# Inspection of Coated Hydrogen Transportation Pipelines
## 5 Novel Inspection Methods  
Some novel and upcoming inspection methods are summarised below:  
* A new method for validating GWT that \"enables the operator to combine the ability of FE analysis to predict the signals reflected from a large number of different defect cases with the complex geometric and environmental effects specific to the particular pipe structure which cannot be effectively simulated\" [72]. This method can be extended to methods other than GWT [72]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline \multicolumn{2}{c}{**Metallic Coatings**} & \multicolumn{1}{c}{**Non-Metallic Coatings**} & \multicolumn{1}{c}{**All Coatings**} \\ \hline \hline \multirow{6}{*}{Subject of Inspection: Coating} & As explained earlier, if the coating is conductive, it could interfere with EC inspections of the pipeline [107]. The same is likely to be the case for MFL inspections if the coating is easily magnetised. There appears to be a gap in literature regarding inspecting pipelines with metallic coatings. & A traditional UT inspection of the pipeline material would be hard to perform due to multiple material surfaces and the need for coupling [106]. Non-contact ultrasonic techniques have been developed, but these have their own limitations [108]. For instance, laser ultrasonic requires the sample to have a finish surface “like a mirror” while capacitive ultrasonic transducers involve a complicated sample preparation process [108]. & Visual inspection would be applicable for all types of coating but is subjective [15]. \\ \hline \hline \multirow{6}{*}{Subject of Inspection: Coating} & Traditional UT coils on a change in acoustic impedance to make thickness measurements [85]. Hence, inspection of coatings using traditional UT is likely to produce stronger signals if the pipeline material and coating have differing acoustic impedance. & Traditional UT could potentially be used for thickness measurements of the coating [85]. Techniques that such as MFL will not work for insulators [85]. There is a potential for holiday inspection to be used for insulator coatings [109]. & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: A summary of how NDT method choice is affected by coating type when the subject of inspection changes. * A new method for characterising pitting resistance: \"ultrasonic relative attenuation coefficient of high-order cumulant\" is used to reduce Gaussian noise and tease apart noise caused by changes to the microstructure during corrosion [110]. This finding can then be used for characterising pitting resistance [110]. * A new \"state-of-the-art\" method for monitoring corrosion using permanently installed transducers is proposed that results in \"repeatability values of 23 nm and 46 nm in the thickness measurements of a mild steel sample over the periods of 1 h and 24 h [111]\". * A new method uses electromechanical impedance to track corrosion-induced thickness loss [79]. A piece of PZT (lead zirconate titanate) is attached to a steel plate and, \"using the direct and converse piezoelectricity effect of the PZT transducer, the discrepancy in the mechanical impedance of the stainless steel plate caused by corrosion can be identified by the discrepancy in the admittance signature of the PZT transducer [79]\". The method was found to differ in its measurement of corrosion degrees, from actual corrosion by a maximum of 4.11% [79]. * A new method for simultaneous crack detection and thickness measurement using a single probe is discussed [112]. It was found that \"crack-like defects with depth 0.3 mm (0.2\(\lambda\)) or higher can be detected\" but the \"amplitude drop value cannot be used to estimate the size of defects of 3 mm depth or larger [112]\". * The use of water immersion UT allowed coating thickness to be measured with a lower degree of error. In particular, the time-of-flight (TOF) of the \"reflected echo on the time-domain waveform\" was determined [113].