Investigations towards the development of a novel multimodal fibre optic sensor for oil and gas applications.

Oil and gas (O&G) explorations are moving into deeper zones of earth, causing serious safety concerns. Hence, sensing of critical multiple parameters like high pressure, high temperature (HPHT), chemicals, etc., are required at longer distances. Traditional electrical sensors operate less effectively under these extreme environmental conditions and are susceptible to electromagnetic interference (EMI). Compared to electrical sensors, fibre optic sensors offer several advantages like immunity to EMI, electrical isolation, ability to operate in harsh environmental conditions and freedom from corrosion. Existing fibre optical sensors in the O&G industry, based on step index single mode fibres (SMF), offer limited performance, as they operate within a narrow wavelength window. A novel multimodal sensor configuration, based on photonic crystal fibre (PCF) and utilising a multiwavelength approach, is proposed for the first time for O&G applications. This thesis reports computational and experimental investigations into the new multimodal sensing methodology, integrating both optical phase-change and spectral-change based approaches, needed for multi-parameter sensing. It includes investigations to improve the signal-to-noise ratio (SNR) by enhancing the signal intensity attained through structural, material and positional optimisations of the sensors. Waveguide related, computational investigations on PCF were carried out on different fibre optic core-cladding structures, material infiltrations and material doping to improve the signal intensity from the multimodal sensors for better SNR. COMSOL Multiphysics simulations indicated that structural and material modifications of the PCF have significant effects on light propagation characteristics. The propagation characteristics of the PCF were improved by modifying the geometrical parameters, and microstructuring the fibre core and cladding. Studies carried out on liquid crystal PCF (LCPCF) identified its enhanced mode confinement characteristics and wavelength tenability features (from visible to near infrared), which can be utilised for multi-wavelength applications. Enhancing core refractive index of the PCF improved the electric field confinements and thereby the signal intensity. Doping rare earth elements into the PCF core increases its refractive index and also provides additional spectroscopic features (photoluminescence and Raman), leading to a scope for multi-point, multimodal sensors. Investigations were carried out on PCF-FBG (Fibre Bragg grating) hybrid configuration, analysing their capabilities for optical phase-change based, multipoint, multi-parameter sensing. Computational investigations were carried out using MATLAB software, to study the effect of various fibre grating parameters. These studies helped in improving understanding of the FBG reflectivity-bandwidth characteristics, for tuning the number of sensors that can be accommodated within the same sensing fibre and enhancing the reflected signal for improved SNR. A new approach of FBG sensor positioning has been experimentally evaluated, to improve its strain sensitivity for structural health monitoring (SHM) of O&G structures. Further, experimental investigations were carried out on FBGs for sensing multiple parameters like temperature, strain (both tensile and compressive) and acoustic signals. Various spectroscopic investigations were carried out to identify the scope of rare earth doping within the PCF for photoluminescence and Raman spectroscopy based multimodal sensors. Rare earth doped glasses (Tb, Dy, Yb, Er, Ce and Ho) were developed using melt-quench approach and excitation- photoluminescence emission studies were carried out. The studies identified that photoluminescence signal intensity increases with rare earth concentration up to an optimum value and it can be further improved by tuning the excitation source characteristics. Photoluminescence based temperature studies were carried out using the rare earth doped glasses to identify their suitability for O&G high temperature conditions. Raman spectroscopic investigations were carried out on rare earth (Tb) doped glasses, developed using both melt-quench and sol-gel based approaches. Effect of 785 nm laser excitation on Raman signatures and suitability of rare earth doped materials for fibre-based Raman distributed temperature sensing (DTS) were also studied. Finally, a novel multimodal fibre optic sensor configuration is proposed for the O&G applications, consisting of rare earth doped photonic crystal fibre integrating Bragg gratings and operating in multiple wavelength regimes in a multiplexed fashion. The integrated sensor combination is expected to overcome the limitations of existing sensors with regards to SNR, sensing range and multimodal sensing capability

Optical fibre-based sensors for oil and gas applications.

Oil and gas (O&G) explorations moving into deeper zones for enhanced oil and gas recovery are causing serious safety concerns across the world. The sensing of critical multiple parameters like high pressure, high temperature (HPHT), chemicals, etc., are required at longer distances in real-time. Traditional electrical sensors operate less effectively under these extreme environmental conditions and are susceptible to electromagnetic interference (EMI). Hence, there is a growing demand for improved sensors with enhanced measurement capabilities and also sensors that generates reliable data for enhanced oil and gas production. In addition to enhanced oil and gas recovery, the sensing technology should also be capable of monitoring the well bore integrity and safety. The sensing requirements of the O&G industry for improved sensing in deeper zones include increased transmission length, improved spatial coverage and integration of multiple sensors with multimodal sensing capability. This imposes problems like signal attenuation, crosstalks and cross sensitivities. Optical fibre-based sensors are expected to provide superior sensing capabilities compared to electrical sensors. This review paper covers a detailed review of different fibre-optic sensing technologies to identify a feasible sensing solution for the O&G industry

Investigation of structural parameter dependence of confinement losses in PCF–FBG sensor for oil and gas sensing applications.

Photonic crystal fibre (PCF)–fibre bragg grating (FBG) integration opens up new possibilities in multi-parameter fibre-optic sensing, owing to their active control over light characteristics and mode confinements. Their integration results in a mismatch in their mode field diameters (MFDs), which in turn causes various types of losses such as confinement loss, scattering loss, etc. This paper primarily investigates the effect of geometrical parameters on fibre parameters such as confinement loss and MFD, which plays a significant role in long distance fibre-optic remote sensing. Liquid crystal PCFs (LCPCFs) are utilized in the sensor configuration, exploiting their optical properties for photonic bandgap based tighter mode confinements and wavelength tunability. Furthermore, the LCPCF–FBG combo enables multi-parameter fibre-optic sensing which can be effectively utilized in oil and gas sensing applications. Theoretical study conducted on the fibre sensor revealed that confinement loss and MFD can be reduced by properly optimizing their structural parameters

Fluorescence lifetime assisted enhanced security feature in travel documents for border control and security applications.

Border management and security challenges are increasing considerably in recent years. One of the major concerns is counterfeiting and fraudulent use of identity and other travel documents for crossing border controls. This poses serious threats and safety concerns worldwide, considering the scenario of terrorism and illegal migration across the world. Hence, advanced technologies with improved security features becomes essential to strengthen border security and to enable smooth transits. In this paper, we present a novel dual waveguide based invisible fluorescence security feature with lifetime discrimination and a simple validation system. Molecular fluorescence and lifetimes from the rare earth doped waveguides can be used as additional security features in the identity documents. The validation system consists of a modulated excitation source and fast photo-diodes which helps in the simultaneous detection of multiple security features from the fluorescence waveguides. The rare earth doped fluorescence waveguides are embedded into the identity document as micro-threads or tags which are invisible to the naked eye and are only machine readable. Rare earth fluorescence materials have higher sensitivity and selectivity as they absorb only specific ultraviolet (UV) or visible (VIS) wavelengths to create corresponding fluorescent emissions in the visible or infrared wavelengths. Herein, we present the results based on the fluorescence and fluorescence lifetime spectroscopic studies carried out on the terbium (Tb) and dysprosium (Dy) doped waveguides. The different emission wavelengths and lifetimes of these rare earth elements is a key differentiating feature, providing selectivity and security to the detector systems

Waveguide-based machine readable fluorescence security feature for border control and security applications.

Border security challenges and immigration issues are increasing considerably in recent years. Counterfeiting and fraudulent use of identity and other travel documents are posing serious threats and safety concerns worldwide, ever since the advancement of computers, photocopiers, printers and scanners. Considering the current scenario of illegal migration and terrorism across the world, advanced technologies and improved security features are essential to enhance border security and to enable smooth transits. In this paper, we present a novel dual waveguide based invisible fluorescence security feature and a simple validation system to elevate and strengthen the security at border controls. The validation system consists of an LED (light emitting diode) as excitation source and an array photodetector which helps in the simultaneous detection of multiple features from the fluorescence waveguides. The fluorescence waveguides can be embedded into the identity document as micro-threads or tags which are invisible to the naked eye and are only machine readable. In order to improve the sensitivity, rare earth fluorescence materials are used which absorb only specific ultraviolet (UV) or visible (VIS) wavelengths to create corresponding fluorescent emission lines in the visible or infrared wavelengths. Herein, we present the preliminary results based on the fluorescence spectroscopic studies carried out on the fabricated rare earth doped waveguides. The effect of different rare earth concentrations and excitation wavelengths on the fluorescence intensity were investigated

Computational study of nanostructured composite materials for photonic crystal fibre sensors.

Photonic Crystal Fibres (PCFs) developed using nanostructured composite materials provides special optical properties which can revolutionise current optical sensing technologies. The modal and propagation characteristics of the PCF can be tailored by altering their geometrical parameters and material infiltrations. A drawback of commercially available PCF is their limited operating wavelengths, which is mostly in the infrared (IR) spectral band. Nanostructured composite materials manipulates the optical properties of the PCF, facilitating their operation in the higher sensitivity near infrared (NIR) wavelength regime. Hence, there arises a need to closely investigate the effect of nanostructure and composite materials on various optical parameters of the PCF sensor. This paper presents a hexagonal PCF designed using COMSOL MULTIPHYSICS 5.1 software, with a nanostructured core and microstructured cladding. Propagation characteristics like confinement loss and mode field diameter (MFD) are investigated and compared with various geometrical parameters like core diameter, cladding hole diameter, pitch, etc. Theoretical study revealed that a nanostructured PCF experiences reduced confinement losses and also improved mode field diameter. Furthermore, studies are also carried out by infiltrating the cladding holes with composite materials (liquid crystal and glass). These simulations helped in analysing the effect of different liquid crystal materials on PCF bandwidth and spectral positions

Design of optical fibre based highly sensitive acoustic sensor for underwater applications.

Fibre optic sensing is a key technology for a variety of underwater sensing and monitoring applications. Fibre optic acoustic sensors are mainly based on interferometric detection approach where the acoustic pressure-induced phase shift of light has been used as sensing principle. Recently, fibre optic acoustic sensors based on speciality fibres like Photonic Crystal Fibre (PCF) were reported. However, interferometry based detection approaches amongst all these fibre optics sensors are intensity based and therefore susceptible to light power fluctuations and require a complex instrumentation related to signal detection. Besides, wavelength based detection approach using FBG (Fibre Bragg Grating) offers significant advantages over the conventional approach. FBG sensors were reported to have higher performance for underwater acoustic sensing applications. This paper reports a novel design of an underwater acoustic pressure sensor using a combination of PCF and FBG to provide high sensitivity. Theoretical investigations were carried out on the PCF-FBG sensor to study the effect of applied pressure and induced strain on the FBG inscribed in the core of PCF. Effect of light confinement in PCF was studied for different geometrical parameters and 4-ring PCF structure was reported. Further, sensitivity enhancement was proposed utilizing air hole structure of the PCF to enhance the impact of acoustic pressure on the induced strain in FBG

Optical-fibre based sensors for monitoring offshore floating photovoltaic farms.

Solar photovoltaic farms are seeing increasing expansion into new domains (offshore) in order to harvest the sunlight as society pushes towards reaching Net Zero by 2050. However, photovoltaics are bound by physics, parameters such as temperature and strain varying result in changes in efficiency. Predictive maintenance through remote sensing can also save resources particularly in a domain where equipment and personnel are relatively expensive. This work represents the first investigation into the utilisation of Fibre Bragg Gratings (FBG) for offshore floating solar farms. It proposes a robust multiplexed network of FBG sensors to monitor these parameters in the harsh oceanic environment where equivalent electromechanical equivalents would fail. Using Super luminous LED (Light Emitting Diode), FBG interrogators, multiplexing techniques and the sensors themselves reliable, effective strain and temperature measurement can be made. A series of MATLAB simulations into how FBGs can be engineered to achieve these results were carried out. The findings suggest that applied to a select case study, 2 strain and 49 temperature sensors can be multiplexed on a single line of fibre. An experiment was also carried out that placed FBGs on an aluminum solar panel frame that suggests a bandwidth of 13.32μm would be needed to encompass the range in that specific study

Investigation of positioning of FBG sensors for smart monitoring of oil and gas subsea structures.

Condition monitoring of offshore structures is an indispensable task in the oil and gas industry. Fibre Bragg Grating (FBG) is the key technology used down-hole in order to sense different physical parameters such as strain, vibration, etc. This paper investigates the effect of FBG sensor positions on its reflected signal, in order to optimise the sensor positioning plan in structural health monitoring of subsea structures. Theoretical and experimental study was carried out on FBG sensors, to evaluate its strain sensitivities with varying positions. In addition, micro-displacement based strain analysis of FBG was carried out using a cantilever setup, in order to identify the effects of tensile and compressive strain under various load conditions. Furthermore, the effect of different grating parameters on FBG sensing signal were also analyzed. Theoretical modeling and simulation of FBG was conducted in MATLAB using the coupled mode theory

Theoretical investigation of positional influence of FBG sensors for structural health monitoring of offshore structures.

Fibre Bragg Grating (FBG) is a key technology for condition monitoring of different offshore oil and gas structures. FBG sensors are used to sense different physical parameters such as strain, temperature, vibration, etc. This paper investigates the effect of FBG sensor positions on the reflected sensing signal, to optimise the sensor positioning plan for structural health monitoring of offshore structures. Theoretical investigations were carried out on a cantilever beam to analyze the strain effects. Effect of different cantilever beam shapes, materials and their thickness on strain was investigated. Theoretical studies were also carried out to evaluate the strain sensitivities of FBG sensors. Furthermore, micrometer displacement based strain analysis of cantilever beam was carried out using FBG sensors and electrical strain gauges to study the positional influence and compared it with the theoretical results obtained