# Inspection of Coated Hydrogen Transportation Pipelines
## 3 Pipeline Inspection
### Defect Detection and Identification
#### 3.2.1 Non-Destructive Evaluation (NDE)  
We first consider the NDE methods available and how they work. Integrity management processes propose the need for a combination of periodic in-line inspections and external corrosion monitoring [8]. Before we look into these monitoring methods, current NDE methods are described. Table 6 then summarises and compares the advantages and disadvantages of these methods and whether they can be applied in ILI and/or external corrosion monitoring (ECM).  
Ultrasonic Testing (UT)  
Ultrasonic inspection encompasses the use of longitudinal sound waves ranging from 0.5 MHz to 25 MHz to detect defects in materials, though these longitudinal waves may undergo mode conversion upon interacting with defects [71]. The choice of frequency ensures that the wavelengths are of approximately the same size as the cracks or defects one may expect to find in the material [71]. Ultrasound is typically generated through the deformation of a piezoelectric crystal when an alternating current is applied to it [71]. There are a variety of techniques specifically used for inspecting pipelines:  
Phased Array UT (PAUT)  
A single crystal yields an \"unfocused, divergent\" beam of ultrasound along a straight line [71]. With multiple crystals, in an array, the time delay can be applied to each crystal to change the wave direction and inspect a larger area [15; 71].  
Time of Flight Diffraction (TOFD)  
Time of flight diffraction or TOFD refers to when \"two ultrasound arrays are used separately as a transmitter and receiver [71]\". The receiver measures both the \"effects of wave diffraction due to the edges/tips of defects in the pipe material\"and the reflection of the wave [71].  
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**NDE** & **SHM** \\ \hline Use non-destructive testing (NDT) to determine & condition of a structure \\ Using readings taken by removable transducers and instruments to assess the integrity of structures & Readings are usually taken by permanently attached instruments and transducers \\ Usually conducted when machines are not in operation and measurements are irregular & Regular measurement during operation \\ \hline \hline \end{tabular}
\end{table}
Table 5: A comparison of NDE and SHM.  
Guided Wave Testing (GWT)  
Guided waves testing (GWT) is useful when spot-testing is not necessary and the pipeline coating is challenging to remove [71]. It has a much longer range of up to 100 m compared to spot testing, but there is a trade-off in terms of the resolution [71]. This is due to the usage of lower frequency (10 MHz to 100 KHz) waves, that are able to travel further but are less effective in interacting with smaller defects [71]. There are also other drawbacks to this method, one being that it is not able to detect defects parallel to the length of the pipe [71]. But when considered as an initial screening method to determine where a follow-up is necessary with more precise methods, GWT has significant utility [71]. [PERSON] et al. [72] also validated the use of GWT in a SHM system.  
It is also suggested that GWT could be specifically used to detect corrosion at support sites along the pipelines [73]. It was further demonstrated that after implementing baseline subtraction followed by coherent averaging, the reliable detection of a 2 mm by 1 mm notch one pipe diameter away from the transducers was achieved in a 6-inch diameter steel pipe [74].