Research On Some Key Technologies Of Distributed Fiber Sensing System

Structure Health Monitoring (SHM) is a technology throughout the entire life cycle of the project for the infrastructure. The life cycle which includes construction, operation and destruction needs to be monitored. SHM system refers to non-destructive monitoring methods to get information of the structure, which analyzes various features inside the structure in order to research the changes due to the damage of the structure. Sensor unit is the foundation of the structure health monitoring system. But the present sensors are limited by monitoring distance, durability, accuracy and so on, which are the bottleneck to the further development of SHM system. Optical fiber sensing technology has become one of the greatest inventions in the last century, because of its small size, high durability, absolute measurement and distributed monitoring, it plays an increasingly important role in the sensing area and has a trend of gradually replace part of the electric sensors to be the first choice as the sensitive components for the SHM system. The advantages of the fiber sensor are obvious, but for a distributed fiber system there are still some difficult key technologies to solve. The dissertation studies three key technologies in the distributed fiber system based on optical fiber sensing technologies which are the temperature- compensation technique, design method of direct fiber sensor and design method of indirect fiber sensor. Main works of this dissertation are as follows:The coupling relationship between temperature and strain in the quasi-distributed system and distributed system with fiber bragg grating (FBG), fiber-optic Brillouin optical time-domain analysis (BOTDR) and Raman optical time-domain analysis (ROTDR) is studied. Based on the research of the coupling relationship, a kind of temperature compensation method is proposed which is using ROTDR technique for the strain monitoring in quasi-distributed FBG strain system and full-distributed BOTDR strain system. Some researches are focused on the calculation and error correction method of the strain sensitivity coefficient in the multi-places monitoring system after ROTDR temperature compensation. In addition, many experiments are carried out to validate the method.Improve the design method of FBG direct sensor. For the range or precision problem in the quasi-distributed FBG strain monitoring system, a design method of coefficient-adjustable strain sensor with different structures is proposed. Meanwhile, based on the research of quasi-distributed monitoring system above, full-distributed direct temperature and strain sensors are developed.FBG-based indirect sensing method is discussed. Two pressure sensors of different types of structure for the liquid pressure and solid pressure are proposed which can monitor the pressure and the temperature at the same time. In addition, a FBG temperature self- compensation displacement sensor is designed for the displacement monitoring on millimeter. Based on the research of pressure sensor and displacement sensor above, for the scouring problem of the bridge pier, two types of FBG scouring sensors are developed and the pie-scour monitoring system is also studied.Upon the related studies above and the project of the parameters monitoring in the wells of oil field, and as it's difficult for the temperature distributed online monitoring of oil well casing annulus in the oil industry, a distributed ROTDR absolute temperature monitoring system is proposed, ROTDR temperature sensor is also developed and applied to practical engineering. For the casing damage in the oil industry, a new method based on fiber optic sensing technology which can monitor the casing deformation in the casing annulus is proposed. An optical fiber sensing system for casing deformation and casing annulus pressure is developed based on FBG, BOTDR and ROTDR, and many tests are made in the oil field. Data of casing deformation in different stages of production are obtained. Combined with the existing information of the oil field, comprehensively analyze the status of casing deformation to predict the occurrence of casing damage and contribute to the study of the reason of casing damage.