Novel applications of distributed fiber optic sensors for pipeline structural health monitoring
thesisposted on 02.03.2017, 03:12 by Lim, Kenneth Wei Wern
This thesis aims to develop novel applications of distributed fiber optic sensors for pipeline structural health monitoring. Specifically, this thesis aims to show that distributed fiber optic sensors can potentially detect the onset of out-of-roundness in pipes and monitor local stiffness changes on the out-of-round pipe, show the theoretical response of the distributed fiber optic sensor when instrumented on an out-of-round pipe subjected to axial and bending loads in addition to internal pressure and demonstrate the potential of distributed fiber optic sensors for monitoring pipe bursts in a laboratory-based burst test while highlighting the importance of sensor spatial and temporal resolution. The ability of distributed fiber optic sensors to detect out-of-roundness in pipes was demonstrated through an experimental program where fiber sensors were instrumented helically around a PVC pipe. The onset of out-of-roundness in the pipe was induced by filling it with water. Strain measurements made by the fiber sensor after the pipe was filled with water showed an oscillatory strain profile with maxima strains along the mid-line of the pipe and minima strains along the top and bottom. This suggested the onset of out-of-roundness. The measurements agreed with an analytical model derived by Brazier describing the deformation of a cylindrical shell due to a bending moment. Internal pressure was then applied to the filled pipe. Strain measurements made with the fiber sensors showed a mean increase in strain and an oscillatory strain profile with maxima strains at the top and bottom of the pipe and minimas along the mid-line. This agreed with an analytical model derived by Haigh describing the expansion of an out-of-round pipe under internal pressure. Local stiffness changes were introduced to the PVC pipe by bonding steel shims to its surface. The distributed fiber optic sensors were found to be capable of locating the shim when the out-of-round pipe was pressurized. These results demonstrate that distributed fiber sensors can potentially be used to detect the onset of out-of-roundness in pipes and monitor local wall stiffness changes in the out-of-round pipe. The theoretical response of the distributed fiber sensor when helically instrumented on an out-of-round pipe subjected to axial and bending loads in addition to internal pressure was investigated through finite element methods. An algorithm converting finite element point strains to distributed fiber sensor strains and a finite element model of the out-of-round pipe were developed. The strain distributions predicted by the combined algorithm and finite element pipe were found to show good agreement with the experimental results of Chapter 7. The validated algorithm and model were then used to study the theoretical response of the distributed fiber optic sensor when instrumented helically on an out-of-round pipe subjected to axial and bending loads in addition to internal pressure. The results of this study showed that the theoretical distributed fiber optic sensor strains of the out-of-round pipe subjected to internal pressure, internal pressure and axial loads and internal pressure and bending loads are different suggesting that a single strand of fiber sensor instrumented helically around the pipe can potentially distinguish between the three loading cases. The potential of distributed fiber sensors for monitoring pipe bursts was demonstrated in a laboratory-based study. Distributed fiber optic sensors were instrumented on a cast iron pipe. Various damages sizes were introduced to the pipe to initiate burst and the pipe was pressurized. It was observed that higher spatial and temporal distributed fiber optic sensor resolutions allowed the expansion of the burst site to be monitored until the pipe failed. Spatial and temporal resolutions that were too low however could not detect the burst site. The findings of this thesis are expected to contribute towards the development of smart pipes capable of relaying information on its structural health to pipeline operators.