Fundamental Study On Distributed Optical Fiber Sensing Technology For Submarine Pipeline

The safety of submarine pipeline has been the focus question. Present inspection techniques can not be used to monitor submarine pipeline in real time, so new inspection techniques need to be developed. In recent years, distributed optical fiber sensing technology has been applied more and more widely in structural health monitoring because of many advantages. In order to apply the distributed optical fiber sensing technology to submarine pipeline, a joint project group have been form to apply for the subject of distributed optical fiber sensing technology for long distance submarine pipeline from Hi-Tech research and development program of china by Zhejiang University and CNOOC, and hope to develop distributed optical fiber sensing system that monitors long distance submarine pipeline in real time. Based on this subject, some fundamental studies are performed in this paper. Studies mainly include analysis for submarine pipeline accident cause, test for distributed optical fiber sensor prototype, construction method research for optical fiber sensing system, residual strength assessment for corrode pipeline, vortex-induced fatigue assessment for span pipeline under current, fatigue and fracture assessment for pipeline with weld crack. These assessments can offer reference for setting safety assessment standard of distributed optical fiber sensing system. The major contents are summarized as follows:(1) Submarine pipeline accident cause is ascribe to corrosion, wave and current action, third party activity or machinery damaging, seabed movement, material and welding defect, equipment and accessory failure and man mistake by analyzing data of submarine pipeline accidents. On the basis of analyzing disadvantages of present inspection techniques for submarine pipeline, it is pointed that new inspection techniques should be developed to monitor submarine pipeline in real time and improve inspection efficiency. Current application situation of distributed optical fiber sensing technology in health monitoring of submarine pipeline is given, and some problems need to be solved, such as optical fiber embedment safety, monitoring information procession, optical fiber reliability et al.(2) Distributed optical fiber sensing technology in series is put forward to solve health monitoring of long distance submarine pipeline, and its work principle and makeup are introduced. Prototype of distributed optical fiber sensor is tested and this isfound that its performance is stable, but its measure scope is smaller and needs to be adjusted further. It is also found that optical fiber clad 'effect on measure result is neglectable if slide don't occur inside optical fiber. Strain increment relation curve between optical fiber and pipeline is established by test, it can offer reference for evaluating pipeline strain in practical engineering.(3) Residual strength of pipeline with groove shape corrosion defect is assessed by utilizing finite element method. It is found that corrosion depth change has a large effect on pipeline residual strength, but the effect of corrosion width change may be neglectable .when corrosion length and corrosion distance along axial direction is longer than one value, they may also be neglectable. When pipeline is acted by axial force, bending moment and torsional moment, their effect on pipeline residual strength should be considered. By Comparing assessment result from finite element, B31G, and DNV RP101, it is found that assessment results from B31Gand DNV RP101 are conservative than result from finite element assessment with the exception of larger axial force, bending moment and torsional moment. Assessment result from B31G is less 15-20% than result from finite element assessment under only inside pressure, and it is about 10% for DNV. RP101. Assessment results from B31G and DNV RP101 approach pressure of failure probability as 50%, these can offer reference for realizing correctly assessment results from B31G and DNV RP101.(4) Formation and development Mechanism of submarine pipeline span and VIV is expla...