# Inspection of Coated Hydrogen Transportation Pipelines
## 3 Pipeline Inspection
### Electromagnetic Acoustic Transducer (EMAT)
\begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**NDT Method** & **Advantages** & **Disadvantages** & **Applicable to ILI** & **ECM** \\ \hline \multirow{3}{*}{UT} & “High penetration depth” and can measure internal and external coating as well as wall thickness [15]. Also allows estimation of external corrosion [15]. Has a higher confidence level than MFL [11]. & Requires liquid coupling between transducer and pipeline, making it difficult to conduct ILI with gas pipelines [11]. UT signals also need to be de-noised to coordinate level than MFL [11]. & \(\checkmark\) & \(\checkmark\) \\ \hline \multirow{3}{*}{MW} & Able to detect and image defects in metals under dielectric coatings [76]. Changes in the thickness of coatings can also be monitored [75]. & Current methods require significant interpretation of data, display unclear “defect geometry”, and have a low “spatial resolution” [76]. & \(\checkmark\) \\ \hline \multirow{3}{*}{MFL} & The required resolution can be chosen by varying parameters such as sensor spacing [11]. & Only works well in easily magnetised metals and may cause the pipe to become magnetised indefinitely resulting in product flow restrictions [11, 15]. & \(\checkmark\) \\ \hline \multirow{3}{*}{EMAT} & No need for couplant, making it ideal for gas pipelines [15]. Literature provides validation of EMAT for identifying and sizing SCC cracks and corrosion in gas pipelines [11]. & The transducer needs to be less than 1 mm from the specimen and even so, its “detection ability and efficiency” is lower compared to UT [11]. This could present a challenge for coated pipelines. & \(\checkmark\) \\ \hline \multirow{3}{*}{GWT} & Not necessary to remove pipeline coating across the entire area inspection [71]. Useful as an initial screening method and for SHM [71, 74]. & Unable to detect defects oriented along the length of pipe and there is a trade-off between range and resolution [71]. & \(\checkmark\) \\ \hline \multirow{3}{*}{EC} & Responsive to a variety of parameters and usable over a wider temperature range [15]. It is also lightweight and cheap to deploy [15]. & Can only be used on materials that conduct electricity, is highly dependent on “lift-off distance” and is unable to detect “external defects” through the pipeline wall [11, 15]. & \(\checkmark\) \\ \hline \multirow{3}{*}{RT} & No preparation of the surface is necessary and the insulation does not have to be removed before inspection [15]. & Potential danger from radiation to living things nearby [15]. & \(\checkmark\) \\ \hline \multirow{3}{*}{ECIT} & Non-contact and capable of “sub-millimetre/millimetre crack” detection [77]. & Possibly necessary to paint the specimen to improve its “optical properties” and obtain better sensitivity [77]. & \\ \hline EMI & Sensitive to localised corrosion and suitable for SHM pipes [79]. & Currently only demonstrated to work for stainless steel plates [79]. & \\ \hline \multirow{3}{*}{IRT} & Could be incorporated with technology such as drones to quickly inspect large sections of the pipeline for CUI [82]. & Ability to detect CUI depends on the temperature of the pipeline and how close water is located to the surface [81]. & \(\checkmark\) \\ \hline \multirow{3}{*}{MBN} & Able to detect microstructure and stress present in materials quickly without any harm to the operator [15]. & Challenging to find “a consistent behaviour of the MBN signal”. The signal “can only be picked up near the surface of the materials” [15]. & \\ \hline \multirow{3}{*}{VT} & Operation is cheap and simple [15]. Could potentially be automated [84]. & Highly dependent on the operator and only suitable for defects on the surface [15]. & \(\checkmark\) & \(\checkmark\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: NDT methods and a comparison of their advantages and disadvantages in the context of coated pipelines. hydrogen pipelines. Separately, it was suggested that UT can be used to determine the health of a polymer internal coating by detecting \"removed or improper coating\" [85].