As an indispensable part of people's life, water resources can ensure stable economic development. Although the total amount of water resources is enormous in China, their spatiotemporal distribution is very uneven. For the sake of resolving the dilemma of uneven distribution of water resources in time and space, a series of large-scale water diversion projects are being built in China. Prestressed concrete cylinder pipe (PCCP) has been widely used in long-distance water delivery projects due to its advantages of high strength, good seismic resistance, and anti-leakage. However, with the increase in service life, there could be an occurrence of prestressed steel wire broken in PCCP pipelines, which may cause pipe burst accidents when broken wires accumulate to a certain extent. Therefore, it is urgent to carry out real-time online PCCP broken wire monitoring. Compared with traditional monitoring methods, the PCCP broken wire monitoring technology based on distributed optical fiber acoustic sensing (DAS) has obvious advantages such as continuously distributed sensing, long monitoring distance, and anti-electromagnetic interference. Meanwhile, the sensing optical cable can be put into the pipelines with water inside, which means that a broken wire monitoring system based on DAS can be deployed even if the PCCP pipeline is in operation. Therefore, DAS has broad application prospects in PCCP broken wire monitoring. As the broken wire monitoring system based on converged DAS gradually develops in recent years, the operation safety of PCCP pipelines will be better guaranteed.

Traditional nondestructive PCCP broken wire detection methods include manual echo detection, electromagnetic detection, and acoustic monitoring. Manual echo detection utilizes the echoes from knocking on the pipes to determine the internal defect position, so its maintenance efficiency is low, and the accuracy is uncertain depending on personal work experience. Electromagnetic detection is based on the remote field eddy current (RFEC), which was proposed by MacLean in 1951. The magnetic field signal will distort when propagating through broken wires so that the location of broken wires can be acquired. However, electromagnetic detection is still inefficient and only practical during the pipeline maintenance period. Subsequently, acoustic monitoring which is able to capture acoustic signals of broken wires through electrical or optical sensors has attracted attention. In the 1990s, Mark Holley, Robert Diaz, and Michael Giovanniello first proposed and commercialized acoustic monitoring in a section of a pipeline in Maryland. Hydrophone arrays manufactured with sensors were adopted in the program to provide comprehensive information of the pipelines so that pipe burst accidents could be predicted and handled in time. Although acoustic monitoring resolves the problems of inefficiency and non-real-time detection, traditional acoustic monitoring methods are still susceptible to ambient noises because there is a certain distance between two adjacent sensors so that the acoustic signals of broken wires will be distorted before being acquired by the sensors. In order to reduce the signal distortion brought by ambient noises, researchers turn their attention to DAS. Michael S. Higgins and Peter Paulson from Pure Technologies first utilized phase‑sensitive optical time domain reflectometry (Ф-OTDR) for PCCP broken wire monitoring and the monitoring device detected 11 broken wire incidents during four months. The positioning uncertainty of the device is about ±1.5 m. However, the frequency response of Ф-OTDR is restricted by monitoring distance so that it is hard to distinguish the broken wire signals with high frequency from other ambient noises along a long monitoring distance. Therefore, a fiber optic interferometer (FOI) can come in handy for the monitoring of high-frequency signals. FOI can detect broadband vibration signals by measuring the phase difference between the sensing light and the reference light while it is hard for FOI to locate the accurate position of the vibration. Based on the advantages and disadvantages of Ф-OTDR and FOI, there have been some converged DAS systems of these two technologies to acquire the accurate position and broadband frequency characteristics of broken wires at the same time. In 2023, Beijing Aqua Intelligent Technology Co., Ltd, Nanjing University, and Nanjing Fiber Photonics Technology Co., Ltd jointly developed a broken wire monitoring system converged by Ф-OTDR and FOI, which achieved single-ended monitoring along a section of a 20 km-long pipeline. The positioning uncertainty of the converged system is ±2 m, and the top frequency response can reach 20 kHz. Compared with the broken wire monitoring devices of foreign companies, there may still exist some gaps in the performance indicators and monitoring accuracy of domestic devices. While the industrialization of the correlation technique in China is at an early stage, relevant construction experience and characteristic data of broken wires still need to be accumulated. With the increasing construction of large-scale water diversion projects in China, broken wire monitoring based on DAS will get further development and innovation.

Broken wire monitoring is an indispensable part of PCCP pipeline health monitoring, which can guarantee people's normal lives. DAS can take advantage of the whole section of optical fiber to locate any faint disturbance along a fiber of tens of kilometers accurately. In recent years, converged DAS system has been used in PCCP broken wire monitoring to give early warning of PCCP pipelines that are at risk of bursting. However, the relevant technology has not been applied in actual projects on a large scale in China. In the future, it is essential to carry out further engineering applications, gather characteristic data of broken wire events, and improve signal processing methods.