# Inspection of Coated Hydrogen Transportation Pipelines
## 2 Background
### Brief Description of a Hydrogen Pipeline
#### 2.1.3 The Question of Blended Hydrogen Pipelines  
Blended hydrogen-natural gas pipelines are of interest due to the existing infrastructure of natural gas pipelines; there is the possibility of re-purposing it to transport and store hydrogen to realise the benefits discussed earlier [17]. Considering the difficulties arising from the differing properties of natural gas and hydrogen, a reasonable approach that has been suggested is to first use the existing infrastructure to transport methane-hydrogen blends, understand the challenges arising from this, implement operational modifications as required and then progressively increase the percentage of hydrogen in the blend to 100% [17]. It is found that as the percentage of hydrogen in the blend increases, leakage flow rate increases, lower heating value increases and the energy cost of transporting the blend increases [17]. It is also suggested that up to 25% of hydrogen in the mixture by volume does not appear to markedly \"increase the risk of explosion\" [17]. Separately, it is further suggested that there are different levels of risk associated with the percentage of hydrogen in the blended pipeline [18]. Table 2 illustrates these findings. \begin{table}
\begin{tabular}{c l l} \hline \hline
**Reference** & **Defect** & **Description \& Cause** \\ \hline \multirow{2}{*}{1} & Hydrogen Induced & HIC occurs when hydrogen recombines in the steel to form gaseous molecules in voids [14]. \\  & Cracking (HIC) [8] & \\ \hline \multirow{2}{*}{2} & Hydrogen Stress Cracking & \multirow{2}{*}{Occurs at surface or near-surface [14]. Forms as a result of tensile static load and a corrosive medium being present together [15].} \\  & (HSC) [8] & \\ \hline \multirow{2}{*}{3} & Hydrogen Induced Blister & \multirow{2}{*}{Occurs when the pressure exerted by molecular and atomic hydrogen in the material is high enough to form blisters [14].} \\  & Cracking (HBC) [14] & \\ \hline \multirow{2}{*}{4} & Stress—Oriented HIC & \multirow{2}{*}{SOHIC is also known as Type I SSC and occurs when HIBC is parallel to the applied stress [14].} \\  & (SOHIC) [8; 16] & \\ \hline \multirow{2}{*}{5} & Hydrogen Emprittlement & \multirow{2}{*}{HE is the result of hydrogen entering the steel either during manufacturing or due to exposure to hydrogen during service. This manifests in the form of deteriorated mechanical properties [14; 15].} \\  & (HE) [8] & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The defects caused by hydrogen in pipeline steels and their cause. Figure 1: A summary of the types of hydrogen damage seen in pipeline steel. There are further factors to consider when hydrogen degradation of pipelines are discussed. Firstly, a distinction is made between higher pressure transmission pipelines and lower pressure distribution pipelines, with added hydrogen of less than or equal to 50 per cent in natural gas transmission pipelines needing \"generally insignificant\" modifications [18]. This figure, however, needs to be verified for every case [18]. Hydrogen degradation was found to be especially prevalent when the system is at high concentrations and pressures--common conditions where hydrogen is introduced into the pipeline at high concentrations [18]. Hydrogen was also found to reduce the pipes' endurance towards cyclical pressure changes (fatigue) and further crack growth from existing defects [18]. The material of the pipes factored heavily into the extent of hydrogen degradation observed. Common low-strength steels used in pipelines (API 5L A, B, X42, and X46) mainly experienced a reduction in ductility [18]. High-strength steels (having a yield strength greater than 100 ksi) tended to be more susceptible to HIC, while low strength steels were more likely to suffer from a loss in tensile ductility [18]. Faster crack growth and lowering of \"fatigue endurance limits\" were seen in carbon and low alloy steels, even at low pressures of hydrogen exposure [18]. Crack growth was more significant at ambient temperature as opposed to higher temperatures [18].