The electrochemical characteristics of corrosion are elevated with the help of electrochemical sensors, which make use of techniques such as galvanic current measurement, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and electrochemical noise (EN) [10,67,[69][70][71]. Electrochemical sensors are fairly advantageous; they can include the measured corrosion rates, and they also have the ability to investigate the mechanism responsible for the resultant corrosion. There are a myriad of electrochemical techniques, but the LPR-based corrosion technique has been used as a commercial method, since it is simple to operate, and the interpretation of data here is easier as compared to other methods. In most LPR-based investigations ( Figure 3) [72], the traditional concept of having of having a three-electrode system is not followed. Rather, the elec-

Electrical Resistance Probe
Owing to corrosion, there is a reduction in the material's mass, and this fact can be advantageous, since the resistance is increased due to a lowered material mass, and this resistance can be measured to get to the proportional relationship between mass loss and resistance in the material. Since resistance has the ability to fathom the amount of corrosion in the material, it can be termed as a real-time corrosion coupon. With the help of this technique, it is possible to remotely log in data. Since it is possible for them to operate in conducting and non-conducting surroundings, it can be considered to be an additional advantage. Water-based corrosion is responsible for a conducting environment, while atmospheric corrosion is responsible for non-conducting. Electrical resistance-based investigations are plagued with primarily one disadvantage, which is that they are not equipped to identify variable corrosion, which means that if the amount of corrosion occurring at various places is not uniform, then an electrical resistance probe will give a result that is averaged throughout. If there is a requirement for an increased sensitivity toward variable corrosion, then multiple probes will have to be installed, and thus, it would complicate the system and increase the monetary investment needed.

Electrochemical Sensors
The electrochemical characteristics of corrosion are elevated with the help of electrochemical sensors, which make use of techniques such as galvanic current measurement, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and electrochemical noise (EN) [10,67,[69][70][71]. Electrochemical sensors are fairly advantageous; they can include the measured corrosion rates, and they also have the ability to investigate the mechanism responsible for the resultant corrosion. There are a myriad of electrochem-ical techniques, but the LPR-based corrosion technique has been used as a commercial method, since it is simple to operate, and the interpretation of data here is easier as compared to other methods. In most LPR-based investigations ( Figure 3) [72], the traditional concept of having of having a three-electrode system is not followed. Rather, the electrodes are made from the same material. In the case of electrochemical sensors, the disadvantage lies in the fact that because of external potential, the corrosion rates can go up. Thus, in order to avoid this, it is necessary that parameters such as overpotential, scan rate, and Tafel slopes are chosen with accurately. There is an additional requirement of an electrolyte that is ionically conducting, making them unfriendly for usage in a non-conductive environment. There is an electrochemical corrosion sensor that can operate even at a temperature of 300 • C and at a pressure of 5000 psi. This sensor makes use of the galvanic current in a coupled multi-electrode array [73,74]

Ultrasonic Testing Sensor
One of the most common methods that monitors the extent of corrosion and the structural health of the pipes is the method where ultrasonic testing (UT) wall thickness is measured. Acoustic waves that have a high frequency, of the order of MHz, are generated from a piezoelectric transducer. The direction in which these waves are emitted is in a direction perpendicular to the pipe wall. The transducer receives the waves, which are reflected from various surfaces such as the internal and external surface of the pipe. The wall thickness is calculated by measuring the time difference between two consecutive echoes from the outer and inner surfaces of the pipe [78][79][80]. The data of wall thickness and stand-off signal are combined together to understand the difference between internal and external material losses in the pipe, as shown in Figure 4 [32,81]. The UT sensors are available in fixed as well as portable forms [71]. In addition, they can be added together with in-line inspection devices. This method has the capability to inspect corrosion by accessing only one side of the pipe. Highly attenuating mud and casing scales affect the acoustic sensors in the most direct fashion [3].

Ultrasonic Testing Sensor
One of the most common methods that monitors the extent of corrosion and the structural health of the pipes is the method where ultrasonic testing (UT) wall thickness is measured. Acoustic waves that have a high frequency, of the order of MHz, are generated from a piezoelectric transducer. The direction in which these waves are emitted is in a direction perpendicular to the pipe wall. The transducer receives the waves, which are reflected from various surfaces such as the internal and external surface of the pipe. The wall thickness is calculated by measuring the time difference between two consecutive echoes from the outer and inner surfaces of the pipe [78][79][80]. The data of wall thickness and stand-off signal are combined together to understand the difference between internal and external material losses in the pipe, as shown in Figure 4 [32,81]. The UT sensors are available in fixed as well as portable forms [71]. In addition, they can be added together with in-line inspection devices. This method has the capability to inspect corrosion by accessing only one side of the pipe. Highly attenuating mud and casing scales affect the acoustic sensors in the most direct fashion [3].

Magnetic Flux Leakage Method
This is yet another marvelous method that does not severely damage the system for monitoring; instead, it uses the magnetic properties of the substance. It can detect a change in flux. The flux changes simply because the flux line always exist in ferromagnetic substances. Normally, the undeformed structures will have a simple continuous and will produce magnetic lines in a fixed direction. However, when a defect is encountered, it surely would have caused some misalignment of the magnetic flux lines due to the deformation. This magnetic field change can be used to interpret a variety of information about the damage ( Figure 5). The only problem that it faces is the differentiation in location of the damage and the inability of the confirmation of a reason for the damage. In addition, it can only detect damage; it cannot predict it [82,83]. echoes from the outer and inner surfaces of the pipe [78][79][80]. The data of wall thickness and stand-off signal are combined together to understand the difference between internal and external material losses in the pipe, as shown in Figure 4 [32,81]. The UT sensors are available in fixed as well as portable forms [71]. In addition, they can be added together with in-line inspection devices. This method has the capability to inspect corrosion by accessing only one side of the pipe. Highly attenuating mud and casing scales affect the acoustic sensors in the most direct fashion [3].

Magnetic Flux Leakage Method
This is yet another marvelous method that does not severely damage the system for monitoring; instead, it uses the magnetic properties of the substance. It can detect a change in flux. The flux changes simply because the flux line always exist in ferromagnetic substances. Normally, the undeformed structures will have a simple continuous and will produce magnetic lines in a fixed direction. However, when a defect is encountered, it surely would have caused some misalignment of the magnetic flux lines due to the deformation. This magnetic field change can be used to interpret a variety of information about the damage ( Figure 5). The only problem that it faces is the differentiation in location of the damage and the inability of the confirmation of a reason for the damage. In addition, it can only detect damage; it cannot predict it [82,83].

Point OFS for Corrosion
It can be considered that point corrosion OFS is an optical version of corrosion coupons. At a particular section of optical fibers, the point OFS generally consists of coated sensing layers. Generally, the optical fibers are metallic films. It can be seen in Figure 6 that as there is corrosion of the metallic films at the ends of the fiber, there is a reduction in the reflected light, and at the other end of the fiber, this light is detected [84,85]. On the other hand, there is a coat of Fe-C film over a particular section of the fiber, and accordingly, there is an increment in the transmission of light along the optical fiber as the corrosion of Fe increases [86]. The long-period grating (LPG) is an alternative method to design a point sensor that allows the interaction of light and the neighboring medium with the help of cladding modes. The periodicity of LPG (Λ) ranges from 100 to 1000 µm, and this is greater than the periodicity range of fiber Bragg grating (FBG), and light from the guided mode is coupled into the core, which then moves toward the forward propagating cladding modes at particular values of wavelengths. Due to this, there is an evident dip in the transmission spectrum, as can be seen in Figure 7. Resonant wavelengths show strong changes toward even smaller changes in the temperature and strain of the cladding modes [87,88]. In a comparison between FBG and LPG, it is found that LPG exhibits higher magnitudes of spectral shifts and thus, owing to the larger values of periodicity, fabrication becomes simpler [87]. Various studies have been carried out that are focused on LPG sensors consisting of an Fe-C coat, and the studies have been carried out to observe the

Point OFS for Corrosion
It can be considered that point corrosion OFS is an optical version of corrosion coupons. At a particular section of optical fibers, the point OFS generally consists of coated sensing layers. Generally, the optical fibers are metallic films. It can be seen in Figure 6 that as there is corrosion of the metallic films at the ends of the fiber, there is a reduction in the reflected light, and at the other end of the fiber, this light is detected [84,85]. On the other hand, there is a coat of Fe-C film over a particular section of the fiber, and accordingly, there is an increment in the transmission of light along the optical fiber as the corrosion of Fe increases [86]. The long-period grating (LPG) is an alternative method to design a point sensor that allows the interaction of light and the neighboring medium with the help of cladding modes. The periodicity of LPG (Λ) ranges from 100 to 1000 µm, and this is greater than the periodicity range of fiber Bragg grating (FBG), and light from the guided mode is coupled into the core, which then moves toward the forward propagating cladding modes at particular values of wavelengths. Due to this, there is an evident dip in the transmission spectrum, as can be seen in Figure 7. Resonant wavelengths show strong changes toward even smaller changes in the temperature and strain of the cladding modes [87,88]. In a comparison between FBG and LPG, it is found that LPG exhibits higher magnitudes of spectral shifts and thus, owing to the larger values of periodicity, fabrication becomes simpler [87]. Various studies have been carried out that are focused on LPG sensors consisting of an Fe-C coat, and the studies have been carried out to observe the loss of mass due to corrosion [89,90].
Appl. Syst. Innov. 2021, 4, x. https://doi.org/10.3390/xxxxx www.mdpi.com/ this is greater than the periodicity range of fiber Bragg grating (FBG), and light f guided mode is coupled into the core, which then moves toward the forward prop cladding modes at particular values of wavelengths. Due to this, there is an evi in the transmission spectrum, as can be seen in Figure 7. Resonant wavelengt strong changes toward even smaller changes in the temperature and strain of the c modes [87,88]. In a comparison between FBG and LPG, it is found that LPG exhibi magnitudes of spectral shifts and thus, owing to the larger values of periodicity, tion becomes simpler [87]. Various studies have been carried out that are focused sensors consisting of an Fe-C coat, and the studies have been carried out to obs loss of mass due to corrosion [89,90].

Quasi-Distributed OFS for Corrosion
It can be considered that sensors based on Fiber Bragg grating are equivalent to point sensors. For every FBG sensor, there is a shift in the Bragg wavelength owing to the changes in the environment such as changing temperature and strain. On the basis of this principle, it becomes possible for the FBG sensors to be able to monitor the changes in temperature and strain parameters which are closely related to the corrosion that happens in pipelines and wellbores (Figure 8).

Quasi-Distributed OFS for Corrosion
It can be considered that sensors based on Fiber Bragg grating are equivalent to point sensors. For every FBG sensor, there is a shift in the Bragg wavelength owing to the changes in the environment such as changing temperature and strain. On the basis of this principle, it becomes possible for the FBG sensors to be able to monitor the changes in temperature and strain parameters which are closely related to the corrosion that happens in pipelines and wellbores (Figure 8).
The FBG-based pressure sensors that have been developed can be utilized in finding the point of leakage in the pipelines by using the method of negative pressure wave (NPW). When a leak in a pipeline initially happens, there is a pressure drop that is induced, and it travels in either direction from the point of leakage. The NPW reaches the FBG pressure sensors that are fixed on the pipe, which records the time taken for the wave to reach it, and this information is utilized to evaluate the point of leakage [91,92]. It has been found that the strain sensors based on FBG are attached directly to the pipeline surface in order to evaluate the strain of the refurbished pipes [93]. In field tests, the installation of sensors based on FBG are done over the risers for watching the riser stress during drilling-based operations such as subsea drilling (Figure 9b) [94][95][96]. When a coat of hygroscopic polymers is done over the FBG-based structures, these structures can then be utilized as an H sensor, since there is a mechanical expansion that happens inside the polymeric structure [97][98][99].
The FBG-based pressure sensors that have been developed can be utilized in finding the point of leakage in the pipelines by using the method of negative pressure wave (NPW). When a leak in a pipeline initially happens, there is a pressure drop that is induced, and it travels in either direction from the point of leakage. The NPW reaches the FBG pressure sensors that are fixed on the pipe, which records the time taken for the wave to reach it, and this information is utilized to evaluate the point of leakage [91,92]. It has been found that the strain sensors based on FBG are attached directly to the pipeline surface in order to evaluate the strain of the refurbished pipes [93]. In field tests, the installation of sensors based on FBG are done over the risers for watching the riser stress during drilling-based operations such as subsea drilling (Figure 9b) [94][95][96]. When a coat of hygroscopic polymers is done over the FBG-based structures, these structures can then be utilized as an H sensor, since there is a mechanical expansion that happens inside the polymeric structure [97][98][99].

Distributed OFS for Physical Sensing
Brillouin scattering is a kind of an inelastic scattering that happens due to acoustic waves generated by the lattice vibrations. This kind of scattering is also extremely sensitive to the temperature and deformation value of the optical fiber. Another kind of inelastic scattering is the Raman scattering, which happens due to the exchange of energy with the molecular vibrations of the fiber. Technologies such as distributed temperature (DTS), distributed strain (DSS), etc. have grown massively in previous decades [100], and thus, their adaptation for monitoring the corrosion and structural health has been carried out in the oil and gas industry. Apart from monitoring the temperature, strain, etc. during operations such as well logging [101], the utilization of distributed OFS has been done in order to monitor various physical parameters that are associated with corrosion, failure, and leak detection.

Distributed OFS for Physical Sensing
Brillouin scattering is a kind of an inelastic scattering that happens due to acoustic waves generated by the lattice vibrations. This kind of scattering is also extremely sensitive to the temperature and deformation value of the optical fiber. Another kind of inelastic scattering is the Raman scattering, which happens due to the exchange of energy with the molecular vibrations of the fiber. Technologies such as distributed temperature (DTS), distributed strain (DSS), etc. have grown massively in previous decades [100], and thus, their adaptation for monitoring the corrosion and structural health has been carried out in the oil and gas industry. Apart from monitoring the temperature, strain, etc. during operations such as well logging [101], the utilization of distributed OFS has been done in order to monitor various physical parameters that are associated with corrosion, failure, and leak detection.
The thermal properties of the fluids flowing inside a pipeline are the ones to detect any kind of leakage in the pipeline, and DTS is largely used for this purpose. One of the methods to lower the viscosity of the flowing fluids, such as crude oil, is heating transportation for a more efficient flow of the crude in the pipes. Any leakage of these hot oils would lead to a considerable amount of temperature change on the outer surface of the pipelines, which is easily detectable with the help of DTS [102]. It can be explained from the Joule-Thomson effect that when a gas exhibiting higher values of pressure leaks, there is a reduction in the resultant temperature. The leakage of liquids leads to an increment in the temperature due to which the DTS detects a leakage in the pipelines [101].
The DAS that makes use of coherent Rayleigh backscattering was studied to see if they are capable of detecting vibrations in a pipeline that are induced due to leakage. The investigation was carried out with the help of optical fibers, which are wound over the pipes in a helical fashion ( Figure 10) [103,104].  The thermal properties of the fluids flowing inside a pipeline are the ones to detect any kind of leakage in the pipeline, and DTS is largely used for this purpose. One of the methods to lower the viscosity of the flowing fluids, such as crude oil, is heating transportation for a more efficient flow of the crude in the pipes. Any leakage of these hot oils would lead to a considerable amount of temperature change on the outer surface of the pipelines, which is easily detectable with the help of DTS [102]. It can be explained from the Joule-Thomson effect that when a gas exhibiting higher values of pressure leaks, there is a reduction in the resultant temperature. The leakage of liquids leads to an increment in the temperature due to which the DTS detects a leakage in the pipelines [101].
The DAS that makes use of coherent Rayleigh backscattering was studied to see if they are capable of detecting vibrations in a pipeline that are induced due to leakage. The investigation was carried out with the help of optical fibers, which are wound over the pipes in a helical fashion ( Figure 10)

Distributed OFS for Physical Sensing
Brillouin scattering is a kind of an inelastic scattering that happens due to acoustic waves generated by the lattice vibrations. This kind of scattering is also extremely sensitive to the temperature and deformation value of the optical fiber. Another kind of inelastic scattering is the Raman scattering, which happens due to the exchange of energy with the molecular vibrations of the fiber. Technologies such as distributed temperature (DTS), distributed strain (DSS), etc. have grown massively in previous decades [100], and thus, their adaptation for monitoring the corrosion and structural health has been carried out in the oil and gas industry. Apart from monitoring the temperature, strain, etc. during operations such as well logging [101], the utilization of distributed OFS has been done in order to monitor various physical parameters that are associated with corrosion, failure, and leak detection.
The thermal properties of the fluids flowing inside a pipeline are the ones to detect any kind of leakage in the pipeline, and DTS is largely used for this purpose. One of the methods to lower the viscosity of the flowing fluids, such as crude oil, is heating transportation for a more efficient flow of the crude in the pipes. Any leakage of these hot oils would lead to a considerable amount of temperature change on the outer surface of the pipelines, which is easily detectable with the help of DTS [102]. It can be explained from the Joule-Thomson effect that when a gas exhibiting higher values of pressure leaks, there is a reduction in the resultant temperature. The leakage of liquids leads to an increment in the temperature due to which the DTS detects a leakage in the pipelines [101].
The DAS that makes use of coherent Rayleigh backscattering was studied to see if they are capable of detecting vibrations in a pipeline that are induced due to leakage. The investigation was carried out with the help of optical fibers, which are wound over the pipes in a helical fashion ( Figure 10) [103,104].

Distributed OFS for Chemical Sensing
Using sensors outside the pipelines to identify and evaluate the early signs of corrosion after it has occurred and once the structural integrity has worsened is the most appropriate method. A less advanced version of sensors when compared to DTS, DSS, and DAS are DCS (distributed chemical sensing), which demonstrates a remarkable capability by tracking corrosive conditions to promote corrosion prevention before or at the initial diagnosis of corrosion. The feasibility of sensors made up of optical fibers is achieved by activating the operational chemical coating (polymers, metal-organic frameworks, nanomaterials, metal-oxide films, etc.) over their core or cladding. To enable the association of light with these chemically sensible layers over the core or cladding and with the external medium [105][106][107], the optical fibers may be carved, trimmed, stitched, or edge-polished.
Since its emergence in the 1990s, a special class of fiber structure for OFS with tremendous capability for DCS has been provided by micro-structured optical fibers [108][109][110], which features the air holes extending along the whole length of the fiber parallel to the longitudinal axis known as photonic crystal fibers (PCF) [108][109][110][111], unless the air holes are regularly organized within the cladding matrix. Figure 11 shows the internal and external look of an evanescent field-based semiconductor [112].
Appl. Syst. Innov. 2021, 4, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/asi propriate method. A less advanced version of sensors when compared to DTS, DSS, and DAS are DCS (distributed chemical sensing), which demonstrates a remarkable capability by tracking corrosive conditions to promote corrosion prevention before or at the initial diagnosis of corrosion. The feasibility of sensors made up of optical fibers is achieved by activating the operational chemical coating (polymers, metal-organic frameworks, nanomaterials, metal-oxide films, etc.) over their core or cladding. To enable the association of light with these chemically sensible layers over the core or cladding and with the external medium [105][106][107], the optical fibers may be carved, trimmed, stitched, or edge-polished. Since its emergence in the 1990s, a special class of fiber structure for OFS with tremendous capability for DCS has been provided by micro-structured optical fibers [108][109][110], which features the air holes extending along the whole length of the fiber parallel to the longitudinal axis known as photonic crystal fibers (PCF) [108][109][110][111], unless the air holes are regularly organized within the cladding matrix. Figure 11 shows the internal and external look of an evanescent field-based semiconductor [112]. While microstructured fibers provide highly sensitive and versatile fiber structures, large production and marketing are still limited, demanding the cost-effective long-distance production of such fibers. By the direct association of light with gas in holes, few PCF sensors (index guided and hollow-core) were observed to track highly sensitive gases such as methane, hydrogen sulfide, carbon dioxide, and acetylene (ppm level) [113][114][115][116][117]. Specific detection for Cl and humidity control with a Cl − sensitive material that shows fluorescent behavior filled in the gaps; a suspended-core fiber PCF sensor has been developed [118]. The use of proxy materials incorporated with the dispersed OFS network to specifically track corrosion as a decentralized optical coupon and provide information about them is a new idea for corrosion diagnosis, which is planned to be mounted along the inner surface of the pipelines to control internal corrosion. For decentralized corrosion control, while examined using OFDR, metallic film-coated optical fibers have been presented where the loss of mass is tracked on the basis of the change in intensity or change in strain along with the optical fiber [71,119,120] (Figure 12). In the corroded area, the light intensity and the strain increase due to the release of compressive internal stress followed by electrodeless deposition of Ni film as the light absorption of the metallic film reduces and the film becomes thinner. Decentralized monitoring of important environmental variables such as quality of water, the conductivity of the electrolyte, and acidic gases (carbon dioxide and hydrogen sulfide) will assess the corrosivity of the atmosphere and thus track corrosion indirectly. While only a few research studies have examined DCS [119,[121][122][123], While microstructured fibers provide highly sensitive and versatile fiber structures, large production and marketing are still limited, demanding the cost-effective long-distance production of such fibers. By the direct association of light with gas in holes, few PCF sensors (index guided and hollow-core) were observed to track highly sensitive gases such as methane, hydrogen sulfide, carbon dioxide, and acetylene (ppm level) [113][114][115][116][117]. Specific detection for Cl and humidity control with a Cl − sensitive material that shows fluorescent behavior filled in the gaps; a suspended-core fiber PCF sensor has been developed [118]. The use of proxy materials incorporated with the dispersed OFS network to specifically track corrosion as a decentralized optical coupon and provide information about them is a new idea for corrosion diagnosis, which is planned to be mounted along the inner surface of the pipelines to control internal corrosion. For decentralized corrosion control, while examined using OFDR, metallic film-coated optical fibers have been presented where the loss of mass is tracked on the basis of the change in intensity or change in strain along with the optical fiber [71,119,120] (Figure 12). In the corroded area, the light intensity and the strain increase due to the release of compressive internal stress followed by electrodeless deposition of Ni film as the light absorption of the metallic film reduces and the film becomes thinner. Decentralized monitoring of important environmental variables such as quality of water, the conductivity of the electrolyte, and acidic gases (carbon dioxide and hydrogen sulfide) will assess the corrosivity of the atmosphere and thus track corrosion indirectly. While only a few research studies have examined DCS [119,[121][122][123], chemical sensing materials are being studied for a wider variety of optical fiber applications, which could theoretically be used for the production of DCS to track corrosive external factors. For compatibility with OFS, there are a number of pH-sensing materials, including localized surface plasmon resonance (LSPR) Au or Ag nanoparticles (NP) integrated composites (Figure 13a) [124][125][126], organic dyes [127][128][129][130][131][132], fluorescent molecules [133][134][135][136][137], polymers [138][139][140][141], pH-sensitive hydrogel [142][143][144][145], etc. The pH value of the solution independent of the material embedded in the matrix is observed to be more integrated with the silica matrix coating [124,146]. For more information regarding the sensitive pH products and optical fiber pH sensors (Figure 14), refer to these references [145,147,148]. To measure the emergence of water, ion intensity, and temperature, graphene oxide coatings and polymers have been tested, and at the same time, with no covering by measuring phase transition in all ways (Figure 13b), a multi-sensor OFS has been designed that can also help in monitoring the internal corrosion within natural gas pipelines [107]. In the case of acid gas detection, salinity tracking, gas-sensitive coatings, or gas absorption sheets are also used in OFS, which are based on the changes in the refractive index using tapered optical fibers [118,[149][150][151]. MOFs that can absorb CO 2 have shown rapid, reversible, and promising results for monitoring CO 2 . Along with the employment of pH, indicators are a must in case of CO 2 monitoring, as it can reduce solution pH value. Some examples of OFS chemical sensing layers for corrosivity control are mentioned in Table 2.
Appl. Syst. Innov. 2021, 4, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/asi including localized surface plasmon resonance (LSPR) Au or Ag nanoparticles (NP) integrated composites (Figure 13a) [124][125][126], organic dyes [127][128][129][130][131][132], fluorescent molecules [133][134][135][136][137], polymers [138][139][140][141], pH-sensitive hydrogel [142][143][144][145], etc. The pH value of the solution independent of the material embedded in the matrix is observed to be more integrated with the silica matrix coating [124,146]. For more information regarding the sensitive pH products and optical fiber pH sensors (Figure 14), refer to these references [145,147,148]. To measure the emergence of water, ion intensity, and temperature, graphene oxide coatings and polymers have been tested, and at the same time, with no covering by measuring phase transition in all ways (Figure 13b), a multi-sensor OFS has been designed that can also help in monitoring the internal corrosion within natural gas pipelines [107]. In the case of acid gas detection, salinity tracking, gas-sensitive coatings, or gas absorption sheets are also used in OFS, which are based on the changes in the refractive index using tapered optical fibers [118,[149][150][151]. MOFs that can absorb CO2 have shown rapid, reversible, and promising results for monitoring CO2. Along with the employment of pH, indicators are a must in case of CO2 monitoring, as it can reduce solution pH value. Some examples of OFS chemical sensing layers for corrosivity control are mentioned in Table 2.      For oil and gas pipelines as well as minor cracks detection for civil infrastructures In the absence of any temperature variation between the gas and the environment, the leaks can be detected The pulsed laser is used for methane detection as a lighting source for pipelines Methane detection light source for gas pipelines

Challenges of OFS Application in the O&G Industry
The extreme conditions that can be found in the oil and gas wel lbore pose a certain difficulty in the monitoring of the downhole. This occurs by the presence of carbon dioxide, hydrogen, and mechanical stresses. The OFS can be relied upon for downhole sensing but with temperatures as high as 300 °C and hydrogen ingress, the silica fibers are dangerously affected. The presence of water can be instrumental in this also, resulting in longterm instability [101]. The silica hydrogen bond with adsorption of hydroxyl can result in

Challenges of OFS Application in the O&G Industry
The extreme conditions that can be found in the oil and gas wel lbore pose a certain difficulty in the monitoring of the downhole. This occurs by the presence of carbon dioxide, hydrogen, and mechanical stresses. The OFS can be relied upon for downhole sensing but with temperatures as high as 300 • C and hydrogen ingress, the silica fibers are dangerously affected. The presence of water can be instrumental in this also, resulting in long-term instability [101]. The silica hydrogen bond with adsorption of hydroxyl can result in extrinsic attenuation. As a counter-action to this, a hermetic carbon layer can be added for protection of the silica, but this idea would also fail at the unusually high temperatures encountered, resulting in an increased importance of protective coatings [152,153]. The alternatives for silica include sapphire cladding, which is highly expensive. If the cost can be overlooked, it normally eliminates the problem and can still work at 1800 degrees. Another challenge is the transfer across long distances and across the hundreds of thousands of miles of pipeline infrastructure with high resolution. There is a common compromise between high-distance transmission and the resolution received. OFS aims to decrease this problem and lead to an optimized system [104,[154][155][156][157][158][159][160][161][162].

Supervisory Control and Data Acquisition (SCADA) and Content Management Systems (CMSs) for Health Monitoring
SCADA and CMSs are the two most recognized health monitoring systems and come very handy when offshore wind farms are considered. Being too distant from the mainland, these wind farms require constant monitoring. They monitor the elementary components and also take care of the difference in input factors including the [163] wind speed and outputs. It is ensured that the sampling frequency can be managed at 0.02 Hz at an interval of 10 min for monitoring. This small interval allows for analysis before it is too late to find the error that has occurred and the details such as the extent and location where it has happened. The program also provides real-time solutions or suggestions. Researchers also find the log data to be very useful in their work, which in turn creates an environment for the creation of even better systems of monitoring. Further studies provided an insight into the combination of different systems and SCADA monitoring to present even reliable combined machines. Notable are the uses of the neuro fuzzy system [164,165], Bayesian frameworks [166], and Yang's varying conditions [167]. Another important tool in the same line is the CMS, which has the capability to identify damages through vibrations at higher frequencies, normally 50 Hz or higher, and the response system collects data at an even increased frequency [168][169][170]. CMSWind and the ADAPT.wind system are the newer and modified forms of the traditional monitoring systems. The only logical next step was to combine these two major systems, SCADA and CMS, together. It is a huge task but could combine the best features of the two. Examples of this combination/integration are the Intrusion Detection System (IDS) and Wait Time Information System (WTIS) systems. A combined basic setup is depicted in Figure 15 below.

Health Monitoring System of Blades
The most important part of the wind energy generation and also the ones that can be most easily affected by damages are the blades of a wind turbine. They are made up of a wide range of elements that are combined together. They also need to maintain high flexibility, and thus, any damage that they face gets easily transmitted to the whole blade, causing them to wear easily. The damage spreads as cracks and delamination. The damage can be harmful because it can cause imbalance, which will not only disrupt the readings in general but also cause major catastrophic events leading to destruction of the wind farm.
Appl. Syst. Innov. 2021, 4, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/asi tem collects data at an even increased frequency [168][169][170]. CMSWind and the ADAPT.wind system are the newer and modified forms of the traditional monitoring systems. The only logical next step was to combine these two major systems, SCADA and CMS, together. It is a huge task but could combine the best features of the two. Examples of this combination/integration are the Intrusion Detection System (IDS) and Wait Time Information System (WTIS) systems. A combined basic setup is depicted in Figure 15 below.

Health Monitoring System of Blades
The most important part of the wind energy generation and also the ones that can be most easily affected by damages are the blades of a wind turbine. They are made up of a wide range of elements that are combined together. They also need to maintain high flexibility, and thus, any damage that they face gets easily transmitted to the whole blade, causing them to wear easily. The damage spreads as cracks and delamination. The damage can be harmful because it can cause imbalance, which will not only disrupt the readings in general but also cause major catastrophic events leading to destruction of the wind farm.
These are the reasons that make the monitoring of the damage of the blades so important. The first stage is to find out the way for perfect monitoring, which would include testing and analysis. Sutherland in 1994 analyzed the blade with non-destructive techniques: acoustic emission and the coherent optical technique. The technique for blade analysis continued including neural network systems and wireless sensor networks [171][172][173].
One of the major problems in creating monitoring systems for a blade is the dynamic conditions faced by blades. Static systems can in no way emulate the ever-changing conditions a blade has to face. It almost makes it necessary to have a monitoring system for the monitoring system itself. The wind harvester is an acoustic emission-based telemetric system, e.g., NEG-MICON NM 48/750, which can collect data for about half a year [174][175][176].
Furthermore, the wind turbine changes a lot of its properties when it faces damage, and thus, the damage receptor and interpreter will also have to be very adaptive while looking at the damage and thus inherently change the parameters for reading the damage These are the reasons that make the monitoring of the damage of the blades so important. The first stage is to find out the way for perfect monitoring, which would include testing and analysis. Sutherland in 1994 analyzed the blade with non-destructive techniques: acoustic emission and the coherent optical technique. The technique for blade analysis continued including neural network systems and wireless sensor networks [171][172][173].
One of the major problems in creating monitoring systems for a blade is the dynamic conditions faced by blades. Static systems can in no way emulate the ever-changing conditions a blade has to face. It almost makes it necessary to have a monitoring system for the monitoring system itself. The wind harvester is an acoustic emission-based telemetric system, e.g., NEG-MICON NM 48/750, which can collect data for about half a year [174][175][176].
Furthermore, the wind turbine changes a lot of its properties when it faces damage, and thus, the damage receptor and interpreter will also have to be very adaptive while looking at the damage and thus inherently change the parameters for reading the damage as the damage increases. Based on this observation, a system to adapt to changes was introduced known as the SMART Wind Turbine Rotor project, which integrated a number of condition-analyzing tools that could provide real-time analysis on the blade conditions; it was developed by the Sandia National Laboratory [177]. An even better version known as the fibrous Bragg grating sensor attracted attention due to being unaffected by the electromagnetic radiations and thunder; it also was more durable and required lesser maintenance [178]. Modified forms of the above ideas integrated into different materials and locations were utilized for better and more accurate results.
Thermal and electromagnetic technologies are also not lagging behind when it comes to monitoring. Apart from the mechanical methods, Ozbek has made use of strain gauges, photogrammetry, and laser interferometry to identify the blades according to several conditions tested at different speeds [179]. A laser thermographic system also became popular in being able to analyze the damage and simulate it in a different place [180,181].

Health Monitoring System of the Tower and Foundation
The problems and threats that the support pillars of the wind turbine face are pretty different from the ones that the blades face. The major reason is that the pillar has to bear a specific weight on the same place for a long time. It has to be robust to continue to carry the maximum weight without faltering, because even though its monitoring may require a different method, its safety is as important as that of the blade. If there is even a small imbalance in the support or a change in its stress-bearing capacity, it could lead to a serious flaw in the calculations and subsequently the wind energy productions. In addition, monitoring of the blades will be affected by a fault in the support.
The wind turbines that are used in off-shore applications were developed much later, and due to that reason, their monitoring systems are much more effective. This is because with older projects, the structural designs and aerodynamic properties could not be optimized to fit in a proper monitoring system. However, due to being built later with the idea of the advanced monitoring system already in the works, their design is good and accommodating. The SCADA system has been utilized, but it is not very effective when it comes to a decreased sampling frequency [182,183].
For overcoming this problem, the wireless network sensors were utilized that had the range of 50 to 150 Hz in three intervals for sampling purposes, which allowed total offline modal analysis of the towers. The SESHMS is a comparatively more economical system that was released as a further development. They have established small programmable Object-Oriented Technology using software to wirelessly transfer data to neighboring receiver nodes. It smartly used the DSL system for the study of dynamic data of displacement, temperature, acceleration, and wind speed during the same time period for even better monitoring and estimation of lifetime of the tower and foundation [184][185][186][187][188].
In latest trend is the smart wireless networks, which have different modules for identification, authentication, and evaluation. This provides information to detect the extent and location of the damage very precisely. The foundation and tower mostly act in the same way, and therefore, it has been observed that the newer technology develops a monitoring system to be able to synchronously detect the conditions of both the components. [189][190][191][192]. These discussed systems aim to determine the integrity of the tower and foundation of the wind turbine, as shown in Figure 15.

Issues of Concern and their Mitigation in Wind Turbines
Since the wind turbine blades are directly exposed to the harsh environment, they are vulnerable to failures. In these environments, wind loads vary constantly, and undergo cyclic fatigue loads under their own weight. Erosion and corrosion experienced by the wind turbines are the results of extreme temperature and humidity changes. Blade failures could also lead to a major economic loss. For example, €1.25 million was exhausted due to the significant downtime and repair costs attributed to the failure of blades in an onshore wind turbine at Dunbar, Scotland in 2005 [193]. The major modes of failure of wind turbine blades include failure of adhesive joints and load-carrying laminates being delaminated. Several non-destructive testing techniques are used to detect these failures. When triggered by the external forces, the vibrational analysis evaluates the dynamic response of blades and hence investigates their health condition [186]. If multiple vibration transducers are used in sequence, that can also be used to detect the damage locations, which is attributed to the reason that fiberglass composite material is used to manufacture the blades of the wind turbine [194]. For the purpose of commercial testing of WT blades, strain gauges are popularly used, which are made up of piezoelectric ceramic materials [195]. These gauges are attached to the surface of the blades for measuring the localized strain arising due to bending and stretching loads [194]. Another technique to monitor structural health monitoring is acoustic emission. When a crack is originated and propagates in the blade, acoustic emission perceives the energy of the elastic waves, which helps in its identification [196]. Other SHM techniques include TFRF, which is developed recently and is trustworthy in both faults as well as damage detection. Along the span direction of the blade, TFRF-based blade SHM measures the dynamic response of the blades. To fulfill this measurement, a distributed strain sensor and high-resolution stereo imaging camera are the two prospective tools, but they require more research for being applicable in the future [194] (Figure 16). gates in the blade, acoustic emission perceives the energy of the elastic waves, which helps in its identification [196]. Other SHM techniques include TFRF, which is developed recently and is trustworthy in both faults as well as damage detection. Along the span direction of the blade, TFRF-based blade SHM measures the dynamic response of the blades. To fulfill this measurement, a distributed strain sensor and high-resolution stereo imaging camera are the two prospective tools, but they require more research for being applicable in the future [194] (Figure 16). Figure 16. Health monitoring systems of wind turbines [185,189,190,192]. SESHMS: server health monitoring system; WSN: wireless sensor network; DSL: digital subscriber line; DAS: data acquisition system.

UAV Systems for Health Monitoring
The Functioning of UAV sensors and receivers and LIDAR data collected from RGIPT Amethi of pipe leakages are shown in Figure 17.  [185,189,190,192]. SESHMS: server health monitoring system; WSN: wireless sensor network; DSL: digital subscriber line; DAS: data acquisition system.

UAV Systems for Health Monitoring
The Functioning of UAV sensors and receivers and LIDAR data collected from RGIPT Amethi of pipe leakages are shown in Figure 17. Leakages in pipelines lead to massive losses; even a leak as small as 1% in a 20-inchthick pipeline can lead to a loss of more than 500,000 barrels of oil every year. For a solution to this, it is important that these leaks are detected at the right time. One of the few techniques is to use UAV for inspecting a pipeline; this is not only a detection method but Leakages in pipelines lead to massive losses; even a leak as small as 1% in a 20inch-thick pipeline can lead to a loss of more than 500,000 barrels of oil every year. For a solution to this, it is important that these leaks are detected at the right time. One of the few techniques is to use UAV for inspecting a pipeline; this is not only a detection method but is also a method to monitor the environmental conditions in the vicinity of the pipeline. The benefit with using UAVs is that they are capable of covering larger areas in a smaller amount of time as compared to manual inspection. This makes it a cost-effective and time-efficient detection method. The use of UAVs is not only limited to pipelines, and they are now being used in nuclear power plants, chemical plants, etc. Several toxic gases are processed each day in chemical plants across the world, and it is important that not even a pinch of such gases leak out into the atmosphere. Thus, UAVs come in handy, as they efficiently sense even the smallest amount of leakage through the sensors mounted on top of it, which sends signals to the controllers who react accordingly. A payload can be installed on various platforms, which is used to detect gas leakage, and the installation of payload can be seen as a possible solution to detect and then mitigate gas leakage. However, the solution provided by the payload installation is not just concealed to UAVs. In a research study, a UAV was operated at a height of about 40 m from the ground at a speed of 30 km/h for a time period of about 2 h, and the maximum weight of the payload was kept at 5 kg.
At first glance, it was found that only methane gas in the temperature range of 15 to 20 degrees Celsius was leaked out in a pipeline of pressure 1500 psi. To track muffled frequencies and waves, leaks, and different frequencies in metallic machine frameworks, industrial air, vacuum systems, and pressurized gas pipelines, a digital ultrasonic test platform known as SONOCHEK was designed. In particular, the leakage of pressurized air and gas equipment can be very expensive, because more electricity is to be extracted to account for the failure. The frequency of detection is inaudible to human beings and varies from 20 to 100 kHz. Nevertheless, they are easily sensed by SONOCHEK, which is integrated into a complete study, making them audible and noticeable when determining the leakage and assessing the overall amount of damage. To track the leakage problem and other abnormalities, SONOCHEK was designed with two applications: Sonoleak and Sonolevel. In a various range of product configurations, highly engineered airborne and framework-borne detectors and apparels are accessible. Airborne DBS10 sound detector with three separate horns detects and assesses the leakage, insulation damage, partial discharge in pipes and walls, etc. The framework-borne and temperature detector DBS20 with a magnet adapter and 150 mm waveguide (optional) helps in monitoring the ultrasound waves generated by spinning machine parts e.g., gears, tracking of flow conditions. Parabolic detectors help in tracking leakage up to 25 m. A DBT10 ultrasonic emitter can be used to conduct air infiltration tests on windows, doors, and containers. Sometimes, it is used in the building projects of ships and airplanes, rail, and road means of transport. The signal amplitude can be changed based on the sound quality of the environment.

Concluding Remarks
The energy industry uses various infrastructures such as reactor, tank, pump, pipeline, plates, turbine, etc. Many of these parts are metallic and undergo wear and tear with time, as they carry fluids of different chemical composition as well as pass through various environmental conditions. The monitoring, repair, and maintenance of this infrastructure is extremely challenging and complex. Corrosion coupons, electrical resistance probes, etc. offer technologies to observe the damages and leaks through gadgets placed inside the conduits. They pass on certain signals and analyze the data to indicate any damage in the infrastructure or help to indicate the rate of corrosion. Generally, the techniques are average in nature and fail to precisely indicate the position of leak or damage. If it works in post processing mode, it fails to indicate the time and extent of leak. The limitation in resolutions for these techniques also restrict them to be used as predictors of damages for various parts of infrastructure. Acoustic emission or ultrasonic-type sensors offers better precision in terms of location and time of leaks; however, the technology is better applicable to see within. Point OFS, distributed OFS, etc. techniques are novel and can precisely locate the positions of leaks or damages from external positions. They also allow the prediction of damages in time ahead. However, many of these techniques fail to indicate the changes happening within. SCADA and CMS have come up with improved techniques to monitor the external damages at very high frequency. These data are required to be processed and communicated to quickly determine damages to external surfaces. These techniques are used to monitor the structural health of wind farms. UAV-based techniques have come up recently, as they offer the opportunity to scan the infrastructure externally from very close proximity. Various sensors (acoustic emission, thermal, laser, etc.) can be mounted on this airborne platform and used to determine the location of leaks or damages ( Table 2).
Over-ground pipelines, reactors, and tanks are easy to monitor; however, complexities increase for underground infrastructures. The extreme conditions that can be found in the oil and gas wellbore pose a certain difficulty in the monitoring of the downhole. This occurs by the presence of carbon dioxide, hydrogen, and mechanical stresses. The OFS can be relied upon for downhole sensing, but with temperatures as high as 300 degrees and hydrogen ingress, the silica fibers are dangerously affected. In addition, instrumental in this can be the presence of water, resulting in long-term instability [101]. The silica hydrogen bond with the adsorption of hydroxyl can result in extrinsic attenuation. As a counter-action to this, a hermetic carbon layer can be added for protection of the silica, but this idea would also fail at the unusually high temperatures encountered, resulting in increased importance of protective coatings [152,153]. The alternatives for silica include sapphire cladding, which is highly expensive. If the cost can be overlooked, it normally eliminates problem and can still work at 1800 degrees. Another challenge is the transfer across long distances and across the hundreds of thousands of miles of pipeline infrastructure with high resolution. There is a common compromise between high-distance transmission and the resolution received. OFS aims to decrease this problem and lead to an optimized [104,[154][155][156][157][158][159][160][161][162].
Acoustic, OFS, and UAV-based techniques offer certain opportunities along with some challenges. It is not possible to monitor the deformations from inside as well as outside using one technology. It is required to place a sensor inside to get an early indication for impending damages or leaks expected in the future. It is required to communicate all the information of deviation or deformation in real time so that the data can be processed and additional or detailed monitoring of leaks (and damage controlling measures) can be instigated instantaneously. A hybrid UAV-OFS-acoustic emission-based monitoring technique can be designed to meet these demands and set up a comprehensive solution for SHM in the energy sector.