The Incredible Fiber-Optic Flowmeter

Online, Real-Time Multiphase Measurements Improve Safety of Oil and Gas Production

By Béla Lipták

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The subsea multiphase flowmeters are “marinized,” packaged and deployed by specialist subsea companies to replace topside well test separators and serve not only the management of individual wells, but also reservoir management and allocation metering.

Fiber-Optic Flowmeters

The latest technology in subsea flow metering uses downhole fiber-optic (FO) cables mounted on the surface of the production pipe. On the right of Figure 3, the FO cable connecting the distributed optical temperature sensors (DTS) is shown in red, and the cable connecting the distributed optical pressure sensors (DPS) is shown in blue. They interrogate multiple pressure and temperature sensors mounted on the outside surface of the production pipe.

These optical sensors take advantage of the fact that light in vacuum travels at velocity C, and when it reaches the surface of a substance, it slows to velocity V. The refractive index n of a particular substance equals the ratio of these two speeds (n = C/V). Therefore, the refractive index determines how much the light is bent (or refracted back when reaching the interface with a substance).

The refractive index also determines the critical angle of reflection, the angle at which total reflection occurs, and the material starts behaving like a mirror. Therefore, if one is able to prepare an optical filter grating element that transmits all wavelengths except one, a wavelength-specific mirror is obtained.

The method, allowing a number of sensors to be interrogated by a single FO cable, uses a fiber Bragg grating (FBG). A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. A fiber Bragg grating can therefore be used to provide inline optical filters, each of which blocks or reflects a different specific wavelength.

This system is usually referred to as a Distributed Bragg reflector. Figure 4 shows the structure of a fiber Bragg grating system. The refractive index profile of the fiber core shows the change of the refractive indexes (n0, n1, n2...) along the core, and the spectral response at the bottom shows how the incident broadband signal is split into the transmitted and reflected components at the Bragg wavelength, λB.

Optical Pressure and Temperature Sensors

The fluid (a mix of gas, water and oil) passing through the production pipe travels at some average temperature and pressure. Both of these variables oscillate around some average value. These fluctuations (the noise superimposed over the average values of the pressure and temperature of the fluid) carry valuable information, because they are caused by eddy currents, gas bubbles, specific gravity changes and composition variations, etc., that occur very quickly.

The differential pressure between two detectors, for example, is related to the volumetric flow passing through the pipe, while the time it takes for a particular fluctuation to travel from one detector to another relates to the velocity of the fluid. The extremely fast optical pressure and temperature detectors (“1” in Figure 5) pick up these oscillations and forward them to the sophisticated algorithms at the receiving end of the FO cable, which interpret them – this “superimposed noise” traveling up the production pipe—into flow rate and composition.

In my next column I will describe other new flowmeter designs, unless in the meanwhile, an even more interesting topic comes up.

Béla Lipták, PE, automation, safety and energy consultant, is also editor of the Instrument Engineers' Handbook. He can be reached at liptakbela@aol.com.

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