Distributed Fibre Optic Sensors for the Detection of Liquid Spills

A range of distributed fibre optic sensors for the detection and location of aqueous, chemical and hydrocarbon fluid spills is presented. The sensors incorporate polymers that convert the swelling into a localised loss on an optical fibre when fluid exposure occurs. Optical Time Domain Reflectometry (OTDR) techniques are employed to rapidly detect and locate target liquids and chemicals at multiple positions along the sensor length. Sensor exposure to fluid can typically be located within 30 seconds to an accuracy of ± 2 m over a total length of 10 km. Once the polymer has dried out, the sensor returns to a nonactivated state where further spill events may be detected. A brief description of the basic sensor construction and of the underlying technology utilised in its operation is given. Results from experimental tests of prototype sensors manufactured to detect water, humidity, hydrocarbon fuels and organic solvents are then discussed. The response characteristics of the sensors in a range of varying environmental conditions and their performance during practical field trials are outlined. We conclude with a summary of the important advantages of the sensor design and the range of applications where it can be effectively implemented. INTRODUCTION The ability to rapidly detect and locate liquid spills is an important capability in industrial sectors where large volumes of liquids are regularly stored or transported. In the water and petrochemical industries this capability is critical to ensure that supplies are safely and efficiently controlled. Early warning of water or hydrocarbon fuel leaks can reduce downtime costs and more importantly minimise the potentially damaging impact on the surrounding environment. Distributed fibre optic sensors are well suited to such applications since they are passive, do not require an electrical supply at the sensing location and provide multiple spill detection capability over the entire sensor length. Optical Time Domain Reflectometry (OTDR) techniques are employed to detect the occurrence of such events and locate the positions anywhere along the length of the sensor. The distributed nature of the OTDR measurement method offers distinct installation and cost advantages over conventional point sensor solutions to monitoring many separate locations. In this paper we describe a range of distributed optical fibre microbend sensors designed for the detection of water, solvents or hydrocarbon fuels. The sensors incorporate polymers that transduce their swelling into a localised microbending on the fibre when activated by the target liquid. The localised bending is detected on the OTDR trace as a change in the attenuation for that position. Experimental results showing the polymer response to the target liquids are described, followed by experimental evaluation of the sensor response to a range of fluids in field trials. PRINCIPLES OF SENSOR OPERATION The basic sensor design consists of a glass reinforced plastic (GRP) central strength member coated in a thin layer of the liquid sensitive material, typically 100 to 200μm thick. Two distinct polymer types are used; a water swelling hydrogel polymer for water detection and a silicone based material for organic solvents and hydrocarbon fluid detection. A 62.5/125 graded index multimode optical fibre with an outer diameter of 250 μm is held against the coated GRP core by a helical Kevlar thread that is wrapped along the entire sensor length. A section of the sensor detailing its constituent components is shown in Figure 1. The polymer coating swells when exposed to the target fluid, the extent being dependent on the activating liquid, the particular type and thickness of the polymer coating. Swelling of the polymer causes the optical fibre to be squeezed against the Kevlar thread, thus inducing a periodic lateral deformation in the fibre at the exposed location. Figure 1. Section of fibre optic sensor element Light passing through the optical fibre at the activated location will suffer a microbend loss caused by mode coupling between the guided and radiative modes. Energy is lost as it is coupled from the highest guided mode to the first radiation mode and subsequently attenuated in the cladding. The period of the deformation has a critical effect on the degree of attenuation that occurs. It was found by Fields 1 that maximum mode coupling occurs in a parabolic index multimode fibre for the specific periodic spacing Λ, given by Equation 1, Λ ∆ = 2 2 πa ( ) (1) where a is the core radius and ∆ is the maximum relative difference between the refractive indices of the core and the cladding of the fibre. Both Fields 1 and Deimeer 2 determined that a multiple of this spacing could also be used to induce high loss. For the multimode fibre used in the sensor, Λ is equivalent to a value of ~1 mm, thus periodic spacings of 2, 3, & 4mm may be used. The Kevlar wrap may therefore be applied at these values, thus reducing the sensitivity of the sensor to small variations in the pitch. Sensor lengths can range from tens of metres to several kilometres, the maximum length that can be detected being limited by the dry loss of the sensor and dynamic range of the OTDR instrument. The current limit is 2.5 km although it is anticipated that this can be extended to 10km with the use of longer interrogation wavelengths. The OTDR operates by sending short pulses of light into the optical fibre and monitors the Rayleigh back-scattered light that is returned as a function of distance along the fibre. When a localised section of the sensor is exposed to the activating fluid, the increase in attenuation causes a reduction in intensity of the back-scattered light for that fibre section. The OTDR trace shows the loss event as an increase in the gradient of the trace that can be readily identified if compared with a dry reference trace. A schematic of a typical set-up and OTDR trace for two separate events on the same sensor is shown in Figure 2.