# Review of Structural Health Monitoring Techniques in Pipeline and Wind Turbine Industries
## 3 Health Monitoring in Corrosion
### Distributed OFS for Chemical Sensing  
Using sensors outside the pipelines to identify and evaluate the early signs of corrosion after it has occurred and once the structural integrity has worsened is the most appropriate method. A less advanced version of sensors when compared to DTS, DSS, and DAS are DCS (distributed chemical sensing), which demonstrates a remarkable capability by tracking corrosive conditions to promote corrosion prevention before or at the initial diagnosis of corrosion. The feasibility of sensors made up of optical fibers is achieved by activating the operational chemical coating (polymers, metal-organic frameworks, nanomaterials, metal-oxide films, etc.) over their core or cladding. To enable the association of light with these chemically sensible layers over the core or cladding and with the external medium [105; 106; 107], the optical fibers may be carved, trimmed, stitched, or edge-polished. Figure 10: The wires are helically wrapped over a pipe of this type to result in the ability to measure acoustically the health of the system [87]. Figure 9: (**a**) Photograph of FBG hoop strain sensors that are wrapped around a pipe. (**b**) Field demonstration of the sensors based on FBG that monitor the riser stress for subsea drilling and operations [71]. Since its emergence in the 1990s, a special class of fiber structure for OFS with tremendous capability for DCS has been provided by micro-structured optical fibers [108; 109; 110], which features the air holes extending along the whole length of the fiber parallel to the longitudinal axis known as photonic crystal fibers (PCF) [108; 110; 111], unless the air holes are regularly organized within the cladding matrix. Figure 11 shows the internal and external look of an evanescent field-based semiconductor [112]. While microstructured fibers provide highly sensitive and versatile fiber structures, large production and marketing are still limited, demanding the cost-effective long-distance production of such fibers. By the direct association of light with gas in holes, few PCF sensors (index guided and hollow-core) were observed to track highly sensitive gases such as methane, hydrogen sulfide, carbon dioxide, and acetylene (ppm level) [113; 114; 115; 116; 117]. Specific detection for Cl and humidity control with a Cl\({}^{-}\) sensitive material that shows fluorescent behavior filled in the gaps; a suspended-core fiber PCF sensor has been developed [118]. The use of proxy materials incorporated with the dispersed OFS network to specifically track corrosion as a decentralized optical coupon and provide information about them is a new idea for corrosion diagnosis, which is planned to be mounted along the inner surface of the pipelines to control internal corrosion. For decentralized corrosion control, while examined using OFDR, metallic film-coated optical fibers have been presented where the loss of mass is tracked on the basis of the change in intensity or change in strain along with the optical fiber [119; 120; 71] (Figure 12). In the corroded area, the light intensity and the strain increase due to the release of compressive internal stress followed by electrodeless deposition of Ni film as the light absorption of the metallic film reduces and the film becomes thinner. Decentralized monitoring of important environmental variables such as quality of water, the conductivity of the electrolyte, and acidic gases (carbon dioxide and hydrogen sulfide) will assess the corrosivity of the atmosphere and thus track corrosion indirectly. While only a few research studies have examined DCS [119; 121; 122; 123], chemical sensing materials are being studied for a wider variety of optical fiber applications, which could theoretically be used for the production of DCS to track corrosive external factors.