Optical fibre sensors for flow and pressure measurement in harsh environment

Abstract

Optical fibre sensor can work in harsh environments, which makes it an ideal candidate for many industrial applications. Flow and pressure sensor are two important types of sensors for oil and gas industry to monitor and control the production of their products. The temperature can reach as high as 200{493}C and located hundreds of meters or even over a kilometer away from the measuring devices. In these applications, remote and harsh environment are often the conditions that the sensors are working in. In this thesis, a number of schemes for implementing thermal anemometer for flow measurement and pressure sensor are proposed and investigated experimentally and theoretically. The proposed fibre optic thermal anemometer has a pure silica structure, which has never been reported before. Pure silica structure makes it possible for the sensor to work in harsh environment like elevated temperature and corrosive gas measurement. The pure silica structure also makes the sensor structure simpler, making it easier to be fabricated. The sensor is based on a section of high absorption fibre that absorbs the pumping laser light carried in the core and convert it into thermal energy. As the flow rate of the fluid changes, the rate of convective heat transfer changes and so the temperature of the sensor varies. A pumping laser with constant power output is required. A fibre Bragg grating (FBG) is inscribed in the section of the high absorption fibre to measure the temperature variation. By measuring the temperature of the sensor, the flow rate of the fluid can be found. Other than flow rate, ambient temperature and optical power absorption also affect the measured temperature. Fluctuation of these two parameters causes measurement errors. Another method is proposed to reduce the sensor's dependency on those two parameters. Instead of applying constant optical power, the pump laser output is modulated on and off. Measuring the time constant of the falling temperature of the sensor when the pumping laser output is off, the flow rate of the fluid can be found. This method is proved to be much less sensitive to absorbed optical power and ambient temperature change. One drawback of such operation is that it only measures the average flow speed during a period of time while constant power operation can tell the instant flow speed. Serial multiplexing of the sensors is realized by shifting the FBGs' wavelength to around 850 nm where the attenuation is low. Multiplexing of three sensors has been realized successfully experimentally. Another sensor studied in the thesis is a Sagnac-loop based pressure sensor. The fibre used is Blaze Photonics PM-1550-01 polarization maintaining photonic crystal fibre (PM-PCF). The benefit of such design is the low temperature sensitivity. By changing the operation wavelength to 850 nm comparing with previous proposed schemes of operating at 1550 nm, simulation and experimental studies have shown that much higher sensitivity can be achieved. Moving to 850 nm can also enable the use of low cost CCD based optical fibre interrogator. This potentially enables much higher measurement speed and lower system cost comparing with system operating at 1550 nm. Instead of measuring the spectral shift of the sensor's transmission spectrum as in the previous studies, the phase change of the sinusoidal waveform like spectrum obtained from a CCD optical fibre interrogator is employed for pressure measurement. This method enables constant sensitivity to be sustained throughout the whole measurement range. The proposed new method also offers slightly better accuracy than previous proposed spectral shift technique.