Study On Super-Long Distance Distributed Optical Fiber Sensing Technology And Its Application

The safety and durability of grand engineering projects and infrastructures was widely focused by the public, due to many fetal engineering accidents occurred every year. Now many grand engineering projects and infrastructures which focused attention upon the world have been build or are being build in this period in China. Such as San-xia project, South-to-north transfers project, offshore flat forms and submarine pipeline in Bohai Sea, and Olympics projects in Beijing etc. It is essential to develop efficient healthy monitoring technique and damage diagnosis technique to ensure the security of 1 grand projects and infrastructures, such as bridges, tunnels, dams, high builds, offshore flat forms and submarine pipelines. A project named 'Technique of Long-Distance Distributed Optical Fiber Sensor to Monitor Submarine Pipeline" was taken on by Zhejiang University (ZJU), which is sponsored by Hi-Tech Research and Development Program of China (863) (Grant No. 2004AA602210-1), which is planed to develop a super-long-distance distributed optical fiber sensor system to monitoring the healthy state of submarine pipeline. Researches done in this paper was based on the national "863" program. Strain field data can be measured by distributed optical fiber sensor. It provided the foundation to evaluate the structure statement. Developing distributed optical fiber sensor technique is essential to ensure grand engineering projects and infrastructures.Firstly, B-DOFS(Distributed Optical Fiber Sensor based on Brillouin back scattering ) sensing mechanism was analyzed. The relationship between Brillouin scattering frequency and sensing optical fiber strain was setup. Then the pivotal technique used in B-DOFS was studied, especially how to realize 250km sensing distance. B-DOFS model machine system was successfully developed for the first time by ZJU in China. The 250km super-long-sensing-distance was solved using WDM technique. According to the information be lookup, the same index was no more than 100km overseas. The system enables one-end measurement with a strain measurement scope -2500με ~ +2500με, a strain resolution 50με, a spatial resolution 5m.A series of experiments were done to validate the performance of B-DOFS'. The feasibility to monitoring the submarine pipeline for B-DOFS was also demonstrated. The performance of different type sensing optical fibers was studied through experiment based on I -shaped steal. Repeated performance was also demonstrated through experiment. It is indicated that press strain and tensile strain can be well measured with good stability and repeated performance.The factors which influenced optical fiber sensor measurement precision brought by encapsulation and installation were analyzed. The strain transfer relationship between structure body and sensing optical fiber was studied theoretically in allusion to the embedded sensor and the surface-mounted sensor. The analytic solution was obtained for optical fiber sensor transfer relationship, which was verified by FEM simulation. The FBG sensor measurement precision was influenced by fiber coating thickness, length and material performance. The measurement precision of surface-mounted sensor was influenced by glue. Coating thickness, length and materialperformance have little influence to the distributed optical fiber sensor precision if sensing optical fiber was fixed well on the structure body. Also, micro-pipe has little influence to the distributed optical fiber sensor if installed by micro-pipe construction method.Synoptic strain data can be measured by B-DOFS, but exact data can not for some pivotal component. This shortage can be reformed by FBG fiber sensor through B-DOFS and FBG syncretizing technique. Thus, whole strain field information can be obtained by B-DOFS and FBG syncretizing technique. Depend on the advantage of magnanimity strain data, damage recognized method based on strain model and method based on static loading experiment structure parameters were studied. Damage can be easily recognized through calculation examples.A...