Fibre As A Sensor

Hugh Ujhazy

Vice President, IoT and Telecommunications

Every point on a fibre optic cable is a sensor. Non-coherent optical time domain reflectometry (OTDR) technology has been used for a long time for long-range monitoring of the quality and the integrity of fiber optic cable infrastructure and this is used to locate break points in subsea underwater fiber cables. By embedding fiber close to or atop infrastructure elements and combining with suitable algorithms including AI-enhanced algorithms, it is possible to greatly improve the accuracy of detection (95%+) while offering high location accuracy. While collection of sensing data from fibre optic cables is not new, generate accurate results by embedded self-learning through AI capabilities means that this technology can be applied to a broad range of use cases.

IDC forecasts that the number of connected devices across utilities, oil& gas, water resources, transportation infrastructure and street lighting will grow at a CAGR of 9.15% for the period 2021-2025 in Asia Pacific (excluding Japan) alone. This growth reflects a change from 1.7 billion connected devices to 2.5 billion in the period of 5 years. This total represents over 20% of the total connected endpoints in the region by 2025. The importance of protecting and maintaining infrastructure has been highlighted in recent years as people globally were forced to modify their working and lifestyle practices to accommodate the COVID-19 pandemic. The need for a resilient and reliance infrastructure across transportation, energy and communications led to an explosion in the urgency of connecting enterprises and residences to networks. Fibre optics have become the infrastructure of choice to enable this endless demand for communications. This demand is reflected in a forecast growth in traffic data across mobile and fixed networks of 13.9% CAGR over the period 2021-2025, representing total traffic of 801.6PB by 2025, up from 476PB in 2021. The majority of this traffic will traverse fibre optic networks.

The fiber optic sensors (FOS) market is expected to grow exponentially over the period from 2021 to 2025. Factors such as significant demand from infrastructure, utilities and surveillance will substantially boost the adoption of distributed fiber optic sensors. The abilities of fibre optics to survive in challenging environments is another factor propelling the market growth.

The rapid acceptance of DFOS has promoted manufacturers and suppliers to increase R&D expenditure to offer better products to their customers. Service providers are trying to regulate efficiencies and optimize their production process to capture maximum market share and eliminate all other substitutes of the fiber optics technology.

However, fibre alone is not sufficient to create highly scalable, widespread solutions with low error rates. The core competitiveness of fiber optic sensing lies in the tight integration of optical device hardware and the optical sensing algorithm.

Why Monitoring Makes Sense

Traditional monitoring techniques range from physical inspection (either through video or personnel onsite) and restriction of access through fencing, isolation and burying of infrastructure. For example, in railway monitoring, the traditional method is to physically install sensors along the trackside. With fiber-optic based sensing all that is needed physically is an interrogator which injects waveforms into the fiber cable and then measure the returning signal since vibrations and temperature directly impact the traveling waves. While these solutions can be expensive, their cost and limited scope of coverage make them spot solutions at best. Embedding monitoring sensors along the infrastructure can help, but this can require deployment of multiple sensors to gain a comprehensive view of vibration, audio, strain and temperature. Moreover, a communications path has to be provided for sensors installed along infrastructure. As networks of infrastructure expand over ever greater geographical areas, a comprehensive solution which covers the entire network at low cost is a high requirement.

An example of this need for coverage comes from the mining and oil & gas industries. As these industries have struggled to cope with the impacts of COVID-19 over the past 18 months, they are increasingly looking to digital tools to assist with continuous site operation. For miners, the ore extraction (i.e., mining) component of the mining value chain was identified as the area in which the greatest value creation from digitalisation is possible. To create this value, mining organisations are looking to invest in autonomous mining equipment and mining optimisation systems, such as short-interval control systems and advanced forecasting tools for mine design optimisation.

The Reliance on Communications Infrastructure

The monitoring of fibre cable integrity and the prediction of the onset of failure and damage is critical to the reliability of fibre communication systems. Most current techniques for monitoring fibre optic cable integrity would benefit from obtaining real-time, quasi-static and dynamic information about disturbances to the fibre cable. This would have the further advantage of monitoring any structure or material near the cable or to which the cable is attached, such as the fencing, material transport pipelines (such as oil, gas or water) and infrastructure (such as roads, bridges and buildings). Such a capability enables simultaneous, real-time fibre optic communications and sensing applications such as structural integrity monitoring, leak detection, ground monitoring, machine condition monitoring and intrusion detection.

This is possible because optical fibres can be more than mere signal carriers. Light that is launched into and confined to the fibre core propagates along the length of the fibre unperturbed unless acted upon by an external influence. Specialised sensing instrumentation may be configured such that any disturbance of the fibre which alters some of the characteristics of the guided light (ie., amplitude, phase, wavelength, polarisation, modal distribution, and time-of-flight) can be monitored and related to the magnitude of the disturbing influence. Such modulation of the light makes possible the measurement of a wide range of events and conditions, including: strain/residual strain; displacement; damage; cracking; vibration/frequency; deformation; impact; acoustic; emission; liquid levels; pressure; temperature; load.

Capabilities and Applications for Fibre Optic Sensing (FOS)

Fibre optic sensors (FOSs) may be intrinsic or extrinsic, depending on whether the fibre is the sensing element or the information carrier, respectively. They are designated "point" sensors when the sensing gauge length is localised to discrete regions. If the sensor is capable of sensing a measurand field continuously over its entire length, it is known as a "distributed" sensor; "quasi-distributed" sensors utilise point sensors at various locations along the fibre length.

Therefore, FOSs are a class of sensing device. They are not limited to a single configuration and operation unlike many conventional sensors such as electrical strain gauges and piezoelectric transducers. Hence fibres are now replacing the role of conventional electrical devices in sensing applications to the extent where we are now seeing a multitude of sensing techniques and applications.

Given the forecast growth in the volume of connected infrastructure foreseen in the AP region in coming years, the use of FOS can be expanded to encompass a number of critical use cases. Some of the use cases around pipeline monitoring include the following:

1. Vibration Sensing

2. Securing infrastructure

3. Absolute displacement / Strain sensing

4. Distributing Temperature Sensing for Oil & Gas pipelines

5. Audio sensing for pipeline leak detection

6. Flow Sensing

An increasing number of pipelines are constructed in remote regions affected by harsh environmental conditions where pipeline routes often cross mountain areas which are characterized by unstable grounds and where soil texture changes between winter and summer increase the probability of hazards. Third party intentional interference or accidental intrusions are a major cause of pipeline failures leading to large leaks or even explosions. Due to the long distances to be monitored and the linear nature of pipelines, distributed fiber optic sensing techniques offer significant advantages and the capability to detect and localize pipeline disturbance with great precision. Furthermore, pipeline owner/operators lay fiber optic cable parallel to transmission pipelines for telecommunication purposes and at minimum additional cost monitoring capabilities can be added to the communication system.

As noted earlier, pipelines can transport oil, gas, water and other materials and are critical to the smooth operation of modern society. Another important use case for FOS is perimeter monitoring. Boundaries need to be managed to control borders, access to sensitive or critical environments such as airports, power stations and communications infrastructure and manage the entry of authorised persons in an area.

Boundary control can be challenging as factors such as wind levels, fencing materials and level of wildlife activity can act as localised factors which increase vibrations on the fencing material. The cooperation between AI software tools and the FOS can work to detect the ‘normal’ level of disturbance at each point along a monitored boundary. Using this approach, the number of false positives can be significantly reduced by eliminating acceptable traffic across the boundary.

Linked with periodic patrols, video surveillance and drone-based surveillance, FOS + AI can significantly decrease how permeable a boundary is and increase the level of safety and security within the monitored precinct.

The use of optical fibre sensing has been in place for decades. The main problem has been the accuracy of alarm reporting. The combination of deeply integrated optical hardware and software algorithms is a requirement of broadening the application of FOS to a range of industry applications.

An Integrated FOS Solution

In 2021, Huawei officially launched the Sensing OptiX Oil&Gas Pipeline Inspection Solution. This solution includes DAS devices and sensing algorithm servers deployed at stations with a maximum distance between two sites of 100 km. The centralized management platform deployed in the command and dispatch center can interconnect with other platforms such as GIS, video surveillance, drones, and pipeline inspection systems to form a complete, integrated solution in the production process. This ultimately eliminates pipeline security risks in a closed-loop manner. The solution integrates optic fibre and sensing algorithms to create a dynamic, adaptive environment.

The solution uses enhanced oDSP algorthms to eliminate monitoring blind spots and improved signal-to-noise ratios which lead to better data and zero false negatives. This is augmented by a 32-dimensional vibration waveform analysis algorithm which is used to filter out background interference and enhance weak signals. Finally, the sensing algorithm supports self-learning and self-optimization. Local samples can be learned online and algorithm models can be quickly adjusted to adapt to new environments and scenarios. These three key technologies applied in conjunction with the fibre sensing provide adaptive intelligence across the span of the monitored infrastructure.

Conclusion

The capabilities which can be deployed using FOS can also be applied to monitoring bridges, tunnels, roadmaps and railways for strain and increased vibrations and strain which could indicate an imminent failure. Similarly, pipelines can be used to carry oil, gas, water and other flow designated materials meaning that leak detection and temperature variations can also be applied.

The use of fibre optic sensing technology for the monitoring and diagnosis of the condition and performance of pipelines could provide a sound engineering and economic basis for the major decisions which will have to be made concerning the operation, maintenance, refurbishment or replacement and life extension of these items. The savings made by avoiding or delaying refurbishment or replacement could be substantial.

Furthermore, their high resolution, insensitivity to electromagnetic interference, real-time monitoring capabilities and relatively low cost are characteristics expected to improve pipeline safety and operation and provide great potential benefit to the industry and society as a whole.

Communications using optical fibres have a number of attractive features and advantages over conventional communication means, and their performance has been proven over the past two decades. The value offered by these systems has now been augmented by the ability to simultaneously monitor, in real-time, the integrity of the cable, as well as any structure or material near the cable or to which the cable is attached.

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