**1. Introduction**

Petroleum oil refining is an essential industry and an important element of the economic infrastructure. Refineries are large compared to other industrial plants because their production and storage capacities are designed to assure volume profitability. The industry deals with considerable amounts of flammable and toxic substances and is thus inherently hazardous. If loss of containment is not prevented or controlled, it can have serious economic and environmental consequences. The reduction of accidents is driving the development of better control technologies and risk management strategies. Corrosion remains one of the challenges which is further elevated because of ageing infrastructure and variation in concentration of crude oil.

According to a report from eMARS (Major Accident Reporting System) [1], corrosion of equipment is an important source of accidents in refineries, being responsible for one in five major refinery accidents occurring in the EU since 1984. The magnitude of a refinery unit and the complexity of the processes are great and a wide variety of equipment such as trays, drums and towers are subject to corrosion problems. The pipeline infrastructure and storage tanks are particularly vulnerable

and have high risk profiles due to the volumes they may contain. The same report analysed 99 corrosion failures, 71% of them originated in pipe works and 15% of them occurred in storage tanks.

Pipelines serve as basic components of refinery infrastructure as well as the chief transmission line between refineries and remote sites delivering the products to distribution points and customers. They are generally constructed from a variant of carbon steel and so are naturally susceptible to corrosion. The intense temperatures and temperature fluctuations, and presence of corrosive agents can accelerate the corrosion process. Corrosion can cause oil leaks which may lead to explosion with severe consequences. One example is an underground oil pipeline operated by Sinopec, China's largest oil refiner [2], which exploded following an oil leak due to corrosion. The blast killed 44 people and injured 136, and led to disruption in electricity and water supply and evacuation of around 18,000 people.

Failure of storage tanks is not as prevalent as pipe work failures but due to the hazardous substances stored, they are well represented in major accidents in the process industries. Storage tanks are extensively used in refineries to store fossil fuel, acids, solvents, benzene, sour water, asphalt and related products (heated storage). Both types of storage tanks are vulnerable to corrosion. Crude oil storage tanks suffer more aggressive corrosion compared to other refinery equipment due to the oil sulphur content. Another study on storage tank accidents [3] showed that 74% of accidents involving them occurred in Petro-chemical refineries with 85% of the accidents leading to fire and explosions. One such incident happened at a fuel storage facility in Brazil in 2015 [4] which took more than 4 days to bring under control with 110 firefighters, road blockages and the shut-down of ports (**Figure 1**).

Over the years, numerous non-destructive testing (NDT) techniques have been used to inspect the condition of pipelines and storage tanks, e.g. penetrant testing, magnetic particle testing, radiography, eddy current, thermography, acoustic emission and conventional ultrasonic testing [5]. Many of these techniques only offer localised inspection. Pipe inspection using these techniques requires removal of insulation to access pipe surfaces and may even require erection of scaffolding for difficult-to-access locations. For storage tanks, exterior corrosion, whether general or localised at crevices, is easy to detect using the aforementioned inspection methods. But for inspection of internal tank floors from exposure to corrosive agents in the product, requires the tank to be emptied and cleaned to gain access. These operations are both time-consuming and expensive and cannot be used in-service.

#### **Figure 1.**

*The damage from (a) oil leakage of a corroded buried pipeline in China [2] and (b) tank at a fuel storage facility in Brazil [4], which led to explosions with severe consequences and put human in danger.*

**57**

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves*

Less than rigorous inspection is considered a major cause of corrosion failure [1]. For this reason, there has been increased emphasis on the development of damage prognosis systems that inform the operator of a structure's health and of any developing damage. This will enable accurate estimation of the remaining useful life of the structures and can transform maintenance procedures from schedule-driven to condition-based implementation. These systems will significantly decrease the time these structures are offline, hence cutting life-cycle costs and labour requirements. Structural Health Monitoring (SHM) serves an essential part of any damage prognosis system. It monitors the structures whilst they are in-service and provides

The integration of Guided Wave Testing (GWT) technology into SHM is growing rapidly as it offers a remote solution with the ability to screen large structures. This chapter will detail the advances in SHM technologies using GWT for the two most critical metallic components in the Oil & Gas industry: pipelines and storage tanks. A brief description of GWT and the underlying physics of Ultrasonic Guided Waves (UGW) for tubular and plate like structures is provided. Its application to SHM of pipelines and storage tanks is described and the state-of-the-art in the enabling technologies including transducers and their coupling (transducer system) and data processing is presented. The design, operation and performance of SHM devices for pipelines and storage tanks are presented, and their current limitations

Much research has been conducted on the use of UGWs to inspect elongated engineering structures, i.e. pipes, plates, rails and cables, because of their inherent long range propagation [6]. Commercial GWT systems have evolved vastly over the past two decades to fulfil many industrial inspection requirements. For pipes, initial realisation of UGW propagation in cylindrical structures by Gazis et al. [7], Zemanek [8] and Silk and Bainton [9], led to initial development of a GWT system [10–12] for pipes which were commercialised [13, 14] and rapidly adopted by the Oil and Gas industry. Worlton [15] and Viktorov [16] originally explored the potential of UGW for NDT of plate-like structures. Based on this, Mažeika et al.

Rayleigh waves [16] are surface waves that exist in half-space, a surface backed by a semi-infinite volume. These waves have an elliptical vibration with the major axis of vibration perpendicular to the direction of propagation. They can penetrate to a depth of 1.5λ below the surface. In contrast, Lamb waves fill the entire volume of the plate provided its thickness is less than 2λ. These waves were first analysed on plates by Horace Lamb [18] and can be considered as Rayleigh waves bounded by two parallel surfaces. In plates, there are three fundamental wave modes in the operating frequency range for GWT: namely, the fundamental Symmetric Lamb mode, S0, the Asymmetric Lamb mode, A0, and the Shear Horizontal (SH) mode,

Just like plates, hollow cylindrical tubes also have a thin cross section bounded by two surfaces. Lamb wave theory of plates assumes an infinite plate extent, whereas in cylinders, the circumferential curvature results in a periodic boundary condition in one dimension. This increases the complexity of Lamb waves in tubes, and many more modes of wave propagation occur in tubes than in plates. In pipes, three

are highlighted to direct future research and development activities.

**2. Background of guided wave technology**

[17] studied the potential for GWT of tank floors.

**2.1 Ultrasonic guided waves**

SH0, as illustrated in **Figure 2**.

*DOI: http://dx.doi.org/10.5772/intechopen.83366*

information about any detected damage.

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves DOI: http://dx.doi.org/10.5772/intechopen.83366*

Less than rigorous inspection is considered a major cause of corrosion failure [1]. For this reason, there has been increased emphasis on the development of damage prognosis systems that inform the operator of a structure's health and of any developing damage. This will enable accurate estimation of the remaining useful life of the structures and can transform maintenance procedures from schedule-driven to condition-based implementation. These systems will significantly decrease the time these structures are offline, hence cutting life-cycle costs and labour requirements. Structural Health Monitoring (SHM) serves an essential part of any damage prognosis system. It monitors the structures whilst they are in-service and provides information about any detected damage.

The integration of Guided Wave Testing (GWT) technology into SHM is growing rapidly as it offers a remote solution with the ability to screen large structures. This chapter will detail the advances in SHM technologies using GWT for the two most critical metallic components in the Oil & Gas industry: pipelines and storage tanks. A brief description of GWT and the underlying physics of Ultrasonic Guided Waves (UGW) for tubular and plate like structures is provided. Its application to SHM of pipelines and storage tanks is described and the state-of-the-art in the enabling technologies including transducers and their coupling (transducer system) and data processing is presented. The design, operation and performance of SHM devices for pipelines and storage tanks are presented, and their current limitations are highlighted to direct future research and development activities.

## **2. Background of guided wave technology**

Much research has been conducted on the use of UGWs to inspect elongated engineering structures, i.e. pipes, plates, rails and cables, because of their inherent long range propagation [6]. Commercial GWT systems have evolved vastly over the past two decades to fulfil many industrial inspection requirements. For pipes, initial realisation of UGW propagation in cylindrical structures by Gazis et al. [7], Zemanek [8] and Silk and Bainton [9], led to initial development of a GWT system [10–12] for pipes which were commercialised [13, 14] and rapidly adopted by the Oil and Gas industry. Worlton [15] and Viktorov [16] originally explored the potential of UGW for NDT of plate-like structures. Based on this, Mažeika et al. [17] studied the potential for GWT of tank floors.

#### **2.1 Ultrasonic guided waves**

Rayleigh waves [16] are surface waves that exist in half-space, a surface backed by a semi-infinite volume. These waves have an elliptical vibration with the major axis of vibration perpendicular to the direction of propagation. They can penetrate to a depth of 1.5λ below the surface. In contrast, Lamb waves fill the entire volume of the plate provided its thickness is less than 2λ. These waves were first analysed on plates by Horace Lamb [18] and can be considered as Rayleigh waves bounded by two parallel surfaces. In plates, there are three fundamental wave modes in the operating frequency range for GWT: namely, the fundamental Symmetric Lamb mode, S0, the Asymmetric Lamb mode, A0, and the Shear Horizontal (SH) mode, SH0, as illustrated in **Figure 2**.

Just like plates, hollow cylindrical tubes also have a thin cross section bounded by two surfaces. Lamb wave theory of plates assumes an infinite plate extent, whereas in cylinders, the circumferential curvature results in a periodic boundary condition in one dimension. This increases the complexity of Lamb waves in tubes, and many more modes of wave propagation occur in tubes than in plates. In pipes, three

*Advances in Structural Health Monitoring*

them occurred in storage tanks.

sive and cannot be used in-service.

and have high risk profiles due to the volumes they may contain. The same report analysed 99 corrosion failures, 71% of them originated in pipe works and 15% of

Pipelines serve as basic components of refinery infrastructure as well as the chief transmission line between refineries and remote sites delivering the products to distribution points and customers. They are generally constructed from a variant of carbon steel and so are naturally susceptible to corrosion. The intense temperatures and temperature fluctuations, and presence of corrosive agents can accelerate the corrosion process. Corrosion can cause oil leaks which may lead to explosion with severe consequences. One example is an underground oil pipeline operated by Sinopec, China's largest oil refiner [2], which exploded following an oil leak due to corrosion. The blast killed 44 people and injured 136, and led to disruption in

Failure of storage tanks is not as prevalent as pipe work failures but due to the hazardous substances stored, they are well represented in major accidents in the process industries. Storage tanks are extensively used in refineries to store fossil fuel, acids, solvents, benzene, sour water, asphalt and related products (heated storage). Both types of storage tanks are vulnerable to corrosion. Crude oil storage tanks suffer more aggressive corrosion compared to other refinery equipment due to the oil sulphur content. Another study on storage tank accidents [3] showed that 74% of accidents involving them occurred in Petro-chemical refineries with 85% of the accidents leading to fire and explosions. One such incident happened at a fuel storage facility in Brazil in 2015 [4] which took more than 4 days to bring under control with 110 firefighters, road blockages and the shut-down of ports (**Figure 1**). Over the years, numerous non-destructive testing (NDT) techniques have been used to inspect the condition of pipelines and storage tanks, e.g. penetrant testing, magnetic particle testing, radiography, eddy current, thermography, acoustic emission and conventional ultrasonic testing [5]. Many of these techniques only offer localised inspection. Pipe inspection using these techniques requires removal of insulation to access pipe surfaces and may even require erection of scaffolding for difficult-to-access locations. For storage tanks, exterior corrosion, whether general or localised at crevices, is easy to detect using the aforementioned inspection methods. But for inspection of internal tank floors from exposure to corrosive agents in the product, requires the tank to be emptied and cleaned to gain access. These operations are both time-consuming and expen-

*The damage from (a) oil leakage of a corroded buried pipeline in China [2] and (b) tank at a fuel storage facility in Brazil [4], which led to explosions with severe consequences and put human in danger.*

electricity and water supply and evacuation of around 18,000 people.

**56**

**Figure 1.**

#### **Figure 2.**

*Displacement of the fundamental symmetric (S0) and asymmetric (A0) wave mode. Note the displacement from the line of symmetry (red dashed line).*

families of modes based on their displacement patterns are present. Axially symmetric wave modes—Longitudinal (L) and Torsional (T); and non-axially symmetric —Flexural (F) modes are illustrated in **Figure 3**. The L and T modes in cylindrical structures are analogous to the Lamb waves and SH modes of vibration in plates, respectively. The wave mode designation is defined by Meitzler [19] and includes two numbers, for example L(0,1), where the first number is the circumferential wavenumber (also known as the order) and the second number represents the sequential mode. All axially symmetric torsional and longitudinal modes are zero order modes. Flexural modes are non-axially symmetric and of order higher than zero.

Phase velocity (*vp*) and group velocity (*vg*) are two important terms in UGW propagation. *vp* is the speed at which a continuous wave propagates. For GWT, it is important to discriminate propagating wave modes by exciting them as a discrete wave pulse with a finite number of cycles. This pulse is controlled by a window function (e.g. hamming) which comprises a bandwidth of frequencies. The speed at which this envelope of discrete pulse propagates is *vg*. Variation of phase velocity with frequency leads to dispersion occurring as the UGW propagates in the structure.

At any given frequency, a number of wave modes may be present in the structure. The wave modes with frequency dependent velocities are called dispersive as they spread in space over time. Dispersion curves illustrate guided waves and their behaviour with frequency for each possible mode in the given structure. Commercial software packages [20, 21] are available to generate dispersion curves for multi-layered plates and cylindrical structures. **Figure 4** shows the dispersion curves computed for a 6 inch Schedule 40 pipe (168.3 mm outer diameter, 7.11 mm wall thickness) and a

#### **Figure 3.**

*Displacement of the axisymmetric L(0,1); L(0,2) and T(0,1) wave modes. Note the dominant radial, axial and circumferential displacements from the central axis, respectively.*

**59**

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves*

1 mm thick steel plate [material properties used, density (ρ)—7830 kg/m3

*Dispersion curves for 6 inch schedule 40 steel pipe (left) and 1 mm thick steel plate (right); showing the* 

For the pipe, axisymmetric L(0,1), T(0,1) and L(0,2) modes are highlighted and their respective associated flexural modes, F(n,1), F(n,2) and F(n,3) are coloured red. It should be noted that L(0,2) and T(0,1) in pipes are analogous to A0 and SH0 wave modes in plates. It can be seen that the T(0,1) wave mode is completely non-dispersive for all frequencies of interest for GWT as the phase velocity dispersion curve is flat. L(0,2) is relatively non-dispersive above a certain frequency and L(0,1) is relatively dispersive in comparison to the other two axisymmetric modes. Compared to a pipe, relatively low numbers of modes are present in plates, which makes mode separation and signal interpretation much less challenging. For GWT, it is desirable to use non-dispersive wave modes for easy data interpretation.

In contrast to conventional ultrasonic testing (UT), where high frequencies are used to examine the material directly under the test location, in GWT, low frequency ultrasound is guided through the structural boundaries and can travel tens of metres. A transducer can excite all modes that exist within its frequency bandwidth and this can complicate the received signals, making their interpretation difficult. Dispersion and the presence of multiple guided wave modes are the two main problems for GWT [22], and for practical applications, it is important for the transducer system to excite a single, non-dispersive wave mode [23]. A procedure for identifying suitable modes for a particular inspection task has been proposed by Wilcox [24] which considers the properties of the structure (dispersion, attenuation

and sensitivity) and transducer (excitability, detectability and selectivity). There are a number of different transduction technologies for excitation and detection of UGW, including Electromagnetic Acoustic Transducer (EMAT) [25], magnetostrictive devices [26], laser [27], piezoelectric and piezocomposite transducers [28]. Piezoelectric transducers offer the most promising solution due to their stability and reliability, and cost-effectiveness with simple and light-weight construction [29]. Lead zirconate titanate (PZT) has been a popular choice for UGW as it shows good electromechanical properties (electromechanical coupling, k > 0.7) which is essential to achieve large coverage. Linear and circular PZT arrays on plates have achieved inspection range of 3000 times the dimensions of the array. Application of PZT material is however limited to temperature below ~150°C (1/2 Tc) above which it experiences accelerated performance degradation over time [30]. Piezoelectric materials for SHM at higher temperatures are available [31, 32] for steamlines.

modulus (E)—207 GPa and Poisson's ratio (*μ*) = 0.3].

*relationship between group velocity and frequency for different modes.*

**2.2 Guided wave excitation**

**Figure 4.**

, Young's

*DOI: http://dx.doi.org/10.5772/intechopen.83366*

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves DOI: http://dx.doi.org/10.5772/intechopen.83366*

**Figure 4.**

*Advances in Structural Health Monitoring*

*from the line of symmetry (red dashed line).*

**Figure 2.**

families of modes based on their displacement patterns are present. Axially symmetric wave modes—Longitudinal (L) and Torsional (T); and non-axially symmetric —Flexural (F) modes are illustrated in **Figure 3**. The L and T modes in cylindrical structures are analogous to the Lamb waves and SH modes of vibration in plates, respectively. The wave mode designation is defined by Meitzler [19] and includes two numbers, for example L(0,1), where the first number is the circumferential wavenumber (also known as the order) and the second number represents the sequential mode. All axially symmetric torsional and longitudinal modes are zero order modes.

*Displacement of the fundamental symmetric (S0) and asymmetric (A0) wave mode. Note the displacement* 

Flexural modes are non-axially symmetric and of order higher than zero.

Phase velocity (*vp*) and group velocity (*vg*) are two important terms in UGW propagation. *vp* is the speed at which a continuous wave propagates. For GWT, it is important to discriminate propagating wave modes by exciting them as a discrete wave pulse with a finite number of cycles. This pulse is controlled by a window function (e.g. hamming) which comprises a bandwidth of frequencies. The speed at which this envelope of discrete pulse propagates is *vg*. Variation of phase velocity with frequency leads to dispersion occurring as the UGW propagates in the structure.

At any given frequency, a number of wave modes may be present in the structure. The wave modes with frequency dependent velocities are called dispersive as they spread in space over time. Dispersion curves illustrate guided waves and their behaviour with frequency for each possible mode in the given structure. Commercial software packages [20, 21] are available to generate dispersion curves for multi-layered plates and cylindrical structures. **Figure 4** shows the dispersion curves computed for a 6 inch Schedule 40 pipe (168.3 mm outer diameter, 7.11 mm wall thickness) and a

*Displacement of the axisymmetric L(0,1); L(0,2) and T(0,1) wave modes. Note the dominant radial, axial* 

*and circumferential displacements from the central axis, respectively.*

**58**

**Figure 3.**

*Dispersion curves for 6 inch schedule 40 steel pipe (left) and 1 mm thick steel plate (right); showing the relationship between group velocity and frequency for different modes.*

1 mm thick steel plate [material properties used, density (ρ)—7830 kg/m3 , Young's modulus (E)—207 GPa and Poisson's ratio (*μ*) = 0.3].

For the pipe, axisymmetric L(0,1), T(0,1) and L(0,2) modes are highlighted and their respective associated flexural modes, F(n,1), F(n,2) and F(n,3) are coloured red. It should be noted that L(0,2) and T(0,1) in pipes are analogous to A0 and SH0 wave modes in plates. It can be seen that the T(0,1) wave mode is completely non-dispersive for all frequencies of interest for GWT as the phase velocity dispersion curve is flat. L(0,2) is relatively non-dispersive above a certain frequency and L(0,1) is relatively dispersive in comparison to the other two axisymmetric modes. Compared to a pipe, relatively low numbers of modes are present in plates, which makes mode separation and signal interpretation much less challenging. For GWT, it is desirable to use non-dispersive wave modes for easy data interpretation.

#### **2.2 Guided wave excitation**

In contrast to conventional ultrasonic testing (UT), where high frequencies are used to examine the material directly under the test location, in GWT, low frequency ultrasound is guided through the structural boundaries and can travel tens of metres. A transducer can excite all modes that exist within its frequency bandwidth and this can complicate the received signals, making their interpretation difficult. Dispersion and the presence of multiple guided wave modes are the two main problems for GWT [22], and for practical applications, it is important for the transducer system to excite a single, non-dispersive wave mode [23]. A procedure for identifying suitable modes for a particular inspection task has been proposed by Wilcox [24] which considers the properties of the structure (dispersion, attenuation and sensitivity) and transducer (excitability, detectability and selectivity).

There are a number of different transduction technologies for excitation and detection of UGW, including Electromagnetic Acoustic Transducer (EMAT) [25], magnetostrictive devices [26], laser [27], piezoelectric and piezocomposite transducers [28]. Piezoelectric transducers offer the most promising solution due to their stability and reliability, and cost-effectiveness with simple and light-weight construction [29]. Lead zirconate titanate (PZT) has been a popular choice for UGW as it shows good electromechanical properties (electromechanical coupling, k > 0.7) which is essential to achieve large coverage. Linear and circular PZT arrays on plates have achieved inspection range of 3000 times the dimensions of the array. Application of PZT material is however limited to temperature below ~150°C (1/2 Tc) above which it experiences accelerated performance degradation over time [30]. Piezoelectric materials for SHM at higher temperatures are available [31, 32] for steamlines.

For pipes, excitation of axisymmetric wave modes [L(0,2) and T(0,1)] using piezoelectric transducers requires a circumferential ring of transducers. The circumferential spacing between the transducers in the array should be even for a high level of mode purity. All transducers in the ring are excited equally and concurrently to launch these axisymmetric modes. Apart from being non-dispersive, both of these modes provide uniform stress over the whole pipe cross section area and provide 100% coverage. Two rings of dry-coupled piezoelectric shear transducers [33] can be used to obtain unidirectional propagation of the L(0,2) mode with propagation distances approaching 50 metres. The second axisymmetric mode, L(0,1), is excited alongside L(0,2) (**Figure 5**), and can complicate the interpretation of results [34]. Therefore, an additional ring of transducers is required to suppress this undesired L(0,1) mode. This however adds to the cost of the system, significantly for larger diameter pipes. On the contrary, the T(0,1) mode is the only axisymmetric torsional mode in the frequency range of interest for GWT, so to obtain a single mode and unidirectional excitation, only two rings of transducers are required. The torsional mode requires an excitation force in the circumferential direction. This can be achieved by displacing the shear transducer used for axial longitudinal excitation by 90°. To cancel the propagation of non-axisymmetric Flexural modes, the number of transducers in a circumferential ring should be greater than the highest order of flexural mode present in the chosen frequency range [35].

For plates, the A0 Lamb mode is the easiest omnidirectional mode to excite as it only requires a point-source exerting a pure out-of-plane force on the surface of the plate. It is also the mode which has the smallest wavelength for a given frequency, therefore offering better resolution to defects compared to the S0 mode. However, due to the attenuation and higher dispersion characteristics, this mode has been predominantly neglected in favour of S0 and SH0. **Figure 6** shows the propagation of these three modes excited using uniaxial in-plane vibration.

Commercially available in-plane thickness shear transducers can generate all fundamental plate modes in the GWT operating frequency range. Both Lamb modes are generated in the axis of vibration while the SH0 mode is generated perpendicular to the axis of vibration.

#### **Figure 5.**

*Displacement patterns and waveforms generated by array of shear transducers aligned (a) circumferentially and (b) axially. U1, U2 and U3 represent radial, circumferential and axial displacement caused by transducer vibration measured using a 3D vibrometer.*

**61**

**Figure 7.**

*Architecture of a typical guided wave inspection system [36].*

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves*

A typical GWT architecture in **Figure 7** shows the key components of the system. Apart from the transducers, the system comprises of a portable computer (PC) to control the test, and a pulser-receiver connected to the transducers to transmit and receive the ultrasonic signal to and from the structure under test. Narrow band signals such as several cycles of sine wave modulated with a window function (e.g. hamming), are generally used. These narrow band signals offer good signal strength

*Propagation of ultrasonic guided wave modes in a 3.5 m diameter, 10 mm thick steel plate from a uniaxial* 

*DOI: http://dx.doi.org/10.5772/intechopen.83366*

**2.3 Guided wave inspection**

**Figure 6.**

*in-plane vibration.*

*Monitoring of Critical Metallic Assets in Oil and Gas Industry Using Ultrasonic Guided Waves DOI: http://dx.doi.org/10.5772/intechopen.83366*

#### **Figure 6.**

*Advances in Structural Health Monitoring*

For pipes, excitation of axisymmetric wave modes [L(0,2) and T(0,1)] using piezoelectric transducers requires a circumferential ring of transducers. The circumferential spacing between the transducers in the array should be even for a high level of mode purity. All transducers in the ring are excited equally and concurrently to launch these axisymmetric modes. Apart from being non-dispersive, both of these modes provide uniform stress over the whole pipe cross section area and provide 100% coverage. Two rings of dry-coupled piezoelectric shear transducers [33] can be used to obtain unidirectional propagation of the L(0,2) mode with propagation distances approaching 50 metres. The second axisymmetric mode, L(0,1), is excited alongside L(0,2) (**Figure 5**), and can complicate the interpretation of results [34]. Therefore, an additional ring of transducers is required to suppress this undesired L(0,1) mode. This however adds to the cost of the system, significantly for larger diameter pipes. On the contrary, the T(0,1) mode is the only axisymmetric torsional mode in the frequency range of interest for GWT, so to obtain a single mode and unidirectional excitation, only two rings of transducers are required. The torsional mode requires an excitation force in the circumferential direction. This can be achieved by displacing the shear transducer used for axial longitudinal excitation by 90°. To cancel the propagation of non-axisymmetric Flexural modes, the number of transducers in a circumferential ring should be greater than the highest order of flexural mode present in the chosen frequency range [35]. For plates, the A0 Lamb mode is the easiest omnidirectional mode to excite as it only requires a point-source exerting a pure out-of-plane force on the surface of the plate. It is also the mode which has the smallest wavelength for a given frequency, therefore offering better resolution to defects compared to the S0 mode. However, due to the attenuation and higher dispersion characteristics, this mode has been predominantly neglected in favour of S0 and SH0. **Figure 6** shows the propagation

of these three modes excited using uniaxial in-plane vibration.

to the axis of vibration.

Commercially available in-plane thickness shear transducers can generate all fundamental plate modes in the GWT operating frequency range. Both Lamb modes are generated in the axis of vibration while the SH0 mode is generated perpendicular

*Displacement patterns and waveforms generated by array of shear transducers aligned (a) circumferentially and (b) axially. U1, U2 and U3 represent radial, circumferential and axial displacement caused by transducer* 

**60**

**Figure 5.**

*vibration measured using a 3D vibrometer.*

*Propagation of ultrasonic guided wave modes in a 3.5 m diameter, 10 mm thick steel plate from a uniaxial in-plane vibration.*

#### **2.3 Guided wave inspection**

A typical GWT architecture in **Figure 7** shows the key components of the system. Apart from the transducers, the system comprises of a portable computer (PC) to control the test, and a pulser-receiver connected to the transducers to transmit and receive the ultrasonic signal to and from the structure under test. Narrow band signals such as several cycles of sine wave modulated with a window function (e.g. hamming), are generally used. These narrow band signals offer good signal strength

**Figure 7.** *Architecture of a typical guided wave inspection system [36].*

and avoid dispersion while propagating long distances. The centre frequency of these signals are chosen based on the desired wave mode to achieve low dispersion over the frequencies in the narrow band.

There are two modes of operation: pulse-echo and pitch-catch. Pulse-echo mode is more common and utilises the same transducers to excite the UGW and receive the reflected signals as illustrated in **Figure 7**. Pitch-catch mode uses two sets of transducers, one to excite the UGW and the other to receive, and is only used if high resolution or a high inspection range are required. As the UGW propagates in the structure, a proportion of the energy contained in the propagating wave front will be reflected when an acoustic impedance change occurs at a feature or discontinuity in the structure. This enables full coverage of the cross section of the plate or pipe, detecting and locating both internal and external defects without disrupting operation.

Since the initial developments of GWT of pipes in late 1990s, several studies have been carried out to understand the interaction of T(0,1) and L(0,2) guided wave modes with pipe features (flanges and pipe supports) [37] and defects [35], and the effect of different defect characteristics and excitation frequencies has also been reported [38, 39]. This has led to definitions and standards for GWT instrumentation, data collection and analysis in ISO 18211:2016 [40]. When an axisymmetric mode is incident on an axisymmetric pipe feature such as a uniform weld or a flange, axisymmetric modes are reflected. With a non-axisymmetric feature such as corrosion, a non-axisymmetric wave will also be reflected back to the transducer array. The presence and axial location of defects can thus be determined by analysing these reflections and their time of arrival. Although the L(0,2) mode has shown ~2.5 times more flaw sensitivity compared to T(0,1) [34], it is difficult to excite in pure form and requires complex signal processing due to its dispersive nature. It is also affected by fluid in the pipe, so the torsional mode is more commonly used in practice. GWT using T(0,1) is most effective on straight sections achieving several tens of metres of inspection range but recent studies have evaluated its performance on bends [41].
