Background

This invention relates to optical monitoring systems, and in particular distributed sea bed optical sensing systems. Optical sensors provide a convenient method of monitoring a range of physical properties of a location. The relative simplicity and robustness of optical sensors and the ability to locate the sensors significant distances from more complex interrogation hardware make optical systems particularly attractive where the sensors are to be located in hostile environments.

A particular family of optical sensing systems utilises a light source and detector (interrogator) located at a convenient interrogator location some distance from the actual sensors, with a fibre optic connection between. A particular application of such sensing systems is the marine oil and gas industry where the sensors are located on the sea floor and the interrogation location is on a surface vessel.

Conventional sensing systems utilise point sensors to sense parameters at a particular discrete location. For example a sensor may comprise mirrors and utilise optical interference as a sensing mechanism. Although such sensors can provide high sensitivity, measurements are only performed at a discrete location and the sensor is relatively complex. Achieving a high resolution thus requires many sensors making the system expensive and complex.

There are also alternative approaches using distributed sensing, in which measurements can be made at any point along a fibre without requiring any mirrors. This approach can give lower cost and higher spatial resolution, but the sensitivity and quality of signal at any position along the fibre is lower than with point sensors.

There is therefore a requirement for optical sensing systems providing improved resolution and/or utilising sensors with reduced complexity but still with the sensitivity and signal quality of point sensors..

Summary

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. There is provided an optical sensing system, comprising an optical sensing fibre comprising at least one optical sensor, the optical components of the sensor consisting of only optical fibre, the sensor utilising mechanical enhancement of the phase sensitivity to the measurand of interest, an optical source optically coupled to the sensing fibre to transmit optical pulses into that sensing fibre, an optical receiver optically coupled to the sensing fibre to receive optical signals from the optical fibre, wherein the at least one optical sensor of the optical sensing fibre is interrogated by transmitting a pair of optical pulses into the optical fibre and sampling a returning composite optical signal comprising light scattered from at least a region of the at least one sensor. The optical sensor may be positioned within an optical sensor body comprising a plurality of the optical sensors connected optically in series.

Each of the optical sensors may be configured to be sensitive to acceleration in a different direction.

Each sensor may comprise 10 - 50m of optical fibre, The pair of optical pulses may be of the same optical frequency. The pair of optical pulses may be of different optical frequencies. The polarisation of each pulse may be different.

The time between adjacent pulses may be greater than the optical time transit time of the pulses through the sensor. A plurality of pairs of pulses may be utilised to interrogate the sensor, the pulses being of a plurality of wavelengths to provide an improved sampling rate.

The optical fibre between optical sensors may be interrogated by transmitting a pair of optical pulses into the optical fibre and sampling a returning composite optical signal comprising light scattered from at least a region of the optical fibre between optical sensors. The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which: Figure 1 shows a distributed sensing system, and

Figure 2 shows the propagation of pulses in a sensing system.

Detailed Description

Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples. Figure 1 shows a schematic diagram of an optical sensing system providing decreased sensor costs. An interrogator 10 is located at a convenient location, for example on a surface vessel and is connected to a sensor cable 11 by a riser cable 12. The sensor cable 11 and riser cable 12 may comprise a plurality of optical fibres, and may comprise a number of branches to cover an area of the sea bed. The interrogator 10 comprises a light source 13 and an optical receiver 14, each coupled to a sensing and control system 15. Light source 13 and optical receiver 14 are coupled to an optical fibre in the riser cable 12 by an optical circulator 16 such that the light source 13 can transmit optical signals into an optical fibre of the riser cable 12 and optical receiver 14 can receive optical signals returning from that fibre. As will be appreciated other optical connection types may be utilised to connect the light source 13 and optical receiver 14 to the optical fibre.

Sensor cable 1 1 comprises a plurality of optical sensors 17a, 17b formed in an optical fibre of that cable to which signals from the interrogator system are guided. Two sensors 17a, 17b are shown in Figure 1 but as will be appreciated the number of sensors may be much larger and will be selected dependent on the requirements of each sensing system. Each sensor comprises a length of fibre coiled into a package and mechanically mounted to be sensitive to certain parameters. In a typical sensor four coils of fibre are formed, with three coils being sensitive to movement (e.g. acceleration) in only one of the x, y, and z axes, and the fourth coil being sensitive to pressure. In a typical sensor the fibre length will be 10 - 50m coiled into a sensor package 5cm long. The structure may be the same as conventional discrete sensors but without the mirrors used to form an interferometer in conventional sensors. The fibre may be continuous through the sensors and connecting cable, or it may be optically fusion spliced to the fibre within the cable The sensor construction can be designed to mechanically increase the sensitivity of the coil to the desired parameter, while minimising sensitivity to other parameters. As an example, an accelerometer can be constructed according to techniques such as described in European Patent No. 1 ,704,394. This sensor is mechanically designed to be sensitive to acceleration along its axis, while being insensitive to acceleration along other axes, or to pressure. That is, the phase sensitivity to the measurand of interest is increased.

Sensors are formed in series along an optical fibre of the sensor cable such that the sensors and interconnections are formed by a single contiguous length of fibre. Compared to conventional discrete optical sensors, optical sensors 17a, 17b are simpler and have reduced cost as no mirror or interferometer structure is required. The lack of such components may remove the requirement for a pressure housing thereby reducing the cost of the sensing system. A further advantage of this sensor system is that the optical loss of the sensors is significantly lower than conventional sensors thereby improving optical loss budgets and allowing greater numbers of sensors to be concatenated. Sensors 17a, 17b are interrogated utilising distributed sensing techniques. In contrast to discrete measurement techniques, distributed sensing techniques allow measurements to be taken at arbitrary locations along a fibre and do not require any particular optical structure. Distributed techniques perform measurements by monitoring light scattered along the length of a fibre as an optical signal propagates along that fibre. One or more pulses of light are launched into an optical fibre and a proportion of the propagating light is scattered as it propagates through the fibre. Various types of scattering occur in optical fibre, including Rayleigh, Brillouin, and Raman. Rayleigh scattering is considered the most appropriate for the type of sensors considered in this disclosure as the magnitude of scattering has a relatively straightforward relation to the physical parameters being sensed and has a low dependence on temperature. As will be appreciated other types of scattering can also be utilised without departing from the general principles described herein. Distributed techniques are typically applied to continuous lengths of fibre where the requirement is to detect disturbance of a distributed fibre length at a low level of sensitivity.

In order to interrogate sensors 17a, 17b pulses of light are launched from light source 13 and propagate to and through sensors 17a, 17b. As the pulses propagate along the fibre they are continuously scattered. A portion of the scattered light is captured within the reverse-direction numerical aperture of the optical fibre and propagates to the receiver 14 via circulator 16.

The time delay between the pulse being transmitted and the scattered light returning to the receiver 14 is dependent upon the distance between the interrogation system and the point of scatter. Since scattering is a continuous, distributed, process a continuous signal will be received at receiver 14. By sampling at defined points in time it is thus possible to sample the scatter from particular locations along the sensor fibre. It is therefore possible to sense the light scattered in the fibre within the sensors by sampling at an appropriate time, and in particular to localise this measurement to the physical location of the discrete sensor Samples of light scattered in the fibre within the sensors can be utilised to measure physical properties to which the sensors 17a, 17b are sensitive.

In a particular sensing technique two optical pulses are launched separated by a time interval T. These pulses may be either of the same optical frequency (a homodyne system) or a different optical frequency (a heterodyne system). Typically a heterodyne system may be preferred and simpler to implement. The light returning at a particular time will consist of light scattered from the first pulse at a first distance from the interrogator, and light scattered from the second pulse at a second distance from the interrogator.

Figure 2 shows two pulses launched at times separated by time T (pulse 1 being launched first). Scatter arriving at a receiver 20 at a particular time will have been scattered from points along the fibre separated by a distance D. Distance D is equal to half of the distance travelled by the light in time T (because pulse 1 travels this distance twice - on the way out and on the return after scattering). Within each pulse there will be multiple reflections from various points within the fibre. The width of the pulse in time should be less than the time taken for the light to transit through the fibres and return. The pulse 21 arriving at the receiver is thus a composite of the two pulses returning from different points along the fibre at different times, and overlapping at the receiver. The instantaneous magnitude of the composite pulse is dependent on the phase differences between reflectors within each pulse and the phase difference due to the distance between the reflectors within both pulses. The phase differences which each pulses are typically small and will be disregarded compared to the phase differences between the 2 pulses, which will dominate the composite response. This is especially true when light is captured from the moment when the first pulse has exited the fibre sensor, and second pulse is yet to enter the fibre sensor, because the fibre between sensors is broadly insensitive. In order to ensure the returning signals are from a single sensor, the fibre length between sensors should be longer than the pulse width. Changes in the effective optical length of fibre between the two reflection points will lead to a change in the optical phase difference between those points, which can be sensed via changes in the composite pulse. Analysis of the composite pulse thus allows sensing of effects occurring in a particular length of fibre.

The sampling rate of each sensor is effectively controlled by the requirement that further pulses cannot be launched into the fibre until all the scattered light has returned from all the sensors along the fibre. This limits the sampling rate and therefore the highest frequency that each sensor can detect, and the dynamic range (the ratio between the amplitude of the smallest and largest detectable signal) at the highest frequency.

The sampling rate can be increased by launching a series of pulses at different optical wavelengths into the fibre (the different wavelengths can be generated by using a series of different lasers, and combining the outputs from the lasers). The pulses should be staggered in time so that each sensor is sampled at different times by the different wavelengths. Also this ensures that the transmitted pulses do not overlap in the fibre (this reduces the potential for non-linear optical effects, if the optical power level becomes too high). The pulses at each optical wavelength will independently be scattered as described above for a single wavelength. If the different wavelength returns are independently demodulated and then added, an effectively higher sampling rate can be obtained - so if there are N wavelengths, the effective sampling arte with be N times the single wavelength sampling rate. A given change in sensor length will lead to different phase change at the different wavelengths, this can be used to increase the dynamic range of the system further.

Another potential issue is polarisation fading, by which the polarisation of the composite returning pulses at each wavelength can vary in a random way, and this means that the interference efficiency is reduced. This can be alleviated by varying the polarisation of the input optical pulses. The input pulse polarisation can be varied using a polarisation controller, which is a well known optical device and exists in various forms. If the polarisation varies between successive pulses at each wavelength, or between wavelengths, then the level of polarisation fading will be different for each set of scatterers, and so for the time averaged signal the polarisation fading will produce a more stable average.

As described above, each sensor 17a, 17b comprises coils of fibre each sensitive to movement in a different direction, or to some other parameter. The pulse separation T and sampling time can be set such that the sampled pulse comprises scattering from the first pulse at the far end of a particular coil of fibre, and scattering from the second pulse at closest end of that coil fibre. The composite pulse thus provides information on the optical phase difference along the length of the coil of fibre and hence can be used to detect changes in the sensor.

Pulse separation T and the sampling time are parameters of the interrogation system and thus can be altered dynamically to sense different coils of fibre within each sensor and within different sensors. It may also be desirable to set T such that more than one coil of fibre within a sensor is interrogated by a particular measurement. The sensing technique described above may also be utilised to interrogate the lengths of the fibre between sensors, or regions thereof. The sensitivity of these fibres may be lower than the sensors described above which are configured to raise their sensitivity by the coiling of lengths of fibre into shorter packages and by the addition of mechanical amplification.

However, useful information may still be obtained from those regions of fibre, especially as they are distributed over a larger spatial extent than the point sensors. This may allow the possibility of spatial integration of signals along the distributed cable. In addition the outputs of the point and sensors and the cable between may be summed optically or electrically to give a composite response, which combines the spatial extent of the cable with the higher sensitivity at the point sensor. To increase the sensitivity multiple fibres within a particular cable may be interrogated and the results summed.

Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages.

Any reference to 'an' item refers to one or more of those items. The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art.

Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.