Sewerage tunnel leakage detection using a fibre optic moisture-detecting sensor system

•For the first time the design of a fibre optic moisture sensor system is reported to detect leakages in sewerage tunnels.•A Fibre Bragg Grating based moisture sensor has been developed.•A distributed fibre optic swellable polymeric sensor has been developed.•The resistance of both sensors to highly alkaline environments (pH 13.4) was verified.•The distributed fibre optic swellable polymeric sensor developed shows a light attenuation of 34.2dB in the presence of water, which is superior to similar sensors reported to date. The design and development of a new fibre optic sensor system for the optical detection of leakages in sewerage tunnels is reported. The system developed overcomes the disadvantages of the usually employed camera based inspection systems which are relatively complex and, in addition, require cleaning of the structures to be monitored beforehand. The sensor concept created combines a Fibre Bragg Grating (FBG)-based humidity sensor and a swellable polymeric fibre optic sensor. Both sensors are located along the sewerage tunnel so that they can response immediately to any leakages that may occur. The swellable polymeric fibre optic sensor shows a response of 34.2dB in the presence of water, a performance which is superior to that seen form other swellable polymeric fibre optic sensors reported so far. Furthermore, the resistance of both sensors to highly alkaline environments (pH 13.4), an important feature of such sensors was verified. Consequently, when compared to the use of conventional inspection techniques, the novel fibre optic sensor system provides a robust, relatively low-cost and continuous monitoring system well suited to use in sewerage tunnels.