Robust and real-time condition monitoring of the underground gas distribution network (ca. 3000 km) in Singapore is critical for interruption-free power generation.. For this purpose, in this paper, we investigate optical techniques, based on distributed fiber optic sensors, recently proposed in the context of the gas pipeline monitoring. A detailed review of these techniques and their applications in terms of sensing range, spatial, temperature and strain resolutions has been generalized. During gas leaks, Joule-Thomson cooling effect takes places. The temperature drop due to Joule-Thomson cooling effect for different gases, at various operating conditions are analyzed. The temperature change due to Joule-Thomson cooling effect could be detected by the DTS, for monitoring leaks in the underground gas network.