WO2012089816A2

WO2012089816A2 - Fibre bragg grating measurement method and system

Info

Publication number: WO2012089816A2
Application number: PCT/EP2011/074248
Authority: WO
Inventors: Johannis Josephus Den Boer; Andre Franzen; Paul Gerard Edmond LUMENS
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2010-12-30
Filing date: 2011-12-29
Publication date: 2012-07-05
Also published as: WO2012089816A3

Abstract

The sensitivity of a fiber optical sensing assembly (2) with Fibre Bragg Gratings ( FBGs) (3, 4) to temperature variations in the associated light source (6) is reduced by using an interrogation system (1) with first and second sensors ( 12&15) that are connected to the sensing assembly (2) and to a reference element ( 14 ) to generate a measurement signal (M1, M2) and a reference signal, and a signal processing device (13), which identifies peaks (R1, R2) of the reference signal, compares a pattern of the identified peaks (R1, R2) with a known pattern of the reference spectrum to determine an order of wavelengths in the reference signal, and calculates for at least one peak an associated wavelength using the order of wavelengths in the reference signal, thereby avoiding that a different order of the peaks (R1, R2) in the reference signal, for instance due to temperature variations at the light source (6), results in false calculation of a wavelength in the measurement signal (M1, M2).

Description

FIBRE BRAGG GRATING MEASUREMENT METHOD AND SYSTEM

BACKGROUND OF THE INVENTION

The invention relates to a method and measurement system for monitoring one or more parameters using an optical fiber having at least one Fibre Bragg

Grating (FBG) .

It may be desirable to monitor parameters, such as for example pressure, temperature and chemical

composition at various locations. Such method may be used in relation to production of marketable hydrocarbons, for instance for monitoring well parameters in an oil or gas reservoir from which the hydrocarbons are extracted.

Fibre Bragg gratings may be interesting for this goal, since fibre Bragg gratings have high resistance to harsh environments, are compliant to electro-magnetic

interference standards and have low signal losses over long lengths.

In a Fibre Bragg Grating ( FBG) , a periodic variation of the refractive index is provided at a measurement location in an optical fibre. This periodic variation of the refractive index reflects light of a specific

wavelength. This specific wavelength is proportional to the spatial period of the variation in refractive index. Predominantly longitudinal strain will change this spatial period and as a result the specific wavelength reflected by the periodic variation will change.

Since local strain in the fibre at the measurement location may change, for example, due to changes in temperature, pressure, and chemical composition, these parameters may be observed by an interrogation system arranged to detect the specific wavelength reflected by the Fibre Bragg Grating (FBG) . US 6,573,489 discloses temperature compensated techniques for tunable filter calibration in fibre Bragg grating interrogation systems.

In these interrogation systems, a tunable filter such as a Fabry-Perot filter is used to provide a series of narrow-band light transmission windows scanning through a relevant wavelength range. The wavelengths at which the resulting light beam will be reflected by the Fibre Bragg Grating ( FBG) , is dependent on the strain in the fibre at the location of the Fibre Bragg

Grating (FBG) . When the wavelength of this one or few reflected transmission windows is known, the strain at the measurement location may be deducted, and on the basis of this strain, one or more relevant parameters at the measurement location causing this strain may be determined. Since measurement of the wavelength reflected by the Fibre Bragg Grating (FBG) is temperature dependent, a reference element may be required to compensate for temperature differences.

In an embodiment of US 6,573,489, it is proposed to provide a hydrogen-cyanide absorption cell as a reference element. Such absorption gas cell absorbs the light at discrete wavelengths corresponding to the molecular vibrational mode frequencies of the gas in the absorption cell. These discrete wavelengths are highly temperature independent. Therefore, the reference spectrum of the gas cell provides a reliable temperature independent

reference for the measurement obtained from the Fibre Bragg Grating (FBG) .

Known interrogation systems, such as the

interrogation system of US 6,573,489 are typically designed for labatory environments, and may not reliably function under more extreme conditions which may for instance be present in or near an oil or gas well. In the latter application both the Fibre Bragg Grating (FBG) in the well as the interrogation system arranged near the well may be subject to large temperature differences and other extreme environmental conditions. For instance, the interrogation system may be placed in a desert

environment or below sea level and the Fibre Bragg

Gratings ( FBGs ) in the well may be subject to high

temperatures and pressures.

The output of a tunable filter such as a Fabry-Perot filter may also be highly temperature dependent.

The processing of the measurements as proposed in US patent 6,573,489 is not designed to deal with this effect. The processing of the measurements in this known interrogation system is based on the assumption that the peaks in the reference signal all have peaks with

increasing wavelength over time, i.e. scanning the control signal over the relevant control range results in scanning of the narrow band light beam of the filter device over the relevant wavelength range from the lowest relevant wavelength to the highest relevant wavelength. Using the method of US patent 6,573,489 in more extreme environmental conditions may, as a result, lead to false calculations .

There is a need for improved method for monitoring one or more parameters using a Fibre Bragg Grating ( FBG) , which is economic and robust and which reliably functions under harsh conditions such as in or near wells that traverse crude oil and/or natural gas containing earth formations. SUMMARY OF THE INVENTION

The invention provides a method for monitoring one or more parameters with an optical fiber having at least one fibre Bragg grating, comprising the steps of:

providing a measurement system comprising:

- a light source system configured to transmit a narrow band light beam through the optical fiber, wherein the wavelength of said narrow band light beam is dependent on a control signal,

- an optical measurement path running through the optical fiber to receive at least part of the narrow band light beam and comprising at least one Fibre Bragg

Grating (FBG) ,

- an optical reference path to receive at least part of the narrow band light beam and comprising a reference element having a substantially temperature independent reference spectrum at least over a relevant wavelength range,

- a first sensor to receive a first output light beam of the measurement path and to provide a measurement signal dependent on the first output light beam,

- a second sensor to receive a second output light beam of the reference path and to provide a reference signal dependent on the second output light beam, and

- a processing device to determine a wavelength of a peak of the first output light beam on the basis of the measurement signal and the reference signal,

scanning the control signal over a relevant control range to provide a narrow band light beam scanning over the relevant wavelength range,

measuring the first output light beam and the second output light beam to obtain the measurement signal and the reference signal, determining the wavelength of at least one peak in said measurement signal, the wavelength of said peak being representative for one or more of the parameters, wherein the step of determining the wavelength of the at least one peak in the measurement signal comprises:

identifying peaks of the reference signal,

comparing a pattern of said identified peaks with a known pattern of the reference spectrum to determine an order of wavelengths in the reference signal, and

calculating for the at least one peak of the measurement signal an associated wavelength using said order of wavelengths in the reference signal.

With the method of the invention, the peaks in the reference signal, for instance the transmission lines of a gas absorption cell, are identified and a pattern of these identified peaks is compared with the pattern of the known reference spectrum of the reference element. By this comparison the order of the wavelengths emitted in the course of time can be reliably determined and used for determining the wavelength of a peak in the

measurement signal.

By using the method according to the invention it can be avoided that a different order of the peaks in the reference signal, for instance due to significant

temperature differences at the location of the light source system, results in false calculation of a

wavelength in the measurement signal.

Since the method of US 6,573,489 assumes that the wavelengths in the reference signal always have the same order, this method does not comprise the step of

comparing a pattern of said identified peaks in the reference signal with a known pattern of the reference spectrum to determine the order of wavelengths in the reference signal.

It is remarked that time may be expressed in number of samples or measurements, values of the control signal or in any other value representative for time in which measurements are made. Further, it is remarked that the peaks of the reference signal and/or measurement signal may also be negative peaks, i.e. dips, in the respective signal. Thus, the reference spectrum may be a

transmission or a reflection spectrum, and the

measurement signal may be based on reflection or

transmission of the Fibre Bragg Grating (FBG) .

The pattern of the identified peaks in the reference signal may be determined on the basis of time intervals between the peaks and/or the values of the peaks. Any other way of determining a pattern in the identified peaks may also be applied.

In an embodiment, the step of comparing identified peaks with a known pattern of the reference spectrum comprises pattern recognition in the identified peaks of the reference signal. By using pattern recognition, for instance by using a pattern recogition algorithm, to determine the pattern of the peaks in the reference signal, this pattern can easily be compared with the known pattern of the reference spectrum.

In an embodiment, the method comprises the step of selecting spectral parts of the reference signal and/or arranging spectral parts of the reference signal to provide a calibration spectrum with increasing

wavelengths. Once the pattern of the peaks in the reference signal is known, the order of the wavelengths in the reference signal can be determined. This order of the wavelengths in the reference signal provides a relation between wavelength and time.

When the order of the wavelengths in the reference signal is known, the wavelengths emitted in the course of time can be determined on the basis of the reference signal. Then, spectral parts of the reference signal comprising wavelengths in the relevant wavelength range can be selected and arranged with increasing wavelengths to obtain the calibration spectrum. When the wavelengths in the reference signal are not in increasing order, this arranging step may comprise stitching of the selected spectrals parts of the reference signal.

By stitching of the spectral parts of the reference signal a useful calibration spectrum with increasing wavelengths may be obtained. This calibration spectrum may be used in further calculation of the wavelength of a peak in the measurement signal.

Also, the measurement signal may be recalculated on the basis of the relation between wavelength and time into a measurement spectrum with increasing wavelengths.

This recalculation may involve selecting spectral parts of the reference signal and/or arranging spectral parts of the reference signal, and may be based on the

corresponding spectral parts of the measurement signal, i.e. the spectral parts in the corresponding time

intervals as the selected and rearranged spectral parts of the reference signal.

In an embodiment, the step of scanning the control signal over the relevant control range results in the narrow band light beam scanning over more than one cycle of the relevant wavelength range. By scanning more than one cycle over the relevant wavelength range more

information in the reference signal is obtained with respect to the reference spectrum. This results in a more reliable recognition of the pattern in the reference signal .

Furthermore, by scanning more than one cycle it can be assured and checked that all relevant wavelengths are transmitted by the light source system. Scanning more than one cycle may for instance be realized by a Fabry- Perot which shows periodic light transmission windows. By scanning the input voltage, for instance a control voltage over a control range, two subsequent periodic transmission windows of the Fabry-Perot filter may together scan more than one cycle in the relevant

wavelength range, even when a first transmission window starts in the middle of the relevant wavelength range.

In such embodiment, the step of scanning the

relevant control range results in two or more subsequent narrow band light transmission windows in the relevant wavelenlength range, each of the two or more narrow band light transmission windows scanning at least a part of the relevant wavelength range, and the two or more narrow band light transmission windows together scanning over the complete relevant wavelength range. This may not always be the case when only one transmission window is provided by the light source system, since due to

temperature effects this transmission window may start with a wavelength in the middle of the relevant

wavelength range. In the latter case a substantial part of the relevant wavelength range may not be scanned by the light source system.

In an embodiment, the method comprises spatial frequency filtering of the reference signal to cancel background transmission fluctuations and noise. By eliminating background transmission fluctuations and noise, the further processing of the reference signal can be done more accurately.

In an embodiment, the light source system comprises a broad band light source to provide a broad band light beam and a tunable filter device, for instance a Fabry-

Perot filter, arranged to receive the broadband light beam and configured to let pass the narrow band light beam dependent on the control signal. In an alternative embodiment, the narrow band light beam may be produced by a tunable laser device, which can subsequently emit narrow band light beams with different wavelengths in dependence of a control signal. The first and second sensors may be photodetectors .

Preferably, the measurement path and the reference path are arranged in parallel, wherein the first sensor is arranged to receive the first output light beam and the second sensor is arranged to receive the second output light beam. In an alternative embodiment, the measurement path and the reference path are arranged serially, the first and second are provided as a single sensor to receive a combined first and second output light beam configured to provide a combined measurement and reference signal.

It is remarked that the method according to the invention may be used in any application, wherein Fibre

Bragg Gratings (FBGs) are used to determine one or more parameters such as temperature or pressure. The method is in particular suitable for aplications where robustness of the measurement method and system is required.

The method according to the invention is in

particular suitable for determining one or more

parameters in a method for converting hydrocarbons in an oil or gas well into a marketable hydrocarbon composition, the method comprising the steps of obtaining hydrocarbons out of the well, transporting the

hydrocarbons to a facility in which the hydrocarbons are converted into a marketable hydrocarbon composition, and converting the hydrocarbons into the marketable

hydrocarbon composition.

In such method, the Fibre Bragg Grating (FBG) may for example be arranged in an oil or gas well to measure well parameters such as temperature, pressure or chemical composition in the well, on or in a transport pipeline for hydrocarbons to measure parameters of hydrocarbons transported through the transport pipeline, or for measuring different parameters in the facility, for example a refinery, in which the hydrocarbons are

converted into a marketable hydrocarbon composition.

The method may also be used to measure parameters before, during, after, or in between the production of a marketable hydrocarbon, and/or to measure parameters in observation wells.

The invention further relates to a fiber optical measurement system comprising:

a light source system configured to transmit a narrow band light beam through an optical fiber, wherein the wavelength of said narrow band light beam is dependent on a control signal,

an optical measurement path running through the optical fiber to receive at least part of the narrow band light beam and comprising at least one fibre Bragg grating, an optical reference path to receive at least part of the narrow band light beam and comprising a reference element having a substantially temperature independent reference spectrum at least over a relevant wavelength range, a first sensor to receive a first output light beam of the measurement path and to provide a measurement signal dependent on the first output light beam,

a second sensor to receive a second output light beam of the reference path and to provide a reference signal dependent on the second output light beam, and

a processing device to determine a wavelength of a peak of the first output light beam on the basis of the measurement signal and the reference signal,

wherein the measurement system is configured to scan the control signal over a relevant control range to provide a narrow band light beam scanning over the relevant

wavelength range,

wherein the processing device is configured to determine the wavelength of at least one peak in the measurement signal by:

identifying peaks of the reference signal,

calculating for the at least one peak in the

measurement signal an associated wavelength using said order of wavelengths in time in the reference signal.

The measurement system can be used to determine a wavelength of a peak reflected by a Fibre Bragg

Grating (FBG) . This wavelength is representative for strain in the Fibre Bragg Grating (FBG) . This strain is a measure for one or more parameters at the measurement location, such as pressure and/or temperature.

In further embodiments, the processing device may be configured to carry out steps in accordance with the dependent method claims. These and other features, embodiments and advantages of the method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of non-limiting

embodiments depicted in the accompanying drawings, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be

described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which :

- Figure 1 shows schematically an embodiment of the measurement system of the invention;

- Figure 2 shows a diagram of a control signal of a tunable filter device;

- Figure 3 shows a diagram of a measurement signal measured by a first photodetector;

- Figure 4 shows the reference spectrum of the a

hydrogen-cyanide gas absorption cell;

- Figure 5 shows a diagram of a reference signal measured by a second photodetector;

- Figure 6 shows a diagram of the reference signal after spatial filtering;

- Figure 7 shows a diagram of wavelength versus the reference signal;

- Figure 8 shows a diagram of wavelength versus the measurement signal; and

- Figure 9 shows schematically the main steps of the method according to the invention DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Figure 1 shows a measurement system according to the invention. The measurement system comprises an

interrogation system 1 and an optical fibre 2 comprising fibre Bragg gratings 3, 4. The fibre is arranged in a measurement position, for example in a well 5. In the fibre Bragg gratings 3, 4, a periodic variation of the refractive index is provided at the respective locations of the fibre Bragg gratings 3, 4. In practice, multiple fibre Bragg gratings may be provided in the well and interrogated by the interrogation system 1. These

multiple fibre Bragg gratings may for instance be used to determine well parameters at different measurement locations in the well 5 or to differentiate between strains resulting from temperature, pressure or other effects .

By measuring the wavelength which is reflected by the fibre Bragg grating 3, 4 the strain to which the fibre Bragg gratings 3, 4 are subject, can be determined. On the basis of this strain, parameters in the well, such as temperature, pressure and chemical composition, may be deducted .

The interrogation system 1 comprises a light source system 6 to provide a narrow band light beam, wherein the wavelength of said narrow band light beam is dependent on a control signal. The light source system 6 comprises a broad band light source 7, for instance a SLED, and a tunable filter device 8, for instance a Fabry-Perot filter .

The broadband light source 7 emits a broad band light beam comprising all wavelengths of the relevant wavelength range. This relevant wavelength range may comprise the wavelengths which may be reflected by the Fibre Bragg Gratings ( FBGs ) 3, 4 when different strains are exerted on the fibre 2 at the measurement locations, and wavelengths of the reference spectrum that is used. The tunable filter device 8 is arranged to receive the broadband light beam from the broadband light source 7 and configured to let pass a narrowband light beam through a light transmission window. The wavelength of this narrow band light beam is dependent on a control signal provided by a control system 9.

During measurements, the control system 9 will provide a scanning control signal which scans through a relevant control range to provide a series of narrow band light beams with different wavelengths. Such scanning control signal in the form of an increasing control voltage Cv in time T is shown in Figure 2.

Via a coupler 10 and a circulator 11, a part of each narrow band light beam is directed to the fibre 2 which is arranged in the well 5. In the well 5 each of the fibre Bragg gratings 3, 4 will reflect one of the narrow band light beams. The wavelength of the respective reflected light beam is dependent on the strain in fibre Bragg gratings 3, 4. The reflected narrow band light beams will come back into the interrogation system 1 and be directed via the circulator 11 towards a first

photosensor 12 configured to measure the intensity of the light received by the first photosensor 12. The

photosensor 12 provides a measurement signal dependent on the first output light beam. This measurement signal is guided to a processing device 13 to determine the

wavelength of one or more peaks in the measurement signal. Figure 3 shows an example of a measurement signal over time in response to the control signal shown in Figure 2. In practice, such measurement signal may for

instance comprise 50.000 to 150.000 measurement points measured in a time period of 1 second, while the tunable filter device 8 scans over the relevant wavelength range. This relevant wavelength range may for instance be 80 to

150 nm scanned with a resolution of 1 to 3 pm.

Since the output of the tunable filter device 8 is highly dependent on temperature and other circumstances, there is no constant relationship between control signal and the wavelength of a narrow band light beam that passes the tunable filter device 8 with this control signal. The same course of the control signal may result in that the wavelengths of the relevant wavelength range are scanned in a different order due to the

circumstances. As a consequence, the wavelength of the respective peaks in the measurement signal cannot be directly deducted from the measurement signal shown in Figure 3.

Therefore, the interrogation unit 1 comprises an optical reference path to calibrate the measurement signal. At the coupler 10 the narrow band light beam coming from the tunable filter device 8 is splitted in two parts. As indicated above one part of the narrowband light beam is directed towards the circulator 11 to follow the optical measurement path towards the first photo sensor 12. The other part of the narrow band light beam is guided towards a gas absorption cell 14, which is, in this embodiment, used as a reference element.

The gas absorption cell 14 is for instance a

hydrogen-cyanide gas cell, in which the gas is held in a pressurized cylinder with an optical input and output. The gas absorption cell 14 aborbs light at discrete wavelengths corresponding to the molecular vibrational mode frequencies of the gas. The reference spectrum of the gas absorption cell 14 is highly temperature

independent, and can therefore be used as a reference element for the measurements with the fibre Bragg

gratings 3, 4. An example of the reference spectrum of the gas absorption cell 14 is shown in Figure 4.

The light beam coming out of the gas cell is

directed to a second photosensor 15 configured to measure the intensity of the light received by the second

photosensor 15. The second photosensor 15 provides a reference signal dependent on intensity of the reference light beam. Figure 5 shows an example of the reference signal over time in response to the control signal shown in Figure 2.

The peaks, in the form of transmission lines or dips, in the reference signal of Figure 5 correspond to particular wavelengths independent of the temperature. However, since the output wavelength of the tunable filter device 8, is highly dependent on temperature, it is not known in which order the relevant wavelength range is scanned, and therefore the peaks cannot directly be coupled to wavelengths from the lowest wavelength to the highest wavelength in the reference spectrum.

To make sure that all wavelengths in the relevant wavelength range are present in the measurement signal and the reference signal, the tunable filter device is configured such that one cycle of scanning the control signal over the control range results in more than one cycle, for instance one and a half cycle of scanning through the relevant wavelength range. This can for instance be obtained by the periodic light transmission windows of a Fabry-Perot filter. When a first light transmission window of the filter runs out of the relavant wavelength range a new light transmission window may enter the relevant wavelength range.

In an alternative embodiment, wherein the light source system does not produce periodic light

transmission windows, but only one light transmission window resulting from a input voltage, the relevant control range may be scanned more than one cycle to ensure that the whole wavelength range is covered by the scanning narrow band lightbeam of the light source system.

Figure 5 shows a diagram of the reference signal comprising intensity of the reference signal obtained from the gas absorption cell 14 versus time. To use this reference signal to determine the wavelength of peaks in the measurement signal shown in Figure 3, the following processing steps are taken in the processing device 13.

As a first step spatial frequency filtering of the reference signal is applied to cancel background

transmission fluctuations and noise. The resulting signal is shown in Figure 6. Any other form of signal processing to cancel background transmission fluctuations and/or noise may also be applied. In some cases, it may be possible to omit the step of spatial frequency filtering.

Thereafter, the peaks of the reference signal are identified. For example, the magnitude of the peaks of the reference signal and the associated points in time are determined using a peak detection algorithm. On the basis of the identified peaks, a pattern of the peaks in the reference signal is identified and compared with a known pattern of the reference spectrum of the gas absorption cell 14, as shown in Figure 4. By matching these patterns, the relation between time and

corresponding wavelengths is obtained. This relation between time and wavelength can be used as a basis to determine the wavelength of the peaks in the measurement signal .

Figure 7 shows a calibration diagram wherein the pattern of the reference signal is shown in relation to the associated wavelengths. The solid line between wavelengths of 1545 and 1547.5 nm indicates the beginning of the measurement signal in time. From this start, the order of the wavelengths in the measurement signal increases till the end of the relevant wavelength range, as indicated by an arrow. A subsequent periodic light transmission window of the tunable filter device 8 then starts to scan the relevant wavelength range from the lowest wavelength up to and beyond the wavelength shown in solid line, since, as explained above, the relevant wavelength range is scanned more than one cycle.

It is observed that in an alternative embodiment of the method and system according to the invention the relevant wavelength range may only be scanned once.

When the pattern of the reference signal is

recognized and matched with the associated wavelengths, the calibration diagram of Figure 7 can be obtained by selecting spectrum parts Rl, R2 in the relevant

wavelength range from the reference signal of Figure 6, and arranging the selected parts Rl, R2 with an ascending order of the wavelengths. The spectral parts Rl, R2 are stitched at a stitching wavelength. The stitching

wavelength is in the example of Figure 7 indicated by the solid line at 1547 nm, but may also be at another

wavelength since the parts obtained by the subsequent light transmission windows of the tunable filter device 8 have a substantial overlapping part. When required any wavelength of measurement points between the identified peaks of the reference signal may be calculated by interpolation between the identified peaks of the reference signal.

Once the relation between time and wavelength is known, the time axis in the measurement signal can be replaced by the corresponding wavelength resulting in a relationship between the peaks in the measurement signal and the associated wavelengths.

Figure 8 shows a diagram wherein the relation between measurement signal and wavelengths is depicted as a measurement spectrum with increasing wavelengths.

For the measurement spectrum of Figure 8, spectral parts Ml, M2 of the measurement signal of Figure 3 are selected corresponding to the spectral parts Rl, R2 of the reference signal, i.e. the spectral parts Ml, M2 are selected from the same time intervals as the spectral parts Rl, R2 used to determine the calibration spectrum of Figure 7. The two spectral parts Ml, M2 are selected and arranged with increasing wavelengths, whereby the spectral parts Ml, M2 are stitched at the stitching wavelength indicated by the solid line.

By using a peak identification algorithm, for instance fitting to a quadratic function, the peaks in the measurement signal can be found and matched with a corresponding wavelength.

The above-described method and measurement system is robust as it is less sensitive for the effects of

temperature differences in the light source system 6.

It will be understood that the above-described method and system significantly reduce the sensitivity of a fiber optical sensing assembly (2) with Fibre Bragg Gratings ( FBGs ) (3,4) to temperature variations in the associated light source (6) by using an interrogation system(l) with first and second sensors ( 12&15) that are connected to the sensing assembly (2) and to a reference element ( 14 ) to generate a measurement signal (Ml , M2 ) and a reference signal, and a signal processing device (13), which identifies peaks of the reference signal (Rl , R2 ) , compares a pattern of the identified peaks (R1,R2) with a known pattern of the reference spectrum to determine an order of wavelengths in the reference signal, and

calculates for at least one peak an associated wavelength using the order of wavelengths in the reference signal, thereby avoiding that a different order of the

peaks (R1,R2) in the reference signal, for instance due to temperature variations at the light source (6), results in false calculation of a wavelength in the measurement signal (M1,M2) .

Therefore, the FBG measurement method and system according to the invention are robust and cost effective and are therefore suitable for monitoring at a wellhead downhole parameters in a well for producing hydrocarbons that are subsequently converted into a marketable

hydrocarbon composition.

Figure 9 shows schematically the main steps of this method. Step A indicates the step of obtaining

hydrocarbons out of an oil or gas well. This may be performed by any suitable method. During the step A, parameters in the well such as temperature, pressure or chemical composition may be monitored, and possibly be used to control the parameters in the well. In accordance with the invention, fibre Bragg gratings are arranged in a well related to the producing reservoir and

interrogated to monitor the well parameters, as shown in Figure 1. In step B the hydrocarbons obtained from the oil or gas well are transported to a facility in which the hydrocarbons can be converted into a marketable

hydrocarbon composition. Transport may be carried out in any suitable way, such as an oil tanker or pipeline.

Before transport the hydrocarbons may be processed to make them more suitable for transport. During transport one or more parameters may be monitored by a method or system according to the invention. The fibre Bragg grating may for instance be arranged on or in a transport pipeline .

In step C the hydrocarbons are in the facility converted into a marketable hydrocarbon, for instance by refining the hydrocarbons in a refinery. Also in the facility parameters may be monitored by a method and system according to the invention.

Claims

C L A I M S

1. A method for monitoring one or more parameters with an optical fiber having at least one Fibre Bragg

Grating ( FBG) , comprising the steps of:

providing a measurement system comprising:

- an optical measurement path running through the optical fiber to receive at least part of the narrow band light beam and comprising at least one fibre Bragg grating,

- a processing device to determine a wavelength of a peak in the first output light beam on the basis of the measurement signal and the reference signal,

measuring the first output light beam and the second output light beam to obtain the measurement signal and the reference signal, determining the wavelength of at least one peak in said measurement signal, the wavelength of said peak being representative for one or more of the parameters, characterized in that the step of determining the wavelength of the at least one peak in the measurement signal comprises:

identifying peaks of the reference signal,

calculating for the at least one peak an associated wavelength using said order of wavelengths in the reference signal.

2. The method of claim 1, wherein comparing identified peaks with a known pattern of the reference spectrum comprises pattern recognition in the identified peaks of the reference signal.

3. The method of claim 1 or 2, comprising the step of selecting and/or arranging and/or stitching one or more spectral parts of the reference signal to provide a calibration spectrum with increasing wavelengths.

4. The method of any of preceding claims, comprising the step of selecting and/or arranging and/or stitching one or more spectral parts of the measurement signal to provide a measurement spectrum with increasing

wavelengths .

6. The method of any of preceding claims, wherein the step of scanning the relevant control range results in the narrow band light beam scanning over more than one cycle of the relevant wavelength range.

7. The method of any of preceding claims, wherein the step of scanning the relevant control range results in two or more narrow band light transmission windows, each of the two or more narrow band light transmission windows scanning at least a part of the relevant wavelength range, and the two or more narrow band light transmission windows together scanning over the complete relevant wavelength range.

8. The method of any of preceding claims, wherein the method comprises spatial frequency filtering of the reference signal to cancel background transmission fluctuations and/or noise.

9. The method of any of preceding claims, wherein identifying peaks of the reference signal and/or

measurement signal is based on a peak detection

algorithm.

10. The method of any of the preceding claims, wherein the light source system comprises a broad band light source to provide a broad band light beam and a tunable filter device arranged to receive the broadband light beam and configured to let pass one or more narrow band light transmission windows dependent on the control signal.

11. The method of any of the preceding claims, wherein the reference element is a reference gas cell.

12. The method of any of the preceding claims, wherein the measurement path and the reference path are arranged serially, and wherein the first sensor and the second sensor are a single sensor to receive a combined first and second output light beam to provide a combined measurement and reference signal.

13. The method of any of the preceding claims, wherein the optical measurement path comprises multiple fibre

Bragg gratings in series or parallel, and wherein the processing device is configured to determine the wavelength of at least one peak in measurement signal of each of the multiple fibre Bragg gratings.

14. A method for converting hydrocarbons in an oil or gas well into a marketable hydrocarbon composition, the method comprising the steps of obtaining hydrocarbons out of the well, transporting the hydrocarbons to a facility in which the hydrocarbons are converted into a marketable hydrocarbon composition, and converting the hydrocarbons into the marketable hydrocarbon composition,

wherein the method comprises the step of monitoring one or more parameters using the method of any of the claims 1-13.

15. A fiber optical measurement system comprising:

an optical measurement path running through the optical fiber to receive at least part of the narrow band light beam and comprising at least one Fibre Bragg

Grating (FBG) ,

an optical reference path to receive at least part of the narrow band light beam and comprising a reference element having a substantially temperature independent reference spectrum at least over a relevant wavelength range, a first sensor to receive a first output light beam of the measurement path and to provide a measurement signal dependent on the first output light beam,

a second sensor to receive a second output light beam of the reference path and to provide a reference signal dependent on the second output light beam, and a processing device to determine a wavelength of a peak of the first output light beam on the basis of the measurement signal and the reference signal,

wavelength range,

characterized in that the processing device is configured to determine the wavelength of at least one peak in the measurement signal by:

identifying peaks of the reference signal,

calculating for the at least one peak in the measurement signal an associated wavelength using said order of wavelengths in the reference signal.